Asian Origin of Interannual Variations of Summer Climate over the Extratropical North Atlantic Ocean

Ping Zhao * State Key Laboratory of Severe Weather, and National Meteorological Information Centre, China Meteorological Administration, Beijing, China

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Song Yang Climate Prediction Center, NOAA/NWS/NCEP, Camp Springs, Maryland

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Renguang Wu Institute of Space and Earth Information Science, Chinese University of Hong Kong, Hong Kong, China

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Zhiping Wen Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, China

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Junming Chen Chinese Academy of Meteorological Sciences, Beijing, China

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Huijun Wang ** Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

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Abstract

The authors have identified an interannual relationship between Asian tropospheric temperature and the North Atlantic Ocean sea surface temperature (SST) during summer (May–September) and discussed the associated features of atmospheric circulation over the Atlantic–Eurasian region. When tropospheric temperature is high (low) over Asia, positive (negative) SST anomalies appear in the extratropical North Atlantic. This relationship is well supported by the changes in background atmospheric circulation and ocean–atmosphere–land thermodynamic processes. When heat transfer from the land surface to the atmosphere over Asia strengthens, local tropospheric temperature increases and positive temperature anomalies propagate westward from Asia to the North Atlantic, leading to an increase in summer tropospheric temperature over the Atlantic–Eurasian region. Accordingly, a deep anomalous ridge occurs over the extratropical North Atlantic Ocean, with low-level southerly anomalies over the western portion of the ocean. Sensitivity experiments with climate models show that the interannual variations of the North Atlantic–Eurasian atmospheric circulation may not be forced by the extratropical Atlantic SST. Instead, experiments with changing Asian land surface heating capture the above observed features of atmospheric circulation anomalies, westward propagation of tropospheric anomalies, and Atlantic SST anomalies. The consistency between the observational and model results indicates a possible impact of Asian land heating on the development of atmospheric circulation and SST anomalies over the Atlantic–Eurasian region.

Corresponding author address: Dr. Ping Zhao, National Meteorological Information Centre, Beijing 100081, China. E-mail: zhaop@cma.gov.cn

Abstract

The authors have identified an interannual relationship between Asian tropospheric temperature and the North Atlantic Ocean sea surface temperature (SST) during summer (May–September) and discussed the associated features of atmospheric circulation over the Atlantic–Eurasian region. When tropospheric temperature is high (low) over Asia, positive (negative) SST anomalies appear in the extratropical North Atlantic. This relationship is well supported by the changes in background atmospheric circulation and ocean–atmosphere–land thermodynamic processes. When heat transfer from the land surface to the atmosphere over Asia strengthens, local tropospheric temperature increases and positive temperature anomalies propagate westward from Asia to the North Atlantic, leading to an increase in summer tropospheric temperature over the Atlantic–Eurasian region. Accordingly, a deep anomalous ridge occurs over the extratropical North Atlantic Ocean, with low-level southerly anomalies over the western portion of the ocean. Sensitivity experiments with climate models show that the interannual variations of the North Atlantic–Eurasian atmospheric circulation may not be forced by the extratropical Atlantic SST. Instead, experiments with changing Asian land surface heating capture the above observed features of atmospheric circulation anomalies, westward propagation of tropospheric anomalies, and Atlantic SST anomalies. The consistency between the observational and model results indicates a possible impact of Asian land heating on the development of atmospheric circulation and SST anomalies over the Atlantic–Eurasian region.

Corresponding author address: Dr. Ping Zhao, National Meteorological Information Centre, Beijing 100081, China. E-mail: zhaop@cma.gov.cn

1. Introduction

The interaction between oceans and the overlying atmosphere has been considered fundamental to the dynamical processes governing climate variations. In the North Atlantic Ocean, where most of the northward heat transport by Northern Hemispheric oceans occurs (Talley 1984), the atmosphere communicates with deep oceanic water masses through the amount of heat extracted from the ocean by the atmosphere, the freshwater supplied by precipitation, continental runoff, and the melting of sea ice and alters the stability of water column (Kushnir 1994). Elements of the extratropical ocean–atmosphere system in the Atlantic exhibit significant and coherent fluctuations on a broad range of time scales, and documenting and understanding these fluctuations has been a primary goal of climate research.

Bjerknes (1964) investigated the interannual variability of North Atlantic sea surface temperature (SST) and sea level pressure (SLP) to understand the nature of local ocean–atmosphere interaction. Interannual variability of SST is governed by a response of the oceanic mixed layer to the local exchange of heat with the atmosphere. The anomalous warm water to the north of 40°N is associated with anomalous high SLP and warm air, while the anomalous cold water to the south (20°–30°N) accompanies anomalous low SLP and cold air. These features were further confirmed by Kushnir (1994). The spatial scales of interannual SST anomalies and their relationships to atmospheric anomalies were also investigated by other studies (e.g., Wallace et al. 1990; Cayan 1992a, b). The correspondence observed between wintertime SST and SLP variability patterns supports the theory that interannual SST fluctuations are forced by the changing pattern of atmospheric circulation (Wallace et al. 1990; Cayan 1992a) and that large-scale atmospheric anomalies lead to oceanic anomalies, consistent with the concept of initial atmospheric forcing on oceans (Zorita et al. 1992). Cayan (1992b) has extensively examined the relationship between surface heat flux and SST anomalies and found significant correlation between heat flux and extratropical ocean temperature tendency. Battisti et al. (1995) have also forced ocean models with observed anomalous surface conditions in the extratropical Atlantic and concluded that surface heat flux forcing could explain most of the oceanic interannual variability. Huang and Shukla (1996, 1997) simulated the characteristics of the interannual variability in the Atlantic using a general circulation model. On the other hand, while being responsive to the changes in the atmosphere, oceans exhibit an inherently long time-scale variation due to their large heat capacity and inertia. This long-term scale variation in turn leaves marked signature on atmospheric circulation. Changes in SST due to ocean dynamics (advection) have been considered important for climate anomalies of decadal or longer time scales (e.g., Sutton and Hodson 2005; Dong et al. 2006; Lu et al. 2006; Wang et al. 2009; Feliks et al. 2011). These studies have implied a difference in Atlantic atmosphere–ocean interaction between interannual and decadal time scales. The interannual variability of the extratropical Atlantic SST is possibly a response to local atmospheric circulation anomalies. Where do the atmospheric circulation anomalies over the extratropical Atlantic Ocean originate?

Previous studies have investigated the origins of atmospheric circulation anomalies over the Atlantic. Latif et al. (2000) and Hoerling et al. (2001) found that the oceanic forcing of the long-term change in the North Atlantic Oscillation (NAO) was in the tropical Pacific and the Indian Oceans. Using atmospheric general circulation models, Hoerling et al. (2001, 2004), Bader and Latif (2003), and Hurrell et al. (2004) documented that the progressive warming of the Indian Ocean might be a principal contributor to the recent change in NAO. Bader and Latif (2005) further used a coupled ocean–atmosphere model to examine the impact of winter Indian Ocean SST anomalies on the dominant pattern of atmospheric variability in the North Atlantic through the South Asian jet stream, which acts as a waveguide of the Northern Hemisphere circum-global teleconnection. Li et al. (2006, 2010) investigated the extratropical Northern Hemisphere atmospheric responses to the Indian Ocean SST. They found that the Indian Ocean warmth could force the positive polarity phase of the NAO and the northern annular mode. Moreover, Foltz and McPhaden (2008) suggested an impact of the interannual variability of Saharan dust content on the tropical North Atlantic SST. Thus, the Indian Ocean and the North African land might be the origins of the atmospheric circulation anomalies over the Northern Atlantic Ocean.

Moreover, intrinsic relationships in atmospheric circulation occur between Asia and other areas (Rodwell and Hoskins 1996, 2001; Webster and Fasullo 2003; Gong and Ho 2003; Fasullo 2004). Branstator (2002) and Zhao et al. (2007) showed that a disturbance generated over the southern Tibetan Plateau could widely reach separated points within the jet stream waveguide during winter, spring, and summer. Ding and Wang (2005) found a summertime atmospheric teleconnection pattern that links the variations of rainfall and temperature among west Europe, European Russia, India, East Asia, and North America. Recently, Linderholm et al. (2011) presented an interannual relationship between the East Asian monsoon and Atlantic climate during summer. Zhao et al. (2010a, 2011) identified a Northern Hemisphere teleconnection between Eurasia and the North Pacific–Atlantic sector through which the heating over Asia modulates the ocean–atmosphere interaction over the Pacific. Numerical simulations have also suggested the important impacts of heating over Asia (including the Tibetan Plateau) on the Northern Hemisphere atmospheric circulation (e.g., Ose 1996; Song et al. 2010; Zhao et al. 2011). These results have indicated a close link in atmospheric circulation between Asia and the Atlantic Ocean. Will an Asian climate anomaly influence the Atlantic atmospheric circulation and in turn SST through this link? If yes, what is the physical process responsible for this impact? This study addresses the above issues about the relationship between Asia and extratropical North Atlantic on interannual time scales and explores the Asian origin of the Atlantic atmospheric anomalies to gain a better understanding of the ocean–atmosphere–land interaction over the Atlantic–Eurasian sector and associated mechanisms.

The main features of datasets, analysis methods, and models are described in section 2. In section 3, we depict the link between Asian temperature and Atlantic SST. In section 4, we explain the physics for this link using both observational data and output from model experiments. The summary and discussion are provided in section 5.

2. Data, model, and analysis methods

We apply the 1958–2001 monthly data from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40; Uppala et al. 2005), the 1870–2001 global monthly SST from the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST; Rayner et al. 2003), and the 1850–2001 SLP analysis data of the Hadley Centre (HadSLP2; Allan and Ansell 2006). The ERA-40 is in a horizontal resolution of 2.5° × 2.5° latitude and longitude, the HadISST dataset has a horizontal resolution of 1° × 1°, and the HadSLP2 SLP data has a horizontal resolution of 5° × 5°.

The National Center for Atmospheric Research (NCAR) Community Atmospheric Model, version 3 (CAM3; Collins et al. 2004), is forced by prescribed SST to investigate the possible impacts of Atlantic SST and Asian land heating on atmospheric circulation. The NCAR Community Climate System Model, version 3 (CCSM3), which is a coupled model, is also used in this study. The CCSM3 includes CAM3 as its atmospheric component, an ocean component, a land model, and a sea ice model (Collins et al. 2006), which is used to understand the impact of Asian land heating on the anomalies of Atlantic–Eurasian atmospheric circulation and Atlantic SST in this study. Previous studies have shown that the CAM3 and CCSM3 can capture the major features of global atmospheric circulation and climate and that these models have been applied extensively to simulate the Northern Hemispheric circulation and climate (e.g., Hack et al. 2006; Meehl et al. 2006a,b; Deser and Phillips 2009; Kim and Wang 2007; Zhao et al. 2010a, 2011).

The technique of singular value decomposition (SVD) is conducted to detect a relationship between two variables. Correlation and composite analyses are also applied to examine the relationships between different variables. The statistical significance of correlation coefficients, composite differences, and nonzero trends is assessed at the 95% confidence level (Student’s t test) unless otherwise stated. In this study, the means of May–June–July–August–September (MJJAS) are used to represent summer conditions.

3. Relationship between Asian temperature and Atlantic SST

Tropospheric temperature over Asia (the North Pacific) was persistently high (low) before 1980 and low (high) from the 1980s to early 1990s, exhibiting a strong negative (positive) trend during 1958–2001 (figures not shown). To detect the relationship between Asian tropospheric temperature and Atlantic SST on interannual time scales, we remove these linear trends, which may be associated with global warming (Zhao et al. 2010b), and analyze the detrended MJJAS tropospheric temperature in the following analyses. Since the average altitude of the Tibetan Plateau exceeds 4000 m, we use the 500–200-hPa vertically averaged temperature to measure Asian tropospheric temperature. We first perform an SVD analysis on both the MJJAS 500–200-hPa mean temperature over the Northern Hemisphere (0°–90°N, 180°–180°) and the simultaneous detrended SST over 0°–70°N, 80°W–0° during 1958–2001. It is found that the leading SVD mode (Figs. 1a,b) explains 52% of the total variance for SST and 45% for tropospheric temperature. The squared covariance fraction between the leading tropospheric temperature and SST modes accounts for 77% of the total covariance. The correlation between the time series of tropospheric temperature and SST modes (Fig. 2a) is 0.71, significantly exceeding the 99.9% confidence level. Figures 1a,b display the leading SST and tropospheric temperature modes, respectively. When there are large positive SST anomalies over 40°–60°N of the Atlantic Ocean (Fig. 1a), positive tropospheric temperature anomalies occur over the extratropical Northern Hemisphere, with maximum values over Asia (Fig. 1b). This relationship suggests a close link between the Asian tropospheric temperature and the extratropical North Atlantic SST. Compared to the leading SVD mode, the second mode shows different patterns. The squared covariance fraction of the second mode accounts for 13% of the total covariance. For the second SST mode, there are positive SST anomalies over the North Atlantic between 30° and 50°N and over the Arctic to the north of 60°N, with negative anomalies over the tropical North Atlantic (Fig. 1c). The positive values of the second SVD tropospheric temperature mode mainly appear over the extratropics of Eurasia and northern Africa and over the Arctic, while negative values mainly appear over the mid–low latitudes from the North Pacific to North America, and the Atlantic (Fig. 1d). Evidently, these centers of the second SVD mode almost correspond to the small positive or negative values of the most leading mode, showing a difference between the two modes.

Fig. 1.
Fig. 1.

Leading SVD modes of SST (a) and tropospheric temperature (b) during 1958–2001. (c),(d) As in (a),(b), but for the second modes.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Fig. 2.
Fig. 2.

(a) Times series of leading SVD tropospheric temperature (red) and Atlantic SST (blue) modes during 1958–2001. (b) Standardized time series of detrended MJJAS ATT (red) and Atlantic SST (blue) indices, in which the ATT index is referred to as the MJJAS Asian tropospheric (500–200 hPa) temperature averaged over 25°–50°N, 60°–120°E and the Atlantic SST index is defined as the MJJAS SST averaged over 38°–50°N, 80°–40°W.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

To measure the variability of summer tropospheric thermal condition over the Asian continent, referring to the position of positive anomalies in Fig. 1b, we define an index using the MJJAS Asian tropospheric (500–200 hPa) temperature averaged over 25°–50°N, 60°–120°E (see Fig. 3a), referred to as the Asian tropospheric temperature (ATT) index. The ATT index (Fig. 2b) has a significant correlation of 0.85 with the time series of the leading tropospheric temperature SVD mode during 1958–2001. Such a high correlation suggests the robustness of the ATT index for measuring the variations of the dominant mode. To depict the variation of atmospheric circulation associated with the ATT index, we select seven high ATT-index (HI) years (1958, 1960, 1961, 1994, 1998, 1999, and 2000) and seven low ATT-index (LI) years (1965, 1972, 1974, 1976, 1982, 1987, and 1992) based on the MJJAS ATT index displayed in Fig. 2b. The ATT-index anomalies in all HI and LI cases are beyond one standard deviation. In these extreme years, consistent anomalies are seen in the time series of the leading tropospheric temperature mode (Table 1). When the high and low value cases are selected based on the time series of the leading tropospheric temperature mode, five different years (1959, 1968, 1986, 1991, and 1993) are identified. In these five years, however, the signs of both the ATT index and the time series of the leading mode are consistent. Thus, it is appropriate to use the ATT index to represent the variations of the leading SVD tropospheric temperature mode.

Fig. 3.
Fig. 3.

(a) Composite difference in 250-hPa temperature (°C) between HI cases and LI cases. (b) As in (a), but for longitude–height cross section along 40°N. (c) As in (a), but for surface air temperature. In (a), the box represents the region for Asia and shaded areas indicate the values significantly exceeding the 95% confidence level.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Table 1.

The seven highest and seven lowest years with the normalized MJJAS ATT index beyond one standard deviation σ during 1958–2001 and the corresponding time series of the leading SVD tropospheric temperature mode and the corresponding extratropical Atlantic SST index.

Table 1.

Composite analysis of the summer 250-hPa temperature shows positive temperature anomalies over Eurasia in the HI cases, with central values exceeding 1.2°C over northeastern Asia, and negative anomalies over extratropical east Europe and Asia in the LI cases, with central values below −1.2°C (figures not shown). On the composite difference map of 250-hPa temperature between the HI and LI cases (Fig. 3a), significant positive anomalies exceeding 1°C cover most of the Eurasian land. These positive anomalies are observed in the entire troposphere (Fig. 3b), with maximum values exceeding 1.5°C in the mid- and upper troposphere. Positive temperature anomalies also appear at the surface in most of Asia, with maximum values of about 2°C (Fig. 3c). Thus, high (low) values of the ATT index measure high (low) Asian tropospheric and surface temperatures.

Corresponding to the variation of the ATT index, there are significant anomalies in the Atlantic SST. Figure 4a shows the composite difference in North Atlantic SST between the HI and LI cases. Significant positive anomalies are observed in the central-western North Atlantic to the north of 25°N, with anomalies exceeding 1°C between 45° and 55°N. Meanwhile, the positive anomalies of SST also correspond to a large standard deviation above 0.6°C (Fig. 4b). We define the summer detrended SST averaged over 38°–50°N, 80°–40°W, an area within the positive SST anomalies in Fig. 4a, as an index of extratropical Atlantic SST (see Fig. 2b). The Atlantic SST index has a highly significant correlation with the time series of the leading Atlantic SST mode during 1958–2001 (R = 0.59, significant at the 99.9% confidence level). The ATT index and the Atlantic SST index also vary similarly, with a correlation coefficient of 0.50 during 1958–2001, significant at the 99.9% confidence level. Corresponding to the seven HI (seven LI) cases of the ATT index, there are positive (negative) anomalies of the Atlantic SST index in 7 (5) years (Table 1). For 1972 and 1976 (LI cases of the ATT index), SST anomalies are positive, but smaller than those of the HI cases. The above results further support a positive correlation between the ATT and Atlantic SST indices.

Fig. 4.
Fig. 4.

(a) Composite difference in MJJAS SST (°C) between HI cases and LI cases. (b) Standard deviation of MJJAS SST (×0.1°C). (c) Time–longitude cross section of composite difference in SST (°C) between HI cases and LI cases along 45°–55°N. (d) As in (c), but for the regression of MJJAS SST against the detrended Atlantic SST index during 1958–2001. Shaded areas indicate the values significantly exceeding the 95% confidence level and in (a), thick dashed lines indicate the area of the extratropical Atlantic Ocean.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

To examine the possible persistence of the positive SST anomalies, we analyze the longitude–time cross section of composite SST difference along 45°–55°N (Fig. 4c). There is hardly any large-scale significant anomaly of SST in the previous winter, and significant positive anomalies of SST above 1°C does not appear near 45°W until May. Figure 4d shows the longitude–time cross section of the regression of SST against the MJJAS Atlantic SST index during 1958–2001. Significant positive anomalies also mainly begin in May, being more extensive gradually, and become insignificant since October. These features are consistent with those shown in Fig. 4c and with the results of Bhatt et al. (1998), who showed that the anomalies of North Atlantic surface air and ocean temperatures during late winter are weakly correlated with those during the following summer. The weak link of summer SST with previous winter SST implies that the summer SST anomalies associated with the ATT index do not necessarily originate from the previous winter.

In short, the above results from SVD, correlation, and composite analyses show that, on interannual time scales, the variability of Asian tropospheric temperature is closely associated with that of extratropical North Atlantic SST. When Asian temperature is high (low), the Atlantic SST is also high (low). What physical processes are responsible for this relationship?

4. Physical explanation for relationship between Asian temperature and Atlantic SST

a. Asian surface heating and tropospheric temperature

Figure 5 shows the regression of heat transfer Hs from the surface to the atmosphere against the ATT index. The heat transfer, calculated from the ERA-40 reanalysis, consists of sensible heat flux, latent heat flux, and radiation at the surface. Positive values of Hs appear over a large part of Asian land, between 25° and 50°N, indicating strengthened heat transfer from the surface to the atmosphere. Scattered negative anomalies are seen over Europe and North Africa. Thus, an increase in Asian tropospheric temperature seems to result from the increase in local surface heat transfer, instead of the surface heat transfer over Europe and North Africa. Model simulations in section 4d will further demonstrate that Asian surface heating anomalies can force significant variations in the Northern Hemispheric tropospheric temperature.

Fig. 5.
Fig. 5.

Regression of MJJAS Hs (W m−2) against the ATT index. Shaded areas indicate the values significantly exceeding the 95% confidence level.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

b. Atmospheric circulation anomalies associated with Asian tropospheric temperature

Previous studies have concluded that a tropospheric disturbance over the Tibetan Plateau may be connected with the hemispheric atmospheric anomalies along the Asian westerly jet stream waveguide during winter, spring, and summer (e.g., Branstator 2002; Zhao et al. 2007). As pointed out by Branstator (2002), the jet stream can influence the organization of atmospheric variations by producing zonally elongated disturbance patterns. Within the Asian westerly jet stream waveguide, a disturbance generated over Asia can reach widely separated points. For example, through this effect of the jet stream waveguide, the perturbation of the Asian jet stream is carried into the North Atlantic sector and leads to a NAO-like response (Bader and Latif 2005). Following the previous studies, we examine the propagation of Asian tropospheric temperature anomalies along the summertime westerly jet stream over Eurasia.

Figure 6a, which shows the climatology of summer zonal wind at 250 hPa, indicates that westerlies exceeding 15 m s−1 are mainly located over the extratropical Northern Hemisphere between 30° and 55°N, with maximum values exceeding 25 m s−1. Comparing Fig. 6a with Fig. 3a reveals that over the North America–Eurasia region, significant positive centers of tropospheric temperature anomalies occur near the westerly jet stream, suggesting a zonally covarying feature along the westerlies. Figure 6b shows the time–longitude cross section of composite difference in 250-hPa temperature between the HI and LI cases along 25°–50°N. Significant positive anomalies exceeding 1°C exhibit a westward propagation from Eurasia in May–September, and the contour of 1°C extends westward to 120°W via the Atlantic Ocean in September. Similar features are observed at other levels of the upper troposphere.

Fig. 6.
Fig. 6.

(a) Climatology of MJJAS 250-hPa zonal wind (m s−1; shaded areas are ≧ 15 m s−1). (b) Time–longitude cross section of composite difference in 250-hPa temperature (°C) along 25°–50°N (shaded areas indicate the values significantly exceeding the 95% confidence level).

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Zhao et al. (2007) have showed that tropospheric anomalies with a striking wavenumber-2 feature often exhibit a westward propagation during spring and summer and this westward propagation may be explained by the westward propagations of phase velocity and anomalous wave energy in the midlatitudes. Results similar to these features are obtained in this study. For example, Fig. 7a, which presents the composite difference in MJJAS 250-hPa geopotential height between the HI and LI cases along 25°–50°N, shows that the geopotential height anomalies exceeding 35 m mainly appear between 0° and 140°E and the anomalies below 35 m mainly appear over the other parts of the Northern Hemisphere, indicating a striking wavenumber-1 feature. A spectrum analysis on the spatial series of the geopotential height anomalies in Fig. 7a further demonstrates the predominant peak at wavenumber 1 (significant at the 99% confidence level). The horizontal group velocity Cg of a three-dimensional stationary wave can be defined as (Plumb 1985)
eq1
Here, Fh=(Fx, Fy), with
eq2
All parameters in the above equations are the same as in Plumb (1985). When the direction of Cg is toward the east (west), wave energy propagates eastward (westward), favoring an eastward (westward) migration of the wave train. Following Plumb (1985), our calculation shows that the monthly and zonal mean values of buoyancy frequency are always positive in the mid- and upper troposphere. Accordingly, is positive, and the direction of Cg is determined by that of Fh. Figure 7b shows the July–August mean 250-hPa Fh. It appears that the direction of Fh is almost toward the west over the extratropical Atlantic–Eurasian sector, which indicates a westward propagation of anomalous wave energy, supporting the above claim of westward propagation of the tropospheric anomalies shown in Fig. 6b.
Fig. 7.
Fig. 7.

(a) Composite difference in MJJAS 250-hPa geopotential height (m) along 25°–50°N. (b) Smoothed July–August mean 250-hPa Fh (×10−12 s−3).

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Therefore, the Eurasian tropospheric temperature anomalies may be connected to the North Atlantic atmospheric circulation variations along the westerly jet stream, and the westward propagation of the positive anomalies results in an increase of tropospheric temperature over the North America–Atlantic region. As seen later from section 4d, this westward propagation may be forced by Asian surface heating anomalies. Accordingly, significant positive anomalies appear in the troposphere over North America and the Atlantic, with significant positive anomalies above 0.8°C over the North Atlantic between 20° and 50°N (Fig. 3a). Thus the tropospheric temperature over North America and the Atlantic is high (low) when Asian tropospheric temperature is high (low), suggesting a consistently varying feature of tropospheric temperature between Asia and the North America–Atlantic sector.

Figure 8a shows the longitude–height cross section of composite difference in geopotential height between the HI and LI cases along 35°N. Positive anomalies appear nearly in the entire troposphere, with maximum values near 150 hPa. Particularly, significant anomalies exceeding 30 m cover most of the Northern Hemisphere between 20° and 60°N at this level, with a central value of 60 m mainly over Eurasia (Fig. 8b). The values exceeding 40 m appear over the extratropical North Atlantic. The similar positive anomalies are also observed in the other levels of the troposphere over the extratropical North Atlantic (figure not shown). Figure 8c shows the composite difference in summer 1000-hPa winds. An anomalous anticyclonic center covers much of the extratropical North Atlantic, with its center near 39°N, consistent with the local tropospheric positive height anomalies, and southerly wind anomalies to the west of the anomalous anticyclonic center emerge from the central-western Atlantic between 35° and 60°N. Since a high pressure system is located over the subtropical Atlantic climatologically, the low-level anomalous anticyclonic circulation actually indicates the enhanced subtropical high pressure system over the ocean. These results indicate that on interannual time scales the local positive SST anomalies mainly corresponds to a deep anomalous high pressure system over the extratropical North Atlantic when the ATT index is high.

Fig. 8.
Fig. 8.

(a) Longitude–height cross section of composite difference in geopotential height (m) between HI cases and LI cases along 35°N. (b) As in (a), but for 150-hPa geopotential height (m). (c) As in (a), but for 1000-hPa winds (m s−1; vector), in which A and C indicate anomalous anticyclonic and cyclonic circulation centers, respectively. (d) Regression of MJJAS HadSLP2 SLP against the simultaneous detrended Atlantic SST index during 1958–2001. In (a),(b), and (d), shaded areas indicate the values significantly exceeding the 95% confidence level.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

The above-described SST–pressure relationship associated with the ATT index is consistent with the feature of regional ocean–atmosphere interaction in the Atlantic. On the regression map of MJJAS 1000-hPa winds against the Atlantic SST index for 1958–2001 (figure not shown), a large-scale anomalous anticyclone is found over the extratropical Atlantic, with anomalous southerly wind over the western Atlantic, while a weak anomalous cyclone is observed to the south of the anomalous anticyclone. This warm SST–ridge relationship can also be seen in Fig. 8d. This figure exhibits a positive correlation between SLP and SST over the extratropical North Atlantic. Clearly, the lower-tropospheric atmospheric circulation features associated with the Atlantic SST index are similar to those shown in Fig. 8c. This warm SST–ridge relationship is also similar to the results of Bjerknes (1964), Wallace et al. (1990), and Kushnir (1994), who have showed that a warm SST anomaly often underlies an anomalous high pressure system over the extratropical Atlantic during winter. In this study, we further demonstrate that the warm SST–ridge relationship is associated with the Asian thermal condition in summer. Does this relationship reflect an impact of the Atlantic SST on the atmospheric circulation over the Atlantic–Eurasian sector?

c. Modeling impact of summer SST on atmospheric circulation over the Atlantic

Here we conduct two experiments using the CAM3 model to investigate the possible impact of Atlantic SST anomalies on atmospheric circulation for summer. One experiment is CAM3_Control, in which forcing is the monthly climatological SST (figures not shown) from the original CAM3 model (Collins et al. 2004) and is consistent with the monthly climatological SST of the HadISST data. The other is CAM3_Atlantic, which is similar to CAM3_Control, but with an increase in SST by 1°C over 38°–50°N, 80°–40°W (shown in Fig. 4a). For both experiments, the model is integrated for 50 yr and results for the last 30 years are analyzed. The 30-yr mean values are equivalent to the values from an ensemble of 30 sensitivity experiments with changes in SST in the extratropical Atlantic by means of different initial atmospheric and land surface conditions.

Corresponding to the high MJJAS SST over the extratropical Atlantic in the CAM3_Atlantic, positive anomalies of 1000-hPa temperature appear over the extratropical Atlantic, with a central value of 2°C (Fig. 9a). These temperature anomalies indicate heated lower troposphere over the extratropical Atlantic in experiment CAM3_Atlantic. Also, as a response to the warm SST, significant positive SLP anomalies appear over the northwestern Atlantic and large-scale significant negative SLP anomalies appear to the south of 45°N (figure not shown), with an anomalous cyclonic pattern over the extratropical Atlantic (Fig. 9b). Accordingly, anomalous northeasterly winds prevail over the Atlantic between 35° and 50°N, which is different from the observed southerly wind anomalies (shown in Fig. 8c). These features indicates that a “warm SST–trough” type of response dominates over the mid- and lower latitudes of the North Atlantic, similar to the results of Conil and Li (2005), Sutton and Hodson (2005), Dong et al. (2006), Lu et al. (2006), and Deser et al. (2007). Their studies showed that a direct response of lower-tropospheric atmosphere to positive (negative) Atlantic SST anomalies is mainly the development of an anomalous low (high) pressure system over the North Atlantic. They also showed that a vertically baroclinic structure is accompanied by an anomalous ridge (trough) aloft. These features are different from the observed warm SST–ridge link near the surface over the extratropical Atlantic as shown in Figs. 4a and 8. This difference between observations and models suggests that the warm SST–ridge link over the Atlantic is not mainly forced by local SST anomalies. Figure 9c further shows the time–longitude cross section of composite difference in 250-hPa temperature between CAM3_Atlantic and CAM3_Control. It can be seen that corresponding to the positive Atlantic SST anomalies, significant positive anomalies appear mainly in the local troposphere and the modeled temperature anomalies do not propagate westward from Asia, which is different from the observed feature shown in Fig. 6b.

Fig. 9.
Fig. 9.

(a) Composite difference in MJJAS 1000-hPa temperature (°C) between CAM3_Atlantic and CAM3_Control. (b) As in (a), but for 1000-hPa winds (m s−1; vector), in which C indicates an anomalous cyclonic circulation center. (c) As in (a), but for time–longitude cross section of 250-hPa temperature along 25°–50°N. In (a) and (c), shaded areas indicate the values significantly exceeding the 95% confidence level.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

The apparent differences between model experiments and observations indicate that the observed atmospheric anomalies over the Atlantic–Eurasian region discussed in section 4b may not be necessarily forced by extratropical Atlantic SST anomalies. In the next section, we further investigate whether the heating over Asian land exerts an influence on the Atlantic ocean–atmosphere interaction.

d. A possible impact of Asian heating on the Atlantic ocean–atmosphere interaction

Reduction of surface albedo increases surface air temperature in climate models and this feature has been used to understand the impact of strengthened land surface heating on atmospheric circulation (Wang et al. 2008). Here, we carry out two experiments (CCSM3_Control and CCSM3_Asia) to investigate the possible impact of Asian land heating on the atmospheric circulation over the Atlantic–Eurasian region and the SST in the Atlantic. One experiment, CCSM3_Control, is an unforced experiment (or a free run) with the ocean–atmosphere–land coupled CCSM3 model (Collins et al. 2006). The other experiment, CCSM3_Asia, is the same as experiment CCSM3_Control but with a change in surface vegetation over Asia. Referring to the distribution of surface heating anomalies over Asia (in Fig. 5), the surface vegetation type at each grid of Asia (25°–50°N, 60°–120°E; see Fig. 10a) is set to the needle leaf evergreen temperate tree from the original type of grass, or shrub, or bare area in experiment CCSM3_Control. Thus, there is a lower albedo value over Asia in experiment CCSM3_Asia relative to experiment CCSM3_Control. It will soon be seen that this change in vegetation type strengthens land surface heating to the atmosphere through adjusting vegetation albedo. For each experiment, the CCSM3 model is integrated for 100 years and the mean values over the last 50 years are used in the analysis.

Fig. 10.
Fig. 10.

(a) Composite difference in MJJAS Hs (W m−2) between CCSM3_Asia and CCSM3_Control. (b) As in (a), but for surface air temperature (°C). (c) As in (a), but for temperature (°C) along 40°N. Shaded areas indicate the values significantly exceeding the 95% confidence level.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Figure 10a shows the composite difference in MJJAS Hs between CCSM3_Asia and CCSM3_Control. Significant positive values exceeding 2 W m−2 appear mainly over extratropical Asia, with a maximum value of 4 W m−2. This result indicates a significant increase in heat transfer from the surface to the atmosphere over this region in CCSM3_Asia relative to CCSM3_Control. That is, when the Asian surface vegetation type is set to be the needle leaf evergreen temperate tree, Hs is larger over Asia, consistent with the result of Wang et al. (2008), and it is comparable to that shown Fig. 5.

Corresponding to the increased Hs over Asia, significant positive anomalies of surface air temperature exceeding 2°C appear over extratropical Asia (Fig. 10b), indicating an increase in temperature in CCSM3_Asia relative to CCSM3_Control. Positive temperature anomalies are also observed in the troposphere as seen in Fig. 10c, which shows composite difference in tropospheric temperature between CCSM3_Asia and CCSM3_Control along 40°N. Positive temperature anomalies appear in the entire troposphere, with a maximum value of 4°C in the mid–upper troposphere over Asia. At 250 hPa, significant positive temperature anomalies exceeding 2°C cover the extratropics of Eurasia and North Africa (figure not shown). These simulated anomalies are generally similar to the observed features shown in Fig. 3.

Figure 11 shows the longitude–time cross section of composite difference in 250-hPa temperature between CCSM3_Asia and CCSM3_Control along 25°–50°N. Significant positive anomalies propagate westward from Asia to the Atlantic in May–August. The 1°C contour reaches the Atlantic in July, generally consistent with the feature shown in Fig. 6b. It is evident that the atmospheric anomalies forced by Asian surface heating anomalies exhibit a westward propagation, indicating that the observed westward propagation in Fig. 6b may be forced by the Asian land heating.

Fig. 11.
Fig. 11.

Longitude–time cross section of composite difference in 250-hPa temperature (°C) between CCSM3_Asia and CCSM3_Control along 25°–50°N. Shaded areas indicate the values significantly exceeding the 95% confidence level.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Figure 12a shows the composite difference in MJJAS 150-hPa geopotential height between CCSM3_Asia and CCSM3_Control. Significant positive anomalies appear over much of the midlatitudes, with maximum values over Asia. Meanwhile, positive geopotential height anomalies also appear in the entire troposphere over the extratropical North Atlantic (figures not shown). At the lower troposphere, an anomalous high pressure (Fig. 12b) and an anomalous anticyclonic circulation pattern appears over the extratropical Atlantic, with its center near 50°N, 30°E (Fig. 12c). These results show that a deep anomalous high dominate the extratropical Atlantic in CCSM3_Asia relative to CCSM3_Control. Accordingly, northeasterly wind anomalies appear in a more southwestward position, compared to Fig. 9b, and southerly wind anomalies prevail over the extratropical western Atlantic.

Fig. 12.
Fig. 12.

As in Fig. 10, but for (a) 150-hPa geopotential height (m), (b) SLP (hPa), and (c) 1000-hPa winds (m s−1). In (c), A indicates the anomalous anticyclonic circulation center.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Because the CCSM3 model is an ocean–atmosphere coupled model and experiment CCSM3_Asia includes a feedback of ocean–atmosphere interaction, we further conduct experiment CAM3_Asia using the CAM3 model with prescribed SST to separate the effect of Asian land heating from ocean–atmosphere interaction. Experiment CAM3_Asia is the same as experiment CAM3_Control, but with a change in Asian surface vegetation like experiment CCSM3_Asia. In experiment CAM3_Asia, the CAM3 model is also integrated for 50 years and the mean values over the last 30 years are analyzed. Significant positive anomalies of surface air temperature between CAM3_Asia and CAM3_Control appear over extratropical Asia (figure not shown), indicating a direct response to the local surface heating anomalies, which is consistent with both the results from the CCSM3 model and the observed features. Meanwhile, positive temperature anomalies also appear over the entire Northern Hemisphere, with temperature anomalies exceeding 2°C in the entire troposphere over Asia (Fig. 13a). Significant positive anomalies of geopotential height occur over much of the midlatitudes in the upper troposphere, with maximum values over Asia (Fig. 13b). Positive height anomalies also occur in the entire troposphere over the extratropical North Atlantic (figure not shown), with a low-level anomalous high pressure over the extratropical Atlantic (Fig. 13c). It is evident that these simulated atmospheric circulation anomalies with the CAM3 model are generally consistent with the features simulated by the CCSM3 model, and with the observed features. Because there is no change in SST between experiments CAM3_Asia and CAM3_Control, this analysis suggests an effect of Asian land heating on the atmospheric circulation over the North Atlantic. That is, the atmospheric circulation anomalies over the Atlantic shown in Fig. 12 are forced by Asian land heating.

Fig. 13.
Fig. 13.

(a) Longitude–height cross section of composite difference in MJJAS temperature (°C) between CAM3_Asia and CAM3_Control along 40°N. (b) As in (a), but for 150-hPa geopotential heights (m). (c) As in (a), but for SLP (hPa). Shaded areas indicate the values significantly exceeding the 95% confidence level.

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Since shortwave radiation and sensible and latent heat flux anomalies at the surface might contribute substantially to the anomalies of Atlantic SST (Battisti et al. 1995; Cayan 1992b; Foltz and McPhaden 2006a,b), we analyze the anomalies of model shortwave radiation and sensible and latent heat fluxes at the surface over the Atlantic. Figure 14a shows the composite difference in surface net shortwave radiation flux between CCSM3_Asia and CCSM3_Control and reveals significant negative anomalies of surface net shortwave radiation over most of the Atlantic north of 30°N, which indicates the heat flux from the atmosphere to the ocean surface and may contribute to an increase in the local SST. Meanwhile, negative surface sensible and latent heat flux anomalies also generally cover the extratropical North Atlantic (Fig. 14b), indicating the downward heat flux, which also plays an important role in increasing the local SST. Accordingly, positive anomalies of SST exceeding 1°C occur in the extratropical North Atlantic (Fig. 14c), which is comparative with the standard deviation of the Atlantic SST shown in Fig. 4b. These results are consistent with the features observed by previous studies (e.g., Battisti et al. 1995; Foltz and McPhaden 2006a,b).

Fig. 14.
Fig. 14.

As in Fig. 10, but for (a) surface net shortwave radiation flux (W m−2), (b) surface sensible and latent heat flux (W m−2), and (c) SST (°C).

Citation: Journal of Climate 25, 19; 10.1175/JCLI-D-11-00617.1

Therefore, the features of horizontal distribution, propagation, and vertical structure of the atmospheric circulation anomalies over the Atlantic–Eurasian region (Figs. 1012) and the SST anomalies in the extratropical Atlantic (Fig. 14c) produced by CCSM3_Asia are similar to the observed features associated with the Asian tropospheric temperature anomalies (Figs. 3, 4a, 5, 6b, and 8). The similarity between the model results and observations exhibits the capability of the CCSM3 in simulating the ocean–atmosphere–land interaction over the Atlantic–Asian sector, also suggesting that the relationship between Asian temperature and Atlantic SST shown in section 3 may reflect an impact of the heating over Asia on the Atlantic climates.

5. Summary and discussion

Using monthly data from the ERA-40 and HadISST SST datasets and applying the SVD, correlation, and composite analysis methods, we have identified a summertime relationship between Asian tropospheric temperature and Atlantic SST on interannual time scales and investigated the associated variations of atmospheric circulation over the Atlantic–Eurasian region. The first SVD component shows a coupled mode between Asian tropospheric temperature and North Atlantic SST on interannual time scales. When the tropospheric temperature is high (low) over Asia in summer, there are synchronously significant positive (negative) anomalies of SST in the extratropical Atlantic Ocean. The SST anomalies associated with Asian tropospheric temperature mainly develop in summer and cannot be observed in the previous winter. Moreover, high Asian tropospheric temperature is closely related to an increase in local heat transfer from the land surface to the atmosphere. Tropospheric atmospheric anomalies have a zonal wavenumber-1 structure, propagating westward from Asia to the Atlantic over the extratropical Atlantic–Eurasian region with a westward propagation of anomalous wave group velocity, which may cause an increase in tropospheric temperature over the Atlantic–European sector and a deep anomalous high in the troposphere over the Atlantic. That is, the ocean–atmosphere interaction over the extratropical Atlantic exhibits a “warm SST–ridge” relationship.

Sensitivity experiments of changing summertime Atlantic SST in the CAM3 model do not produce the observed atmospheric circulation anomalies over the Atlantic. In the model, positive anomalies of extratropical Atlantic SST mainly generate an anomalous low or cyclone, instead of an anomalous high or anticyclone, over the extratropical Atlantic Ocean. This feature is consistent with the response of “warm SST–low pressure” type revealed by Sutton and Hodson (2005) and is different from the observed in this study. Meanwhile, the Atlantic SST anomalies do not produce large-scale significant atmospheric anomalies over extratropical Asia or a westward propagation of tropospheric anomalies from Asia to the Atlantic as found in the observation. These differences between model and observation suggest a weaker impact of Atlantic SST on the summer atmospheric circulation over the Atlantic–Eurasian sector. That is, the variations of summer Atlantic–Eurasian atmospheric circulation associated with Asian tropospheric temperature may not be mainly forced by Atlantic SST.

On the contrary, sensitivity experiments using the CCSM3 with changing Asian surface heating generate many features that are similar to those observed in the atmosphere and SST over the Atlantic–Eurasian region. An increase in Asian land heating raises surface and tropospheric temperatures over Asia, triggering tropospheric temperature anomalies propagating westward from Asia to the Atlantic and an increase in the tropospheric temperature over the extratropical Atlantic. Corresponding to this increase in tropospheric temperature, an anomalous high appears over the extratropical Atlantic Ocean. The atmospheric circulation anomalies over the extratropical Atlantic Ocean may also be forced by the CAM3 model with prescribed climatological mean SST, which excludes a possible influence of the atmosphere–ocean interaction over the North Atlantic on the atmosphere. Accordingly, the anomalous high may enhance the downward shortwave radiation and sensible and latent heat fluxes at the surface over the ocean. Positive SST anomalies appear in the extratropical western-central North Atlantic. The resemblance of these modeling features associated with Asian land heating to the observed features suggests that the ocean–atmosphere–land coupled relationship over the Atlantic–Eurasian region may be forced by the heating over Asian land during summer.

Therefore, at least on interannual time scales, the summertime oceanic and atmospheric anomalies over the Atlantic–Eurasian region may be mainly produced by the anomalies of Asian land heating, instead of the Atlantic SST anomalies. The response of extratropical Atlantic shows a warm SST–ridge feature. On decadal time scales, however, the coupled relationship between the atmosphere and the Atlantic Ocean shows a warm SST–trough feature, which may be forced by the Atlantic SST anomalies (e.g., Sutton and Hodson 2005). The interdecadal variability of the Atlantic SST may be further linked to or act as forcing on the Asian tropospheric warming (Lu et al. 2006; Wang et al. 2009; Luo et al. 2011), due to the strong feedback of transient eddies (Peng et al. 1997). Therefore, the decadal impact of North Atlantic SST on Asian tropospheric warming cannot be excluded. These results exhibit a remarkable difference in the ocean–atmosphere–land interaction over the North Atlantic–Asian sector between different time scales. Moreover, this study suggests the importance of further investigations into the teleconnection patterns of atmospheric circulation over the North Atlantic–Asian region for improved understanding of the relationships between Asian and Atlantic climate anomalies and the physical processes of these relationships on various time scales. It is also interesting to investigate the climatic links between Asia and the Atlantic for other seasons.

Acknowledgments

We thank the two anonymous reviewers for constructive comments that are helpful for improving the overall quality of the paper. We also thank the ECMWF for providing the ERA-40 reanalysis from its data server, the U.K. Hadley Centre for providing the monthly mean SST and SLP data on its home page, and NCAR for providing the CCSM3 and CAM3 on its home page. This work was jointly sponsored by the National Natural Science Foundation of China (Grant 40921003), the National Key Basic Research Project of China (Grant 2009CB421404), and the foundation of State Key Laboratory of Severe Weather of China.

REFERENCES

  • Allan, R. J., and T. J. Ansell, 2006: A new globally complete monthly historical mean sea level pressure data set (HadSLP2): 1850–2004. J. Climate, 19, 58165842.

    • Search Google Scholar
    • Export Citation
  • Bader, J., and M. Latif, 2003: The impact of decadal-scale Indian Ocean sea surface temperature anomalies on Sahelian rainfall and the North Atlantic Oscillation. Geophys. Res. Lett., 30, 2169, doi:10.1029/2003GL018426.

    • Search Google Scholar
    • Export Citation
  • Bader, J., and M. Latif, 2005: North Atlantic Oscillation response to anomalous Indian Ocean SST in a coupled GCM. J. Climate, 18, 53825389.

    • Search Google Scholar
    • Export Citation
  • Battisti, D. S., U. S. Bhatt, and M. A. Alexander, 1995: A modeling study of the interannual variability in the wintertime North Atlantic Ocean. J. Climate, 8, 30673083.

    • Search Google Scholar
    • Export Citation
  • Bhatt, U. S., M. A. Alexander, D. S. Battisti, D. D. Houghton, and L. M. Keller, 1998: Atmosphere–ocean interaction in the North Atlantic: Near-surface climate variability. J. Climate, 11, 16151632.

    • Search Google Scholar
    • Export Citation
  • Bjerknes, J., 1964: Atlantic air-sea interaction. Advances in Geophysics, Vol. 10, Academic Press, 1–82.

  • Branstator, G., 2002: Circumglobal teleconnections, the jet stream waveguide, and the North Atlantic Oscillation. J. Climate, 15, 18931910.

    • Search Google Scholar
    • Export Citation
  • Cayan, D. R., 1992a: Latent and sensible heat flux anomalies over the northern oceans: The connection to monthly atmospheric circulation. J. Climate, 5, 354369.

    • Search Google Scholar
    • Export Citation
  • Cayan, D. R., 1992b: Latent and sensible heat flux anomalies over the northern oceans: Driving the sea surface temperature. J. Phys. Oceanogr., 22, 859881.

    • Search Google Scholar
    • Export Citation
  • Collins, W. D., and Coauthors, 2004: Description of the NCAR Community Atmosphere Model (CAM3). National Center for Atmospheric Research Tech. Rep. NCAR/TN-464_STR, 226 pp.

  • Collins, W. D., and Coauthors, 2006: The Community Climate System Model version 3 (CCSM3). J. Climate, 19, 21222143.

  • Conil, S., and L. Z. X. Li, 2005: Linearity of the atmospheric response to North Atlantic SST and sea ice anomalies. J. Climate, 18, 19862003.

    • Search Google Scholar
    • Export Citation
  • Deser, C., and A. S. Phillips, 2009: Atmospheric circulation trends, 1950–2000: The relative roles of sea surface temperature forcing and direct atmospheric radiative forcing. J. Climate, 22, 396413.

    • Search Google Scholar
    • Export Citation
  • Deser, C., R. A. Tomas, and S. L. Peng, 2007: The transient atmospheric circulation response to North Atlantic SST and sea ice anomalies. J. Climate, 20, 47514767.

    • Search Google Scholar
    • Export Citation
  • Ding, Q. H., and B. Wang, 2005: Circumglobal teleconnection in the Northern Hemisphere summer. J. Climate, 18, 34843505.

  • Dong, B., R. T. Sutton, and A. A. Scaife, 2006: Multidecadal modulation of El Niño–Southern Oscillation (ENSO) variance by Atlantic Ocean sea surface temperatures. Geophys. Res. Lett., 33, L08705, doi:10.1029/2006GL025766.

    • Search Google Scholar
    • Export Citation
  • Fasullo, J., 2004: A stratified diagnosis of the Indian monsoon—Eurasian snow cover relationship. J. Climate, 17, 11101122.

  • Feliks, Y. M., M. Ghil, and A. W. Robertson, 2011: The atmospheric circulation over the North Atlantic as induced by the SST field. J. Climate, 24, 522542.

    • Search Google Scholar
    • Export Citation
  • Foltz, G. R., and M. J. McPhaden, 2006a: Unusually warm sea surface temperatures in the tropical North Atlantic during 2005. Geophys. Res. Lett., 33, L19703, doi:10.1029/2006GL027394.

    • Search Google Scholar
    • Export Citation
  • Foltz, G. R., and M. J. McPhaden, 2006b: The role of oceanic heat advection in the evolution of tropical North and South Atlantic SST anomalies. J. Climate, 19, 61226138.

    • Search Google Scholar
    • Export Citation
  • Foltz, G. R., and M. J. McPhaden, 2008: Impact of Saharan dust on tropical North Atlantic SST. J. Climate, 21, 50485060.

  • Gong, D., and C. H. Ho, 2003: Arctic Oscillation signals in the East Asian summer monsoon. J. Geophys. Res., 108, 4066, doi:10.1029/2002JD002193.

    • Search Google Scholar
    • Export Citation
  • Hack, J. J., J. M. Caron, S. G. Yeager, K. W. Oleson, M. M. Holland, J. E. Truesdale, and P. J. Rasch, 2006: Simulation of the global hydrological cycle in the CCSM Community Atmosphere Model version 3 (CAM3): Mean features. J. Climate, 19, 21992221.

    • Search Google Scholar
    • Export Citation
  • Hoerling, M. P., J. W. Hurrell, and T. Xu, 2001: Tropical origins for recent North Atlantic climate change. Science, 292, 9092.

  • Hoerling, M. P., J. W. Hurrell, T. Xu, G. T. Bates, and A. Phillips, 2004: Twentieth century North Atlantic climate change. Part II: Understanding the effect of Indian Ocean warming. Climate Dyn., 23, 391405.

    • Search Google Scholar
    • Export Citation
  • Huang, B. H., and J. Shukla, 1996: A comparison of two surface wind stress analyses over the tropical Atlantic during 1980–87. J. Climate, 9, 906927.

    • Search Google Scholar
    • Export Citation
  • Huang, B. H., and J. Shukla, 1997: Characteristics of the interannual and decadal variability in a general circulation model of the tropical Atlantic Ocean. J. Phys. Oceanogr., 27, 16931712.

    • Search Google Scholar
    • Export Citation
  • Hurrell, J. W., M. P. Hoerling, A. Phillips, and T. Xu, 2004: Twentieth century North Atlantic climate change. Part I: Assessing determinism. Climate Dyn., 23, 371389.

    • Search Google Scholar
    • Export Citation
  • Kim, Y., and G. Wang, 2007: Impact of initial soil moisture anomalies on subsequent precipitation over North America in the Coupled Land–Atmosphere Model CAM3–CLM3. J. Hydrometeor., 8, 513533.

    • Search Google Scholar
    • Export Citation
  • Kushnir, Y., 1994: Interdecadal variations in North Atlantic sea surface temperature and associated atmospheric conditions. J. Climate, 7, 141157.

    • Search Google Scholar
    • Export Citation
  • Latif, M., K. Arpe, and E. Roeckner, 2000: Oceanic control of decadal North Atlantic sea level pressure variability in winter. Geophys. Res. Lett., 27, 727730.

    • Search Google Scholar
    • Export Citation
  • Li, S., M. P. Hoerling, and S. Peng, 2006: Coupled ocean-atmosphere response to Indian Ocean warmth. Geophys. Res. Lett., 33, L07713, doi:10.1029/2005GL025558.

    • Search Google Scholar
    • Export Citation
  • Li, S., J. Perlwitz, M. P. Hoerling, and X. Chen, 2010: Opposite annular responses of the Northern and Southern Hemispheres to Indian Ocean warming. J. Climate, 23, 37203738.

    • Search Google Scholar
    • Export Citation
  • Linderholm, H. W., T. Ou, J. H. Jeong, C. K. Folland, D. Gong, H. Liu, Y. Liu, and D. Chen, 2011: Interannual teleconnections between the summer North Atlantic Oscillation and the East Asian summer monsoon. J. Geophys. Res., 116, D13107, doi:10.1029/2010JD015235.

    • Search Google Scholar
    • Export Citation
  • Lu, R., B. Dong, and H. Ding, 2006: Impact of the Atlantic multidecadal oscillation on the Asian summer monsoon. Geophys. Res. Lett., 33, L24701, doi:10.1029/2006GL027655.

    • Search Google Scholar
    • Export Citation
  • Luo, F., S. Li, and T. Furevik, 2011: The connection between the Atlantic multidecadal oscillation and the Indian summer monsoon in Bergen Climate Model version 2.0. J. Geophys. Res., 116, D19117, doi:10.1029/2011JD015848.

    • Search Google Scholar
    • Export Citation
  • Meehl, G. A., J. M. Arblaster, D. M. Lawrence, A. Seth, E. K. Schneider, B. P. Kirtman, and D. Min, 2006a: Monsoon regimes in the CCSM3. J. Climate, 19, 24822495.

    • Search Google Scholar
    • Export Citation
  • Meehl, G. A., and Coauthors, 2006b: Climate change projections for the twenty-first century and climate change commitment in the CCSM3. J. Climate, 19, 25972616.

    • Search Google Scholar
    • Export Citation
  • Ose, T., 1996: The comparison of the simulated response to the regional snow mass anomalies over Tibet, Eastern Europe, and Siberia. J. Meteor. Soc. Japan, 74, 845866.

    • Search Google Scholar
    • Export Citation
  • Peng, S., W. A. Robinson, and M. P. Hoerling, 1997: The modeled atmospheric response to midlatitude SST anomalies and its dependence on background circulation states. J. Climate, 10, 971987.

    • Search Google Scholar
    • Export Citation
  • Plumb, R. A., 1985: On the three-dimensional propagation of stationary waves. J. Atmos. Sci., 42, 217229.

  • Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. A. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108, 4407, doi:10.1029/2002JD002670.

    • Search Google Scholar
    • Export Citation
  • Rodwell, M. J., and B. J. Hoskins, 1996: Monsoons and the dynamics of deserts. Quart. J. Roy. Meteor. Soc., 122, 13851404.

  • Rodwell, M. J., and B. J. Hoskins, 2001: Subtropical anticyclones and summer monsoons. J. Climate, 14, 31923211.

  • Song, J. H., H. S. Kang, Y. H. Byun, and S. Y. Hong, 2010: Effects of the Tibetan Plateau on the Asian summer monsoon: A numerical case study using a regional climate model. Int. J. Climatol., 30, 743759.

    • Search Google Scholar
    • Export Citation
  • Sutton, R. T., and D. L. R. Hodson, 2005: Atlantic Ocean forcing of North American and European summer climate. Science, 309, 115118.

    • Search Google Scholar
    • Export Citation
  • Talley, L. D., 1984: Meridional heat transport in the Pacific Ocean. J. Phys. Oceanogr., 14, 231241.

  • Uppala, S. M., and Coauthors, 2005: The ERA-40 Re-Analysis. Quart. J. Roy. Meteor. Soc., 131, 29613012.

  • Wallace, J. M., C. Smith, and Q. R. Jiang, 1990: Spatial patterns of atmosphere–ocean interaction in the northern winter. J. Climate, 3, 990998.

    • Search Google Scholar
    • Export Citation
  • Wang, B., Q. Bao, B. Hoskins, G. Wu, and Y. Liu, 2008: Tibetan Plateau warming and precipitation changes in East Asia. Geophys. Res. Lett., 35, L14702, doi:10.1029/2008GL034330.

    • Search Google Scholar
    • Export Citation
  • Wang, Y., S. Li, and D. Luo, 2009: Seasonal response of Asian monsoonal climate to the Atlantic multidecadal oscillation. J. Geophys. Res., 114, D02112, doi:10.1029/2008JD010929.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., and J. Fasullo, 2003: Monsoon: Dynamical theory. Encyclopedia of Atmospheric Sciences, J. Holton and J. A. Curry, Eds., Academic Press, 1370–1386.

  • Zhao, P., Z. J. Zhou, and J. P. Liu, 2007: Variability of Tibetan spring snow and its associations with the hemispheric extratropical circulation and East Asian summer monsoon rainfall: An observational investigation. J. Climate, 20, 39423955.

    • Search Google Scholar
    • Export Citation
  • Zhao, P., Z. H. Cao, and J. M. Chen, 2010a: A summer teleconnection pattern over the extratropical Northern Hemisphere and associated mechanisms. Climate Dyn., 35, 523534.

    • Search Google Scholar
    • Export Citation
  • Zhao, P., S. Yang, and R. Yu, 2010b: Long-term changes in rainfall over eastern China and large-scale atmospheric circulation associated with recent global warming. J. Climate, 23, 15441562.

    • Search Google Scholar
    • Export Citation
  • Zhao, P., S. Yang, M. Q. Jian, and J. M. Chen, 2011: Relative controls of Asian–Pacific summer climate by Asian land and tropical–North Pacific sea surface temperature. J. Climate, 24, 41654188.

    • Search Google Scholar
    • Export Citation
  • Zorita, E., V. V. Kharin, and H. von Storch, 1992: The atmospheric circulation and sea surface temperature in the North Atlantic area in winter: Their interaction and relevance for Iberian precipitation. J. Climate, 5, 10971108.

    • Search Google Scholar
    • Export Citation
Save
  • Allan, R. J., and T. J. Ansell, 2006: A new globally complete monthly historical mean sea level pressure data set (HadSLP2): 1850–2004. J. Climate, 19, 58165842.

    • Search Google Scholar
    • Export Citation
  • Bader, J., and M. Latif, 2003: The impact of decadal-scale Indian Ocean sea surface temperature anomalies on Sahelian rainfall and the North Atlantic Oscillation. Geophys. Res. Lett., 30, 2169, doi:10.1029/2003GL018426.

    • Search Google Scholar
    • Export Citation
  • Bader, J., and M. Latif, 2005: North Atlantic Oscillation response to anomalous Indian Ocean SST in a coupled GCM. J. Climate, 18, 53825389.

    • Search Google Scholar
    • Export Citation
  • Battisti, D. S., U. S. Bhatt, and M. A. Alexander, 1995: A modeling study of the interannual variability in the wintertime North Atlantic Ocean. J. Climate, 8, 30673083.

    • Search Google Scholar
    • Export Citation
  • Bhatt, U. S., M. A. Alexander, D. S. Battisti, D. D. Houghton, and L. M. Keller, 1998: Atmosphere–ocean interaction in the North Atlantic: Near-surface climate variability. J. Climate, 11, 16151632.

    • Search Google Scholar
    • Export Citation
  • Bjerknes, J., 1964: Atlantic air-sea interaction. Advances in Geophysics, Vol. 10, Academic Press, 1–82.

  • Branstator, G., 2002: Circumglobal teleconnections, the jet stream waveguide, and the North Atlantic Oscillation. J. Climate, 15, 18931910.

    • Search Google Scholar
    • Export Citation
  • Cayan, D. R., 1992a: Latent and sensible heat flux anomalies over the northern oceans: The connection to monthly atmospheric circulation. J. Climate, 5, 354369.

    • Search Google Scholar
    • Export Citation
  • Cayan, D. R., 1992b: Latent and sensible heat flux anomalies over the northern oceans: Driving the sea surface temperature. J. Phys. Oceanogr., 22, 859881.

    • Search Google Scholar
    • Export Citation
  • Collins, W. D., and Coauthors, 2004: Description of the NCAR Community Atmosphere Model (CAM3). National Center for Atmospheric Research Tech. Rep. NCAR/TN-464_STR, 226 pp.

  • Collins, W. D., and Coauthors, 2006: The Community Climate System Model version 3 (CCSM3). J. Climate, 19, 21222143.

  • Conil, S., and L. Z. X. Li, 2005: Linearity of the atmospheric response to North Atlantic SST and sea ice anomalies. J. Climate, 18, 19862003.

    • Search Google Scholar
    • Export Citation
  • Deser, C., and A. S. Phillips, 2009: Atmospheric circulation trends, 1950–2000: The relative roles of sea surface temperature forcing and direct atmospheric radiative forcing. J. Climate, 22, 396413.

    • Search Google Scholar
    • Export Citation
  • Deser, C., R. A. Tomas, and S. L. Peng, 2007: The transient atmospheric circulation response to North Atlantic SST and sea ice anomalies. J. Climate, 20, 47514767.

    • Search Google Scholar
    • Export Citation
  • Ding, Q. H., and B. Wang, 2005: Circumglobal teleconnection in the Northern Hemisphere summer. J. Climate, 18, 34843505.

  • Dong, B., R. T. Sutton, and A. A. Scaife, 2006: Multidecadal modulation of El Niño–Southern Oscillation (ENSO) variance by Atlantic Ocean sea surface temperatures. Geophys. Res. Lett., 33, L08705, doi:10.1029/2006GL025766.

    • Search Google Scholar
    • Export Citation
  • Fasullo, J., 2004: A stratified diagnosis of the Indian monsoon—Eurasian snow cover relationship. J. Climate, 17, 11101122.

  • Feliks, Y. M., M. Ghil, and A. W. Robertson, 2011: The atmospheric circulation over the North Atlantic as induced by the SST field. J. Climate, 24, 522542.

    • Search Google Scholar
    • Export Citation
  • Foltz, G. R., and M. J. McPhaden, 2006a: Unusually warm sea surface temperatures in the tropical North Atlantic during 2005. Geophys. Res. Lett., 33, L19703, doi:10.1029/2006GL027394.

    • Search Google Scholar
    • Export Citation
  • Foltz, G. R., and M. J. McPhaden, 2006b: The role of oceanic heat advection in the evolution of tropical North and South Atlantic SST anomalies. J. Climate, 19, 61226138.

    • Search Google Scholar
    • Export Citation
  • Foltz, G. R., and M. J. McPhaden, 2008: Impact of Saharan dust on tropical North Atlantic SST. J. Climate, 21, 50485060.

  • Gong, D., and C. H. Ho, 2003: Arctic Oscillation signals in the East Asian summer monsoon. J. Geophys. Res., 108, 4066, doi:10.1029/2002JD002193.

    • Search Google Scholar
    • Export Citation
  • Hack, J. J., J. M. Caron, S. G. Yeager, K. W. Oleson, M. M. Holland, J. E. Truesdale, and P. J. Rasch, 2006: Simulation of the global hydrological cycle in the CCSM Community Atmosphere Model version 3 (CAM3): Mean features. J. Climate, 19, 21992221.

    • Search Google Scholar
    • Export Citation
  • Hoerling, M. P., J. W. Hurrell, and T. Xu, 2001: Tropical origins for recent North Atlantic climate change. Science, 292, 9092.

  • Hoerling, M. P., J. W. Hurrell, T. Xu, G. T. Bates, and A. Phillips, 2004: Twentieth century North Atlantic climate change. Part II: Understanding the effect of Indian Ocean warming. Climate Dyn., 23, 391405.

    • Search Google Scholar
    • Export Citation
  • Huang, B. H., and J. Shukla, 1996: A comparison of two surface wind stress analyses over the tropical Atlantic during 1980–87. J. Climate, 9, 906927.

    • Search Google Scholar
    • Export Citation
  • Huang, B. H., and J. Shukla, 1997: Characteristics of the interannual and decadal variability in a general circulation model of the tropical Atlantic Ocean. J. Phys. Oceanogr., 27, 16931712.

    • Search Google Scholar
    • Export Citation
  • Hurrell, J. W., M. P. Hoerling, A. Phillips, and T. Xu, 2004: Twentieth century North Atlantic climate change. Part I: Assessing determinism. Climate Dyn., 23, 371389.

    • Search Google Scholar
    • Export Citation
  • Kim, Y., and G. Wang, 2007: Impact of initial soil moisture anomalies on subsequent precipitation over North America in the Coupled Land–Atmosphere Model CAM3–CLM3. J. Hydrometeor., 8, 513533.

    • Search Google Scholar
    • Export Citation
  • Kushnir, Y., 1994: Interdecadal variations in North Atlantic sea surface temperature and associated atmospheric conditions. J. Climate, 7, 141157.

    • Search Google Scholar
    • Export Citation
  • Latif, M., K. Arpe, and E. Roeckner, 2000: Oceanic control of decadal North Atlantic sea level pressure variability in winter. Geophys. Res. Lett., 27, 727730.

    • Search Google Scholar
    • Export Citation
  • Li, S., M. P. Hoerling, and S. Peng, 2006: Coupled ocean-atmosphere response to Indian Ocean warmth. Geophys. Res. Lett., 33, L07713, doi:10.1029/2005GL025558.

    • Search Google Scholar
    • Export Citation
  • Li, S., J. Perlwitz, M. P. Hoerling, and X. Chen, 2010: Opposite annular responses of the Northern and Southern Hemispheres to Indian Ocean warming. J. Climate, 23, 37203738.

    • Search Google Scholar
    • Export Citation
  • Linderholm, H. W., T. Ou, J. H. Jeong, C. K. Folland, D. Gong, H. Liu, Y. Liu, and D. Chen, 2011: Interannual teleconnections between the summer North Atlantic Oscillation and the East Asian summer monsoon. J. Geophys. Res., 116, D13107, doi:10.1029/2010JD015235.

    • Search Google Scholar
    • Export Citation
  • Lu, R., B. Dong, and H. Ding, 2006: Impact of the Atlantic multidecadal oscillation on the Asian summer monsoon. Geophys. Res. Lett., 33, L24701, doi:10.1029/2006GL027655.

    • Search Google Scholar
    • Export Citation
  • Luo, F., S. Li, and T. Furevik, 2011: The connection between the Atlantic multidecadal oscillation and the Indian summer monsoon in Bergen Climate Model version 2.0. J. Geophys. Res., 116, D19117, doi:10.1029/2011JD015848.

    • Search Google Scholar
    • Export Citation
  • Meehl, G. A., J. M. Arblaster, D. M. Lawrence, A. Seth, E. K. Schneider, B. P. Kirtman, and D. Min, 2006a: Monsoon regimes in the CCSM3. J. Climate, 19, 24822495.

    • Search Google Scholar
    • Export Citation
  • Meehl, G. A., and Coauthors, 2006b: Climate change projections for the twenty-first century and climate change commitment in the CCSM3. J. Climate, 19, 25972616.

    • Search Google Scholar
    • Export Citation
  • Ose, T., 1996: The comparison of the simulated response to the regional snow mass anomalies over Tibet, Eastern Europe, and Siberia. J. Meteor. Soc. Japan, 74, 845866.

    • Search Google Scholar
    • Export Citation
  • Peng, S., W. A. Robinson, and M. P. Hoerling, 1997: The modeled atmospheric response to midlatitude SST anomalies and its dependence on background circulation states. J. Climate, 10, 971987.

    • Search Google Scholar
    • Export Citation
  • Plumb, R. A., 1985: On the three-dimensional propagation of stationary waves. J. Atmos. Sci., 42, 217229.

  • Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. A. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. J. Geophys. Res., 108, 4407, doi:10.1029/2002JD002670.

    • Search Google Scholar
    • Export Citation
  • Rodwell, M. J., and B. J. Hoskins, 1996: Monsoons and the dynamics of deserts. Quart. J. Roy. Meteor. Soc., 122, 13851404.

  • Rodwell, M. J., and B. J. Hoskins, 2001: Subtropical anticyclones and summer monsoons. J. Climate, 14, 31923211.

  • Song, J. H., H. S. Kang, Y. H. Byun, and S. Y. Hong, 2010: Effects of the Tibetan Plateau on the Asian summer monsoon: A numerical case study using a regional climate model. Int. J. Climatol., 30, 743759.

    • Search Google Scholar
    • Export Citation
  • Sutton, R. T., and D. L. R. Hodson, 2005: Atlantic Ocean forcing of North American and European summer climate. Science, 309, 115118.

    • Search Google Scholar
    • Export Citation
  • Talley, L. D., 1984: Meridional heat transport in the Pacific Ocean. J. Phys. Oceanogr., 14, 231241.

  • Uppala, S. M., and Coauthors, 2005: The ERA-40 Re-Analysis. Quart. J. Roy. Meteor. Soc., 131, 29613012.

  • Wallace, J. M., C. Smith, and Q. R. Jiang, 1990: Spatial patterns of atmosphere–ocean interaction in the northern winter. J. Climate, 3, 990998.

    • Search Google Scholar
    • Export Citation
  • Wang, B., Q. Bao, B. Hoskins, G. Wu, and Y. Liu, 2008: Tibetan Plateau warming and precipitation changes in East Asia. Geophys. Res. Lett., 35, L14702, doi:10.1029/2008GL034330.

    • Search Google Scholar
    • Export Citation
  • Wang, Y., S. Li, and D. Luo, 2009: Seasonal response of Asian monsoonal climate to the Atlantic multidecadal oscillation. J. Geophys. Res., 114, D02112, doi:10.1029/2008JD010929.

    • Search Google Scholar
    • Export Citation
  • Webster, P. J., and J. Fasullo, 2003: Monsoon: Dynamical theory. Encyclopedia of Atmospheric Sciences, J. Holton and J. A. Curry, Eds., Academic Press, 1370–1386.

  • Zhao, P., Z. J. Zhou, and J. P. Liu, 2007: Variability of Tibetan spring snow and its associations with the hemispheric extratropical circulation and East Asian summer monsoon rainfall: An observational investigation. J. Climate, 20, 39423955.

    • Search Google Scholar
    • Export Citation
  • Zhao, P., Z. H. Cao, and J. M. Chen, 2010a: A summer teleconnection pattern over the extratropical Northern Hemisphere and associated mechanisms. Climate Dyn., 35, 523534.

    • Search Google Scholar
    • Export Citation
  • Zhao, P., S. Yang, and R. Yu, 2010b: Long-term changes in rainfall over eastern China and large-scale atmospheric circulation associated with recent global warming. J. Climate, 23, 15441562.

    • Search Google Scholar
    • Export Citation
  • Zhao, P., S. Yang, M. Q. Jian, and J. M. Chen, 2011: Relative controls of Asian–Pacific summer climate by Asian land and tropical–North Pacific sea surface temperature. J. Climate, 24, 41654188.

    • Search Google Scholar
    • Export Citation
  • Zorita, E., V. V. Kharin, and H. von Storch, 1992: The atmospheric circulation and sea surface temperature in the North Atlantic area in winter: Their interaction and relevance for Iberian precipitation. J. Climate, 5, 10971108.

    • Search Google Scholar
    • Export Citation
  • Fig. 1.

    Leading SVD modes of SST (a) and tropospheric temperature (b) during 1958–2001. (c),(d) As in (a),(b), but for the second modes.

  • Fig. 2.

    (a) Times series of leading SVD tropospheric temperature (red) and Atlantic SST (blue) modes during 1958–2001. (b) Standardized time series of detrended MJJAS ATT (red) and Atlantic SST (blue) indices, in which the ATT index is referred to as the MJJAS Asian tropospheric (500–200 hPa) temperature averaged over 25°–50°N, 60°–120°E and the Atlantic SST index is defined as the MJJAS SST averaged over 38°–50°N, 80°–40°W.

  • Fig. 3.

    (a) Composite difference in 250-hPa temperature (°C) between HI cases and LI cases. (b) As in (a), but for longitude–height cross section along 40°N. (c) As in (a), but for surface air temperature. In (a), the box represents the region for Asia and shaded areas indicate the values significantly exceeding the 95% confidence level.

  • Fig. 4.

    (a) Composite difference in MJJAS SST (°C) between HI cases and LI cases. (b) Standard deviation of MJJAS SST (×0.1°C). (c) Time–longitude cross section of composite difference in SST (°C) between HI cases and LI cases along 45°–55°N. (d) As in (c), but for the regression of MJJAS SST against the detrended Atlantic SST index during 1958–2001. Shaded areas indicate the values significantly exceeding the 95% confidence level and in (a), thick dashed lines indicate the area of the extratropical Atlantic Ocean.

  • Fig. 5.

    Regression of MJJAS Hs (W m−2) against the ATT index. Shaded areas indicate the values significantly exceeding the 95% confidence level.

  • Fig. 6.

    (a) Climatology of MJJAS 250-hPa zonal wind (m s−1; shaded areas are ≧ 15 m s−1). (b) Time–longitude cross section of composite difference in 250-hPa temperature (°C) along 25°–50°N (shaded areas indicate the values significantly exceeding the 95% confidence level).

  • Fig. 7.

    (a) Composite difference in MJJAS 250-hPa geopotential height (m) along 25°–50°N. (b) Smoothed July–August mean 250-hPa Fh (×10−12 s−3).