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    Time series of the SCS TCGF from June to August during 1979–2011 derived from the (a) JTWC, (b) JMA, and (c) CMA datasets. The dashed curves are the mean TCGF of three periods (1979–93, 1994–2002, and 2003–11), and the respective epochal mean values are denoted above the curves.

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    Statistics of Lepage test for the SCS TCGF derived from the three datasets in the period 1979–2011. The horizontal dashed lines indicate the test statistic critical values of 9.21 and 5.99, which correspond to the 99% and 95% confidence levels, respectively.

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    TC genesis locations (red TC symbols) and tracks (lines) from June to August recorded in the (a),(b) JTWC, (c),(d) JMA, and (e),(f) CMA datasets during (top) 1994–2002 and (bottom) 2003–11. The blue TC tracks denote those TCs enter the East China Sea. The green contours indicate 5865 gpm averaged from June to August in the respective periods. The numbers at the top left in each panel denote total and annual TCGFs.

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    Epochal differences in (a) vertical pressure velocity (shading; Pa s−1) averaged from 850 to 300 hPa, (b) relative humidity (shading; %) averaged from 850 to 300 hPa, (e) 850-hPa wind (vector; m s−1), and (f) vertical wind shear of 200 minus 850 hPa (shading; m s−1) in summer between the periods of 2003–11 and 1994–2002, and differences enclosed by solid contours and thick wind vectors denote that the anomalies are significant above the 95% confidence level based on the Mann–Whitney U statistic. Time series of (c) vertical pressure velocity [Pa s−1; area-weighted mean over the box of 15°–25°N, 105°–120°E in (a)] and (d) relative humidity [%; area-weighted mean over the box in (b), as in (a)]. The red lines and numbers in (c) and (d) denote the epochal mean values. The decadal shift of omega (relative humidity) index in 2002/03 is significant above the 99% (99%) confidence level based on the Lepage test.

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    Simultaneous correlation between SST and the (a) JTWC TCGF, (b) JMA TCGF, (c) CMA TCGF, (d) omega (in Fig. 4c), and (e) relative humidity (in Fig. 4d) indices in summer. Calculations cover the period 1994–2011. Shading indicates correlation coefficients are statistically significant above the 95% confidence level. The boxes indicate the area for the NIO (5°S–20°N, 60°–95°E) and WPO (10°S–10°N, 145°–165°E), where time series of SST are calculated.

  • View in gallery

    (a) Epochal differences in SST (contour; °C) in summer between the periods of 2003–11 and 1994–2002. Shaded areas indicate where the anomalies are significant above the 95% confidence level based on the Mann–Whitney U statistic. Time series of the (b) NIO SST [area-weighted mean over 5°S–20°N, 60°–95°E, box in (a)] and (c) WPO SST [area-weighted mean over 10°S–10°N, 145°–165°E, box in (a)]. The dashed lines in (b) and (c) denote the linear trends of SST change and the epochal means; trend values [°C (18 yr)−1] and epochal mean values are also shown. The decadal shift of the NIO (WPO) SST index in 2002/03 is significant above the 99% (95%) confidence level based on the Lepage test.

  • View in gallery

    Epochal differences in 200-hPa velocity potential (contour; 106 m2 s−1) and divergent wind (vector; m s−1) in summer between the periods of 2003–11 and 1994–2002. “Con” and “Div” indicate convergent and divergent centers, respectively.

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    Cross sections of (a) the epochal differences in velocity (streamline), relative humidity (shading; %), and SST (red line; °C) for zonal circulation averaged from the equator to 20°N, and (b) differences in velocity (streamline) and relative humidity (shading; %) for Hadley circulation averaged from 105° to 125°E in summer between the periods of 2003–011 and 1994–2002. The relative humidity is available in the NNRP data only from 1000 to 300 hPa.

  • View in gallery

    (a) Epochal differences in mean standard deviation of OLR anomalies (contour; W m−2) on a 30–60-day time scale in summer between the periods of 2003–11 and 1994–2002. Anomalies enclosed by solid contours denote those are significant above the 95% confidence level based on the Mann–Whitney U statistic. (b) Time series of ISV index [area-weighted mean over 7.5°–20°N, 110°–122.5°E, box in (a)] in the period 1994–2011. The dashed lines denote the epochal mean, and values are also shown.

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    Latitude–time cross section in summer 30–60-day filtered OLR (shading; W m−2) and 850-hPa wind divergence (contour; 10−6 s−1) anomalies averaged from 100° to 125°E during (a) 1994–2002 and (b) 2003–11, overlapped with the formation time and latitude of every TC (TC symbols). The contour interval is 0.3 × 10−6 s−1, and zero contours are omitted. The number in parentheses denotes the summer TCGF in that year.

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    Box plots of ambient 30–60-day filtered OLR anomalies of each TC genesis from (a) 1994–2002 and (b) 2003–11. Plotted are the inner quartile range (box), the median (horizontal line inside the box), the 25th and 75th percentile ±1.5 times the inner quartile range (horizontal line below and above the box, respectively), the mean value of all the samples (thick cross and corresponding number), and the ambient 30–60-day filtered OLR anomalies of each sample (dots).

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    Schematic diagram illustrating the anomalous atmospheric circulation after the early 2000s. Red shaded areas indicate warm SST anomalies. The dark (light) gray arrows indicate anomalous zonal (meridional) circulation branches. “Con” and “Div” indicate convergence and divergence in the upper level, respectively.

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Decadal Change in Tropical Cyclone Activity over the South China Sea around 2002/03

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  • 1 College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing, China
  • | 2 College of Meteorology and Oceanography, PLA University of Science and Technology, and Jiangsu Collaborative Innovation Center for Climate Change and School of Atmospheric Sciences, Nanjing University, Nanjing, China
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Abstract

This study investigates the decadal change in tropical cyclone (TC) activity over the South China Sea (SCS) in the boreal summer (June–August) since the early 1990s and explores possible causes behind it. Results show that the SCS TC activity experienced an abrupt decadal decrease at around 2003/03. Compared to the TC activities from the early 1990s to 2002, the number of TCs formed in the SCS markedly decreased from 2003 through the early 2010s. Moreover, most of the TCs were primarily confined within the SCS basin during this period. The TCs that formed during the period of 2003–11 usually moved west-northwestward and rapidly weakened after making landfall. It is found that a significant decadal-scale sea surface temperature (SST) warming occurred in the northern Indian Ocean and the western Pacific Ocean after 2002 while convection intensified over the tropical regions between 60° and 80°E and around 150°E, respectively. The warm SST anomalies induced an anomalous subsiding flow over the SCS basin via the Walker-like (zonal) circulation. Meanwhile, anomalously dry, sinking air around 5°–20°N derived from local Hadley (meridional) circulation reinforced the subsiding flow of the zonal circulation. The above circulation patterns suppressed TC genesis over the northern SCS, leading to the decadal decrease in TC activity that occurred around 2002/03. In addition, in conjunction with the local anomalous easterly flow, the intraseasonal atmospheric variability over the SCS has decreased since the early 2000s. This is unfavorable for the development of synoptic-scale disturbances and may also contribute to the decadal decrease in TC activity.

Corresponding author address: Zhong Zhong, College of Meteorology and Oceanography, PLA University of Science and Technology, No. 60, Shuanglong Road, Nanjing 211101, China. E-mail: zhong_zhong@yeah.net

Abstract

This study investigates the decadal change in tropical cyclone (TC) activity over the South China Sea (SCS) in the boreal summer (June–August) since the early 1990s and explores possible causes behind it. Results show that the SCS TC activity experienced an abrupt decadal decrease at around 2003/03. Compared to the TC activities from the early 1990s to 2002, the number of TCs formed in the SCS markedly decreased from 2003 through the early 2010s. Moreover, most of the TCs were primarily confined within the SCS basin during this period. The TCs that formed during the period of 2003–11 usually moved west-northwestward and rapidly weakened after making landfall. It is found that a significant decadal-scale sea surface temperature (SST) warming occurred in the northern Indian Ocean and the western Pacific Ocean after 2002 while convection intensified over the tropical regions between 60° and 80°E and around 150°E, respectively. The warm SST anomalies induced an anomalous subsiding flow over the SCS basin via the Walker-like (zonal) circulation. Meanwhile, anomalously dry, sinking air around 5°–20°N derived from local Hadley (meridional) circulation reinforced the subsiding flow of the zonal circulation. The above circulation patterns suppressed TC genesis over the northern SCS, leading to the decadal decrease in TC activity that occurred around 2002/03. In addition, in conjunction with the local anomalous easterly flow, the intraseasonal atmospheric variability over the SCS has decreased since the early 2000s. This is unfavorable for the development of synoptic-scale disturbances and may also contribute to the decadal decrease in TC activity.

Corresponding author address: Zhong Zhong, College of Meteorology and Oceanography, PLA University of Science and Technology, No. 60, Shuanglong Road, Nanjing 211101, China. E-mail: zhong_zhong@yeah.net

1. Introduction

The South China Sea (SCS) is one of the largest semienclosed marginal seas in the western North Pacific (WNP). Almost all the tropical cyclones (TCs) formed over the SCS can make landfall either along the southern China coast or in Vietnam and the Philippines shortly after their formation, resulting in severe damages resulting from TC-caused strong winds, storm surges, floods, and landslides. Because of the limited warning time prior to landfall, these TCs often cause great loss of human life and property destruction in the coastal areas. Understanding the multiscale temporal variation of the SCS TC activity is important for improving the TC prediction skill and thus will have significant socioeconomic impact.

The interannual variability of the SCS TC activity is modulated primarily by El Niño–Southern Oscillation (ENSO; Goh and Chan 2010). Although the SCS TC activity in the summer months (June–August) exhibits unobvious difference between the El Niño and La Niña events, TC frequency during strong El Niño (La Niña) years is significantly below (above) normal in September and October (Chan 2000). Moreover, the influence of ENSO on the SCS TC activity not only exists in the El Niño and/or La Niña years but also lingers after the peak phases. Induced by the El Niño teleconnection, tropical Indian Ocean warming persists throughout the following summer after the warm sea surface temperature (SST) anomalies over the equatorial eastern Pacific has dissipated, exerting its remote forcing by the lower-level equatorial baroclinic Kelvin wave (Wu et al. 2009; B. Wu et al. 2010; Xie et al. 2009). The Kelvin wave with anomalous easterly flow reduces zonal vertical shear over the SCS and western WNP, which may promote the TC activities there in the decaying El Niño summers (Du et al. 2011; Ha et al. 2013a,b).

In recent studies, the equatorial central-eastern Pacific warming has been separated into two regimes based on the spatial distribution of the maximum SST (Ashok et al. 2007). In addition to the traditional El Niño with a maximum warm SST in the cold tongue region of the eastern Pacific, El Niño Modoki, which is a phenomenon with the warm anomalies over the equatorial central Pacific and colder SSTs on its western and eastern sides, has occurred frequently in the past three decades (Lee and McPhaden 2010). Chen (2011) revealed that above-normal TC activity appears over the SCS in the El Niño Modoki summers, whereas suppressed TC activity is observed in the traditional El Niño falls. These distinct features are mainly attributed to differences large-scale circulation triggered by the tropical SST anomalies in the two types of equatorial Pacific warming events.

Many previous studies have focused on the interdecadal/decadal variability of TC activity over the SCS. The SCS TC frequency significantly decreased from the mid-1970s through the early 1990s compared to that in the earlier period from the 1950s to the 1970s, exhibiting a clear interdecadal reduction since the mid-1970s (Chan 2008; Kubota and Chan 2009; Goh and Chan 2010). Various mechanisms have been proposed to interpret this interdecadal change around the mid-1970s. Goh and Chan (2010) attributed it to changes in the large-scale circulation over the western Pacific, which are closely related to the phase shift of the Pacific decadal oscillation (PDO) in the 1970s. Wang et al. (2013) argued that it is associated with the SST warming over the tropical Indian Ocean, which plays a crucial role in intensifying the anomalous descending flow and lower-level anticyclone over the SCS. Concurrent variations in the East Asian jet stream and western Pacific subtropical high also help explain the reduction in the SCS TC genesis since the mid-1970s (X. Wang et al. 2012).

Recent studies have revealed a significant decadal-scale enhancement in the SCS TC activity after the early 1990s (Chen et al. 2012; X. Wang et al. 2012; Ha et al. 2014). Compared to the period 1979–93, the SCS TC number markedly increased from the mid-1990s through the early 2000s. Moreover, TC activities before the mid-1990s were mostly confined within the SCS basin, whereas more TCs form over the SCS and move north-northeastward thereafter, and finally enter the East China Sea. Specifically, Ha et al. (2014) found that the changes in the large-scale circulation and anomalous environmental fields on the decadal time scale before and after 1994 are unable to explain the decadal increase in the SCS TC genesis in the mid-1990s (Ha et al. 2014; please see their Figs. 4, 5, 6, S1, and S2). They further proposed that this decadal variation of TC activity is closely related to the change in the SCS atmospheric intraseasonal variability (ISV) and suggested that the increased TC activity after the mid-1990s can be largely attributed to the simultaneous enhanced convection in the active and wet phases of the local ISV.

It is noted that the TC activity over the SCS exhibits two decadal shifts in the past 30 years. After the significant cyclogenesis increase in the mid-1990s, the SCS TC frequency exhibits an abrupt decrease since the early 2000s (Li and Zhou 2014; Ha et al. 2014; also please see Fig. 1), but the robustness of the decadal change in the early 2000s needs to be further confirmed by different TC best-track datasets as well as by various diagnostic methods. Moreover, it is found that the environmental variables over the northern SCS experienced a simultaneous decadal change around 2002/03. The suppressed TC activity after the early 2000s is closely related to the decadal shift of background condition over the SCS. Considering that the mechanism behind this change around 2002/03 appears to differ from the former one in the mid-1990s, it is necessary to investigate the recent decadal change over the SCS, which still remains an open problem.

Fig. 1.
Fig. 1.

Time series of the SCS TCGF from June to August during 1979–2011 derived from the (a) JTWC, (b) JMA, and (c) CMA datasets. The dashed curves are the mean TCGF of three periods (1979–93, 1994–2002, and 2003–11), and the respective epochal mean values are denoted above the curves.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Note that a remarkable SST warming is found dominant over the tropical central Pacific after the late 1990s (Lee and McPhaden 2010). This SST anomaly arises from the interdecadal change, which is characterized by a grand La Niña–like background pattern with the SST cooling in the equatorial central-eastern Pacific and the SST warming in the western Pacific (Xiang et al. 2013; their Fig. 7). Given the shift of SST anomalies over the tropical Pacific after the late 1990s, corresponding changes in the large-scale circulation over East Asia on the decadal time scale have been found and reported in recent studies (Xiang and Wang 2013; Zhu et al. 2014). For instance, Xiang and Wang (2013) found that the Asian summer monsoon onset became earlier over the Arabian Sea, the Bay of Bengal, and the SCS in the middle-to-late 1990s, and Zhu et al. (2014) noted that the decadal rainfall change over southern China experienced an out-of-phase relationship between boreal spring and summer around the 1990s. These phenomena are attributed primarily to the forcing of the grand La Niña–like SST anomaly over the tropical Pacific. Considering the changes in background SST over the tropical oceans, it is hypothesized that the decadal variation of the SCS TC activity should have a linkage with the anomalous large-scale circulation, which is associated with the remote tropical SST forcing. The physical mechanisms and the interactive process need to be investigated in detail. In addition, according to the close relationship between the SCS TC genesis and the local ISV activity during the period 1975–2010 (Ha et al. 2014), the influence of the SCS ISV on the decadal reduction in TC activity in the early 2000s also needs further examination. Based on the above discussions, the present research is designed to study the variability of the SCS TC activity on the decadal time scale in the recent decades and to understand the physical mechanisms responsible for the decadal TC activity change occurring in the early 2000s.

This paper is organized as follows: The datasets and methodology used in this study are described in section 2. Observed decadal reduction in summer TC activity over the SCS in the early 2000s and its association with the environmental conditions are presented in section 3. The influences of the Indo-Pacific Ocean SST anomalies on the decadal change in the SCS TC activity are analyzed in section 4. The relationship between the cyclogenesis and ISV activity over the SCS is discussed in section 5. Finally, section 6 gives discussion and conclusions.

2. Data and methodology

a. Datasets

Three TC best-track datasets at a 6-h interval are used in this study. They are provided by the Joint Typhoon Warming Center (JTWC 2012), the Regional Specialized Meteorological Center of Japan Meteorological Agency (JMA 2012), and Shanghai Typhoon Institute of China Meteorological Administration (CMA 2013), respectively. The three TC datasets are employed to detect the robustness of the decadal change in TC activity occurring in the early 2000s over the SCS. We examine the TCs that formed over the SCS (10°–22.5°N, 105°–120°E) and the associated environmental variables in summer (June–August) during the period 1979–2011. Only those TCs reaching tropical storm intensity (maximum sustained wind speed ≥17.2 m s−1) are selected for this study.

The monthly Extended Reconstructed SST (ERSST) dataset with a spatial resolution of 2° × 2° (Smith et al. 2008) is used in this study. Another dataset includes the daily outgoing longwave radiation (OLR; Liebmann and Smith 1996). Horizontal winds, vertical velocities, geopotential heights, and relative humidity fields are extracted from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis project (NNRP) dataset (Kalnay et al. 1996). These datasets are available at the Earth System Research Laboratory of the National Oceanic and Atmospheric Administration (NOAA 2014a). To obtain the intraseasonal signal of the 30–60-day atmospheric oscillation, the Lanczos bandpass filter (Duchon 1979) is applied to the daily OLR and 850-hPa wind fields. In addition, we also examine another two SST datasets with higher spatial resolution of 1° × 1°: the HadISST data (Rayner et al. 2003; Hadley Centre for Climate Prediction and Research 2006) and the Centennial In Situ Observation-Based Estimates of the Variability of SST and Marine Meteorological Variables (COBE) SST data (Ishii et al. 2005; NOAA 2014b). It is found that the results based on analyses of the HadISST and COBE datasets are quite close to that from the ERSST (not shown). This indicates that the mechanism of the decadal change in the SCS TC activity is not sensitive to the SST dataset chosen.

b. Methods of changepoint detection for decadal change

Two methods to detect the changepoint for decadal variability—the Lepage test and the minimum description length (MDL) objective function method—are adopted to identify the point of decadal abrupt change. We first test the autocorrelation of TC series derived from the three best-track datasets. Their autocorrelation coefficients with time lags of 1–10 yr are generally insignificant in the interannual-to-decadal time scale during 1979–2011 (not shown). Thus, the individual values in the TC series can be considered as independent ones within the study period. For this reason, the above methods can be applied to these TC series to detect the decadal abrupt change.

The Lepage test is a common technique to detect abrupt changepoints in time series with long-term trends (Lepage 1971). It has been widely adopted to investigate issues related to climate change (Kwon et al. 2007; Liu et al. 2011; Xu and Wang 2014). The Lepage test statistic [HK, the same as used by Yonetani and McCabe (1994)] is represented by a combination of the standardized Wilcoxon’s (W) and Ansari–Bradley’s (A) statistics
eq1
It is a nonparametric test to diagnose the significant difference between two series, even if the distributions of their parent populations are unknown. A series xy (x1, x2, …, , y1, y2, …, ) is composed of two independent series x (x1, x2, …, ) and y (y1, y2, …, ) with sizes n1 and n2, respectively, and assume that ui = 1 (ui = 0) if the ith smallest data point in xy belongs to x (y). The statistics for the HK can be derived based on the following empirical formulas:
eq2
eq3
eq4
eq5
eq6
eq7
The test statistic HK follows a chi-squared distribution with 2 degrees of freedom. Because the decadal time scale generally features a period of 8–13 yr (Mehta et al. 2000), a 9-yr window is chosen to detect the decadal changes by this method. A detailed description and application can be found in Liu et al. (2011).
The traditional MDL theory is rooted in information theory and stochastic complexity (Rissanen 2007). The MDL method is designed to seek integer-valued parameters, such as the number and time of changepoints in a series through minimizing a penalized likelihood objective function based on the MDL principles (Hansen and Yu 2001; Lund et al. 2007). Lu et al. (2010) first employed the MDL criterion in climatic changepoint research. Li and Lund (2012) introduced genetic algorithms into the MDL technique to optimize the objective function, and their improvements have led to an effective detection of the changepoints in annual TC genesis series over the North Atlantic basin. The form of MDL scores is given by
eq8
where Lopt is the optimized model likelihood objective function, P is the penalty term that accounts for the number and type of model parameters, and log2 means logarithm base 2. We adopt the Eq. (3.8) of Li and Lund (2012) to calculate the MDL scores of TC counts. By enumerating different points in the TC series and calculating the associated MDL scores, one can derive the optimum MDL scores for various numbers of segments. More details of the method and its applications can be found in Li and Lund (2012).

c. Mann–Whitney U statistic for significance test in environmental fields

Because the sample size of two decades is small, we use a nonparametric method, the Mann–Whitney U statistic test (Mann and Whitney 1947), to examine the significance of the difference in the large-scale environmental variables between the two periods. This rank-sum test statistic is a function not of the data values themselves, but of their ranks within n observations (n = n1 + n2) that are pooled and ranked under the null hypothesis (Wilks 2006). It is proved effective when the sample size is small, or when the distribution of the sample is unknown (Li and Zhou 2014). Specifically, R1 and R2 are defined as the sum of the ranks held by the members of the two series, respectively, in this pooled distribution, and n1 and n2 are their sizes. The Mann–Whitney U statistic test can be calculated by
eq9
eq10
Here U1 and U2 are the statistics for the two series, respectively. The null distribution of the Mann–Whitney U statistic is approximately a Gaussian distribution with the mean parameter μu = n1n2/2 and variance parameter σu = [n1n2(n1 + n2 + 1)/12]1/2. Within this Gaussian distribution, the observed U1 and U2 correspond to a standard Gaussian value z = (U1μu)/σu and z = (U2μu)/σu, respectively. Based on the tables of critical values for the smaller sample size (Conover 1999), one can estimate whether the difference between two samples is significant at a certain confidence level. More details of this method and an example of a cloud seeding experiment can be found in section 5.3.1 of Wilks (2006).

3. Observation of decadal change in the early 2000s

a. TC activity

Table 1 shows the correlation matrix between the SCS TC genesis frequency (TCGF) in summer derived from the JTWC, JMA, and CMA datasets. It can be seen that the TCGFs from the three best-track datasets are highly correlated in the period 1994–2011, with correlation coefficients above 0.8 and up to 0.92 between the JTWC and JMA. This agreement indicates a consistent feature in the interannual variability of the SCS cyclogenesis recorded by the three different datasets. Figure 1 shows the annual TCGF over the SCS from June to August in the past 30 years. The variation of TC frequency experiences two obvious decadal changes, which occurred in the mid-1990s and the early 2000s, respectively. The result of the Lepage test detection shows that the first abrupt change took place in 1993/94, which exceeds the 99% and 95% confidence levels in the JTWC/JMA and CMA datasets, respectively (Fig. 2). During 1979–93, the JTWC, JMA, and CMA datasets record 0.8, 1.0, and 1.8 TCs annually, whereas the number significantly increases to 3.0, 2.6, and 3.1 TCs during 1994–2002, respectively.

Table 1.

Correlation matrix between the TCGFs, omega, and relative humidity (RH) indices from June to August in the period 1994–2011. The correlation coefficients are all statistically significant above the 99% confidence level (±0.58).

Table 1.
Fig. 2.
Fig. 2.

Statistics of Lepage test for the SCS TCGF derived from the three datasets in the period 1979–2011. The horizontal dashed lines indicate the test statistic critical values of 9.21 and 5.99, which correspond to the 99% and 95% confidence levels, respectively.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Although this abrupt TC increase in the period 1994–2002 is not consistent with changes in the large-scale circulation and anomalous environments over the northern SCS since the early 1990s (R. Wu et al. 2010; Lu et al. 2011; Chen et al. 2012; Ha et al. 2014), the intensified local ISV activity is closely related to the change in TC genesis (Ha et al. 2014). The second abrupt change in TC genesis occurred in 2002/03. This decadal reduction that can be found in all the three TC datasets exceeds the 99% confidence level based on the Lepage test (Fig. 2). The annual TC frequency recorded in the JTWC, JMA, and CMA datasets sharply decreased to 0.9, 0.7, and 1.2 over the SCS during 2003–11, respectively, less than one-third of that during 1994–2002 (3.0, 2.6, and 3.1).

The diagnostic results of MDL method agree well with the above conclusion. Table 2 lists the optimum MDL scores for changepoints of various numbers. For the JTWC dataset, the minimal MDL score with one changepoint is −21.934 and is located at 1994. The optimal MDL score of −21.8136 is very close to the global optimal score, with the two changepoints occurring at 1994 and 2003. For both the JMA and CMA datasets, the global optimal changepoint is located at 2003. The optimal MDL scores for the two changepoints are slightly worse than the global one, with the changepoints located at 1994 and 2003. In general, the abrupt change in 2002/03 is the most distinct decadal change in the SCS TCGF during 1979–2011.

Table 2.

Optimum MDL scores for various numbers of changepoints in the SCS TCGF during 1979–2011 derived from the JTWC, JMA, and CMA datasets.

Table 2.

Figure 3 shows the genesis locations and tracks of TC recorded by the three datasets in the active period (1994–2002) and inactive period (2003–11) based on the abrupt change around 2002/03. On the whole, most TCs during 1994–2011 formed over the central-northern SCS from June to August, which is consistent with the previous studies’ finding that the main TC genesis location in the summer is situated north of 15°N (Wang et al. 2007). More TCs made landfall during 1994–2002 (Figs. 3a,c,e) than during 2003–11, bringing destructive impacts along the coastal areas. Note that these TCs generally moved farther inland and sustained longer after making landfall, implying that the intensity of TCs in the period of 1994–2002 is much stronger than in the period of 2003–11. Moreover, in addition to the traditional west-northwestward movement, some of these TCs followed a northward track and made landfall over southern China. It is particularly noteworthy that 5–6 TCs moved northeastward and passed through either the Luzon Strait or the Taiwan Strait after their formation in the SCS, and then entered the East China Sea and the Philippine Sea, imposing great impacts on the midlatitude areas (Ha et al. 2014).

Fig. 3.
Fig. 3.

TC genesis locations (red TC symbols) and tracks (lines) from June to August recorded in the (a),(b) JTWC, (c),(d) JMA, and (e),(f) CMA datasets during (top) 1994–2002 and (bottom) 2003–11. The blue TC tracks denote those TCs enter the East China Sea. The green contours indicate 5865 gpm averaged from June to August in the respective periods. The numbers at the top left in each panel denote total and annual TCGFs.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Contrarily, in the latter period of 2003–11, most TCs rapidly weakened after making landfall over the coastal areas (Figs. 3b,d,f), and the life span of each individual TC also became short compared to those in the period of 1994–2002. This phenomenon indicates that the intensity of TCs in 2003–11 became dramatically weaker than in 1994–2002. The movement of TCs is largely dominated by the steering flow in the midtroposphere, and TCs formed in the WNP tend to move along the western flank of the western Pacific subtropical high (Liu and Chan 2008). During 1994–2002, the subtropical high became weak, shrunk in size, and shifted eastward compared to that in the latter period (Fig. 3). As a result, more TCs moved north-northeastward and either made landfall along the coastal region in southeastern China or entered the East China Sea. In contrast, fewer TCs took the north-northeastward track during 2003–11 than during 1994–2002 because of the intensified subtropical high.

In addition, it is noted that discrepancies of the TC frequency exist among the three TC best-track datasets shown in Fig. 3. This is because the time interval used for calculating the mean sustained wind speed and determining TC intensity is different in these three datasets (1, 10, and 2 min for the JTWC, JMA, and CMA, respectively). Because of the difference in time interval, the same TC is likely to be recorded with different intensities in these different datasets. As a result, the number of TCs that can reach tropical storm intensity would also be different. Despite the fact that the methods to record TCs in the best-track datasets are slightly different at various tropical analysis centers, the variation of the SCS TC activity based on these different reanalysis datasets in the past 30 years is generally consistent among the databases, and the decadal reduction in the SCS TC activities around 2002/03 is also robust.

b. Environmental conditions

The summer monsoon dominates the local synoptic-scale disturbances and TC activities over the tropical monsoon regions to a great extent (B. Wang et al. 2012; Xu and Wang 2014). To obtain a clear picture of the climatological background of the SCS summer monsoon in the past 20 years, we plot the epochal differences (2003–11 minus 1994–2002) in summer 850-hPa wind (Fig. 4e), which is often utilized as a proxy for the evolution of the SCS summer monsoon (e.g., Wang et al. 2009). It shows that an anticyclonic circulation anomaly is located over southern China, accompanied by the anomalous east-northeasterly flow over the northern SCS and Indochina. Meanwhile, a dynamically coherent suppressed rainfall is found over southern China and the northern SCS (not shown).

Fig. 4.
Fig. 4.

Epochal differences in (a) vertical pressure velocity (shading; Pa s−1) averaged from 850 to 300 hPa, (b) relative humidity (shading; %) averaged from 850 to 300 hPa, (e) 850-hPa wind (vector; m s−1), and (f) vertical wind shear of 200 minus 850 hPa (shading; m s−1) in summer between the periods of 2003–11 and 1994–2002, and differences enclosed by solid contours and thick wind vectors denote that the anomalies are significant above the 95% confidence level based on the Mann–Whitney U statistic. Time series of (c) vertical pressure velocity [Pa s−1; area-weighted mean over the box of 15°–25°N, 105°–120°E in (a)] and (d) relative humidity [%; area-weighted mean over the box in (b), as in (a)]. The red lines and numbers in (c) and (d) denote the epochal mean values. The decadal shift of omega (relative humidity) index in 2002/03 is significant above the 99% (99%) confidence level based on the Lepage test.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Abundant moisture and ascending motion in the lower-to-middle troposphere are favorable for TC formation and intensification (Wang 2009; Wang 2011). Since about 90% of SCS TCs originate from north of 15°N during the period of June–August, we define 15°–25°N, 105°–120°E as the TC domain shown in Figs. 4c and 4d. Table 1 lists the correlation between the TCGF derived from the three datasets and the series of area-weighted vertical velocity (omega) and relative humidity vertically averaged from 850 to 300 hPa. Their correlation coefficients are all significant above the 99% confidence level, implying that the interannual variation of the TC frequency has a close relationship with the ambient environmental variables over the northern SCS. Figures 4a and 4b show the epochal differences (2003–11 minus 1994–2002) in tropospheric omega and relative humidity in the summer, respectively. Similar to the background change in the SCS summer monsoon, the omega and relative humidity fields display a typical dipole pattern, with a significant dry subsidence anomaly and suppressed convection over the northern SCS and southern China, accompanied by an enhanced ascending motion and active convection over the tropical western Pacific (Figs. 4a,b). The anomalous distribution of the OLR field also highly resembles the dipole pattern of omega and relative humidity fields (not shown). These anomalies indicate that the environmental conditions are unfavorable for TC activity over the SCS during 2003–11.

In addition to the increasing (decreasing) trend in the omega (relative humidity) indices over the northern SCS in 1994–2011, significant decadal changes in the two variables are detected around 2002/03. These changes exceed the 99% confidence level by the Lepage test (Figs. 4c,d). This result indicates that the decadal reduction in the SCS TC activity around 2002/03 is under strong influence of the simultaneous anomalous subsiding motion and suppressed convection, which are closely related to the decadal-scale weakening of the summer monsoon over the northern SCS.

Previous studies have revealed that a strong vertical wind shear can play an evidently negative role in TC formation and intensification over the WNP and the SCS (Emanuel 2000; Frank and Ritchie 2001; Wong and Chan 2004; Zeng et al. 2007). Although no obvious shear anomaly has been observed over the northern SCS where most TCs formed, significant positive anomaly of vertical shear is found during 2003–11 along the southern China coast between 20° and 25°N (Fig. 4f). The enhanced vertical shear north of 20°N can lead to rapid TC weakening after the TC makes landfall, which partly explains why the life span of TCs became short in the period of 2003–11. In other words, changes in the vertical wind shear along the coastal region in southern China have contributed to the decadal reduction in the SCS TC intensity and life span around 2002/03 to some extent.

4. Influences of the Indo-Pacific Ocean SST anomalies

To identify the driving force that controls the decadal change in the SCS TC activity, we calculate the simultaneous correlation between the TCGF indices and global SST in the period 1994–2011 (Figs. 5a–c). The significant correlation clearly reflects the impact on TC activity caused by the pronounced SST warming over the northern Indian Ocean (NIO) and the west-southwestern Pacific and the SST cooling over the central-eastern Pacific around 10°–20°N. The distribution of correlation distribution in the Pacific is close to a well-known grand La Niña–like pattern (Xiang and Wang 2013), which is considered to be a new interdecadal paradigm for the predominance of the standing central Pacific warming after the late 1990s (Xiang et al. 2013). Figures 5d and 5e show the simultaneous correlations between global SST and the vertical velocity and relative humidity indices shown in Figs. 4c and 4d, respectively. The most significant correlation between the two environmental variables and SST appears in the NIO and the equatorial western Pacific Ocean (WPO) around 150°–160°E.

Fig. 5.
Fig. 5.

Simultaneous correlation between SST and the (a) JTWC TCGF, (b) JMA TCGF, (c) CMA TCGF, (d) omega (in Fig. 4c), and (e) relative humidity (in Fig. 4d) indices in summer. Calculations cover the period 1994–2011. Shading indicates correlation coefficients are statistically significant above the 95% confidence level. The boxes indicate the area for the NIO (5°S–20°N, 60°–95°E) and WPO (10°S–10°N, 145°–165°E), where time series of SST are calculated.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Results of the above correlation analysis imply that the tropical SST forcing over the NIO and the WPO is a crucial factor for the decadal reduction in summer monsoon and TC activity over the SCS. Figure 6a illustrates the epochal difference (2003–11 minus 1994–2002) in the summer SST pattern. It is shown that an obvious warming appears over the NIO during 2003–11. Meanwhile, the SST anomaly over the tropical Pacific displays a La Niña–like pattern, with a significant warming over the west-southwestern Pacific and a cooling over the northeastern Pacific. The correlation coefficients shown in Fig. 5 clearly indicate that the changes in the TCGF and environmental variables over the northern SCS are closely linked to the SST warming over both the NIO and the WPO not only on the interannual time scale (Fig. 5) but also on the decadal time scale (Fig. 6a).

Fig. 6.
Fig. 6.

(a) Epochal differences in SST (contour; °C) in summer between the periods of 2003–11 and 1994–2002. Shaded areas indicate where the anomalies are significant above the 95% confidence level based on the Mann–Whitney U statistic. Time series of the (b) NIO SST [area-weighted mean over 5°S–20°N, 60°–95°E, box in (a)] and (c) WPO SST [area-weighted mean over 10°S–10°N, 145°–165°E, box in (a)]. The dashed lines in (b) and (c) denote the linear trends of SST change and the epochal means; trend values [°C (18 yr)−1] and epochal mean values are also shown. The decadal shift of the NIO (WPO) SST index in 2002/03 is significant above the 99% (95%) confidence level based on the Lepage test.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Figures 6b and 6c show the time series of the NIO SST (area-weighted mean over 5°S–20°N, 60°–95°E) and the WPO SST (area-weighted mean over 10°S–10°N, 145°–165°E) from 1994 to 2011. One prominent feature in the SSTs averaged over the two regions is the increasing trend in the past 20 years, which reaches 0.34°C (18 yr)−1 [0.23°C (18 yr)−1] for the NIO (WPO) SST index. This implies that the SST warming possibly contributed to the linear trend of the changes in the environmental fields over the northern SCS. Moreover, an abrupt increase in the NIO SST occurring around 2002/03 that exceeds the 99% confidence level by the Lepage test (Fig. 6b) is detected on the decadal time scale. Meanwhile, a sharp increase in the WPO SST occurred from the late 1990s to the early 2000s, and the decadal change in the WPO SST between the time periods of 1994–2002 and 2003–11 is also significant at the 95% confidence level based on the Lepage test (Fig. 6c). The decadal in-phase relationship between the NIO and WPO SST changes implies a potential combined impact of the Indo-Pacific Ocean SST warming on the decadal variation of the SCS TC activity in the early 2000s.

To reveal the physical processes for the decadal reduction in the SCS TC activity that are associated with the anomalous atmospheric circulation induced by the tropical SST anomalies, we assess the epochal differences (2003–11 minus 1994–2002) in 200-hPa velocity potential and divergent wind (Fig. 7), which can reflect the variation of large-scale vertical motions as well as the Walker circulation. It can be seen that the anomaly of the velocity potential displays a tripole wave train pattern over the tropical Indo-Pacific region. Two upper-level mass source (divergence) centers are located at the equatorial western Indian Ocean and the WPO around 60° and 150°E, respectively. The anomalous zonal divergent winds converge toward the upper-level mass sink (convergence) region from the SCS to the west of the Sumatra around 10°N.

Fig. 7.
Fig. 7.

Epochal differences in 200-hPa velocity potential (contour; 106 m2 s−1) and divergent wind (vector; m s−1) in summer between the periods of 2003–11 and 1994–2002. “Con” and “Div” indicate convergent and divergent centers, respectively.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Figure 8a shows the longitude–height cross section of epochal differences (2003–2011 minus 1994–2002) in velocity, relative humidity, and SST averaged from the equator to 20°N. Corresponding to the zonal gradient of tropical SST caused by the warming over the NIO/WPO and the cooling over the SCS (Figs. 6a and 8a), anomalous ascending motions accompanied by the midtropospheric moistening are observed west of 80°E and around 140°–160°E. Meanwhile, the descending branch of the anomalous zonal circulation is dominant over the region around 90°–120°E (Fig. 8a). The anomaly of subsiding flow over the SCS is responsible for the anomalous environmental fields as shown in Fig. 4. These anomalous environmental fields are important factors that suppress the development of synoptic-scale disturbance and may eventually result in the decrease in TC activities over the northern SCS in the period 2003–11.

Fig. 8.
Fig. 8.

Cross sections of (a) the epochal differences in velocity (streamline), relative humidity (shading; %), and SST (red line; °C) for zonal circulation averaged from the equator to 20°N, and (b) differences in velocity (streamline) and relative humidity (shading; %) for Hadley circulation averaged from 105° to 125°E in summer between the periods of 2003–011 and 1994–2002. The relative humidity is available in the NNRP data only from 1000 to 300 hPa.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Although the anomalously warm SST is observed in the northeastern Indian Ocean (Fig. 6a), convection is still suppressed and anomalous descending motion is evident in this region (Figs. 7 and 8a). This phenomenon can be explained by the passive response of the Indian Ocean SST to the remote effects of the WPO and northwestern Indian Ocean SST anomalies. This process is considered to be a particular process of air–sea interaction with a negative SSTA–precipitation relationship over the Asian–Australian summer monsoon region (Wu and Kirtman 2005; Wang et al. 2004, 2005; Wu et al. 2009). Because of the distinct SST warming over the northwestern Indian Ocean and the WPO, anomalous Walker-like (zonal) circulations are induced and the subsiding flows of the anomalous circulations converge and dominate the area over 90°–120°E. As a result, fewer clouds are formed there and more incoming shortwave radiation can reach the sea surface, leading to the passive response of SST increase over the northeastern Indian Ocean.

Figure 8b depicts the latitude–height cross section of epochal differences (2003–11 minus 1994–2002) in velocity and relative humidity averaged from 105° to 125°E. In accord with the anomalous ascending motion over the equatorial region, the enhanced descending dry air dominates north of 5°N, connected by the anomalous local Hadley circulation. Moreover, it is worth noting that the anomalous zonal and Hadley circulations share the same descending branch located over the northern SCS, which can explain why the much stronger anomalous vertical velocity and drier condition are found there. This also explains the more westward extending of the western Pacific subtropical high in the northern SCS. These conditions are likely related to the decadal changes in the environmental variables over the northern SCS, which markedly suppress the TC activity after 2002. Therefore, the SST warming in the NIO and the WPO exerts combined effects on the SCS TC activity through the anomalous zonal and Hadley circulations.

5. Possible role of the SCS ISV

The SCS summer monsoon presents a strong ISV in the East Asian monsoon regions (Kemball-Cook and Wang 2001). The active (inactive) phases of ISV are associated with the enhanced (suppressed) convection and are closely related to the onset and evolution of the SCS monsoon. The ISV activity plays a critical role in weather and climate over the East Asian regions (Zhou and Chan 2005). Moreover, the SCS ISV activity also largely determines the features of the local TC genesis and track (Li et al. 2012; Feng et al. 2013; Zhang 2005, 2013). In this section, the possible role of local ISV activity on the decadal reduction in TC genesis in the early 2000s is examined.

To quantitatively determine the intensity of the SCS ISV activity during 1994–2011, we calculate the mean standard deviation of 30–60-day filtered OLR over the SCS averaged from June to August, which can represent the seasonal intensity of the intraseasonal atmospheric oscillation in summer (Kajikawa and Wang 2012). Figure 9a shows the epochal difference in the standard deviation of OLR anomalies between the periods of 2003–11 and 1994–2002. The evident negative standard deviation of OLR anomalies is found over the central-northern SCS, which is the major area of TC genesis (Fig. 2). The spatial distribution of standard deviation of OLR anomalies indicates that the ISV activity over the northern SCS is greatly suppressed after 2002. Figure 10b shows the time series of the mean standard deviation of the filtered, area-weighted mean OLR over 7.5°–20°N, 110°–122.5°E (box in Fig. 9a). An obvious decrease can be observed around 2002/03, which is consistent with the time of decadal change in the SCS ISV activity.

Fig. 9.
Fig. 9.

(a) Epochal differences in mean standard deviation of OLR anomalies (contour; W m−2) on a 30–60-day time scale in summer between the periods of 2003–11 and 1994–2002. Anomalies enclosed by solid contours denote those are significant above the 95% confidence level based on the Mann–Whitney U statistic. (b) Time series of ISV index [area-weighted mean over 7.5°–20°N, 110°–122.5°E, box in (a)] in the period 1994–2011. The dashed lines denote the epochal mean, and values are also shown.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Fig. 10.
Fig. 10.

Latitude–time cross section in summer 30–60-day filtered OLR (shading; W m−2) and 850-hPa wind divergence (contour; 10−6 s−1) anomalies averaged from 100° to 125°E during (a) 1994–2002 and (b) 2003–11, overlapped with the formation time and latitude of every TC (TC symbols). The contour interval is 0.3 × 10−6 s−1, and zero contours are omitted. The number in parentheses denotes the summer TCGF in that year.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

Figure 10 shows the latitude–time cross section in the 30–60-day filtered OLR and 850-hPa divergence anomalies averaged from 100° to 125°E. It is found that the filtered OLR generally exhibits a stronger intraseasonal oscillation during 1994–2002 than in 2003–11, which agrees well with the epochal difference in the mean standard deviation of the filtered OLR as shown in Fig. 9a. When overlapping with the formation time and latitude of every TC, it can be seen that most TCs in 1994–2002 tended to form in the active/wet phases of ISV. Anomalous lower-level convergence of the filtered 850-hPa winds is also found during the active/wet phases of the ISV (Fig. 10a). This is because the active/wet phases of the ISV often accompany abundant moisture and strong convergence flow in the lower troposphere, which are favorable for cyclogenesis and TC formation. In contrast, fewer TCs form in the latter period of 2003–11 when the ISV activity was suppressed (Fig. 10b). However, some TCs can still form in the inactive/dry ISV phases in this period, for example, 2005 and 2010. This implies that under the suppressed ISV conditions, some other environmental variables might play more important role in cyclogenesis over the SCS when the ISV activity is inactive. Apparently, the variability between the TC activity and the intensity of ISV does not show a linear relationship, and this nonlinear impact of the local ISV on the TC genesis needs further investigation.

To quantitatively determine the impact of ambient ISV activity on the TC genesis, we calculate the ambient ISV signal around individual TC by averaging the 30–60-day filtered OLR in the four grid points that are closest to the TC formation position at the time of TC genesis. Figure 11 shows the box plots of ambient 30–60-day filtered OLR anomalies for each TC genesis in the two periods. A more concentrated distribution of the samples is exhibited during 1994–2002 than during 2003–11. Moreover, the mean value of the TC-ambient filtered OLR between the two periods has changed significantly, and the change is significant at the 95% confidence level based on the Mann–Whitney U statistic. These results suggest that the summer cyclogenesis over the SCS has a close linkage with the TC-ambient environmental condition, which is associated with the ISV activity. Again, the decadal reduction in the SCS ISV after 2002 is accompanied by the suppressed convection and insufficient moisture content in the lower atmosphere, which are unfavorable for TC activity over the SCS.

Fig. 11.
Fig. 11.

Box plots of ambient 30–60-day filtered OLR anomalies of each TC genesis from (a) 1994–2002 and (b) 2003–11. Plotted are the inner quartile range (box), the median (horizontal line inside the box), the 25th and 75th percentile ±1.5 times the inner quartile range (horizontal line below and above the box, respectively), the mean value of all the samples (thick cross and corresponding number), and the ambient 30–60-day filtered OLR anomalies of each sample (dots).

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

We have calculated the correlation between the SCS ISV index and the simultaneous global SST in the summers of 1994–2011 to detect the cause of the ISV variation (results not shown). Significant positive correlation is found over the northern SCS and the Philippine Sea. Meanwhile, a decreasing trend of the SCS SST is observed during 1994–2011 (not shown). The persistent cooling in the local SST might suppress the ISV activity over the SCS during 2003–11. Moreover, the northern SCS is controlled by the anomalous lower-level easterly flow during 2003–11 (Fig. 3a), which is triggered by the zonal SST gradient between the cold SCS and the warm NIO. The anomalous easterly flow suppresses the ISV activity and reduces the synoptic-scale disturbances over the SCS (Xu and Wang 2014), contributing to the decrease in TC activity during 2003–11. The above analysis demonstrates that the suppressed ISV activity is also a critical factor that affects the decadal reduction in the SCS TC activity occurring around the early 2000s.

6. Discussion and conclusions

This study reveals that TC activity over the SCS experienced a significant decadal change in the early 2000s. The TCGF obviously decreased during the period from 2003 to the early 2000s compared to that in the preceding period of the mid-1990s to the early 2000s. During 1994–2002, the TC intensity is much stronger than during 2003–11, and TCs made more frequent landfall along the coastal areas during 1994–2002 than during 2003–11. In particular, several TCs moved northeastward and passed through either the Luzon Strait or the Taiwan Strait after their formation over the SCS, and then entered the East China Sea, imposing great impacts on the midlatitude regions. In contrast, after the early 2000s, TC activities were primarily confined within the SCS basin, and stayed on the northwestward track. Most of them rapidly weakened after making landfall.

It is found that the large-scale circulation as well as the environmental variables over the northern SCS experienced a significant decadal change around 2002/03, simultaneous to the decadal change in TC activities. In 2003–11, a lower-level anomalous anticyclonic circulation became dominant over southern China, and the northern SCS was controlled by the anomalous east-northeasterly flow and subsiding motion with dry air mass. In addition, the intensity of the SCS summer monsoon persistently decreased from 1994 to 2011. Further analyses reveal that the suppressed TC activity after the early 2000s can be largely attributed to the decadal change in background conditions over the northern SCS. Note that the mechanism for the decadal change in the SCS TC activity that occurred around 2002/03 is different from that for the change occurring in the mid-1990s. The increased TC frequency during 1994–2002 can be largely attributed to enhanced convection in the active phases of the local ISV activity over the SCS. However, changes in the large-scale circulation and anomalous environmental variables before and after 1993/94 are unable to explain the decadal increase in TC genesis in the mid-1990s. (Ha et al. 2014).

Figure 12 shows a schematic illustration of the processes that potentially explain how the anomalous atmospheric circulation affects the decadal change in the SCS TC activity occurring around 2002/03. Compared with the preceding period of 1994–2002, obvious SST warming appears over the NIO and the WPO, which is generally considered to be arising from the natural decadal variability (Zhou et al. 2009; Xiang et al. 2013). Induced by the zonal gradient of tropical SST, anomalous ascending motions develop over the NIO around 60°–80°E and over the WPO around 150°E, respectively. The two branches of zonal divergent winds in the upper level converge in the SCS region, and the strong subsiding flow of the anomalous Walker-like (zonal) circulations with dry air is dominant over the region around 90°–120°E. Meanwhile, in accord with the anomalous ascending motion over the equatorial region, the descending dry air of the local anomalous Hadley circulation appears over the region around 5°–20°N, reinforcing the subsiding motion of the anomalous zonal circulation. Apparently, the combined anomalous descending motion in conjunction with the dry atmospheric environmental condition suppress the convective development and TC genesis over the northern SCS, resulting in the decadal reduction in TC activity around 2002/03. In addition, accompanied by the anomalous easterly flow over the SCS, the local ISV activity weakened during 1994–2011. As a result, the suppressed ISV reduced the frequency of local synoptic-scale disturbances, which also leads to the decrease in TCs to a certain extent.

Fig. 12.
Fig. 12.

Schematic diagram illustrating the anomalous atmospheric circulation after the early 2000s. Red shaded areas indicate warm SST anomalies. The dark (light) gray arrows indicate anomalous zonal (meridional) circulation branches. “Con” and “Div” indicate convergence and divergence in the upper level, respectively.

Citation: Journal of Climate 28, 15; 10.1175/JCLI-D-14-00769.1

This study has revealed the observational evidences for the decadal change in the SCS TC activity that occurred around 2002/03. The physical mechanisms and causes for such a decadal change are explored. In a future study, we will further investigate impacts of remote and local SST changes on the large-scale circulation and environmental conditions over the SCS and quantitatively assess their influence by numerical experiments. These studies are expected to enhance our understanding of the decadal change in the SCS TC activity in the early 2000s and provide basis for a better long-term climate change prediction.

Acknowledgments

The authors acknowledge Professor Robert B. Lund in Clemson University for his suggestions on the method of changepoint detection. We are also grateful to Professor Kevin J. E. Walsh and the anonymous reviewers for their constructive comments. This work is sponsored by the National Natural Science Foundation of China (41430426), the R&D Special Fund for Public Welfare Industry (Meteorology) (GYHY201306025), and the Jiangsu Collaborative Innovation Center for Climate Change.

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