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  • View in gallery

    The variance of synoptic westward-propagating (a) GPCP rainfall (mm2 day−2) and (b) IR brightness temperature (°C2) during JJASON from 2000 to 2009. Refer to section 2 for the filter method.

  • View in gallery

    The variance of synoptic westward-propagating PW (mm2) during (a) spring, (b) summer, (c) fall, and (d) winter from 2000 to 2009. Refer to section 2 for the filter method.

  • View in gallery

    Lag correlation of daily PW between the southern band (averaged over 4°–7°N) and the northern band (averaged over 11°–14°N) along longitude during JJASON from 2000 to 2009. Negative lag days represent the time by which the southern band leads the northern band. Shading denotes significant regions at the 95% confidence level using Student’s t test.

  • View in gallery

    Composite of filtered PW anomaly (contour; mm), 850-hPa wind anomaly (vector; m s−1), and 850-hPa divergence anomaly (shaded; 10−7 s−1) over (a) the eastern Pacific and (b) the western Pacific based on the different indices referred to in sections 3 and 4. Values are plotted only when they are significant at the 95% confidence level.

  • View in gallery

    Composite of longitude–height cross sections of filtered pressure vertical velocity anomaly (shaded; 10−2 Pa s−1) and meridional wind anomaly (contour; m s−1) along (a) 9°N over the EP and (b) 13°N over the WNP.

  • View in gallery

    Composite of longitude–height cross section of total meridional wind (blue contour; m s−1), relative humidity (red contour; %), and pressure vertical velocity (shaded; 10−2 Pa s−1) along 9°N.

  • View in gallery

    (a) Composite of total surface wind (vector; m s−1), PW (contour; mm), and IR brightness temperature (shaded; K) obtained from CCMP, NVAP, and ISCCP datasets, respectively. (b) As in (a), but for composite based on the index over the WNP. See section 4 for the index definition.

  • View in gallery

    Composite of latitude–height cross sections of total meridional wind (black contour; m s−1), pressure vertical velocity (shaded; 10−2 Pa s−1), and filtered pressure vertical velocity (colored contour; 10−2 Pa s−1) averaged over 130°–125°W.

  • View in gallery

    Contribution to local tendency of filtered PW in association with (a) horizontal divergence, (b) horizontal advection, (c) vertical divergence, and (d) vertical advection (contour; contour interval of 1.5 × 10−5 kg m−2 s−1).

  • View in gallery

    Climatology of surface wind (vector; m s−1), total PW (contour; mm), and variance of 850-hPa filtered meridional wind (shaded; m2 s−2).

  • View in gallery

    As in Fig. 9, but for composite based on the index over the WNP. See section 4 for the index definition.

  • View in gallery

    Longitude–time diagrams of 850-hPa filtered meridional wind (contour; m s−1) averaged over 8°–10°N and the difference of filtered PW (shaded; mm) between the southern band (averaged over 4°–7°N) and the northern band (averaged over 11°–14°N) in (a) 2004 and (b) 2005. The black dots represent the strongest meridional dipoles at 130°W on 4 Sep 2004 and 10 Sep 2005, respectively.

  • View in gallery

    (a)–(e) Surface wind (vector; m s−1) and vorticity (shaded; ×10−5 s−1) and (f)–(j) IR brightness temperature (shaded; K). (top)–(bottom) Time corresponds to 1200 UTC on 1, 2, 3, 4, and 5 Sep 2004.

  • View in gallery

    As in Fig. 13, but for (top)–(bottom) time corresponding to 1200 UTC on 8, 9, 10, 11, and 12 Sep 2005.

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Synoptic-Scale Dual Structure of Precipitable Water along the Eastern Pacific ITCZ

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  • 1 Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Science, Beijing, China
  • | 2 Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan
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Abstract

Using 10-yr high-resolution satellite and reanalysis data, the synoptic-scale dual structure of precipitable water (PW), in which the southern and northern bands straddled at the ITCZ produce zonally propagating meridional dipoles, is observed over the eastern Pacific (EP) during boreal summer and fall. Composites indicate that the PW dipole, concurrent with the dipole-like filtered divergence, has a shift to the west of the anomalously cyclonic circulation. The vertical structure of filtered meridional wind is characterized by a wavenumber-1 baroclinic mode, and the vertical motion has two peaks situated at 850 and 300 hPa, respectively. To the east of the PW dipole, the shallow convection is embedded within the deep convection, forming a multilevel structure of meridional wind on the ITCZ equatorward side. To the west of the PW dipole, the deep convection tends to be suppressed because of the invasion of midlevel dry air advected by northerly flows. The generation and propagation of the dual PW band can be attributed to the divergence and advection terms related to specific humidity and three-dimensional wind. By comparison, the PW anomalies over the western North Pacific, only exhibiting a single band, coincide with the centers of synoptic disturbances with a barotropic vertical structure. Because of the weakening of lower-level divergence, the vertical motion, and the horizontal gradient of PW, the synoptic-scale PW signal is reduced significantly. The typical cases and statistics confirm that the strong meridional dipoles and westward-propagating disturbances are closely associated with the distortion and breakdown of ITCZ over the EP.

Corresponding author address: Dr. Guanghua Chen, Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, P.O. Box 2718, Beijing 100190, China. E-mail: cgh@mail.iap.ac.cn

Abstract

Using 10-yr high-resolution satellite and reanalysis data, the synoptic-scale dual structure of precipitable water (PW), in which the southern and northern bands straddled at the ITCZ produce zonally propagating meridional dipoles, is observed over the eastern Pacific (EP) during boreal summer and fall. Composites indicate that the PW dipole, concurrent with the dipole-like filtered divergence, has a shift to the west of the anomalously cyclonic circulation. The vertical structure of filtered meridional wind is characterized by a wavenumber-1 baroclinic mode, and the vertical motion has two peaks situated at 850 and 300 hPa, respectively. To the east of the PW dipole, the shallow convection is embedded within the deep convection, forming a multilevel structure of meridional wind on the ITCZ equatorward side. To the west of the PW dipole, the deep convection tends to be suppressed because of the invasion of midlevel dry air advected by northerly flows. The generation and propagation of the dual PW band can be attributed to the divergence and advection terms related to specific humidity and three-dimensional wind. By comparison, the PW anomalies over the western North Pacific, only exhibiting a single band, coincide with the centers of synoptic disturbances with a barotropic vertical structure. Because of the weakening of lower-level divergence, the vertical motion, and the horizontal gradient of PW, the synoptic-scale PW signal is reduced significantly. The typical cases and statistics confirm that the strong meridional dipoles and westward-propagating disturbances are closely associated with the distortion and breakdown of ITCZ over the EP.

Corresponding author address: Dr. Guanghua Chen, Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, P.O. Box 2718, Beijing 100190, China. E-mail: cgh@mail.iap.ac.cn

1. Introduction

Climatologically, a narrow and zonally extending band resides over the central and eastern Pacific (EP) in the Northern Hemisphere for precipitation and cloudiness, though the intensity and meridional extent of the band are seasonally dependent (e.g., Roundy and Frank 2004; Back and Bretherton 2009a). The band has a good correspondence with the intertropical convergence zone (ITCZ) that tends to occur in the Northern Hemisphere year-round and is characterized by the convergence of northeasterly and southeasterly trade winds. In early studies, the ITCZ was described as a deep overturning structure, in which the ascending branch in the ITCZ region and the descending branch south of the equator give rise to a southerly inflow into the ITCZ near the surface and a northerly return flow in the upper troposphere, forming a deep overturning circulation similar to the zonal-mean Hadley circulation (e.g., Grotjahn 1993; Wang and Enfield 2003). More recently, a shallow meridional circulation in the tropical EP has been revealed based on in situ observations and modeling (e.g., Zhang et al. 2004; Nolan et al. 2007). In this shallow meridional circulation, the return flow from the ITCZ can be observed not only at high altitudes, but also at low altitudes immediately above the planetary boundary layer, which is remarkably different from the classic deep overturning circulation. This shallow meridional circulation can be viewed as a sea-breeze-like circulation driven by sea surface temperature (SST) gradients (Nolan et al. 2007).

During summer and fall, the atmosphere in the central-eastern Pacific ITCZ is dominated by synoptic-scale westward-propagating disturbances (WPDs) that can contribute to a large percentage of synoptic variability (e.g., Gu and Zhang 2001, 2002; Magnusdottir and Wang 2008). Satellite observations also show that WPDs with a period of about 5 days can be observed along the latitudinal belt of the ITCZ (e.g., Chang 1970). In general, WPDs consist of easterly waves, equatorial mixed Rossby–gravity (MRG) waves, and even tropical cyclones that can significantly affect the ITCZ variability on the synoptic time scale (e.g., Reed and Recker 1971; Nitta et al. 1985; Wang and Magnusdottir 2006; Serra et al. 2008). For example, easterly waves induced by barotropic and baroclinic instabilities related to a strong meridional surface temperature gradient over West Africa and the eastern Atlantic can rejuvenate over the EP because of the reversal in the background meridional potential vorticity gradient (Molinari et al. 1997) or enhanced barotropic growth of eddies due to MJO-related low-level westerly flows (Maloney and Hartmann 2001). Through the interaction between the ITCZ and WPDs, the vorticity strip in the ITCZ may undulate and roll up, causing ITCZ breakdown and even triggering tropical cyclogenesis (e.g., Carlson 1969; Molinari and Vollaro 2000; Cao et al. 2013). In addition to disturbances external to the ITCZ, the undulation of the ITCZ belt can also break down into a number of WPDs, which continue their coherent westward propagation as they weaken further and finally dissipate.

Along the ITCZ, the cloudy convection and precipitation system can be observed to be advected by prevailing winds or to propagate like waves. Based on the daily rainfall from the Global Precipitation Climatology Project (GPCP) and infrared (IR) brightness temperature from the International Satellite Cloud Climatology Project (ISCCP) during June–November from 2000 to 2009, the maximum variances of synoptic-filtered variables exhibit a latitudinally narrow strip over the central and eastern Pacific but extend northwestward to west of 160°E and have a more spatially broad extent in Fig. 1. The IR brightness temperature is indicative of the temperature of the cloud top that can be used to infer the height of the cloud top as a proxy of deep convection. Therefore, the coincidence of the maximum variances in the GPCP and IR brightness temperature over the western and central Pacific suggests that the precipitation variability is closely related to deep convection mode. There exists another maximum variance in the GPCP precipitation spanning from 140° to 120°W that may be associated with the combined vertical modes of shallow and deep convection (e.g., Back and Bretherton 2009b; Yokoyama and Takayabu 2012a).

Fig. 1.
Fig. 1.

The variance of synoptic westward-propagating (a) GPCP rainfall (mm2 day−2) and (b) IR brightness temperature (°C2) during JJASON from 2000 to 2009. Refer to section 2 for the filter method.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

In contrast to a single variance band of precipitation and convection, if the same filtering domain is applied to the precipitable water (PW) data, the distribution of synoptic PW variance is seasonally dependent (Fig. 2). Different from the single zonally elongated bands of synoptic PW variance in the Northern Hemisphere during spring and winter (Figs. 2a,d), of interest is the existence of a dual structure over the EP in the Northern Hemisphere during summer and fall, and two variance belts are situated on the equatorward and poleward sides of 9°N, respectively (Figs. 2b,c). The variance in the southern part is stronger than that in the northern part, especially during boreal fall. Roundy and Frank (2004) documented a global snapshot of the tropical depression (TD)-type band-filtered PW for 7 December 1992. Figure 21 in their paper hints at two zonally propagating TD-type bands over the EP that have opposite phases to the north and south of 9°N, respectively. But, over the central Pacific, the southern part becomes obscure and the northern part continues to propagate westward, forming a single band over the western North Pacific (WNP). Based on the spectrum power of 3–7-day filtered PW, Yokoyama and Takayabu (2012b) also showed that there are two bands of large spectrum power on both the southern and northern edges of the ITCZ, giving rise to a latitudinally dual structure during boreal fall (see their Fig. 3b).

Fig. 2.
Fig. 2.

The variance of synoptic westward-propagating PW (mm2) during (a) spring, (b) summer, (c) fall, and (d) winter from 2000 to 2009. Refer to section 2 for the filter method.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

The variances related to synoptic waves have a single zonal belt along the EP ITCZ in terms of meteorological variables such as meridional wind, precipitation, and outgoing longwave radiation. By comparison, it is an interesting phenomenon that the synoptic-scale dual structure of PW is straddled over the EP ITCZ, which was not yet revealed systematically in previous studies. Therefore, the following intriguing questions will be addressed in this study: What is the phase relationship between the two zonally elongated bands? What factors are responsible for the generation and propagation of this dual PW band? How do the behaviors of the synoptic PW band over the EP differ from those of its counterpart over the WNP, in which the latitudinally narrow ITCZ is less observed and the convective structure has a distinct feature? And to what extent is this dual PW structure related to WPDs and ITCZ behavior? In the presatellite era, this intriguing structure is hard to identify in conventional measurements, especially in the tropical central to eastern Pacific, where observations are sparse. With the abundance of high-quality satellite datasets and the improvement of global data assimilation and forecast systems, it becomes feasible to comprehensively study the characteristics of the dual PW structure, as well as the corresponding circulation features.

This paper is organized as follows: In section 2, the data sources and wavenumber–frequency filter method used in this study are described. In section 3, an index is defined to represent the meridional dipole pattern of filtered PW, and the three-dimensional circulation structure is studied by composite analysis. In section 4, a comparison with the counterpart over the WNP is made to examine what factors play key roles in the differences in the characteristics of PW and circulation over the two ocean basins. Section 5 is devoted to shedding some light on possible linkages between the dual PW structure, WPDs, and ITCZ breakdown. Finally, the conclusions are given in section 6.

2. Data and methodology

Daily PW data are obtained from the National Aeronautics and Space Administration Water Vapor Project (NVAP) that provide total column PW with a resolution of 1°. The NVAP’s unique methodology of blending data from a variety of sensors to create a cohesive archive included multiple satellite inputs (infrared and microwave). The dataset combines measurements from satellite instruments, such as the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) and Special Sensor Microwave Imager (SSM/I), and was designed to be as model independent as possible (Randel et al. 1996). A small fraction of missing data was filled by temporal linear interpolation.

The other two complementary satellite-derived products include the cross-calibrated multiplatform (CCMP) ocean surface wind dataset, which combines cross-calibrated satellite winds obtained from Remote Sensing Systems (RSS; http://www.remss.com) using a variational analysis method to produce a high-resolution (0.25°) 6-hourly gridded analysis (available at ftp://podaac-ftp.jpl.nasa.gov/allData/ccmp/L3.0/flk/), and IR window channel brightness temperatures provided by the ISCCP B1 data (available at http://www.ncdc.noaa.gov/oa/gridsat/). The data from 28 satellites have been incorporated into the ISCCP B1 data record, which has approximately 10-km and 3-hourly resolution. The above two datasets have been combined to scrutinize the detailed behaviors in the ITCZ strip.

The Japanese 55-yr Reanalysis Project (JRA-55) is used to identify environmental fields, such as wind fields and specific humidity. This dataset is a third-generation reanalysis with some distinct improvements, such as a four-dimensional variational data assimilation system and some updated dynamical and physical processes (Ebita et al. 2011). The reanalysis data have a 1.25° horizontal resolution and 37 vertical levels at a 6-hourly interval. In this study, we focus on the summer and fall spanning from June to November (JJASON) for the 10 years of 2000–09, in which all datasets are available.

Because the synoptic variability in the ITCZ zone over the EP is dominated by the westward-propagating mode (e.g., Gu and Zhang 2002; Magnusdottir and Wang 2008), the wavenumber–frequency filter method in Wheeler and Kiladis (1999) was employed to extract the synoptic westward-propagating signals. The annual cycle was subtracted from the PW time series at each grid point after calculating the mean value for each day of the 10 years. A fast Fourier transform was performed on the zonal and time dimensions at each latitude to isolate the time–longitude behavior of disturbances. The filtering domain covers periods from 3 to 8 days and westward propagation at wavenumbers of −20 to 0. This regime is broad enough to encompass commonly observed MRG wave and TD-type disturbances that have been extensively documented in previous theoretical and observational studies (e.g., Matsuno 1966; Lau and Lau 1990; Takayabu and Nitta 1993). On the other hand, to reduce the contamination from nonsynoptic signals with a period of more than 10 days because of the potential end effect, a 3–8-day filtering window is chosen. In fact, if a 2–10-day filtering window is employed, the results are qualitatively similar to those in this study, suggesting that the results are not sensitive to the selection of the synoptic-scale filtering window. In addition, in the wavenumber–frequency spectrum of PW (not shown), the spectrum power of the synoptic westward-propagating component is much stronger and more robust than that of the eastward-propagating component, which assures that our filtering domain can extract the major synoptic signals. In this study, the filtering is also applied to other variables, including wind field, pressure vertical velocity, and divergence.

3. Characteristics of dual PW band

a. Phase relationship of dual PW band

A question arises as to whether the dual PW bands over the EP are independent of or dependent on each other. If a connection exists, where are the regions with a significant phase relationship? To specify the phase relationship of two PW bands, the longitude–lag correlation of filtered PW between the southern band (averaged over 4°–7°N) and the northern band (averaged over 11°–14°N) during JJASON from 2000 to 2009 is computed. The time series consists of a total of 1830 samples. The two bands correspond to the regions of the maximum synoptic-scale PW variance, as shown in Fig. 2. As shown in Fig. 3, the coherent negative correlation appears simultaneously, whereas the relationship is found to be less robust in the lead and lag days. Significant correlation regions exist to the east of 160°W. In particular, the area encompassed by the contour of −0.4 lies approximately between 145° and 120°W, with a minimum of −0.48 at 130°W. It suggests that there exists a significantly negative correlation of filtered PW between the two bands at the same longitude over the EP.

Fig. 3.
Fig. 3.

Lag correlation of daily PW between the southern band (averaged over 4°–7°N) and the northern band (averaged over 11°–14°N) along longitude during JJASON from 2000 to 2009. Negative lag days represent the time by which the southern band leads the northern band. Shading denotes significant regions at the 95% confidence level using Student’s t test.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

Based on the above longitudinal distribution of the correlation between the two filtered PW bands, an index is defined as the difference of filtered PW at 130°W between the southern part averaged over 4°–7°N and the northern part averaged over 11°–14°N (i.e., the filtered PW in the southern part minus that in the northern part at the base longitude of 130°W), reflecting an opposite phase of southern and northern PW bands. A positive index stands for a positive PW anomaly on the southern side and a negative PW anomaly on the northern side, while a negative index corresponds to the reversed situation. Although the largest negative correlation of the two PW bands occurs at 130°W, the shift of reference longitude within 140°–120°W as the index cannot qualitatively alter the results.

To examine the structure of synoptic-scale disturbances, a composite analysis is performed. A total of 78 cases are chosen for composite from the daily data during JJASON from 2000 to 2009, following the criterion that the standardized index exceeds 1.5 standard deviations.

b. Three-dimensional structure of synoptic dual PW band

Figure 4a depicts the composite of filtered PW, 850-hPa winds, and divergence over the EP, which form a wave train pattern. The PW anomalies clearly lead to a meridional dual structure, with the southern band center situated at 5°N and the northern one at 13°N. The strongest meridional dipole mode lies at 130°W, consistent with the defined index. The 850-hPa convergence (divergence) anomalies have a good correspondence with the positive (negative) regions of filtered PW, except that the dipole centers of divergence anomalies are closer to each other than those of filtered PW. One robust anomalously cyclonic circulation is straddled at 9°N, which is also the climatological ITCZ central latitude. Note that the cyclonic circulation center is shifted about 4° longitude to the east of the strongest PW dipole mode center. As a result, the composited southerly anomalies are enhanced between 125° and 110°W, which can be traced back to the south of the equator, while the northerly anomalies are prevalent in the PW dipole region near 130°W. Farther to the east, there is an anticyclonic but relatively weak anomalous circulation centered at (9°N, 108°W). Furthermore, the regression of filtered variables on the defined index also exhibits the similar wave train structure (not shown), indicating that the dual PW band and its related WPDs are robust.

Fig. 4.
Fig. 4.

Composite of filtered PW anomaly (contour; mm), 850-hPa wind anomaly (vector; m s−1), and 850-hPa divergence anomaly (shaded; 10−7 s−1) over (a) the eastern Pacific and (b) the western Pacific based on the different indices referred to in sections 3 and 4. Values are plotted only when they are significant at the 95% confidence level.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

To elucidate vertical profiles along the ITCZ, the longitude–height composite of filtered pressure vertical velocity and meridional wind at 9°N is shown in Fig. 5a. The wave train in the lower troposphere has a large amplitude at 900 hPa, with a cyclonic synoptic disturbance centered at 126°W. In collaboration with the opposite phase above about 350 hPa, the vertical mode is characterized by the gravest baroclinic structure. Accompanied by the lower-level cyclonic vortex, the robust ascending motion lies below 600 hPa. It is worth noting that the secondary center of upward motion can be observed at 300 hPa, which is related to the vertical dual-cored structure of unfiltered vertical motion in the presence of the shallow circulation. The ascending region between 130° and 125°W is flanked by a weak descent to the east and a strong but single-cored descent to the west, in a good agreement with the composite of total fields as described below.

Fig. 5.
Fig. 5.

Composite of longitude–height cross sections of filtered pressure vertical velocity anomaly (shaded; 10−2 Pa s−1) and meridional wind anomaly (contour; m s−1) along (a) 9°N over the EP and (b) 13°N over the WNP.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

Corresponding to the filtered fields, the total meridional wind, relative humidity, and pressure vertical velocity along 9°N are composited in Fig. 6. The zonal asymmetry of vertical velocity is remarkable to the east and west of 130°W. Specifically, to the east of 130°W, in conjunction with the lower-level southerly flows, a two-deck structure of upward motion is evident, with a primary region located near 850 hPa and a secondary one located at approximately 300 hPa, suggesting that the strong ascending motion can extend up to the upper troposphere. In sharp contrast, to the west of 130°W, the vertical motion is dominated by the lower-level branch but with weak amplitudes, and the upper-level branch almost dissolves. The discrepancy in the vertical convection mode on both sides could be attributed to the intrusion of midtropospheric dry air due to the northerly flows, and it is indicated by a massive pool of dry air at 400 hPa to the west of 130°W, with a low relative humidity value of 30%, whereas there is a zonally uniform distribution of high relative humidity below 700 hPa. Some previous studies have shown that the penetration of midtropospheric dry air can negatively affect the development of deep convection by fostering cold downdrafts, lowering the convective available potential energy and reducing updraft buoyancy near the ascending region (e.g., Dunion and Velden 2004; Takayabu et al. 2010). Based on in situ observations, Jensen and Del Genio (2006) also suggested that the drying at the middle level is more effective to limit congestus cloud-top heights. Likewise, the vertical cross sections along 5°N and 13°N, the central latitudes of the dual PW band, also exhibit the potent dry air in the midtroposphere to the west of 130°W (not shown), except that the contours of lower-level relative humidity have an upward bulge (downward dent) near 130°W in the cross section along 5°N (13°N) because of the anomalous ascending (descending) motion in response to the convergence (divergence) anomaly (as indicated in Figs. 4a and 8).

Fig. 6.
Fig. 6.

Composite of longitude–height cross section of total meridional wind (blue contour; m s−1), relative humidity (red contour; %), and pressure vertical velocity (shaded; 10−2 Pa s−1) along 9°N.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

The abovementioned distinction of convection characteristics on both sides of the PW meridional dipole derives from the reanalysis data. To enhance the credibility of our analysis, the satellite-derived data are used to support the results. Figure 7a depicts the composite of total surface winds, PW, and IR brightness temperature obtained from the CCMP, NVAP, and ISCCP datasets, respectively. It clearly appears that a narrow zonal strip of PW extends from the eastern part of the EP through the central Pacific, with the PW maximum of 60 mm residing near 130°W. The surface southerly winds to the east of 130°W penetrate poleward across the PW band and correspond to the meridional extension of the PW band, whereas the PW band is latitudinally contracted to the west of 130°W. By comparison, to the west of 130°W is the robust convergence of meridional winds along the central latitude of the PW band and the easterly trade winds. Examination of IR brightness temperature, representing the height of cloud top as a proxy of deep convection, indicates that there is a distinct contrast in the vicinity of 130°W, that is, a lower brightness temperature to the east and a higher brightness temperature to the west, suggesting that deep convection tends to be strengthened to the east and suppressed to the west, consistent with the vertical profile of vertical velocity shown in Fig. 6.

Fig. 7.
Fig. 7.

(a) Composite of total surface wind (vector; m s−1), PW (contour; mm), and IR brightness temperature (shaded; K) obtained from CCMP, NVAP, and ISCCP datasets, respectively. (b) As in (a), but for composite based on the index over the WNP. See section 4 for the index definition.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

The composite latitude–height cross section can clarify the meridional overturning structure and validate the vertical convection mode. In Fig. 8, the vertical profile of total meridional wind equatorward of 10°N takes on a multilevel structure: the boundary layer inflow, the shallow return flow, the midlevel inflow, and the upper-level outflow, in accordance with previous observational and modeling studies (e.g., Zhang et al. 2004; Nolan et al. 2007; Yokoyama and Takayabu 2012b). Of interest is the meridional structural asymmetry at about 10°N; that is, the vertical structure poleward of 10°N only has two meridional flows: one pointing equatorward at the midlower level and the other pointing poleward at the upper level. This asymmetry of the meridional overturning circulation may be attributed to the varying strength of the meridional SST gradient. Back and Bretherton (2009a) concluded that the SST gradient is the primary driver of strong shallow circulation over the EP. Based on the idealized simulation, Nolan et al. (2010) pointed out that the shallow return flow is primarily a sea-breeze-like response to large meridional gradients of surface temperature and pressure. The simulation in their study, in which an SST profile modeled after the observed SST of the EP is imposed in the model, also confirmed that the vertical structure of meridional flow on both sides of the ITCZ is quite sensitive to the SST gradient. It was found that, in response to the SST profiles that decrease slowly poleward of the SST maximum at 10°N, the multilevel circulations (especially the shallow return flow and the midlevel inflow) are suppressed on the poleward side, which is remarkably different from the southern counterpart with a “four flow” structure in response to the strong meridional SST gradient due to the existence of the cold tongue equatorward of the ITCZ (cf. Figs. 2b and 13a in Nolan et al. 2010).

Fig. 8.
Fig. 8.

Composite of latitude–height cross sections of total meridional wind (black contour; m s−1), pressure vertical velocity (shaded; 10−2 Pa s−1), and filtered pressure vertical velocity (colored contour; 10−2 Pa s−1) averaged over 130°–125°W.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

In Fig. 8, the filtered vertical velocity has a characteristic of meridional dipole but with the southern ascending anomaly stronger than the northern descending anomaly. This dipole structure agrees well with the lower-level divergence anomalies, as demonstrated in Fig. 4a. The region of unfiltered ascending motion approaches close to that of the filtered ascending motion, bearing a resemblance of vertical pattern in that a primary peak is located at 850 hPa and a secondary one at 300 hPa, also consistent with the vertical mode shown in Fig. 6.

c. Generation and propagation of synoptic dual PW band

In this subsection, we intend to explain why this dipole structure of synoptic PW forms and migrates westward. To address this question, the contribution of WPDs to the PW tendency needs to be investigated. According to the atmospheric water balance equation (e.g., Peixoto and Oort 1992), the local tendency of water vapor is proportional to the vertically integrated moisture flux divergence as follows
eq1
where W, q, V, Vh, and ω stand for PW, specific humidity, three-dimensional wind vector, horizontal wind vector, and pressure vertical velocity, respectively. The vertical integration of moisture flux divergence is estimated from ps = 1000 hPa to pt = 100 hPa. The moisture flux divergence can be decomposed into four terms on the right-hand side of the equation that are related to the horizontal divergence (HD), horizontal advection (HA), vertical divergence (VD), and vertical advection (VA), respectively. Each term is computed first using unfiltered fields and then is filtered in the same wavenumber–frequency domain, as described in section 2. The filtered terms are denoted by the tilde symbol in the equation.
Figure 9 indicates the contribution of each term by compositing individual cases. It clearly displays that the terms of VD and VA are almost identical in magnitude but with an opposite sign (Figs. 9c,d). The mutual cancellation between the VD and VA terms can be interpreted by the following equality
eq2
As a result, the effect related to the HD can contribute to the generation of the dual PW structure (Fig. 9a). Comparatively, the HA term (Fig. 9b) has a crucial role in advecting the dual PW band westward; consequently, the dipole centers are longitudinally shifted approximately 7° to the west. To sum up, the HD effects in association with the WPDs facilitate the local generation of the meridional dipole structure of PW on the synoptic time scale, while the HA effect can dynamically steer the dipole mode of synoptic PW westward.
Fig. 9.
Fig. 9.

Contribution to local tendency of filtered PW in association with (a) horizontal divergence, (b) horizontal advection, (c) vertical divergence, and (d) vertical advection (contour; contour interval of 1.5 × 10−5 kg m−2 s−1).

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

Regarding the preferred formation regions of the PW dipole structure, the climatology of surface wind, PW, and variance of filtered 850-hPa meridional winds are depicted in Fig. 10. Indicated by the variance maximum of filtered 850-hPa meridional winds, there are two regions with strong synoptic wave activity. One maximum is located in a meridionally narrow zone between 130° and 110°W over the EP, and the variance strip extends zonally westward along the ITCZ region through the central Pacific and then reintensifies to form the other maximum variance region with a southeast–northwest orientation. The characteristics of synoptic meridional wind variance imply that westward-propagating tropical disturbances over the central and eastern Pacific can deflect northwestward and transition to TD-type disturbances to the west of 160°E when encountering summer monsoon circulation over the WNP, in good agreement with previous studies (e.g., Lau and Lau 1990; Liebmann and Hendon 1990; Chen and Huang 2009). Similar to the distribution of meridional wind variance, the total PW displays a latitudinally compacted belt with an axis along the ITCZ over the central and eastern Pacific, but over the WNP, the area with large PW values expands spatially, accompanied by the weakening of the PW meridional gradient.

Fig. 10.
Fig. 10.

Climatology of surface wind (vector; m s−1), total PW (contour; mm), and variance of 850-hPa filtered meridional wind (shaded; m2 s−2).

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

The surface winds are characterized by the enhanced meridional convergence near the ITCZ to the east of 140°W, while the easterly trade flows are prevalent over the central Pacific and gradually diminish over the WNP. The strong lower-level convergence over the EP is able to produce the shallow circulation and lower-tropospheric ascending core. On the other hand, from the viewpoint of barotropic energy conversion, Yokoyama and Takayabu (2012b) found that, over the EP, the barotropic conversion term related to meridional convergence plays a primary role in amplifying the eddy kinetic energy, which is quite different from cases over the WNP, where the conversion related to the horizontal shear of zonal wind makes the primary contribution (e.g., Feng et al. 2014). As a result, the active synoptic WPDs over the EP in turn enhance the eddy convergence to prompt the local amplification of the dual PW band near 130°W, as shown in Fig. 9a. In addition, the sharp meridional gradient of PW on both sides of the ITCZ strip assists in augmenting the horizontal advection effect, as described in Fig. 9b, and thus favors the generation and propagation of the dual PW structure over the EP.

4. Comparison with the counterpart over the WNP

The dual PW band is coincident with the maximum synoptic wave activity over the EP, while the similar PW structure is absent over the WNP, where synoptic waves are more active. Therefore, it is imperative to explore what mechanisms are responsible for this discrepancy. For this purpose, the filtered PW averaged over 10°–15°N and 135°–140°E is defined as an index, representative over the WNP. Likewise, a total of 67 cases in which the standardized index exceeds 1.5 standard deviations are picked up for composite. Figure 4b displays the composite of filtered PW, 850-hPa winds, and divergence over the WNP that clearly reveals a northwestward-propagating wave train with a southwest–northeast-tilted cell structure. Compared with the meridional dual bands of filtered PW and divergence over the EP (shown in Fig. 4a), the cyclonic (anticyclonic) circulation is almost coincident with the positive (negative) PW anomaly and lower-level convergence (divergence) anomaly to the west of 150°E, which implies a tight coupling of circulation and convection, only forming a single band of PW and divergence anomalies.

Examination of divergence anomalies at 850 hPa indicates that the magnitudes are less than half of those in Fig. 4a, which can be ascribed to the distinct vertical structure. As demonstrated in Fig. 5b, the vertical cross section of the wave train along 13°N takes on a nearly barotropic structure with a slight westward tilt with height west of 150°E. Another striking feature is that the filtered vertical motion extends upright through the troposphere and possesses a single-cored structure with a maximum center at the upper level, rather than a dual-cored structure over the EP shown in Fig. 5a. Consequently, the lower-tropospheric vertical velocity and divergence are relatively small over the WNP. As for the unfiltered meridional wind and vertical velocity, their cross sections display a similar barotropic structure and vertical profile (not shown), implying that the upright single-cored vertical structure is dominated by deep convection, which is able to tightly couple the synoptic wave and convection, forming a vertically uniform convection mode. Accordingly, the lower-level ascending core and shallow circulation observed over the EP are absent over the WNP, which results from the lack of a sea-breeze-like response due to the weak SST gradient over the WNP (Nolan et al. 2007).

The composite of total fields obtained from the satellite-derived datasets also validates the above results, as shown in Fig. 7b. Based on the defined index over the WNP, there is a vast coverage of deep convection with IR brightness temperature values less than 268 K around the region of interest. The abundant PW has a good correspondence with the deep convection, which is basically coincident with the large-scale cyclonic circulation. However, in contrast to the case over the EP, the composited 850-hPa winds have smaller magnitudes, suggesting that the lower-level dynamic factor over the WNP does not play a significant role in modulating PW behavior as it does over the EP. Corresponding to the dominant deep convection over the WNP, some previous studies also revealed that the northwestward-propagating TD-type disturbance over the WNP is more like a rotational Rossby wave type that primarily modulates convective rain compared with the more divergent waves that modulate relatively more stratiform rain (e.g., Kiladis et al. 2009; Yasunaga and Mapes 2012a,b).

As interpreted in Fig. 9, the individual terms related to HD, HA, VD, and VA make the same qualitative contribution to the generation and propagation of PW anomalies over the WNP (Fig. 11). However, the amplitudes in the HD and VA terms over the WNP are nearly half as much as those over the EP, which can be attributed to the decrease in the lower-level divergence and vertical motion over the WNP leading to the weakening of PW accumulation and vertical transport (Figs. 11a,d). In addition, the relatively flat PW horizontal gradient and weakened lower-level easterly flows over the WNP also decrease significantly the HA effect (Fig. 11b). Therefore, considering that most of the PW is concentrated in the low troposphere, these lower-level atmospheric factors take less effect on the generation and propagation of the synoptic PW band that is responsible for the attenuation of the PW anomaly over the WNP.

Fig. 11.
Fig. 11.

As in Fig. 9, but for composite based on the index over the WNP. See section 4 for the index definition.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

5. Relationship between dual PW band, WPD, and ITCZ behavior

The above composite results show that, accompanied by the synoptic WPDs, the meridional dipole of PW anomalies propagates westward along the ITCZ. In this section, we will investigate whether the dual PW structure and WPDs are closely associated with ITCZ undulation and breakdown through case and statistical analysis. First, two cases in 2004 and 2005 are examined. The case in 2004 corresponds to the ITCZ belt rollup triggering tropical cyclogenesis, while the case in 2005 exhibits the long-lasting WPDs undulating the ITCZ band.

Figure 12 demonstrates the longitude–time diagram of 850-hPa filtered meridional wind averaged over 8°–10°N and the difference of filtered PW between the southern band (averaged over 4°–7°N) and the northern band (averaged over 11°–14°N), which reflect the westward propagation of WPDs and the dual PW band, respectively. In the case in 2004 (Fig. 12a), a series of WPDs seems to be triggered by the eastward energy dispersion of a Rossby wave, given that the centers with maximum and minimum values embedded within the wave packet were clearly shifted eastward with time. Although the WPDs indicated by the meridional wind did not displace a long distance in the longitudinal direction, they induced a robust meridional dipole mode (implied by the difference of filtered PW between the southern and northern bands) that reached its peak at 130°W on 4 September where the standardized index was greater than 1.5 standard deviations.

Fig. 12.
Fig. 12.

Longitude–time diagrams of 850-hPa filtered meridional wind (contour; m s−1) averaged over 8°–10°N and the difference of filtered PW (shaded; mm) between the southern band (averaged over 4°–7°N) and the northern band (averaged over 11°–14°N) in (a) 2004 and (b) 2005. The black dots represent the strongest meridional dipoles at 130°W on 4 Sep 2004 and 10 Sep 2005, respectively.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

The corresponding surface winds, vorticity, and IR images are depicted in Fig. 13. At the initial time, a continuous and narrow surface vorticity and deep convection strip related to the ITCZ stretched through the whole EP, with a strong vortex at 110°W at the easternmost edge of the ITCZ (Figs. 13a,f). In the following 2 days, the vortex became strengthened and moved northwestward, along with the expansion of a deep cloud cluster. With the northwestward deflection of the vortex, the ITCZ belt rolled up, forming a spiral cloud band, and ultimately the vortex intensified into a tropical cyclone. Meanwhile, the enhanced northerly flows to the west of the vortex penetrated the ITCZ strip, suppressing the deep convection in the ITCZ tail and causing the ITCZ breakdown (Figs. 13d,i). In Fig. 13i, the gray–white contrast in the IR image on the eastern and western sides of 130°W indicates that the deep convection was located to the east of the strong PW meridional dipole as a result of the ITCZ rollup, while it was suppressed to the west because of the invasion of dry air advected by northerly flows, consistent with the results in Figs. 6 and 7. Subsequently, the tropical cyclone began to detach from the ITCZ belt, and the ITCZ tail moved westward along with the westward propagation of the dual PW band and WPDs.

Fig. 13.
Fig. 13.

(a)–(e) Surface wind (vector; m s−1) and vorticity (shaded; ×10−5 s−1) and (f)–(j) IR brightness temperature (shaded; K). (top)–(bottom) Time corresponds to 1200 UTC on 1, 2, 3, 4, and 5 Sep 2004.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

Unlike the case in 2004, the case in 2005 was characterized by the long-distance propagation of WPDs that could be traced back to the east of 100°W and had less evidence of Rossby energy dispersion (Fig. 12b). Correspondingly, the PW meridional dipole mode (shaded in red) lasted for about two weeks, starting on 4 September with a maximum magnitude on 10 September near 130°W. In addition, similar to the case in 2004, the strongest meridional dipole of filtered PW corresponded to the northerly anomalies, suggesting that there was a phase lag between the meridional dipole center and the anomalous circulation center in accordance with the longitudinal shift between them, as observed in Fig. 4a. In collaboration with the long-lived WPDs, the ITCZ vorticity strip began to undulate on 8 September, with an evident breakdown at 115°W (Fig. 14a). Unlike the rollup process at the eastern edge of the ITCZ, as displayed in Fig. 13, the vorticity patches continued to propagate westward after the ITCZ broke down into several vortical parts in the following several days (Figs. 14b–e). Careful examination of the IR images also indicates that the dense cloud clusters embedded within the broad convection mass, matching with the compacted vortices, migrated westward, though they did not form entirely isolated cells detaching from the ITCZ band (Figs. 14f–j). With the strongest meridional dipole at 130°W on 10 September (Fig. 13b), the dim area near 130°W in the IR image is indicative of the suppression of deep convection (Fig. 14h), in agreement with the dominance of shallow convection west of 130°W, as discussed in Fig. 6.

Fig. 14.
Fig. 14.

As in Fig. 13, but for (top)–(bottom) time corresponding to 1200 UTC on 8, 9, 10, 11, and 12 Sep 2005.

Citation: Journal of Climate 27, 16; 10.1175/JCLI-D-14-00060.1

We look through all 78 cases in this study with the standardized index larger than 1.5 standard deviations. Based on the subjective identification from the longitude–time diagrams and surface vorticity fields from the CCMP dataset, the dual PW bands are accompanied by the enhanced WPDs in 66 cases, and the zonally elongated ITCZ vorticity strips can be clearly discerned in 69 cases. Further examination reveals that, among these 69 ITCZ strips, 13 cases can be found where the ITCZ rolls up into a strong and isolated vortex, and 51 cases experience the pronounced undulation of the ITCZ strip, which are all related to WPDs. The above results suggest that there are close relationships between the dual PW structure, WPDs, and ITCZ variability on the synoptic time scale. Though examination of the cases with less coherence among them is beyond the scope of this study, it needs further investigation to advance a comprehensive understanding of the PW-related convection in association with synoptic disturbances and ITCZ behavior in future work.

6. Conclusions

Using the high-resolution JRA-55 dataset and satellite-derived datasets from June to November for the 10 years of 2000–09, the synoptic-scale PW features over the EP are examined. Different from the single zonally elongated band in terms of precipitation and IR brightness temperature during summer and fall, the synoptic PW over the EP exhibits a dual structure with meridional dipoles straddled around 9°N that corresponds to the climatological mean ITCZ central latitude. The simultaneous correlation between the two bands is significantly negative with a minimum at 130°W. The difference of PW anomalies at 130°W between the southern and northern bands is defined as an index to reflect the intensity and propagation of the dual PW structure. The composite for the cases, in which the standardized index exceeds 1.5 standard deviations, shows that, in conjuncture with the strongest meridional dipole structure, an anomalously cyclonic circulation appears with a vortex center shifting eastward 4° relative to the longitude of the PW dipole centers. The divergence anomalies also take on a meridional dipole mode, coincident with that of filtered PW. The vertical structure for the filtered meridional winds is primarily characterized by a wavenumber-1 vertical baroclinic mode, and the vertical motion has two peaks, one located at 850 hPa and the other located at 300 hPa. For the total fields, to the east of the meridional dipole at 130°W, the strong southerly flows enhance the ascending motion, leading to the deep convection in which the shallow circulation is embedded, while to the west of it, the deep convection is suppressed because of the invasion of midlevel dry air. On the equatorward side of the ITCZ are the multilevel meridional flows consisting of the boundary layer inflow, the shallow return flow, the midlevel inflow, and the upper-level outflow, while only the lower-level inflow and upper-level outflow appear on the poleward side of the ITCZ, forming a meridionally asymmetric structure with respect to the ITCZ, which may be partly attributed to the varying meridional gradient of SST on the southern and northern sides of the ITCZ.

Based on the tendency of water vapor, it is indicated that the vertical divergence and vertical advection terms are almost identical in magnitude but with an opposite sign. As a result, the horizontal convergence acts as primary contribution to the generation of the dual PW band, while the horizontal advection can affect the propagation of the dual PW band by steering the meridional dipole westward. From a climatological viewpoint, the robustness of the filtered PW meridional dipole around 130°W is well coincident with the enhanced WPD activity over the EP. In contrast, the strong synoptic wave activity over the WNP covers a broad region but produces solely a single wave train of PW anomalies whose centers coincide with those of the rotational-dominant disturbances. This wave train has a nearly barotropic vertical structure, indicative of a uniform deep convection mode. Because of the decrease in the lower-level vertical motion, horizontal convergence, and horizontal gradient of moisture over the WNP, the contribution to the generation and propagation of the PW anomaly is reduced significantly, resulting in the weakening of the filtered PW signal over the WNP.

The meridional dipole of PW anomalies related to WPDs is often observed to accompany ITCZ distortion and breakdown. Two typical cases are chosen to exhibit ITCZ rollup into a tropical cyclone and ITCZ breakdown into several vortices, respectively. In collaboration with the enhancement of WPDs, the PW meridional dipole near 130°W becomes robust, basically corresponding to the joint node of ITCZ distortion and breakdown. The statistical analysis further confirms that the majority of events with a strong dual PW band are concurrent with WPDs and ITCZ strips, among which the ITCZ bands are, to a greater or lesser extent, disturbed, causing the zonally elongated vorticity strip distortion and even breakdown on the synoptic time scale.

Acknowledgments

The authors are grateful to Dr. Atushi Hamada and Nagio Hirota of the University of Tokyo for their helpful comments and technical support in data collection. This study is supported financially by the National Basic Research Program of China (Grant 2014CB953902) and the National Natural Science Foundation of China (Grant 41275001). The first author gives an acknowledgement to the visiting program of the Atmosphere and Ocean Research Institute (AORI), the University of Tokyo.

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