South China Sea Western North Pacific Research Articles

Influences of Tropical Indian and Pacific Oceans on the Interannual Variations of Precipitation in the Early and Late Rainy Seasons in South China

Abstract Because of the seasonal northward migration of the East Asian summer monsoon, the mean-state atmospheric circulation in South China (SC) is remarkably different between the early (May–June) and late (July–August) rainy seasons. This study presents distinct teleconnections between the SC precipitation in the two periods and the sea surface temperatures (SSTs) in the tropical oceans. In the early rainy season when the major monsoon rain belt is located in SC, the increased local precipitation is related to the tropical Indian Ocean Basin warming. The basin warming induces an anomalous anticyclone in the South China Sea–western North Pacific (SCS-WNP). The related southwesterly anomalies transport more moisture to SC and lead to more moisture convergence and precipitation there. In the late rainy season when the major monsoon rain belt migrates northward to the Yangtze River valley, the precipitation increase in SC can be caused by the dipole SST anomalies in the tropical Pacific with the cold anomalies near the Maritime Continent and warm ones near the date line. The dipole SST anomalies generate an anomalous cyclone in the WNP with its center more northward than that of the anomalous anticyclone in the early rainy season. The related northeasterly anomalies along its northwestern flank reduce the climatological northward transport of moisture flux out of SC, and increase the moisture convergence and precipitation there. The distinct teleconnections between the SC precipitation and the tropical SSTs in the early and late rainy seasons can be well reproduced in the sensitivity experiments by an atmospheric general circulation model.

Springtime Convective Quasi-Biweekly Oscillation and Interannual Variation of Its Intensity over the South China Sea and Western North Pacific

This study investigates characteristics of the convective quasi-biweekly oscillation (QBWO) over the South China Sea (SCS) and western North Pacific (WNP) in spring, and the interannual variation of its intensity. Convective QBWO over the WNP and SCS shows both similarities and differences. Convective QBWO over the WNP originates mainly from southeast of the Philippine Sea and propagates northwestward. In contrast, convective QBWO over the SCS can be traced mainly to east of the Philippines and features a westward propagation. Such a westward or northwestward propagation is probably related to n = 1 equatorial Rossby waves. During the evolution of convective QBWO over the WNP and SCS, the vertical motion and specific humidity exhibit a barotropic structure and the vertical relative vorticity shows a baroclinic structure in the troposphere. The dominant mode of interannual variation of convective QBWO intensity over the SCS–WNP region in spring is homogeneous. Its positive phase indicates enhanced convective QBWO intensity accompanied by local enhanced QBWO intensity of vertical motion throughout the troposphere as well as local enhanced (weakened) QBWO intensity of kinetic energy, vertical relative vorticity, and wind in the lower (upper) troposphere. The positive phase usually results from local increases of the background moisture and anomalous vertical shear of easterlies. The latter contributes to the relationship between the dominant mode and QBWO intensities of kinetic energy, vertical relative vorticity, and wind. Finally, a connection between the dominant mode and the sea surface temperature anomalies in the tropical Pacific Ocean is demonstrated.

Climatological characteristics of the synoptic changes accompanying South China Sea summer monsoon withdrawal

This manuscript analyses the climatological characteristics (especially the synoptic phenomena and changes) of the South China Sea (SCS) summer monsoon withdrawal (SCSSMW) based on monsoon retreat dates from the National Climate Center of China Meteorological Administration. The SCSSMW is mainly defined as the westerly to easterly shift in the zonal winds due to the westwards intrusion of the western North Pacific (WNP) subtropical high in the northern SCS region. At the low level (850 hPa), the weakening and retreat of the intertropical convergence zone (ITCZ) and rain belt in the SCS–WNP are pertinent, and the southwesterly winds over the north Indian Ocean and the SCS also weaken and retreat. The anomalous anticyclone centred over the northern SCS resembles a Rossby wave response to the reduced precipitation over the SCS and the Philippine Sea. Changes in the upper‐level (200 hPa) circulations include the deceleration of the tropical easterly stream (from the Indo‐China Peninsula to the Arabian Sea) and northerly cross‐equatorial flow (around the equatorial eastern Indian Ocean and Maritime Continent). At the mid‐level (500 hPa), the ascending centre moves equatorwards, shifting from the northern to the southern SCS. There appears an anomalous vertical circulation cell (descending/ascending in the northern/southern SCS), which links the anomalous horizontal circulation through the Sverdrup balance. In addition, the upper‐level divergent centre is shifted southeastwards, as is the generating centre of tropical cyclones. Moreover, an anomalous convergent centre over Taiwan Island is prominent in the upper level. The time series of several atmospheric variables (e.g., zonal wind, precipitation, convection) also exhibited abrupt changes during the SCSSMW. The reasons for not simultaneous retreat of monsoonal westerlies and rainy season in the SCS are also explored.

Relationship over southern China between the summer rainfall induced by tropical cyclones and that by monsoon

This study investigates the relationship between tropical cyclone–induced precipitation (P TC) and summer monsoon–induced precipitation (P SM) in southern China (SC) during June–August. The spatial patterns of the first interannual mode are uniform in sign over SC, with positive anomalies for P TC and negative anomalies for P SM. The background of an increase in cyclonic vorticity, an increase in RH, and a decrease in vertical wind shear over the South China Sea (SCS)–western north Pacific (WNP) provides favorable conditions for more TC genesis. The positive equatorial central Pacific SST anomaly and negative North Indian Ocean SST anomaly contribute to the anomalous cyclone over the SCS–WNP, which causes decreasing P SM in SC together with an anomalous anticyclone over eastern China–Japan. By contrast, whilst the spatial patterns of the first interdecadal mode are also uniform in sign over SC, there are positive anomalies for both P TC and P SM. The first interdecadal principal component features significant positive correlation with the number of TCs forming in the SCS. There is a significant increase in P TC and P SM after early 1990s. A positive tropical Indian Ocean (TIO) SST anomaly persists from the preceding winter to summer. During the preceding winter and spring after the early 1990s, a positive western Pacific SST anomaly can result in TIO SST warming through vertical circulation. Then, the positive TIO SST anomaly triggers an anomalous WNP anticyclone and contributes to the interdecadal increase in SC P SM in the succeeding summer. The persistent heating source over SC from May to summer related to an earlier onset of the SCS monsoon may strengthen the East Asian subtropical summer monsoon.

Interdecadal variability of early summer monsoon rainfall over South China in association with the Pacific Decadal Oscillation

ABSTRACTThe interdecadal variability of early summer South China (SC) monsoon rainfall (SCMR) is investigated based on several long‐term rainfall data sets from 1926 to 2013. Both correlation and composite analyses suggest a significant out‐of‐phase relationship between interdecadal fluctuations of SCMR and the preceding wintertime Pacific Decadal Oscillation (PDO), with negative (positive) PDO epochs favouring above‐normal (below‐normal) SCMR. The physical mechanism relating the wintertime PDO to the interdecadal variability of SCMR appears to take place via atmospheric and oceanic teleconnections. The below‐normal SCMR is characterized by a meridionally arranged vortex (MAV) pattern across the East Asia–western North Pacific (WNP) sector, with an anomalous cyclone, an anomalous anticyclone and an anomalous cyclone occurring alternately over the South China Sea (SCS)–WNP, Yangtze Basin and Lake Baikal in the early summer. This may respond to the PDO‐related positive sea surface temperature (SST) anomalies locally over the SCS–WNP and remotely over the tropical eastern Pacific, with the latter forcing anomalous Hadley and Walker circulations. SC is located directly under the divergent moisture environment of anomalous northeasterlies between the anomalous cyclone over the SCS–WNP and the anticyclone over the Yangtze Basin. Again, the below‐normal SCMR is preceded by negative (positive) SST anomalies in the midlatitude North Pacific (tropical eastern Pacific) associated with positive PDO in wintertime. Such an anomalous SST pattern subsequently induces a strengthened and northward‐shifted upper‐tropospheric jet in early summer, leading to significant upper‐tropospheric convergence over SC. The out‐of‐phase relationship between SCMR and wintertime PDO as well as related physical processes are validated by coupled model simulations.

Contrast of 10–20-day and 30–60-day intraseasonal SST propagation during summer and winter over the South China Sea and western North Pacific

This study documents the structure and propagation of intraseasonal sea surface temperature (SST) variations and relative contribution of surface latent heat flux and shortwave radiation to the SST propagation in the South China Sea (SCS) and western North Pacific (WNP) regions. The emphasis is on the contrast of intraseasonal SST propagation between summer and winter and between 10–20-day and 30–60-day time scales. The dominant SST pattern during summer displays a tilted southwest–northeast band from the SCS to the subtropical WNP on both time scales, but with a larger value in the subtropical WNP on the 10–20-day time scale and in the SCS on the 30–60-day time scale. The dominant SST pattern during winter resembles that during summer, but with a larger value in the SCS. In summer, the SST anomalies show obvious northwestward and northward propagations in the SCS–WNP region on the 10–20-day and 30–60-day time scales, respectively. The cloud–radiation effect is a dominant factor for the SST propagation on both time scales in the SCS–WNP region, with a supplementary effect from the wind–evaporation effect on the 10–20-day time scale. In winter, the SST anomalies show southward propagation on both time scales in the SCS, while the southward propagation in the WNP is weak and confined to the subtropics on the 10–20-day time scale. The wind–evaporation effect makes a larger contribution to the SST propagation than the cloud–radiation effect on both time scales in the SCS–WNP region.

Coupled intraseasonal variations in the East Asian winter monsoon and the South China Sea–western North Pacific SST in boreal winter

The sea surface temperature (SST) in the South China Sea (SCS) displays prominent intraseasonal variations during boreal winter with a spectrum peak in the 10–30-day time period. These intraseasonal SST variations are closely associated with intraseasonal variations of the East Asian winter monsoon (EAWM). A weak EAWM is preceded by cooler SST and followed by warmer SST in the SCS and subtropical western North Pacific. A coherent southward propagation is seen in the SCS in SST, surface wind, and latent heat flux anomalies. This southward propagation is attributed to the wind-evaporation-SST effect under climatological northerly winds in winter, which differs from summer when climatological winds are westerly. The SST-induced wind speed anomalies are larger to the north side of SST anomalies. This induces larger surface evaporation anomalies to the north side, leading to a southward displacement of large SST anomalies. In turn, wind and evaporation anomalies move southward. There appears to be a positive feedback between circulation and precipitation that leads to amplification of meridional wind anomalies when the SST anomalies are weak. Surface latent heat flux is a dominant factor for the SST change in the SCS and the Yellow Sea. Shortwave radiation has a complementary contribution to the SST change in the SCS, but has a negative effect in the Yellow Sea. The wind-induced Ekman advection appears important for the SST warming in the Yellow Sea.

Signals of the South China Sea summer rainfall variability in the Indian Ocean

The present study investigates signals of the South China Sea (SCS) summer rainfall variability in the Indian Ocean. It is found that the SCS summer rainfall has a negative relationship with December–January–February (DJF) western-equatorial Indian Ocean (WIO) sea surface temperature (SST), a positive relationship with an asymmetric mode of precipitation anomalies in the tropical Indian Ocean during March–April–May (MAM), and a positive relationship with June–July–August (JJA) South Indian Ocean (SIO) SST. The WIO SST anomalies induce same-sign southeast Indian Ocean SST anomalies through an anomalous zonal vertical circulation. The southeast Indian Ocean SST anomalies last from late winter to early summer and induce opposite-sign SCS summer rainfall anomalies via an anomalous meridional vertical circulation. The asymmetric mode influences the SCS summer rainfall variation via the North Indian Ocean (NIO) SST anomalies with significant cloud–radiation and wind–evaporation effect. Positive (negative) SIO SST anomalies drive an anomalous direct circulation between the SIO and the NIO, and an anomalous indirect circulation between the NIO and the SCS which facilitates the occurrence of cyclonic (anti–cyclonic) wind anomalies over the SCS–western North Pacific and results in positive (negative) SCS summer rainfall anomalies. Partial correlation analysis indicates that the influence of DJF WIO SST anomalies and JJA SIO SST anomalies on the SCS summer rainfall is partly ENSO-independent, while the MAM asymmetric mode is mostly related to the preceding DJF eastern Pacific SST anomalies.

Influences of northward propagating 25–90-day and quasi-biweekly oscillations on eastern China summer rainfall

The present study reveals that rainfall anomalies associated with the 25–90-day (quasi-biweekly) tropical convective activity propagates northward (northwestward) into northern China (the Yangtze River). Enhanced rainfall over southern China results directly from enhanced convection propagating northward from the South China Sea (SCS)–western north Pacific (WNP), strengthened northward water vapor transport and intensified upper-level divergence. The anomalous anticyclone over SCS–WNP, which is induced by local lower sea surface temperature, provides a favorable condition for transporting more moisture to southern China. Stronger subtropical westerly jet and South Asia high strengthen upper-level divergence. The northward propagating 25–90-day oscillation over eastern China reaches farther north than the quasi-biweekly oscillation (QBWO). The intensity of QBWO in summer rainfall has significantly strengthened over southern China after 1993 but not for the 25–90-day oscillation. This is mainly contributed by an interdecadal increase in rainfall over southern China and the Yangtze River Basin in the wet phases of QBWO. The interdecadal enhancement of rainfall associated with the QBWO is attributed to three factors. One is lower tropospheric convergence caused by anomalous anticyclone over the WNP and anomalous cyclone over Korean Peninsula and Japan. The second is upper tropospheric divergence resulting from strengthening of subtropical westerly jet and the South Asia high. The last is enhanced ascent over South China through meridional vertical circulation.

An Interdecadal Change in Southern China Summer Rainfall around 1992/93

Abstract The present study documents a pronounced interdecadal change in summer rainfall over southern China around 1992/93 and explores the plausible reasons for this change. The summer rainfall is persistently below normal during 1980–92 and above normal during 1993–2002. Coherent changes in atmospheric circulation are identified over East Asia and the South China Sea (SCS)–western North Pacific (WNP). The increase in rainfall is accompanied by an increase in lower-level convergence, midtropospheric ascent, and upper-level divergence over southern China. The changes in lower-level winds feature two anomalous anticyclones: one over the SCS–subtropical WNP, and the other over north China–Mongolia. The outflows from the two anomalous anticyclones converge over southern China, leading to anomalous moisture convergence, enhanced ascent, and increased rainfall. The development of the northern anticyclone is related to an increase in the Tibetan Plateau snow cover in the preceding winter–spring that leads to a contrast in temperature change between the plateau and the surrounding regions. The relatively small temperature change over the plateau, coupled with increases in temperature to the west and the east, leads to an increase in surface pressure extending northward from the plateau. The development of the southern anticyclone is related to an increase in sea surface temperature in the equatorial Indian Ocean that enhances lower-level convergence and ascent. The accompanying upper-level divergent flows from the tropical Indian Ocean to the SCS–WNP lead to the development of anomalous descent and lower-level anomalous anticyclone over the SCS–WNP.

Impacts of land surface and sea surface temperatures on the onset date of the South China Sea summer monsoon

The present study analyzes the differences in spatial and temporal variations of surface temperatures between early and late onset years of the South China Sea summer monsoon (SCSSM). It is found that when the land surface temperature north of 40°N is lower (higher) and the sea surface temperature over the South China Sea-western North Pacific (SCS-WNP) is higher (lower) in winter, the onset of the SCSSM begins earlier (later). When the land surface temperature north of 40°N is higher (lower) and the sea surface temperature over the SCS-WNP is lower (higher) in spring, the onset of the SCSSM occurs earlier (later). The reason why the anomalies of the land surface temperatures north of 40°N can influence the atmospheric circulation is investigated by analysis of the wind and temperature fields. In order to verify the mechanisms of influence over the land and sea surface temperature distribution patterns and test the ability of the p-σ regional climate model (p-σ RCM9) to simulate the SCSSM onset, three types of years with early, normal, and late SCSSM onset are selected and the SCSSM regimes are numerically simulated. According to the results obtained from five sensitive experiments, when the land surface temperature is higher in the eastern part, north of 40°N, and lower in the western part, north of 40°N, and it rises faster in the eastern coastal regions and the Indian Peninsula, while the sea surface temperatures over the SCS-WNP are lower, the early onset of the SCSSM can be expected.