The increase in southern China summer rainfall around 1993 was accompanied by an increase in tropical cyclones that formed in the South China Sea.This study documents the connection of these two features.Our analysis shows that the contribution of tropical cyclones that formed in the South China Sea to southern China summer rainfall experienced a significant increase around 1993,in particular,along the coast and in the heavy rain category.The number of tropical cyclones that formed in the western North Pacific and entered the South China Sea decreased,and their contribution to summer rainfall was reduced in eastern part of southern China(but statistically insignificant).The increase in tropical cyclone-induced rainfall contributed up to ～ 30% of the total rainfall increase along the coastal regions.The increase of tropical cyclones in the South China Sea appears to be related to an increase in local sea surface temperature.
Previous studies have shown that meridional displacement of the East Asian upper-tropospheric jet stream (EAJS) dominates interannual variability of the EAJS in the summer months.This study investigates the tropical Pacific sea surface temperature (SST) anomalies associated with meridional displacement of the monthly EAJS during the summer.The meridional displacement of the EAJS in June is significantly associated with the tropical central Pacific SST anomaly in the winter of previous years,while displacements in July and August are related to tropical eastern Pacific SST anomalies in the late spring and concurrent summer.The EAJS tends to shift southward in the following June (July and August) corresponding to a warm SST anomaly in the central (eastern) Pacific in the winter (late spring-summer).The westerly anomaly south of the Asian jet stream is a result of tropical central Pacific warm SST anomaly-related warming in the tropical troposphere,which is proposed as a possible reason for southward displacement of the EAJS in June.The late spring-summer warm SST anomaly in the tropical eastern Pacific,however,may be linked to southward displacement of the EAJS in July and August through a meridional teleconnection over the western North Pacific (WNP) and East Asia.
The present study revisited the first two leading modes of tropical Pacific sea surface temperature anomalies (SSTA) during the period of 1979 2008.It is suggested that the so-called El Nio Modoki,which is captured by the second mode,exists objectively and exhibits obvious differences from traditional El Nio,which is captured by the first mode,in terms of its spatial characteristics.Furthermore,the authors found that El Nio Modoki is linearly independent of traditional El Nio;hence,it cannot be described as part of the traditional El Nio evolution,and vice versa.
Spring is the critical period for atmospheric circulation transition from winter to summer.The spring water cycle is very im-portant to agriculture in planting crop and the initial growth of crop.Note that there was a significant abrupt decadal change in the water budget increase during early spring over eastern China in the late 1970s.Studied here are the decadal variations of water budgets over the key regions and the associated change of water cycle over East Asia and atmospheric circulation over Asia-West Pacific region in early spring,using the observed(OBS) precipitation,the ECMWF(ERA) and NCEP/NCAR rea-nalysis(NRA),and the Mantua's Pacific decadal oscillation index(PDOI).The water budget increments from March to April exhibited a sharp decrease over the key region around Huaihe River basin(HHR)(111°-120°E;31°-36°N) after year 1978.Before 1977 the water vapor flux through south boundary of the HHR region increased greatly during March to April by 1.52 mm d 1 in ERA and 1.88 mm d 1 in NRA.Concurrently the moisture convergence and precipitation over the region also in-creased greatly.The increment for the moisture convergence was 1.11 mm d 1 in ERA and 1.22 mm d 1 in NRA,and for the precipitation was 1.08 mm d 1 in observation and 1.05 mm d 1 in ERA.April was the time that the water budgets over HHR increased most rapidly before 1977.But after 1978 the water budgets decreased conversely from Mach to April.The water vapor flux increment through the south boundary was 0.03 mm d 1 in ERA and 0.01 mm d 1 in NRA,the moisture conver-gence increment was 0.91 mm d 1 in ERA and 0.53 mm d 1 in NRA,and precipitation increment was 0.08 mm d 1 in ob-servation and 0.15 mm d 1 in ERA.Further investigation has shown that the large-scale atmospheric circulation in the early spring has correspondingly changed significantly after the late 1970s.During March to April,the weakening of the trough over East Asia became significantly slower,and the strengthening of the ridge over the west China became significantly faster in the
The eight datasets of the summer (June-August) surface sensible heat (SH) flux over the Tibetan Plateau (TP) are compared on the time scales of the climatology,interannual variability and linear trend during 1980-2006.These data sets include five reanalyses (National Center for Environmental Prediction reanalysis,NCEPR1 and NCEPR2,NCEP climate forecast system reanalysis,CFSR,Japanese 25-year reanalysis,JRA,and European Centre for Medium Range Weather Forecasts reanalysis,ERA40),two land surface model outputs (Noah model data of Global Land Data Assimilation System version 2,G2_Noah,and Simple Biosphere version 2 output by Yang et al.,YSiB2),and estimated SH based on China Meteorological Administration (CMA) station observations,ObCh.The results suggest that the summer SH on the TP differs from one dataset to another due to different inputs and calculations.Climatologically,the ERA40 and JRA distribute rather uniformly while the other six products show similar regional disparities,that is,larger in the west than in the east and stronger in the north and the south than in the middle of the plateau.The mean magnitude of the SH averaged over the 76 stations above the TP varies considerably among each dataset with the difference of more than 20 W m?2 between the maximum (G2_Noah) and minimum (ObCh).Nevertheless,they are consistent in the interannual variability and mostly show a significant decreasing trend corresponding to the weakening surface wind speed,in spite of the distinct trend for the ground-air temperature difference among the different data sets.These two consistencies indicate the particular availability of the SH products,which is helpful to the relevant climate dynamics research.
Physical processes associated with onset of the 1998 Asian summer monsoon were examined in detail using multi-source datasets. We demonstrated that strong ocean-atmosphere-land interaction in the northern Indian Ocean and tropical Asian area during spring is a fundamental factor that induces the genesis and development of a monsoon onset vortex over the Bay of Bengal (BOB), with the vortex in turn triggering onset of the Asian summer monsoon. In spring, strong surface sensible heating over India and the Indochina Peninsula is transferred to the atmosphere, forming prominent in situ cyclonic circulation, with anticyclonic circulations over the Arabian Sea and northern BOB where the ocean receives abundant solar radiation. The corresponding surface winds along the North Indian Ocean coastal areas cause the ocean to produce the in situ offshore currents and upwelling, resulting in sea surface temperature (SST) cooling. With precipitation on the Indochina Peninsula increasing from late April to early May, the offshore current disappears in the eastern BOB or develops into an onshore current, leading to SST increasing. A southwest-northeast oriented spring BOB warm pool with SST >31°C forms in a band from the southeastern Arabian Sea to the eastern BOB. In early May, the Somali cross-equatorial flow forms due to the meridional SST gradient between the two hemispheres, and surface sensible heat over the African land surface. The Somali flow overlaps in phase with the anticyclone over the northern Arabian Sea in the course of its inertial fluctuation along the equator. The convergent cold northerlies on the eastern side of the anticyclone cause the westerly in the inertial trough to increase rapidly, so that enhanced sensible heat is released from the sea surface into the atmosphere. The cyclonic vorticity forced by such sensible heating is superimposed on the inertial trough, leading to its further increase in vorticity strength. Since atmospheric inertial motion is destroyed, the flow deviates from the inertial track
In this study,the teleconnection between Indian Ocean sea surface temperature anomalies (SSTAs) and the frequency of high temperature extremes (HTEs) across the southern Yangtze River valley (YRV) was investigated.The results indicate that the frequency of HTEs across the southern YRV in August is remotely influenced by the Indian Ocean basin mode (IOBM) SSTAs.Corresponding to June-July-August (JJA) IOBM warming condition,the number of HTEs was above normal,and corresponding to IOBM cooling conditions,the number of HTEs was below normal across the southern YRV in August.The results of this study indicate that the tropical IOBM warming triggered low-level anomalous anticyclonic circulation in the subtropical northwestern Pacific Ocean and southern China by emanating a warm Kelvin wave in August.In the southern YRV,the reduced rainfall and downward vertical motion associated with the anomalous low-level anticyclonic circulation led to the increase of HTE frequency in August.
Instead of using complicated general circulation models (GCMs), a simple semi-analytical model based on ray theory has been used to study energy evolution and ray path of Rossby waves in slowly varying mean flows. Our model yields similar results to those calculated from barotropic models, and also provides a chance to study Rossby waves in the slowly varying flows with both vertical and meridional shears. The model results show that upward Rossby waves can only grow in westerlies, and decay when further ascend. The baroclinic Rossky waves are restrained by the β effect in lower latitude. In the westerly jet with meridional and vertical shears, the barotropic Rossby waves originated from south of the westerly jet, and these can grow while propagating upper-northward. The baroclinic Rossby waves originated from north of the westerly jet and can grow while propagating upward and southward. Such a semi-analytical model provides a simple forecasting tool to allow study of the local weather anomalies to the heating/topography forcing associated with the global warming.
