Observational evidence of the delayed response of stratospheric polar vortex variability to ENSO SST anomalies

The temporal and spatial relationship between ENSO and the extratropical stratospheric variability in the Northern Hemisphere is examined. In general, there exists a negative correlation between ENSO and the strength of the polar vortex, but the maximum correlation is found in the next winter season after the mature phase of ENSO event, rather than in the concurrent winter. Specifically, the stratospheric polar vortex tends to be anomalously warmer and weaker in both the concurrent and the next winter season following a warm ENSO event, and vice versa. However, the polar anomalies in the next winter are much stronger and with a deeper vertical structure than that in the concurrent winter. Our analysis also shows that, the delayed stratospheric response to ENSO is characterized with poleward and downward propagation of temperature anomalies, suggesting an ENSO-induced interannual variability of the global mass circulation in the stratosphere. Particularly, in response to the growing of a warm ENSO event, there exist warm temperature and positive isentropic mass anomalies in the midlatitude stratosphere since the preceding summer. The presence of an anomalous wavenumber-1 in the concurrent winter, associated with an anomalous Aleutian high, results in a poleward extension of warm anomalies into the polar region, and thus a weaker stratospheric polar vortex. However, the midlatitude warm temperature and positive isentropic mass anomalies persist throughout the concurrent winter till the end of the next summer. In comparison with the concurrent winter, the strengthening of poleward heat transport by an anomalous wavenumber-2 in the next winter results in a much warmer and weaker polar vortex accompanied with a colder midlatitude stratosphere.     

An Introduction to the Coupled Model FGOALS1.1-s and Its Performance in East Asia

The spectral version 1.1 of the Flexible Global Ocean-atmosphere-land System (FGOALS1.1-s) model was developed in the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophys- ical Fluid Dynamics at the Institute of Atmospheric Physics (LASG/IAP). This paper reports the major modifications to the physical parameterization package in its atmospheric component, including the radia- tion scheme, convection scheme, and cloud scheme. Furthermore, the simulation of the East Asian Summer Mon...     

Influences of external forcing changes on the summer cooling trend over East Asia

Observations indicate a surface cooling trend during the East Asian summer in recent decades, against a background of global warming. This cooling trend is re-examined using station data from 1951 to 2007, and atmospheric general circulation model (AGCM) simulations are performed to investigate the possible influence of changes in external forcing. The numerical experiments are designed to investigate the effects of four types of external forcing: greenhouse gases (GHGs), Total Solar Irradiance (TSI), ozone, and the direct effects of aerosols. Results indicate that external forcing contributes to the cooling trend over East Asia. Furthermore, GHGs, and to a lesser degree the direct effects of aerosols, are the main contributors to the cooling trend. The possible linkages between the external forcings and the cooling trend are discussed.     

Barotropic Process Contributing to the Formation and Growth of Tropical Cyclone Nargis

This study reveals the barotropic dynamics associated with the formation and growth of tropical cyclone Nargis in 2008,during its formation stage.Strong equatorial westerlies occurred over the southern Bay of Bengal in association with the arrival of an intraseasonal westerly event during the period 22-24 April 2008. The westerlies,together with strong tropical-subtropical easterlies,constituted a large-scale horizontal shear flow,creating cyclonic vorticity and thereby promoting the incipient disturbance that eventually evolved into Nargis.This basic zonal flow in the lower troposphere was barotropically unstable,with the amplified disturbance gaining more kinetic energy from the easterly jet than from the westerly jet during 25-26 April. This finding suggests that more attention should be paid to the unstable easterly jet when monitoring and predicting the development of tropical cyclones.Energetics analyses reveal that barotropic energy conversion by the meridional gradient of the basic zonal flow was indeed an important energy source for the growth of Nargis.     

20–50-day oscillation of summer Yangtze rainfall in response to intraseasonal variations in the subtropical high over the western North Pacific and South China Sea

The spatio-temporal variability in summer rainfall within eastern China is identified based on empirical orthogonal function (EOF) analysis of daily rain-gauge precipitation data for the period 1979–2003. Spatial coherence of rainfall is found in the Yangtze Basin, and a wavelet transform is applied to the corresponding principal component to capture the intraseasonal oscillation (ISO) of Yangtze rainfall. The ensemble mean wavelet spectrum, representing statistically significant intraseasonal variability, shows a predominant oscillation in summer Yangtze rainfall with a period of 20–50 days; a 10–20-day oscillation is pronounced during June and July. This finding suggests that the 20–50-day oscillation is a major agent in regulating summer Yangtze rainfall. Composite analyses reveal that the 20–50-day oscillation of summer Yangtze rainfall arises in response to intraseasonal variations in the western North Pacific subtropical high (WNPSH), which in turn is modulated by a Rossby wave-like coupled circulation–convection system that propagates northward and northwestward from the equatorial western Pacific. When an anomalous cyclone associated with this Rossby wave-like system reaches the South China Sea (SCS) and Philippine Sea, the WNPSH retreats northeastward due to a reduction in local pressure. Under these conditions, strong monsoonal southwesterlies blow mainly toward the SCS–Philippine Sea, while dry conditions form in the Yangtze Basin, with a pronounced divergent flow pattern. In contrast, the movement of an anomalous anticyclone over the SCS–Philippine Sea results in the southwestward extension of the WNPSH; consequently, the tropical monsoonal southwesterlies veer to the northeast over the SCS and then converge toward the Yangtze Basin, producing wet conditions. Therefore, the 20–50-day oscillation of Yangtze rainfall is also manifest as a seesaw pattern in convective anomalies between the Yangtze Basin and the SCS–Philippine Sea. A considerable zonal shift in the WNPSH is associated with extreme dry (wet) episodes in the Yangtze Basin, with an abrupt eastward (westward) shift in the WNPSH generally leading the extreme negative (positive) Yangtze rainfall anomaly by a 3/8-period of the 20–50-day oscillation. This finding may have implications for improving extended-range weather forecasting in the Yangtze Basin.     

AAS has been accepted for Coverage by the Science Citation Index in 2009

In May 2008, ScienceWatch.com named Advances in Atmospheric Sciences a Rising Star among Geosciences journals. According to Essential Science IndicatorsSM from Thomson Reuters, the journal＇s cur-rent citation record includes 764 papers cited a total of 1,658 times between January 1, 1998 and February 29 2008.     

WAVE-MEAN FLOW INTERACTION AND FORMATION OF BLOCKING--PERSISTENT ANOMALOUS WEATHER IN CHINA IN THE SUMMER OF 1980*

In the summer of 1980,serious persistent abnormal weather occurred over vast areas in China.While record-breaking cold and flood were observed in the reaches of the Changjiang and Huaihe Rivers,severe hot wave and drought dominated the entire northern China.The long-lasting disastrous weather is mainly due to the stable development and maintenance of blocking anticyclone over the northeastern Asia.This study aims at the understanding of the roles of time-varying weather system transport in the formation of the blocking.It was shown that during this period,there appeared continuous generation of synoptic-scale perturbations along the strong baroclinic zone over Europe and the western Asia.While such perturbations propagated eastward,energy conversion occurred.At equivalent barotropic layer,with weak dissipation,such energy conversion was subjected to the so-called bi-directional principle:while the energy of the synoptic-scale system cascaded to smaller scale system,a much larger portion was transferred to the blocking system with larger scale.Potential vorticity diagnoses also revealed that the transient weather systems played the roles of maintaining the mean anticyclonic vorticity to the south,and mean cyclonic vorticity to the north,of the westerly jet,and exciting strong anticyclonic vorticity growth and corresponding geopotential height increase in high latitude area downstream of the westerly diffluence region.The research also showed that,the intensity of the forcing of the blocking formation via wave-mean flow interaction in this Asian case was much stronger than that occurring in the western Europe in the summer of 1976.It was therefore concluded that when persistent abnormal weather in the northern China was studied,in addition to the subtropical weather systems,attention should also be drawn to the development of baroclinic zone over Europe and the western Asia,and the propagation and transfer properties of the synoptic systems embedded in the baroclinic zone.     

MAIN HEATING MODES OVER THE TIBETAN PLATEAU IN JULY AND THE CORRELATION PATTERNS OF CIRCULATION AND PRECIPITATION OVER EAST ASIA*

Based on the 1958-1999 monthly averaged NCEP/NCAR reanalysis data,the REOF analysis is applied to obtain the main spatial modes of normalized atmospheric heating source over the Tibetan Plateau(TP) in July.Results show that the four leading modes are located over the northeast TP,southwest TP.Kashmir and southeast TP respectively,and the cumulative variances are no more than one third of the total.It indicates that the heating source distribution is very complicated over the TP in July.In other words.it is difficult to depict the heating spatial distribution with a few modes.By using wavelet analysis,a 2-4-year variation period is identified in these modes.Moreover,correlation coefficients between each RPC and zonal wind U, meridional wind V.zonal moisture flux Q_v,meridional moisture flux Q_v,and precipitation rate over East Asia are calculated to construct correlation fields,Results show that different heating modes over the TP correspond to different circulation,moisture flux as well as precipitation patterns,Precipitation over North China(or Kashmir) is negatively(or positively) correlated with REOF1.Similarly.notable negative(or positive) correlation can be found between the rainfall over south part of Southwest China.South China,and the Philippines(or Japan) and the REOF3. Due to high localization of diabatic heating over the TP.it is not enough to study the influence of TP thermal forcing on the climate with an area averaged heating index.     

STUDY ON THE VARIATION IN THE CONFIGURATION OF SUBTROPICAL ANTICYCLONE AND ITS MECHANISM DURING SEASONAL TRANSITION-PART Ⅲ:THERMODYNAMIC DIAGNOSES

The mechanisms for the variation in the configuration of subtropical anticyclone during seasonal transition are explored from energy budget using the NCEP/NCAR reanalysis data.Based on the seasonal variations of temperature and heating fields,it is found that the significant diabatic heating associated with spring precipitation over southern China has impacts on subsequent Asian seasonal transition.The reversal of meridional temperature gradient in the vicinity of the WEB (westerly-easterly boundary) in the middle and upper troposphere also depends on the latitudinal position where temperature ridge locates.The northward shift of the warm temperature ridge results from the fact that the local temperature increase to the north of the WEB is more than that in its vicinity.The diagnostic results through thermodynamic equation show that physical mechanism responsible for seasonal transition is different from area to area over the Asian monsoon region.The dominant factors responsible for northward shift of the Bay of Bengal warm ridge are the meridional temperature in initial stages of the onset and the descending motion after the onset. The factors for causing the northward jump of the South China Sea warm ridge involve the zonal temperature advection,meridional temperature advection,and diabatic heating associated with the southern China spring rainfall.The subsidence is the factor leading to the northward migration of the South Asia warm ridge.