No abstract

Extraordinarily frequent and long-lasting snowstorms affected China in January 2008, causing abovenormal precipitation, below-normal temperature, and severe icing conditions over central-southern China. These snowstorms were closely linked to the change in the Middle East jet stream (MEJS), which intensified and shifted southeastward. The change in MEJS was accompanied by southeastward shifts of the ridge and the trough over Europe and western Asia. The intensified MEJS also strengthened the trough embedded in the southern branch of the subtropical westerlies over the southern Tibetan Plateau, enhancing the water vapor transport from western Asia and the Bay of Bengal to China. In the meantime, the subtropical western Pacific high (SWPH) was stronger and its ridgeline was farther north than normal. The anomalous high slowed down the eastward propagation of weather systems to the Pacific and favored convergence of water vapor over central-southern China. The MEJS is usually strong when the Arctic Oscillation (AO) is positive and the SWPH is farther north than normal in La Niñ a winters. Compared to the SWPH and the Niñ o-3.4 sea surface temperature (SST), the MEJS and the AO exert stronger influences on the temperature and the precipitation over central-southern China, despite the fact that these possible impacting factors are not completely independent from each other. Although the La Niñ a event might contribute to the climate anomalies through its relation with the SWPH in January 2008, an analysis of historical events indicates that La Niñ a conditions alone can hardly cause severe and persistent snow conditions over central-southern China. In addition, compared to the Niñ o-3.4 SST and the SWPH, the conditions of December MEJS and AO exhibit stronger precursory signals of the variability of January temperature over central-southern China.

A diagnostic analysis reveals that on the interannual time scale the southeast-northwest movement is a dominant feature of the South Asian high (SAH), and it is closely related to the Indian and East Asian summer monsoon rainfall. The southeastward (northwestward) shift of the SAH is closely related to less (more) Indian summer monsoon rainfall and more (less) rainfall in the Yangtze River valley (YRV) over the East Asian summer monsoon region. An anomalous AGCM is utilized to examine the effect of latent heat anomalies associated with the Asian summer monsoon rainfall on the SAH. The negative latent heat anomalies over the northern Indian Subcontinent associated with a weak Indian summer monsoon stimulates an anomalous cyclone to its northwest and an anticyclone to its northeast over the eastern Tibetan Plateau and eastern China in the upper troposphere, which is responsible for the east-west shift of the SAH and more rainfall in the YRV. The positive latent heat release associated with rainfall anomalies in the YRV excites a southward-located anticyclone over eastern China, exerting a feedback effect on the SAH and leading to a southeast-northwest shift of the SAH.

Diagnostic analyses are performed to investigate the relationship between the upper-level Asian westerly jet stream (AWJS) and the associated rainfall pattern over the AWJS region in boreal summer on interannual time scales. Results show an out-of-phase rainfall variation over central Asia (CA) and north China (NC), which is closely related to the southeast–northwest (SE–NW) shift of the AWJS. The physical mechanisms on the relationship between the AWJS and the rainfall pattern are revealed by exploring the effects of the Indian summer monsoon (ISM) and the South Asian high (SAH). It is found that the SE–NW shifts of the AWJS and SAH associated with the ISM lead to the anomalous circulations over the midlatitudes in the AWJS region and cause rainfall anomalies over CA and NC. A weak ISM results in a southeastward shift of SAH, which is responsible for a southeastward shift of the AWJS. The anomalous atmospheric circulation associated with the southeastward located SAH produces anomalous updrafts (downdrafts) over the western (eastern) AWJS region, resulting in increased rainfall over CA and decreased rainfall over NC. It is proposed that the upper-level system SAH plays a crucial role in the teleconnection among the summer rainfall over the midlatitude AWJS region and ISM region.

[1] A recently derived data set of daily precipitation is used to study the summer precipitation events over Asia and their changes in the decades of . Regional features of the precipitation over entire tropical-subtropical Asia are investigated, exploiting the increased resolution and improved accuracy of the data set relative to other estimates. The changes in precipitation amount and precipitation days for total, extreme, heavy, and light-moderate precipitations are examined. Although the Asian summer monsoon precipitation falls mostly in the form of light-moderate rainfalls, regions of relatively frequent extreme precipitation events are found over South Asia and EastSoutheast Asia. These regions are separated by a narrow zone over the Indo-China peninsula, along 100°E, where extreme precipitation rarely occurs. During the period examined, the amount of total precipitation and light-moderate precipitation exhibits positive trends over southeastern and northwestern China, separated by negative trends over central China and southwestern and northeastern Asia. This sandwich-like pattern, which also appears in the fields of precipitation days and soil moisture content, is associated with the enhanced water vapor supply related to the strengthened monsoon flow over southeastern China and the anomalous easterlies over northwestern China. It is also associated with the decreased water vapor supply linked to the weakened monsoon flows over southern-southwestern Asia and central China and to the anomalous northerly flow over northeastern Asia. Over the entire tropical-subtropical Asia, the largest changes in precipitation, atmospheric circulation, and water vapor transport occur over southern China. On the other hand, the changes over India are much smaller.