Westerlies-dominated Climate Regime’ In The Mid-latitude Asia On Decadal And Interannual Time Scales And Their Physical Mechanisms

The mid-latitude Asian continent can be roughly divided into two distinct climatic regions:the humid eastern-southern part of Asia mainly influenced by monsoon circulation and the inland arid/semiarid part of western Asia dominated by the mid-latitude westerlies. Anti-phase/out-of-phase relationship in precipitation (moisture) changes between the inland arid Asia and eastern-southern part of Asia exists on centennial to millennial timescale during the Holocene. It is thus proposed that there is a ’westerlies-dominated climate regime’in mid-latitude Asia during the modern interglacial period. However, it is not clear that such spatiotemporal precipitation/moisture variations also exist on interannual to decadal time scales. The inland arid Asia is one of the largest arid regions at the middle latitudes. The climate changes in this region are closely linked to the change of atmospheric circulations/sea surface temperature (SST). For example, the precipitation variations in this region are strongly influenced by the westerly circulation and the North Atlantic Oscillation. The temperature also increased rapidly during the past century in the context of global warming. These results cause increasing evaporation to and decreasing soil moisture, which in term significantly affects the change of regional hydrological cycle. Therefore, understanding the variations of precipitation on interannual to decadal scales in mid-latitude Asia is of paramount importance for the regional agriculture and economy. This study analyzed the ’westerlies-dominated climate regime’in mid-latitude Asia on interannual to decadal scales and their physical processes using instrumental and reanalysis datasets.Based on ’westerlies-dominated climate regime’, this study firstly used the surface wind fields, and the boundary of the summer monsoon and arid regions to define the’modern westerly circulation dominated region’, which contains Xinjiang, Hexi Corridor in China and the arid Central Asia (ACA). Additionally, based on the monthly precipitation dataset from the Global Precipitation Climatology Centre (GPCC) and monthly geopotential height, winds and specific humidity on pressure levels from the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, the climatic features associated with the ’modern westerly circulation dominated region’were identified. Those features include1) The precipitation variations are influenced by westerly circulation year round;2) The shifts of surface wind from winter to summer are distinctly different from that in the monsoon dominated middle latitude Eastern Asia;3) The precipitation variations in the area also differ regionally.This study further analyzed the temporal precipitation variations in the ACA and their regional differences during the past80years using monthly gridded precipitation from the Climatic Research Unit (CRU). Our results showed that the annual precipitation in this westerly circulation dominated arid region is generally increasing during the past80years, with an apparent increasing trend in winter. The precipitation variations in ACA also differ regionally, which can be divided into five distinct subregions:I-West Kazakhstan region, II-East Kazakhstan region, Ⅲ-Central Asia Plains region, Ⅳ-Kyrgyzstan region, and Ⅴ-Iran Plateau region. The annual precipitation falls fairly even on all seasons in the subregions I and II, whereas the annual precipitation is falling mainly on winter and spring in the subregions Ⅲ, IV and Ⅴ. The annual precipitation is increasing on all subregions except the subregion V. The long-term trends in annual precipitation in all subregions are determined mainly by trends in winter precipitation. The increasing precipitation in ’modern westerly circulation dominated region’is opposite to that of the summer monsoon dominated regions in China, suggesting that the ’westerly-dominant climate regime’ also existed on decadal scales.The precipitation in ’modern westerly circulation dominated region’ has significant interannual variations, especially Tropospheric Biennial Oscillation (TBO), which is linked to the variations of westerly circulation in the middle troposphere. This study analyzed the spatial differences of the precipitation variations in the mid-latitude Asia and their possible physical mechanisms during1960-2009. The annual precipitation showed an opposite variation between the ’modern westerly circulation dominated region’ and summer monsoon dominated regions, suggesting that the ’westerly-dominant climate regime’ also existed on interannual scales. Given the different contributions of seasonal precipitation to annual total precipitation in both regions, the atmospheric circulation anomalies during the major precipitation seasons were analyzed. In winter, negative North Atlantic Oscillation may cause negative height anomalies over the north side and positive anomalies over the south side of the ’modem westerly circulation dominated region’. Together, the enhanced pressure gradient and anomalous westerly wind brings more water vapor to the region, and leaves less precipitation in summer monsoon dominated regions. In summer, the low-pressure anomalies in Northeast China, along with a weaker summer monsoon and negative height anomalies in Eastern Europe together contribute to reduced (excessive) summer precipitation in summer monsoon dominated regions (’modern westerly circulation dominated arid region’). The interactions between ENSO and NAO may result in the opposite precipitation variations between the two regions.The Taklimakan and the adjacent Gobi Desert (TD in short) in northwestern China is the most arid region in the modern westerly circulation dominated region, where water is scarce year round. As the typical case, this study investigated the variations of summer precipitation in TD and their association with water vapor flux and atmospheric circulation using the monthly precipitation dataset from the Global Precipitation Climatology Centre (GPCC) and monthly geopotential height, winds and specific humidity on pressure levels from the National Center for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) and the new NCEP Climate Forecast System Reanalysis. Though the long-term mean water vapor mostly comes from the west, the variations of summer precipitation in TD are dominated by the water vapor flux from the south, which originates from the Arabian Sea. The anomalous water vapor fluxes are closely associated with the meridional teleconnection pattern around50(?)-80(?) and the zonal teleconnection pattern along the Asian westerly jet in summer. The meridional teleconnection connects the Central Asia and the tropical Indian Ocean; and the zonal teleconnection resembles the’Silk Road pattern’. The two teleconnections lead to negative height anomalies in the Central Asia and the positive height anomalies in the Arabian Sea/India and North Central China. The anomalous pressure gradient force, caused by those height anomalies, leads to anomalous ascending motion in TD and brings more water vapor from the Arabian Sea passing over the Tibetan Plateau to influence the precipitation development in the study region. These mechanisms can well explain the out-of-phase relationship between the TD precipitation and the Indian summer monsoon in the instrumental period and the past2000years. Furthermore, this study analyzed the air mass back trajectory track of heavy precipitation events, suggesting that the water vapor fluxes influencing TD mostly come from the Arabian Sea.