| Central Asia and Western Tibetan Plateau located in the middle latitude of Asia.With complicated topography and below 400 mm precipitation per year,this area is known as a typical arid and semi-arid land.The water resources are scarce and the risks of climate change are more dangerous.Since the precipitation in this area has severe impact on both the local natural environment and human activities,it is highly regarded by researchers.This research analyzes the spatial and temporal distribution of the last 40 years precipitation over Central Asia and Western Tibetan Plateau by observation data.We further discuss the spatial distribution,year cycle,inter-annual variability and intra-seasonal variability of the precipitation over this domain.The regional and large scale wind circulations which influence the oscillation of the precipitation are analyzed by simultaneous reanalysis data.At last,a numerical model is used to complement the observation data.The main results are listed as follow:(1)Central Asia and Western Tibetan Plateau is controlled by the westerlies all the year.The main precipitation season is in boreal winter(December to the next March).The winter precipitation in this domain,specially on the windward slope,is much higher than the other part of Asia.There are two precipitation center,one is on the north of Pamir Plateau and another one is on Karakorum-Himalaya.The maximum precipitation rate can reach to 8mm/d.The total winter precipitation counts about 50%of the whole year.The column water vapour in this area is much lower than the lower latitude.The middle troposphere westerlies are the main transportation path of water vapour.Because of the large slope of western part of Tibetan Plateau,the westerly wind is uplifted.On one hand,the uplift of air leads to condensation of water although there is less water vapour.On the other hand,the mid-latitude westerlies are disturbed due to the block of topography,which leads to more cyclones on the upstream.The cyclones move downstream with the westerlies and induce more precipitation over Central Asia and Western Tibetan Plateau.(2)Empirical orthogonal decomposition(EOF)is applied on the winter daily precipitation over Central Asia and Western Tibetan Plateau and two main mode,the first principle mode(PC1)and the second principle mode(PC2)are decomposed.The variance explanation of PC1 reaches to 24.4%and the mode shows consistent precipitation variation in the domain.The center of anomaly lies on Karakorum-Himalaya.The differences of 500 hPa zonal winds between high and low PC1 illustrate that the westerly winds over arabian sea,southeastern Iran,Pakistan,southwestern India and Tibetan Plateau increase while the precipitation in the domain increases.That means the mode of PC1 comes from meridional oscillation of the whole westerlies and leads to inter-annual precipitation variation.The variance explanation of PC2 reaches 15.6%and the mode shows dipole precipitation anomalies over north Pamir and Karakorum-Himalaya.The differences of 500 hPa zonal winds between high and low PC2 illustrate that the westerly winds over arabian sea,southern Pakistan,northwestern Indian Ocean and India increase while the winds over western Pamir decrease.That means the mode of PC2 comes from the out-of-phase oscillation of north and south branches of the westerlies and leads to intra-seasonal precipitation variation.(3)Singular Variance decomposition(SVD)is applied between winter precipitation and circulation over the domain and two main mode which are similar to the results of EOF analysis.However,the PC1 in SVD resembles the PC2 in EOF and the variance explanation reaches to 55.12%.That means the regional circulation has much stronger influence on the dipole precipitation pattern over north Pamir and Karakorum-Himalaya.The results illustrate that when the precipitation decreases over north Pamir and increases over Karakorum-Himalaya,the southwesterly winds deceaseovernorthPamirandcyclonecirculationenhancedover Karakorum-Himalaya.At the same time,500 hPa negative geopotential height anomalies show up over Tibetan Plateau.The lead regression of 500 hPa gropotential height anomalies illustrate positive anomalies over the mid-latitude Atlantic Ocean and negative anomalies over high-latitude Atlantic Ocean,which resembles the positive phase of North Atlantic Oscillation(NAO).The dipole precipitation anomaly over the domain induced by the intra-seasonal oscillation of westerlies can be related to preceding activities of NAO.(4)Composite analysis is applied on different kind of meteorological factors according to the NAO index.Results shows that NAO can influence the trough-ridge phase of both the north and south branches of westerlies.During the positive NAO phases,north branch trough and south branch ridge forms over the north Atlantic Ocean.The two branches merge to one strong westerly wind belt.On the downstream,the two branches separate over Europe.The north branch ridge and south branch trough forms on 40~oE.The north branch avoids the north part of Tibetan Plateau while the south branch encounters the south part of Tibetan Plateau.When the north branch avoids Tibetan Plateau,the southwesterly winds,water transportation and precipitation decease.When the south branch encounters Tibetan Plateau,the westerly disturbances,cyclone activities and precipitation increase.Vice versa.Thus the dipole precipitation anomalies are influences by the north and south branches separately.The causes of precipitation formation are also different.(5)The Community Atmosphere Model,version 5(CAM5)is applied to simulate the global atmosphere activities from 1990-2014 forced by observed sea surface temperature.The output of model is comparable with the observation data in the domain.The year cycle,inter-annual variability and intra-seasonal variability are reasonable.The results of model data shows the same dipole precipitation anomalies,regional wind field and large scale circulation as the results of observation data. |
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