北太平洋增暖对我国西北秋雨的影响

利用1979—2012年我国160站逐月降水资料、NOAA全球海洋表面温度资料和NCEP-DOE大气环流再分析资料，采用统计分析方法研究了北太平洋海表增暖对我国西北秋雨年代际变化的影响。结果表明：西北秋雨在2000年前后经历了年代际跃变，1986—1999年为少雨期，2000—2012年为多雨期。进一步分析表明：西北秋雨的年代际变化与北太平洋海表增暖关系密切，北太平洋海温偏暖时，东亚—北太平洋地区的大气温度升高，引起东亚地区的南北温差减弱，使东亚西风急流减弱，急流中心偏北，东亚中纬度地区气压升高，导致异常东风水汽输送带偏强，造成西北秋雨异常偏多。

Influences of the North Pacific Warming on Autumn Precipitation in Northwest China

Based on monthly precipitation data from 160 meteorological stations established by National Climate Center in China, NOAA reconstructed extended monthly SST and monthly atmospheric circulation data from NCEP-DOE Reanalysis datasets, the influence of North Pacific sea surface temperature (SST) on autumn precipitation from 1979 to 2012 in Northwest China is studied by using statistical analysis method. An inter-decadal change of the autumn precipitation is found around the year of 2000 by 11-year-running t test. It is in the dry period during 1986-1999 and wet period during 2000-2012 for the region in Northwest China. Further analysis shows that the inter-decadal change of autumn precipitation in Northwest China is connected with the warming SST in the North Pacific after 2000. When the SST in the North Pacific is in warm phase, the troposphere temperature over East Asia-Northwest Pacific is dramatically increased. The contrast of troposphere temperature between the north and the south over the East Asia-Northwest Pacific is directly associated with the strength of westerly jet and the geopotential high anomalies over the region. The warming of troposphere temperature over East Asia-Northwest Pacific weakens the atmospheric thermal contrast between the mid-low latitudes and the polar area in the East Asian region. Influenced by the decreased thermal contrast along East Asia region, the westerly jet over the East Asia-Northwest Pacific is reduced significantly. And the westerly jet center at middle latitudes near the East Asian coast shifts northward. The warming in troposphere circulation and the north shifting of weakened westerly jet tend to make the geopotential high along East Asian coast increase significantly. In other words, the trough at 500 hPa along the East Asian coast is weakened, which is characterized as an anomalous anticyclone at 500 hPa. The anomalous wind flows at 850 hPa in the south of the anticyclone are easterlies. As a result, the water vapor transported by anomalous easterlies from the Northwest Pacific into Northwest China along East Asian coast is increased. Both water vapor and convergence are enhanced, leading to increasing autumn precipitation in northwest China after 2000. Results suggest that autumn precipitation in Northwest China is in the wet phase after 2000 and has a rising chance to bring flood disaster. As the autumn precipitation is much different from summer rainfall, the prediction of the autumn precipitation in the Northwest China is a new challenge to short term climate forecast. The warming background in the North Pacific could be a factor to consider at the decadal timescale.