春季北极涛动对盛夏长江流域地表气温的影响

本文基于1958至2002年的ERA-40 月平均再分析资料,利用年际增量方法分析了春季北极涛动(Arctic Oscillation,简称AO)与我国夏季长江流域地表气温的关系。结果表明,在扣除前期冬季ENSO影响后,5月AO指数与8月长江流域地表气温存在显著正相关。通过回归分析发现,5月AO可通过影响中低纬度的海气相互作用进而影响8月长江流域地表气温。当5月AO处于正位相时,在(10°~15°N)及赤道附近产生异常下沉气流,对应着西太平洋局地Hadley环流减弱,对流层底层出现了异常的反气旋性辐散气流。与之对应,赤道西太平洋地区出现了显著的东风异常。由于该东风异常位于5月气候平均的局地海表面温度(SST)极大值中心位置上,该东风异常可通过平流作用使得高海温不断地向西堆积,最终造成赤道西太平洋SST出现显著正异常。当该SST正异常持续至8月时,它通过Gill响应(Gill,1980)在其西北侧激发出气旋性异常环流,它有利于西太平洋副热带高压在我国长江流域的维持,进而造成长江流域地表气温正异常。反之,则相反。

Influence of the Spring Arctic Oscillation on Midsummer Surface Air Temperature over the Yangtze River Valley

Based on ERA-40 monthly mean reanalysis data over the period 1958-2002, this paper studies the relationship between the spring Arctic Oscillation (AO) and the summer surface air temperature (SAT) over the Yangtze River valley by adopting the year-to-year increment approach. The winter ENSO signal is filtered out in advance. Results show that the AO in May is significantly and positively correlated with the SAT over the Yangtze River valley in August. The possible physical mechanism for this relationship is investigated by means of regression analysis. The air-ocean interaction over the middle and low latitudes might play an important role. Following a stronger positive AO in May, an anomalous subsidence flow appears over the region 10°-15°N and the Equator. At the same time, Hadley circulation over the western Pacific becomes weaker, leading to an anomalous anticyclonic divergent flow in the lower troposphere. Accordingly, significant easterly anomalies appear over the equatorial western Pacific. Since the easterly anomalies are located at the local maximum center of the climatological mean sea surface temperature (SST), the warm SSTs pile up to the west via advection, inducing the above-normal SST over the equatorial western Pacific. The positive SST anomalies persist from May to August. This can trigger an anomalous cyclonic circulation to the northwest through a Gill-like atmospheric response (Gill, 1980), which is conducive to the maintenance of the western Pacific subtropical high over the Yangtze River valley, resulting in the positive SAT anomalies over the region, and vice versa.