春季北极涛动对南海气候的影响

The possible influence of Arctic Oscillation on South China Sea climate during boreal spring

利用1979～2013年月降水量和NCEP再分析大气环流资料,分析了春季（3～5月）北极涛动对南海气候变率的可能影响.结果表明,春季南海多气候要素主导模态的时间系数与同期北极涛动指数相关系数达？0.4,超过95%的信度水平,二者反位相的特征在年际尺度上更加明显.当北极涛动指数为正异常时,西太平洋副热带高压减弱,热带西太平洋有异常气旋式环流,南海地区偏南风减弱,南海大部分地区降水增多,北部到华南地区降水减少,北极涛动指数为负异常时情况则相反.北极涛动可由两条途径影响南海地区气候,一是通过青藏高原南侧西风急流的波列,二是通过北太平洋西风急流区南北侧的偶极型环流异常.南海地区环流和气候的异常与这两条途径的相对强弱和配置有关,当高原南侧波列异常偏强同时北太平洋偶极型异常偏弱时,二者共同作用可增强西太平洋副热带高压和南海地区异常反气旋环流,导致降水的显著减少.当北极涛动指数为正异常时,北太平洋偶极型南支的气旋还可触发热带地区海气相互作用间接影响南海气候.

In this study, the possible influences of the springtime Arctic Oscillation（AO） on climate over South China Sea（SCS） has been studied, by employing monthly precipitation data（GPCP） and atmospheric circulation reanalysis data from NCEP-NCAR during the period of 1979–2013. Prior to analysis, the El Ni？o-Southern Oscillation（ENSO） signals have been linearly fitted and subtracted from the time series under consideration. To capture the major feature of the regional climate variability, multi-variable empirical orthogonal function（MV-EOF） analysis was performed, by taking into account sea surface temperature（SST）, precipitation, and horizontal and vertical winds at 850 h Pa over the domain of 3.75°S–23.75°N and 106.25°E–123.75°E. The first mode explains 27.8% of the total variance. The corresponding temporal variations of the first mode co-changes tightly with the spring AO. Their correlation coefficient is ？0.4, being significant at the 0.05 level. Their out-of-phase relationship is even more dominant on the interannual time scales. During positive（negative） AO years, a cyclonic（anticyclonic） anomalous circulation is observed over the tropical Western Pacific in the mid- and lower troposphere. There are more（less） precipitation in the central SCS, corresponding to the circulation anomalies. There are two possible mechanisms linking AO and SCS climate. The first mechanism is the wave train along the westerly jet stream extending from northern Africa through mid-Asia to northern India. Along this route, there are two anomalous circulation centers in the upstream direction, one being located in the Bay of Bengal, and the other located in the Arabian Sea. The second mechanism seems to be related to the dipole-structure circulation anomalies over the North Pacific, which characterized by a dipole of anticyclone and cyclone located to the north and south of the northern Pacific storm tracks, respectively. Their phase and corresponding circulation center locations are crucial with regard to the AO-SCS links. In cases of a strong positive phase of southern Asian waves and negative phase of northern Pacific dipole, there is an enhanced anticyclone circulation over the SCS and reduced precipitation. In addition to the influences on the SCS climate from the atmosphere, positive feedback from the tropical western Pacific Ocean strengthens the anomalous north winds of SCS. Local SST of SCS mainly responds to（rather than affects） the atmospheric circulation.