海洋锋海表风速最小值及其成因

SURFACE WIND MINIMUM ALONG THE SST FRONTS AND ITS CAUSE

利用2000—2008年AVHRR、QuickSCAT等高分辨率卫星观测资料和CFSR再分析资料,分析了墨西哥湾流区、东海黑潮锋区、巴西-马尔维纳斯合流区和厄加勒斯回流区等全球主要海洋锋区的大气响应特征,发现在上述海洋锋区普遍存在海表矢量风速的最小值分布,并对这一现象的产生原因进行探讨。研究指出:夏季(6—8月)墨西哥湾流区、6月东海黑潮锋区附近有明显的矢量风速最小值分布,而巴西-马尔维纳斯合流区及厄加勒斯回流区海洋锋附近则终年存在矢量风速最小值。产生这一现象的条件是大尺度气压背景场梯度方向与海洋锋附近海表温度梯度方向接近一致,其物理过程为:海洋锋暖(冷)水区一侧上空对应有低(高)气压,由此产生的局地气压梯度与大尺度背景气压梯度方向接近相反,导致锋区附近叠加后的气压梯度最小,海表风速因此也最小。同时,摩擦作用使海表风偏向低压一侧,于是沿锋区走向(跨锋区走向)的风速分量差在暖水区一侧产生气旋性切变涡度(风速辐合),进而造成上升运动和强降水,而该分量差在冷水区一侧则产生相反的大气响应特征。

By using QuickSCAT and AVHRR high resolution satellite data and CFSR reanalysis data during 2000--2008, the atmospheric response to the SST fronts in the Gulf Stream （GS）, the East China Sea Kuroshio （ESK）, the Brazil-Malvinas Confluence （BMC） and the Agulhas Return Current （ARC） is investigated. The minimum surface wind is found to be universal along the above fronts, which occurs in JJA for the GS, June for ESK, year-round both for the BMC and ARC. The pre-condition for generating the minimum surface wind is that the large-scale sea level pressure （SLP） gradient （SLPG） is basically in the same direction with the local sea surface temperature gradient near the fronts. This can be explained as follows according to the SLP adjustment mechanism： the warmer （colder） side of the fronts will produce lower （higher） SLP, and such a local SLPG will counteract with the large-scale SLPG, thus leading to the minimum resultant SLPG as well as the minimum surface wind along the fronts. Furthermore, the surface wind will turn to the lower SLP side due to the friction. The difference between components of surface wind along （across） the fronts will produce cyclonic shear vorticity （convergence in wind speed） over the warmer side of the fronts, thereby forming ascending motion and enhanced precipitation there. The atmospheric response is opposite to that over the colder side of the fronts.