孟加拉湾季风爆发可预测性的分析和初步应用

基于季风区对流层中高层副高脊附近的经向温度梯度能表征季节转换和季风爆发的物理本质这一事实，使用1980—1999年过渡季节期间(3～5月)逐日和月平均的NCEP／NCAR高空温度场再分析资料，对该温度梯度潜在的预报季风爆发进行了分析。结果表明：在已知初始时刻孟加拉湾季风区对流层中高层经向温度梯度的前提下，依据初始时刻的经向温度梯度和气候平均的经向增温率梯度，可以对孟加拉湾季风爆发的迟早做出定性预测。另外，由于孟加拉湾地区季风爆发日期与3月份青藏高原上空对流层中高层气温有显著相关，故前期高原上空对流层中高层的气温高低也是判断孟加拉湾季风爆发迟早的重要因子。对2000年和2001年孟加拉湾季风爆发迟早定性预测的结果表明，这两种预报方法具有潜在的应用价值。

Predictability Analysis and Preliminary Application of the Bay of Bengal Summer Monsoon Onset

Based on thermal-wind relation and the fact that the essence of summer monsoon onset is a result of the change in land-sea thermal contrast, the relation between monsoon onset and the variation of the subtropical anticyclone is studied. The summer monsoon onset appears firstly over the eastern Bay of Bengal (BOB), then propagates regularly eastwards to the western Pacific by early June. Whenever the axis arrives at a region, the in situ lower-layer wind direction and the sign of meridional temperature gradient (MTG) in the upper-layer are changed, and monsoon onset occurs. Furthermore, the time required to overturn the MTG depends on the initial MTG as well as the averaged meridional differential heating before the monsoon onset. Based on these results, the daily and monthly NCEP/NCAR reanalysis data during March to May 1980\_1999 were used to explore the predictability of the BOB monsoon onset. Results show that qualitative prediction of the BOB monsoon onset is possible by using the initial MTG and the climate mean meridional differential temperature variations. Moreover, there is significant correlation between the spring-time monthly-mean air temperature over Qinghai-Xizang Plateau and the BOB summer monsoon onset. This provides additional clue for predicting the monsoon onset. These two methods have been utilized to predict the qualitative BOB monsoon onset in 2000 and 2001 correctly. However, detailed examination reveals that a satisfactory prediction of the BOB monsoon onset requires the consideration of the low-frequency variability of the atmospheric circulation.