孟加拉湾季风对热带气旋生成和发展的调制机制研究

孟加拉湾与其他热带海盆不同,在季风影响下,该地区热带气旋具有双气旋季的独特结构(4—5月的春季转换期和10—11月的秋季转换期)。虽然孟加拉湾气旋频数在10—11月较多,但是4—5月超强气旋(Saffir-Simpson 4,5级)的生成率却远高于10—11月。1981—2016年,春季转换期内孟加拉湾超强气旋都与第一支北传季节内振荡(First Northward-propagating Intra-Seasonal Oscillation,FNISO)相应而生,然而并不是所有伴随FNISO发生的气旋都能发展成为超强气旋。因此本研究以气旋生成指数为基础,利用气旋最佳轨道数据以及NCEP的海气参量数据,诊断指出孟加拉湾夏季风形成的强垂直风速剪切配合低层大气旋度和气旋潜在强度抵消夏季风期间水汽对气旋生成的促进作用,造成双峰分布,而中层大气相对湿度差异双峰不对称的主因。FNISO强度的不同与深对流中心与气旋中心的相对位置的差异,使得部分气旋受季节内振荡影响更大,强深对流的超越作用导致更显著的高低层大气温差,促使气旋具有且达到更大的潜在强度。在年际尺度上大气高低层温差的不同也是引起气旋潜在强度不同的主要原因。当季节内尺度和年际尺度共同作用,使得部分气旋发展成为超强气旋。

Modulation of Tropical Cyclone Activity by the Monsoon Over the Bay of Bengal

The annual cycle of tropical cyclone(TC)over the Bay of Bengal(BoB)exhibits a unique bimodal character,which is different from the single peak in other basins.It may be attributed to the classic monsoonal climate in the region and resulted atmospheric circulation.The TCs over the BoB mainly occur during pre-and post-monsoon transition period,namely April—May and October—November,and TC frequency during pre-monsoon transition period is just about 1/3 of the post-monsoon transition period.The causes of the unusual features were investigated through diagnosis of a genesis potential index(GPI)based on the NCEP Reanalysis dataset.A methodology was developed to quantitatively assess the relative contributions of four environmental parameters.Different from the conventional view that the seasonal change of vertical shear causes the bimodal feature,it was found that the strengthened vertical shear alone from boreal spring to summer cannot overcome the relative humidity effect,but the combined effect of vertical shear,vorticity,and potential intensity term controlled by SST leads to the GPI minimum in boreal summer.It is noted that the TC frequency in October—November is higher than that in April—May,primarily attributed to the difference of mean relative humidity between the two periods.In contrast,more super tropical cyclones(STCs,category 4 or above)occur in April—May than in October—November.These STCs are often synchronized with the first-branch northward-propagating intraseasonal oscillation(FNISO)over the BoB,but not all the TCs synchronizing with FNISO were able to develop into STCs.The study analyzes the distinctive environmental conditions of regular and super TC groups.For intraseasonal background,the most important difference appears in the intensity and relative location of FNISO.It is argued that a stronger and closer ISO would lead to a colder upper-level air temperature and thus a more unstable stratification which promotes a greater TC intensity.Similar to intraseasonal timescale,upper-level temperature on the interannual timescale is also colder in the STC cases.The combined intraseasonal and interannual environmental temperature difference between lower and upper troposphere are responsible for significant difference between the STC and TC cases.