The Climatic Characteristics Of Summer Somali Cross-equatorial Flow Vertical Structure And Its Interdecadal Variation

Based on NECP/NCAR and ERA40reanalysis monthly mean data, the spatial-temporal evolution of Somali cross-equatorial flow (SMJ) in summer at five levels (1000hPa,925hPa,850hPa,700hPa,600hPa) are studied. The results show that SMJ in the middle and lower troposphere has different vertical structures, with interannual and interdecadal change characteristics. Two main vertical structure modes are clarified, one is the consistent mode in the whole region ("A" mode) and the other is the vertically opposite mode ("B" mode), so there are four vertical anomalous types of SMJ in detail, namely consistently strong pattern ("A+" pattern),consistently weak pattern ("A-" pattern), higher-weak and lower-strong pattern ("B+" pattern), higher-strong and lower-weak pattern ("B-" pattern). Then two indices, denoted by VS1and VS2, are defined, which are able to describe the variabilities of the two vertical modes of SMJ. Using the two indices for further analysis, it is mainly analysis of interdecadal variations of SMJ vertical structures and the associated atmospheric circulation features. SST signals. We also discuss the relationship between the indices and the Asia summer monsoon activity. The results show that:(1) VS1and VS2exhibit obvious interdecadal variabilities. In the study time of interest, the former turns positive in the mid-1980s, the later experiences two decadal mutations, one is from positive to negative in the early1970s and the other is from negative to positive in the late1990s. Composite analysis indicates the Asian low and Mascarene high at the sea-surface pressure level, India-Burma trough and the western Pacific subtropical high at500hPa, south Asia high at200hPa affect the interdecadal variability of VS1. It’s worth noting that Antarctic Oscillation (AAO) may be an important factor which effects the interdecadal changes of VS1. The continental low over Asia and Mascarene high at the sea-surface pressure level, India-Burma trough at500hPa, wave trains propagating along the Asian jet at200hPa are important systems influencing the interdecadal variability of VS1.(2) The SSTA in the Indian Ocean could play the regulating and indication role in the interdecadal variations of SMJ vertical structures. VS1is found to be in line with the trend seen in SSTA of the whole Indian Ocean. VS2is related with the Southern Indian Ocean SST anomaly dipole oscillation. (3) The interdecadal characteristic of VS1(VS2) has close relationship with the change of the first (second) spatial mode of south Asian summer monsoon. The vertical structures of SMJ closely relate to the East Asia Summer Monsoon activity on interdecadal timescale. Specifically,"A+" pattern ("A-" pattern) is well accordance to the weak (strong) East Asia Summer Monsoon;"B+" pattern ("B-" pattern) is also well accordance to the weak (strong) East Asia Summer Monsoon.(4) On interdecadal timescale, when VS1i s in a positive phase, the summer monsoon weakens, accordingly, zonal moisture supply over Indian Peninsula to the Indo-China Peninsula reduces, summer rainfall tends to be less than normal. At the same time, the relatively weak East Asia summer monsoon, the related weak moisture transportation and the weak moisture convergence lead less rainfall in North China. But more rainfall occurs in the south Yangtze River area because of the enhancement of moisture convergence. The case of negative VS1is opposite. On interdecadal timescale, when VS1is in a positive phase, the summer monsoon over the southwestern China, South China and Japan declines, moisture transportation is weakened, thereby summer rainfall in these areas tends to be less, and vice versa. Further research suggests the interannual correlations between SMJ vertical structure indices and the precipitation over Asia summer monsoon regions are remarkably different under different decadal backgrounds.