The Three Dimensional Structure Of The Somali Current System And Its Seasonal And Interannual Variations

Based on SODA data from January,1958to December,2007, the structure andits seasonal and interannual variations of the Somali Current system above500m arestudied. The main results show that during the summer monsoon period, thedistinguishing features of the horizontal structure of Somali Current system arenorthward boundary current and two-gyre system that is quasi-stationary, and themain characteristics of the vertical flow is coastal upwelling with strongest branchlocated at8°N~11°N. However, the horizontal structure during the winter monsoon ischaracterized by southward coastal current and an undercurrent that flows northwardacross the equator, and the downwelling is dominant in the vertical direction. Thestratification structure of the Somali Current system is complex. Three-layer structureof northward-southward-northward and southward-northward-southward appear nearthe equator from January to March and3°N from June to September, respectively. Thesignificant periods of the volume transport of coastal current are one year and a halfyear. Besides, as a driving factor, the surface wind stress is closely related to theseasonal changes of the coastal flow and the formation of the Great Whirl (GW) andupwelling. On the other hand, the results of the composite analysis of annual averagevolume transport for each section show that there are significant differences tomeridional flow structure between positive and negative anomaly years. Besides, thelocation of the Great Whirl in positive anomaly years is about three degree northerlycompared to negative cases.Interannual variations of the GW and related mechanisms are emphaticallystudied, based on the above analysis and some newer data such as AVISO data from1993to2010, QuikSCAT and Argo data. The results show that the location andstrength of the GW, as well as its generation and demise times, have significantinterannual variabilities. The GW is weaker and its location tends to southwest in2003, while it is stronger and tends to northeast in2005. The main reasons are as follows.(1) Coupling mechanism between intraseasonal winds and sea surface height(SSH) indicates that interannual variation of the intraseasonal wind results tointerannual variation of GW.(2) Baroclinic Rossby waves are associated with thestrength of GW. The GW will be enhanced with the joining of crests; on the contrary,it will be weakened.(3) The increase of barocilinic energy conversion rate ofthermocline in summer is conducive to the formation of the GW, and the duration ofits big value corresponds to the duration of the GW.