Extended Associate Pattern Analysis On The Evolution Of Sea Surface Salinity In The Bay Of Bengal And ENSO Cycle

The evolution of sea surface salinity (SSS) in the bay is studied using Rotated Empirical Orthogonal Function (REOF) analysis and Extended Associate Pattern Analysis (EAPA). The results show that SSS distribution in the northern bay is featured by eastern-bay and western-bay plumes in the northern bay during different seasons. The western-bay plume begins in early July, peaks in late August, turns into a bay-shaped plume with the two plumes in either side of the bay in late October. The southward extension of the western-bay plume is explained by the southwestward geostrophic flow associated with the cyclonic gyre in the northern bay, which counters the northeastward Ekman drift driven by wind stress. The offshore expansion of the western-bay plume is induced by the offshore Ekman drift which also produces a nearshore salinity front. The bay-shaped plume appears when the cyclonic gyre shifts westward and a weak anticyclonic gyre occupies the northeastern bay. As season advances, the western part of the bay-shaped plume decays while the eastern part persists until next June, which is believed to be associated with the anticyclonic gyre in the northern bay. The circulation of the plumes throughout the year except the eastern part of bay-shaped plume in fall can be partly explained by the seasonal variation of mass transport associated with the Sverdrup balance. The western-bay (eastern-bay) plume appears when surface freshwater flux in the northeastern bay increases (decreases) dramatically, which imply that the plumes are not directly produced by surface freshwater flux. River discharge seems to be the freshwater source for the plumes and has little to do with the evolution of the plumes.Similarly, EAPA is adopted to study the formation mechanism of warm phase of ENSO cycle—El Nino. The results show that El Nino events mean a redistribution of sea surface temperature (SST) covering almost the whole Pacific, which needs a huge amount of energy provided by the air-sea interaction over the Pacific and related regions; It confirms that it is the westerly burst and wind convergence, coming directly from middle latitudes, instead of Kelvin waves, that produce the strong SST warm signal in Nino regions.The results also show that the air and sea behaviors that induce El Nino are quite different. There does not exist a relatively independent tropical atmosphere but does exist a relatively independent tropical Pacific because the atmosphere is heated from the bottom instead of the surface, leading to much stronger baroclinic instability than the ocean and has a very large inter-tropical convergence zone covering the most tropical Pacific. In general, the ocean is forced by the atmosphere during the early stage of El Nino events, while the atmosphere is forced by the ocean during its late stage.The results show that EAPA is effective tool for studing the formation mechanism of oceanic hydrography and ENSO.