Study On Variations And Mechanisms Of The Typical Upwellings In Bay Of Bengal

In this paper,using multi-source satellite remote sensing data with the results of the ROMS(Regional Ocean Modeling System)model,the evolution characteristics and main mechanisms of two typical upwellings in the Bay of Bengal(the southern coast of Sri Lanka and the offshore upwelling of Manang Island in western Myanmar)are studied.Firstly,the variation characteristics of the upwelling along the southern coast of Sri Lanka and its relationship with the Indian Ocean Dipole(IOD)event in 2013 were analyzed.Satellite remote sensing data has revealed that the upwelling event in 2013was the strongest among the years from 1982 to 2019.The upwelling area in 2013 is about 6 times larger than that in a normal year,and lasts from June to August,with the peaks of the cooling anomaly reaching-1.5°C and the positive chlorophyll-a concentration anomaly exceeding 3.1 mg/m³.In 2013,the negative unseasonable IOD(n IOD_JJA)event enhances the southwest monsoon,while the blocking of the monsoon wind by the island results in a stronger westerly/northwesterly wind stress off the southern coast of Sri Lanka and a weaker westerly/northwesterly wind stress over the eastern Sri Lanka waters.Inhomogeneous spatial distribution of the wind speed over its southern and eastern waters and thus results in a strong positive wind stress curl there.These cause stronger offshore transport and positive Ekman pumping off the southern coast,forming a strong upwelling event there.Numerical experiments based on the ROMS model have reproduced the basic characteristics of the upwelling along the southern coast of Sri Lanka.The simulated SST level distribution is basically consistent with the results of OISST,so the model can be used for upwelling research.The analysis of the thermal budget of the mixed layer shows that the vertical entrainment item played a leading role in the strong cooling process in the summer of 2013.The advection term and the heat flux term also contributed to the cooling process in the summer of 2013,but their effects were weak.Further analysis indicates that the interannual variability of the upwelling,as represented by a newly constructed index,is primarily caused by the variations of local wind associated with the IOD.Compared with wind stress curl,the interannual variation of the upwelling has a higher correlation with the coastal wind stress.The upwelling off the southern coast of Sri Lanka weakens(strengthens)in the positive(negative)IOD years.However,an analysis based on 21 IOD events during1982?2019 demonstrates that the effects of the three types of IOD events,including IOD_JJA,prolonged IOD(IOD_LONG)and normal IOD(IOD_SON),on the upwelling are different.Compared to the IOD_SON events,the IOD_JJA and IOD_LONG events tend to have stronger influences due to their earlier developing phases.Finally,the spatial distribution and evolution of the upwelling over the Manang Island in western Myanmar in the Bay of Bengal were analyzed,and the possible dynamic mechanism leading to the evolution process was discussed.Signals of upwelling exist off the coasts of Manaung Island,in the western Myanmar during spring.It appears in February,reaches its peak in March,and decays in May.Low-temperature and high-salinity water at the surface of this upwelling zone is caused by upwelling of seawater from a depth below 100 m.Further research reveals that the remote forcing from the equator predominantly induces the evolution of the upwelling,while the local wind forcing also contributes to strengthen the intensity of the upwelling during spring.