Study On Multi-Scale Variability And Mechanism Of Barrier Layer In The Bay Of Bengal

Based on the Argo and SODA gridded datasets,seasonal variability of barrier layer thickness(BLT)in the Bay of Bengal(BOB)are discussed,and the variability characteristics and mechanism in pure IOD and IOD+ENSO events are analysed.Then the characteristics of a tropical cyclone(TC)induced BLT variability and the possible mechanism of barrier layer(BL)in the development of TCs are explored by taking Phailin in 2013 as an example.In addition,the seasonal and inter-annual variability of diurnal warming of Sea Surface Temperature(d SST)is revealed based on the Seaflux datasets and combined with satellite observations such as shortwave radiation and surface wind data.The main conclusions are summarized as follow:(1)The variability of BLT is dominated by annual cycle and exhibits significant half-year cycle in the BOB.BL is thinner in spring and autumn,thicker in summer and winter.The thinner BL appears in the bay mouth,while thicker BL is concentrated in central-northern part of the bay and the equatorial region.The BL changed significantly in IOD and ENSO years.In the peak of pure p IOD events and pIOD+El Ni?o events,except for the slightly thickening of BL in the southwest of the BOB,BL became thinner in the equatorial region,Andaman Sea and the northern part of the bay.This form disappears as the pIOD weakens.However,the form was maintained until March to May of the following year in pIOD+El Ni?o events.The variability of BL in the nIOD events is roughly opposite to that in pIOD events.Further analyses show that the variability of isothermal layer(IL)caused by the remote forcing of the equatorial wind anomaly in IOD events is one of the main causes of the variability of BL.In IOD+ENSO events,the equatorial wind anomaly wa s maintained for more than 3 months after IOD disappeared,making the anomaly of BL last longer.In addition to the equatorial forcing,the Ekman pumping and the variability of the salt in mixed layer(ML)have a certain impact on the interannual variability of BL.(2)The strong vertical mixing induced by TC deepened the mixed layer and the isothermal layer during Phailin.For the TC's track vicinity,the upward Ekman pumping caused by the positive wind stress vorticity offseted the deepening of the isothermal layer,resulting a thinning of BLT.For the northern bay,the IL deepened more than the ML due to the command of negative wind stress vorticity and the warm water accumulation caused by the shore shelf,causing a slightly thicken of BL.And the BLT does not change significantly in the southern bay.Further analysis shows that the heat storage in the upper ocean is important for TC intensification.The high thermal energy can be stored in the thick IL,and/or thick BLT,or extremely high SST,which provides fuel for TC growth in the BOB.As Phailin moved over a thick BLT region,reducing the TC-induced vertical mixing and entrainment,causing a weak cooling in SST.Therefore,high SST continuously provides heat energy for TC,which favored a rapidly development of Phailin.(3)The dSST varies in annual cycle for the south of 5°N and present with larger(smaller)amplitude in the first(second)half of the year.The semiannual signal occurs in the north of 5°N and presents a unique double-peak structure with lager amplitudes in spring and autumn while small amplitudes in summer and winter.In spring,the range of d SST is higher in the center bay and lower in surrounding.While d SST is lower in the bay mouth and higher in the center bay and equatorial region in autumn.For summer and winter,the distributions of d SST represent a similar pattern of higher in equatorial region and lower in the bay,with different areas of high value centers that locate in the northeast Sri Lanka and western boundary of the bay,respectively.Surface wind is vital for the entire areas,which determines the seasonal and spatial distribution of d SST.Shortwave radiation plays a significant role in the variability of dSST in BOB,while its effect is relative weak for the south of BOB and most of equatorial regions.Besides,the surface wind plays a leading role when d SST gets its largest value in April,and the wind and shortwave radiation are both important in November.And the impact of precipitation on dSST is relatively weak.Further studies show that d SST gets smaller(larger)with the increase(decrese)of wind speed and intensify(weaken)of shortwave radiation under the same circumstances.