Role of interannual equatorial forcing on the subsurface temperature dipole in the Bay of Bengal during IOD and ENSO events
Role of equatorial forcing on the thermocline variability in the Bay of Bengal (BoB) during positive and negative phases of the Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) was investigated using the Regional Ocean Modeling System (ROMS) simulations during 1988 to 2015. Two numerical experiments were carried out for (i) the Indian Ocean Model (IOM) with interannual open boundary conditions and (ii) the BoB Model (BoBM) with climatological boundary conditions. The first mode of Sea Surface Height Anomalies (SSHA) variability showed a west-east dipole nature in both IOM and altimetry observations around 11°N, which was absent in the BoBM. The vertical section of temperature along the same latitude showed a sharp subsurface temperature dipole with a core at ~ 100 m depth. The positive (negative) subsurface temperature anomalies were observed over the whole northeastern BoB during NIOD (PIOD) and LN (EN) composites due to stronger (weaker) second downwelling Kelvin Waves. During the negative phases of IOD and ENSO, the cyclonic eddy on the southwestern BoB strengthened due to intensified southward coastal current along the western BoB and local wind stress. The subsurface temperature dipole was at its peak during October–December (OND) with 1-month lag from IOD and was evident from the Argo observations and other reanalysis datasets as well. A new BoB dipole index (BDI) was defined as the normalized difference of 100-m temperature anomaly and found to be closely related to the frequency of cyclones and the surface chlorophyll-a concentration in the BoB.
KeywordsBay of Bengal Thermocline ROMS Kelvin and Rossby waves Argos Chlorophyll-a Cyclones
Authors thank the Earth System Science Organization (ESSO)–Indian National Centre for Oceanic Information Services (INCOIS) for providing TropFlux datasets freely. Authors also thank the other data providers as mentioned in Sect. 2.2. Authors are thankful the anonymous reviewer and the editor for valuable suggestions. Authors also acknowledge Kiranmayi L for helping on the statistical methods used in the study. All figures are prepared using MATLAB.
Financial supports are from Space Application Centre (SAC), Indian Space Research Organization (Grant No. SAC/EPSA/4.19/2016) Science and Engineering Research Board (SERB, Grant No. SB/S4/AS-155/2014).
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