Inconsistent response of biophysical characteristics in the western Bay of Bengal associated with positive Indian Ocean dipole

Abstract

The Bay of Bengal (BoB) is known to have high primary productivity at its western margin during the Indian summer monsoon season (June–September). This higher coastal productivity is mainly caused due to the near-surface nutrient availability maintained by the local coastal upwelling process. The surface winds in the Indian Ocean significantly vary during El-Niño/La-Niña and Indian Ocean dipole (IOD). The current study examines the sea surface temperature (SST) and Chlorophyll-a (Chl-a) anomalies in the western BoB for the period of 18 years (2000 to 2017), using a coupled regional ocean biophysical model. All considered positive IOD (pIOD) years show discrete behavior of biophysical features in the western BoB. The co-occurrence years of pIOD and El-Niño modes are associated with contrast biophysical anomalies. In the analyzed pIOD events, the years 2006 and 2012 show an enhancement in the Chl-a anomalies whereas, the other two years (2015 and 2017) experience Chl-a decrement. The western BoB was anomalously warmer during the 2015 and 2017 pIOD years compared to the other two pIOD years (2006, 2012). This inconsistent response of biophysical features associated with pIOD years is investigated in terms of local surface flux (momentum, heat, and freshwater) changes over the western BoB. The combined impact of local surface flux changes during the individual years remains the major contributing factor affecting the upper-ocean stratification. Ultimately, the stratification changes are responsible for the observed inconsistent response of biophysical features by significantly altering the upper-ocean mixing, upwelling, and nutrient availability in the western BoB.