Upper Ocean and Subsurface Variability in the Bay of Bengal During Cyclone ROANU: A Synergistic View Using In Situ and Satellite Observations

Abstract

In this study, the upper ocean and subsurface variability during the different phases of the cyclonic storm ROANU along the western Bay of Bengal (BoB) in May 2016 are investigated by using the moored buoys, Argos, HF radar and satellite datasets in the proximity of the cyclone track. The moored buoy observations recorded a decrease of sea surface temperature (SST) by ~ 1 °C all over the track, whereas increase in salinity by ~ 1.5 PSU was detected along with the highest wind speed of 16 m s−1, pressure drop of 14 hPa and air temperature drop of 4 °C. The cooling at the cyclone centers from the satellite data indicated higher (lower) SST drops when translation speed of the cyclone was low (high) and took more (less) time to recover to its pre-cyclone state in southern (northern) BoB. Mostly, higher SST drop was observed along the right side of the cyclone track. Interestingly, the opposite phenomenon occurred before landfall, where SST drop was higher on the left due to upwelling in the head bay as observed both from wave rider buoy (WRB) at Digha and satellite SST. The WRB near Vizag showed the maximum increase in significant wave heights by ~ 2.4 m during the passage of cyclone. Argos also captured cyclone-induced drop in temperature due to upwelling and entrainment reasonably well. In the southwestern bay, significant upwelling was observed from the Argos with drop in temperature and increase in salinity in the upper layers. However, a strong stratification was observed from Argos in the northwestern BoB due to lesser salinity and higher precipitation. The currents from in situ as well as HF radar datasets measured the increase in current magnitude during the passage of ROANU. Rotary spectral analysis showed strong inertial currents with frequency ~ 2.1 days at BD11 location, with higher amplitudes of the clockwise component during the cyclone period.