Role of Mesoscale Eddies on Upper Ocean and Atmospheric Convection in the Bay of Bengal

Abstract

Oceanic mesoscale eddies are omnipresent in the ocean due to their turbulent nature. Understanding oceanic mesoscale eddies are essential because they play a prominent role in transporting heat/salt, circulation, biological activities, and intensification/weakening of tropical cyclones. Bay of Bengal (BoB) is in the northeastern part of the Indian Ocean, where mesoscale eddy activity is high. Very few studies have attempted to understand statistical characteristics, the vertical structure of mesoscale eddies, and air-sea interaction processes related to mesoscale eddies. The autonomous eddy identification method is employed to identify and track mesoscale eddies in the BoB from daily data of sea surface height anomalies for 26 years from 1993 to 2018. Results show that the tracks of anti-cyclonic and cyclonic eddies in the BoB primarily propagate towards the west and southwest during all seasons. The annual occurrence frequency of mesoscale eddies demonstrates that most eddies are dominant over the western BoB, followed by the Andaman Sea. Using composite analysis, the present study investigated the surface features of eddy-induced temperature and salinity changes near anti-cyclonic and cyclonic eddies. Surface eddy-centric composite analysis reveals the existence of warm (cold) and diverse sea surface salinity (SSS) anomalies for anti-cyclonic (cyclonic) eddies. During winter, it is essential to note that the eddy-induced sea surface temperature (SST) and SSS anomalies depict the dipole patterns that show opposite phases for cyclonic and anti-cyclonic eddies. Observed diploe structures are consistent with the eddy rotation and background large-scale meridional gradient of temperature and salinity fields. The composite vertical structure of mesoscale eddies is examined using ARGO profiles collocated near eddies locations, revealing that the anti-cyclonic eddies have a deeper core compared with a core of cyclonic eddies. In addition, the present study investigated mixed layer dynamics near mesoscale eddies and quantified the processes responsible for the eddy-induced temperature and salinity variations using mixed layer diagnostic models near both anti-cyclonic and cyclonic eddies. Results show that the anti-cyclonic eddies deepen the mixed layer depth (MLD), whereas cyclonic eddies decline MLD across all seasons. Mixed layer heat budget analysis near eddy locations depicts that the primary factors responsible for eddy-induced temperature variations are the surface heat flux convergences and vertical entrainment. The salinity budget analysis near eddy locations reveals that horizontal advection and vertical entrainment are the predominant processes responsible for the eddy-induced mixed layer salinity variations.
The impact of mesoscale eddies on the turbulent fluxes and overlying atmosphere are investigated using the composite analysis. In general, it is assumed that anti-cyclonic eddies are related to the positive anomalies of SST, and cyclonic eddies are associated with negative anomalies. However, in the BoB, the composites of SST anomalies reveal the abnormal response of SST near anti-cyclonic and cyclonic eddies. The unusual association of cold SST near anti-cyclonic eddies and warm SST near cyclonic eddies in BoB are investigated by analyzing the mean composites of precipitation, OLR and turbulent fluxes. Results show that the anti-cyclonic eddies significantly enhance precipitation and convection, whereas the cyclonic eddies suppress convection. The composites of latent heat flux, and wind speed anomalies over surface cold anti-cyclonic eddies, reveal that the ocean loses heat, and the presence of stronger winds is responsible for the anomalous negative anomalies observed near anti-cyclonic eddies in the BoB. Vice-versa is evident in near-surface warm cyclonic eddies. The results show that the atmospheric response observed over mesoscale eddies is influenced by the intraseasonal variations (ISV), since both mesoscale eddies and ISV operated at same time scales. Therefore, it is difficult to separate the atmospheric ISV and the mesoscale variability, to understand the role of mesoscale eddies on atmospheric variability. However, the influence of atmosphere is averaged out by combining the surface warm, surface cold eddies together, and the anomaly obtained could be associated with the anti-cyclonic or cyclonic eddy.
The variability of mesoscale eddies and their influence on atmospheric convection during weak (2009) and strong (2013) monsoon seasons is examined in the present study. Results revealed that deep atmospheric (shallow) convection is observed over anti-cyclonic (cyclonic) eddies during the active monsoon season, and no such relationship is observed during the weak monsoon season. The impact of mesoscale eddies on the near-surface atmosphere is evidently observed using composite analysis in the BoB. The existing atmosphere and ocean-coupled models are poor in representing the air-sea flux exchanges or sometimes neglect the influence caused by these mesoscale features. Therefore, the air-sea interactions over the mesoscale eddies need to be considered to improve the accuracy level in the numerical model simulations.