Thermohaline Structures and Heat/Freshwater Transports of Mesoscale Eddies in the Bay of Bengal Observed by Argo and Satellite Data

Abstract

Knowledge of mesoscale eddies in the Bay of Bengal (BOB) is key for further understanding the climate variability in this region and beyond, but little is known about the vertical structure of these eddies. In this study, the three−dimensional structure and transport characteristics of mesoscale eddies in the BOB were comprehensively investigated by the combined use of Argo profiles and satellite data. The composite analysis showed that eddy−induced ocean anomalies are mainly confined to the upper 300 m of the water. The spatial structure of eddy−induced thermohaline perturbations is characterized by a dominant dipole structure in the near surface layer, arising from horizontal advection of the background thermohaline gradient by eddy rotation, and a monopole structure in the subsurface layer, caused by eddy−induced vertical displacements of the isopycnal surfaces. In the eddy core, the maximum temperature anomalies induced by a cyclonic eddy (CE) and an anticyclonic eddy (AE) are about −1.2 °C and +1.2 °C, respectively. The anomalies are located at approximately 100 m. The corresponding salinity anomalies are located at approximately 50 m with a value of −0.1 psu (0.1 psu) for CE (AE). The eddy thermohaline structure has a seasonal character. A deeper temperature and salinity core occurs in both CE and AE in spring compared to that in other seasons, which is primarily caused by the relatively deep thermocline and halocline during that season. In addition, unique warming anomalies induced by CE are present in the mixed layer during winter due to the vertical advection of the BL (Barrier Layer) warmer water by eddies. The total meridional heat transport induced by the composite eddy is poleward (equatorward) south (north) to 10°N with a value of 0.01 PW (−0.013 PW), whereas the total meridional freshwater transport is equatorward with a value of 0.046 Sv over a one−year period. The volume of freshwater export out of the bay is approximately 35% of the annual net freshwater input from local precipitation and river discharge.

Eddy‐Induced Transport and Kinetic Energy Budget in the Arabian Sea

“…Swirl transport occurs when the tracer anomalies and rotational velocities are asymmetric and, therefore, leads to net transport over the eddy wavelength (Chelton et al, 2011; Hausmann & Czaja, 2012); this process is sometimes enhanced by a series of flanking eddies (Zhan et al, 2016). Drift transport is caused by the movement of eddies when tracer anomalies inside an eddy are trapped by interior water parcels and move with the eddy (Dong et al, 2014; Lin et al, 2019). These eddy‐induced transports may exhibit comparable magnitudes (Dong et al, 2014; Sun et al, 2019), and their contribution should be jointly considered.…”

An Intrathermocline Eddy Observed in the Northeastern Bay of Bengal

“…For each Argo profile collocated within an eddy, the steric dynamic height anomaly ( h ′) is computed using the reference level at 1,000 dbar, following the definition by Gill and Niiler (1973): $$${h}^{\prime }={\int }_{-1000}^{0}\delta \rho \,dz$$$ where δρ is the density anomaly of an Argo profile relative to its climatological values obtained from Roemmich–Gilson Argo climatology (Roemmich & Gilson, 2009) using the method proposed by Lin et al. (2019), and dz is the vertical grid resolution. Larger values of h ′ in the subsurface compared to the surface can be representative of the subsurface maximum in velocities associated with the eddy.…”

Seasonal variation of eddy activity and associated heat/salt transport in the Bay of Bengal based on satellite, Argo and 3D reprocessed data

“…𝜃 𝑒 ′ and 𝑠 𝑒 ′ are the eddy-induced potential temperature anomaly and salinity anomaly, respectively. R is the eddy region, D0 is the intergration depth (500 dbar for He and 300 dbar for Se; Lin et al (2019); Gulakaram et al (2020); also, Section 3.2). The unit of eddy heat anomaly He is J, and that of salt anomaly Se is psu•kg.…”

“…Knowledge of the vertical structure of the ocean is vital both for comprehensive understanding of ocean dynamic processes and for analysis of the ocean circulation and energy transport. Based on satellite altimetry and Argo floats, Lin et al (2019) and Gulakaram et al (2020) showed that eddy-induced ocean anomalies in the Bay of Bengal are mainly confined to the upper 300 m and eddy thermohaline structure has a seasonal character. Cui et al (2021) Many studies examined the surface characteristics of eddies in the Bay of Bengal ans some have investigated the vertical eddy properties (Nuncio and Kumar, 2012;Dandapat and Chakraborty, 2016;Chen et al, 2012Cui et al, 2021).…”