The Mixed Layer Heat Budget (MLHB) is based on energy conservation wherein Mixed Layer Temperature (MLT) tendency equals the sum of Net Surface Heat Flux (NSHF), Horizontal Advection (HA), Vertical Entrainment (VE), Vertical Diffusion (VD), and residual. This study utilizes 25 years (1991–2015) of data from a Regional Ocean Modeling System (ROMS) simulation for quantifying the Relative Importance (RI) of MLHB terms, using Johnson’s Relative Weight Analysis (RWA) approach, in the Bay of Bengal (BoB). The RIs obtained from the RWA approach fall within their respective 95% confidence intervals obtained through bootstrapping. The analysis showed that the NSHF, dependent on spatio-temporally varying Mixed Layer Depth (MLD), plays a greater role in modulating the MLT tendency (\(\approx \)50%) over the whole BoB during different seasons. Moreover, HA (\(\approx \)20%) prominently impacts the southern and western BoB, which is related to the boundary current system and the associated thermal gradients. During monsoon, it was found that HA warms the Sri Lanka dome region due to meridional heat transport from around the east coast of India while cooling the southern BoB through cooler south-west monsoon currents. Finally, VE (\(\approx \)20%) and VD (\(\approx \)10%) contribute higher around the Sri Lanka dome due to the presence of a cyclonic eddy during pre-monsoon and monsoon. Also, VE and VD have a higher role during post-monsoon and winter due to the presence of thermal inversion in the northern BoB. Lastly, the residual showed an average RI of about 15%, which is of the same order as HA, VE, and VD. This suggests that the residual is not only composed of errors associated with the MLHB terms but there might also be some unknown physical processes at play that significantly impact MLT. This points toward the need for improvement in the MLHB, which holds a key for a better assessment of the ocean state.

Oceanic eddies exhibit remarkable coherence and longevity compared to other transient features in the surrounding flow. They possess the ability to transport properties over extensive distances while maintaining their material identity intact. The Lagrangian Coherent Structure (LCS) framework has proven effective in capturing these coherent eddies, where they display a solid-body-like rotation. Although various LCS approaches have been employed to investigate different facets of coherent eddies, a comprehensive understanding of their three-dimensional structures and internal dynamics remains elusive. This study aims to advance our comprehension of coherent eddies’ structural characteristics and delve into the precise nature of their internal dynamics by utilizing the Lagrangian Averaged Vorticity Deviation approach. Two eddies, one cyclonic and the other anti-cyclonic, were chosen from a high-resolution simulation carried out in the Bay of Bengal using the Regional Ocean Modeling System (ROMS). The findings unveil that these eddies have three-dimensional coherent cores resembling gently tapered cones that are broader at the surface and gradually narrow towards the bottom. Intriguingly, the dynamically coherent core of these eddies exhibits simultaneous upwelling and downwelling while maintaining their volumes during advection due to persistent material coherence. The three-dimensional trajectories followed by the fluid parcels inside the coherent core are helical. Their two-dimensional horizontal projections show alternating spiral bands of upwelling and downwelling which are the manifestations of Vortex Rossby Waves. These observations lead to a conceptual framework of a three-dimensional helico-spiralling recirculation pattern within the coherent cores of eddies.

The marine environment is a crucial component of global biogeochemical cycles. Recent BGC-Argo observations provide new opportunities to study the profiles of biogeochemical parameters. The study analyzed the diurnal variations of temperature, salinity, chlorophyll-a and dissolved oxygen using a high-frequency (~5 hours) cycle BGC-Argo float in the Bay of Bengal. The hydrography showed the existence of a strong barrier layer with a thickness of around 30 meters, with fresh water on top and an inversion layer within it. Analysis showed that the Mixed Layer Depth (MLD) was dominated by diffuse convection, while the Isothermal Layer Depth (ILD) exhibited salt-fingering regimes. In the upper layer (0-60 meters), temperature showed significant variation on a daily scale; however, notable changes were not observed for salinity. Additionally, temperature and chlorophyll-a were found to be strongly linked to solar insolation. The mean chlorophyll-a in the upper layer increased from 0600 hrs and peaked around 1800 hrs local time. However, surface chlorophyll-a increased only from 1100 hrs to 1800 hrs. It is suggested that this difference between surface and mean chlorophyll-a during high availability of sunlight was due to the process of photoacclimation. The dissolved oxygen cycle closely followed the variability of biomass production. The similarity between dissolved oxygen and the difference between the surface and mean chlorophyll-a further indicated photoacclimation variations on a diurnal scale. The Sverdrup model was used to indicate luminosity and an accumulation time of 14 hours was used to show a strong correlation with diel chlorophyll-a variation. The work highlights the importance of having high-frequency BGC-Argo floats with finer vertical resolution and the need for time-series observations of biological parameters in the Bay of Bengal.

Solar penetration depth is a major factor in modulating the Sea Surface Temperature (SST) and hence influences the air-sea interface processes. This study investigates the influence of solar penetration depth and heat-fluxes on the SST at three RAMA Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction) buoy locations (along 90 E at 15 N, 12 N, and 8 N) in the Bay of Bengal (BoB) using the One-Dimensional Price-Weller-Pinkel (1D-PWP) model. The results show that SST has strong dependence on the solar penetration depths with the shortwave component of solar insolation outweighing its longwave component. We found that spatio-temporally varying penetration depths refine the simulated SST, when compared against constant penetration depths. These spatio-temporally varying penetration depths (ranging from 10 to 75 m for the shortwave component; with higher values during monsoon) can be attributed to spatio-temporally changing biological parameters, wind, precipitation, and suspended particulate matter. Upon examination, an antiphase relationship between the penetration depths and total chlorophyll, total primary production, & total phytoplankton was found. Upon including the effect of horizontal advection and vertical entrainment in the 1D-PWP model results, the SST predictions improved albeit by a small amount. Furthermore, the effects of increased heat-fluxes, under the different Representative Concentration Pathway (RCP) equivalent scenarios, coupled with varying penetration depths were estimated. It was found that for the spatio-temporally varying penetration depths, the mean (maximum) increment was 0.52 ◦C (0.92 ◦C) under the highest emission scenario, i.e., RCP8.5, across all the locations. The results of the study can potentially be used further for 3D ocean modeling studies, and the characterization of air-sea interaction during monsoon and cyclones in the BoB.