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The Monsoon Intraseasonal Oscillations (MISO) is known to be strongly coupled to the upper ocean variability in the Bay of Bengal (BoB). Here, we analyze high-resolution moored observations from the northern BoB (18 °N, in comparison with an array of moorings to the south along 90 °E (8, 12, and 15 °N) to examine the observed temperature and salinity variability during the 2015 summer monsoon season. The heat budget analysis indicates that the mixed-layer (ML) temperature tendency at 18 °N in late summer (August and September) deviated greatly from the tendency that is expected based on surface heat flux alone. Examination of moored ML salinity and satellite sea surface salinity (SSS) fields suggests that southward extension of riverine freshwater plumes to the mooring site in late summer plays an important role in modulating the heat balance. The associated shoaling of the ML results in enhanced penetrative heat loss and warming beneath the surface layer. Price-Weller-Pinkel (PWP) 1-D model simulations, which account for vertical mixing, suggest that the entrainment of this subsurface warm water under monsoon wind improves closure of the temperature balance. Long-term analysis of satellite-based sea surface temperature (SST) and rainfall data indicates that late summer ML shoaling is accompanied by amplified intraseasonal (10–60 day) SST variability and reduced MISO precipitation variability. Overall, these results demonstrate the critical importance of freshwater plumes to improved understanding of the upper-ocean heat budget and air-sea interaction in the BoB.

The upper ocean stratification and circulation in the Bay of Bengal (BoB) plays a key role in the northward propagating monsoon intraseasonal oscillation during the months of June–August. This region is highly influenced by strong, seasonal atmospheric forcing and the oceanic circulation is characterized by dominant mesoscale variability and strong horizontal gradients in salinity and temperature during the monsoon period. Given the role of the ocean in the monsoon circulation, it is important to investigate accurate ocean state estimates and forecasts of the BoB ocean circulation in preparation for coupled ocean-atmosphere modeling and predictions. Hence, we use a mesoscale-permitting regional implementation of Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint-based four-dimensional variational (4DVAR) system to assimilate satellite-derived Sea Surface Height (SSH) and Sea Surface Temperature (SST) data in the BoB for a period of one month (June 1 – 30, 2017). It is shown that the MITgcm-BoB 4DVAR assimilation system is able to significantly improve the model consistency with the assimilated observations in the BoB region, reducing the model-data misfit by 50% and provided a dynamically-consistent BoB ocean circulation for the one-month hindcast period. We performed forecasting experiments using the state estimate to initialize two forecasts for a period of 30-days (July 1 – 30, 2017) from the end of the hindcast period. These forecasts used either atmosphere reanalysis and ocean analysis forcings or monthly climatology of atmosphere reanalysis and ocean analysis forcings. They therefore do not represent a “true” regional ocean forecast, forced using actual atmosphere and ocean forecasts, but bound the performance between climatological and nearly perfect forecasts. The model forecast is a cross-validation against future observations and showed that the initial conditions from the state estimate improves the prediction of the three-dimensional circulation in the BoB. The model hindcast and forecasts were also cross-validated against independent Argo temperature and salinity observations in the BoB. Additional state estimation and forecast experiments for other periods showed similar model performance with improved hindcasts and forecasts for the BoB region.

This study examines the effect of surface current in the bulk formula for the wind stress, referred to as the relative wind (RW) effect, on the energetics of the geostrophic circulation and the upper ocean stratification in the Bay of Bengal (BoB) during the summer monsoon seasons. When the RW effect is taken into account in the high-resolution SCOAR (WRF-ROMS) regional coupled model simulation and compared to the run without such a consideration, the kinetic energy both in the mean (MKE) and eddy (EKE) is reduced by more than a factor of two. The most significant reduction in the kinetic energy is found along the path of the northward East India Coastal Current (EICC) and to the south of its separated latitude. The energetics calculations and spectral analysis reveal that this significant damping of EKE is primarily due to reduced eddy wind work principally at wavelengths close to the first baroclinic Rossby deformation radius, indicating the modulation of the wind work by geostrophic mesoscale eddy fields. Moreover, the mixed layer depth (MLD) is significantly shoaled south of the separated EICC latitude, the area dominated by anticyclonic eddy activity. The shallower mixed layer and enhanced stratification with the RW effect are attributed to doming of the isopycnals by the anomalous upward Ekman velocity, which itself is generated by the interaction of anticyclonic mesoscale surface current and the prevailing southwesterly monsoonal wind. Overall, the geostrophic circulation and upper ocean stratification along the EICC and south of its separated latitude exhibit the most significant dynamical response. This result implies that this southwestern part of the BoB is a hot spot for the momentum exchange between the surface circulation and the monsoonal winds, thus a potential area for focused field measurements for the ocean circulation energetics and air-sea interaction.

The Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air–sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using a regional eddy-resolving coupled ocean–atmosphere (OA) model that downscales the observed large-scale climate variability from 2001 to 2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum and turbulent heat fluxes. We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SSTs), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak seen in the coupling strength in both the model and observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesocale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. Coupling between latent heat flux and SST is found to be significant throughout the entire seasonal cycle in both fully coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is also studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale.

Air–sea interaction at ocean fronts and eddies exhibits positive correlation between sea surface temperature (SST), wind speed, and heat fluxes out of the ocean, indicating that the ocean is forcing the atmosphere. This contrasts with larger scale climate modes where the negative correlations suggest that the atmosphere is driving the system. This paper examines the physical processes that lie behind the interaction of sharp SST gradients and the overlying marine atmospheric boundary layer and deeper atmosphere, using high resolution satellite data, field data and numerical models. The importance of different physical mechanisms of atmospheric response to SST gradients, such as the effect of surface stability variations on momentum transfer, pressure gradients, secondary circulations and cloud cover will be assessed. The atmospheric response is known to create small-scale wind stress curl and divergence anomalies, and a discussion of the feedback of these features onto the ocean will also be presented. These processes will be compared and contrasted for different regions such as the Equatorial Front in the Eastern Pacific, and oceanic fronts in mid-latitudes such as the Gulf Stream, Kuroshio, and Agulhas Return Current.