Characteristics of Submesoscale Compensated/Reinforced Fronts in the Northern Bay of Bengal

Fronts in the Bay of Bengal (BoB) are active and can potentially impact the regional dynamics such as temperature variability, salinity distribution and oceanic circulation. Based on the high resolution model output (LLC4320), this study investigates the characteristics of submesoscale fronts in the northern BoB and associated compensation/reinforcement effects. At sea surface, horizontal gradients of salinity and density are remarkable in the northern BoB, and they are nearly 3 times larger than temperature gradients. As the depth deepens, temperature gradients increase and become comparable to salinity gradients, while density gradients decrease a lot due to the increasing effects of compensation at subsurface. Statistical results show the dominance of salinity-controlled fronts over temperature-controlled fronts, and compensated fronts over reinforced fronts. The surface cooling/heating results in significant temporal variation of compensation at surface, but this variation is limited at subsurface by the blocking of the mixed layer base. The submesoscale-selective feature of compensation is much more pronounced at subsurface layer than surface layer. From statistical analysis and idealized numerical model, we found the slump of salinity-controlled compensated fronts are important in generating temperature inversion and maintaining barrier layer. This study validates the compensation theories originating from observations, and further illustrates the importance of subsurface compensated fronts using spatially continuous, regionally extended and longer-term model output. The subsurface-intensified submesoscale-selective compensation is proved for the first time in this study. With cold, fresh water on the one side and warm, salty water on the other side, roles of temperature and salinity in changing density are opposed across fronts, and they are referred to as compensated fronts. Otherwise, the fronts are reinforced by temperature and salinity. The strength of compensated fronts are thwarted to some extent compared to reinforced fronts, thus the existence of compensation effects are not favorable for forming a sharp front and associated instability processes. The statistics of submesoscale compensated/reinforced fronts in the northern BoB are comprehensively analyzed in this study. Concerning the amount of freshwater input, fronts are dominated by salinity gradients instead of temperature gradients over the northern BoB, and compensated fronts are more likely to form during winter with the existence of both large salinity gradients and heat losses. Both statistical analysis and idealized model simulation suggest that when the salinity front compensated by temperature collapses, a reversed vertical temperature gradient (temperature inversion) is favored to form. The submesoscale salinity (temperature) fronts are pronounced at surface (subsurface), and they tend to be compensated Subsurface layer is featured with more compensation than surface layer, and this subsurface compensation is more submesoscale-selective The slump of salinity-controlled compensated fronts leads to temperature inversion and the retention of barrier layer