Generation of Submesoscale Temperature Inversions Below Salinity Fronts in the Bay of Bengal. Issue 12 (27th November 2020)

• Record Type: Journal Article
• Authors: Ramachandran, Sanjiv; Tandon, Amit
• Is Part Of: Journal of geophysical research. Volume 125:Issue 12(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-27
• Subjects: Bay of Bengal -- subduction -- submesoscale -- temperature inversion; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016278
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: This study uses submesoscale‐permitting simulations to explore the formation of temperature inversions at shallow, salinity‐controlled density fronts representative of conditions in the wintertime Bay of Bengal (BoB). Our simulations complement earlier one‐dimensional studies that have largely used mooring records in the BoB to infer mechanisms for causing temperature inversions, by exploring three‐dimensional frontal processes. We use three different initial conditions with the same lateral and vertical gradients in density, with differing lateral contrasts in temperature within the mixed layer. Our simulations evolve unforced for a few inertial periods, are then forced by weak downfront winds for a few more inertial periods to generate an eddy field, after which the winds are turned off. The formation of inversions proceeds via the subduction of low potential vorticity (PV) fluid from the surface during the forced phase. The inversions result from tilting of horizontal temperature gradients into the vertical direction. Tilting is simultaneously the dominant source of negative vertical vorticity in the Lagrangian vorticity budget. Turning off the winds caps the subducted fluid by overlying stratified fluid, creating temperature inversions with O(1–10 km) lateral scales and O(10 m) thickness. The inversions are pycnostads with anticyclonic rotation. The increase in temperature for the strongest inversion in each simulation is comparable to the initial lateral contrast in temperature. The strongest inversions are associated with anticyclonic rotation and low PV. Our results have potential implications for the vertical thermal structure of the upper BoB in coarse‐resolution models that do not resolve subduction at submesoscale fronts. Plain Language Summary: The ocean and atmosphere communicate at the air‐sea interface. This communication occurs through exchange of heat, momentum, and other properties. The exchange of heat, in particular, shapes the coupled interplay of the ocean and atmosphere over periods ranging from hours to years. The change in ocean temperature versus depth crucially impacts how much heat is available for exchange with the overlying atmosphere. Typically, temperature decreases with depth but in regions like the Bay of Bengal (BoB), it can increase with depth for some distance before continuing to decrease at greater depths. Such increases in temperature are called "inversions." In this study, we use high‐resolution numerical modeling to explain the formation of inversions in the BoB that have a thickness of 10–30 m and a horizontal size of 1–10 km (submesoscale). Observations show frequent occurrence of such inversions in this region. We identify mechanisms illustrating how such inversions might be formed. Our results have potential implications for climate models where the grid spacing is too coarse to capture such mechanisms. The study demonstrates the value of high‐resolution modeling in identifying new processes missing in today's climate models. Key Points: Submesoscale fronts are sites of temperature inversions in the Bay of Bengal The inversions have anticyclonic vorticity and anomalously low potential vorticity The inversions have a lateral scale O(1–10 km) and a maximum thickness O(10 m)