Multi-platform observations of small-scale lateral mixed layer variability in the northern Bay of Bengal

Abstract

Observations of the ocean surface boundary layer in the Northern Bay of Bengal were collected simultaneously from multiple platforms during the late summer of 2015. A spatial survey, consisting of a ∼4 km triangle, was repeated for 2 days by R/V Roger Revelle. The shipboard observations included profiles of temperature, salinity, velocity, and microstructure, and a towed bow-chain. Concurrently, an autonomous surface vessel and a drifting vertical profiler collected high resolution temperature, salinity, velocity and turbulence measurements nearby. An air-sea flux mooring provided continuous atmospheric and upper ocean data. The observed ocean surface boundary layer (SBL) was very shallow (∼ 10 m) and salinity stratified, with frequent observations of subsurface temperature maxima. Freshwater filaments strongly influenced SBL depth on horizontal scales of one to tens of kilometers. Our measurements showed a complex pattern in the strength and vertical structure of shear, stratification, and turbulent heat fluxes within and just below the SBL. SBL heat flux was impacted by surface buoyancy loss, shear at the SBL base from wind-driven near-inertial oscillations, and, at times, vertically spiraling Ekman currents. The phase of near-inertial currents displayed significant submesoscale lateral variability, as observed by the multiple high-resolution synoptic measurements, with horizontal differences in vertical turbulent fluxes in the SBL of the ocean varying by an order of magnitude over only a few kilometers. Integrated air-sea heat fluxes diverged by about 7–15 % over a few days within a series of one-dimensional simulations initialized with simultaneously observed SBL profiles only a few kilometers apart. Taken together, our results document the variability of ocean-atmosphere coupling on scales far smaller than those used in coupled ocean-atmosphere forecast models.