Decay Mechanisms of Near-Inertial Mixed Layer Oscillations in the Bay of Bengal

Shaun Johnston,Debasis Sengupta,Daniel Rudnick,Harper Simmons,R Venkatesan,Amit Tandon,Dipanjan Chaudhuri,Manikandan Mathur

doi:10.5670/oceanog.2016.50

Shaun Johnston, Debasis Sengupta + Show 6 more

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https://doi.org/10.5670/oceanog.2016.50

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Abstract

Winds generate inertial and near-inertial currents in the upper ocean. These currents dominate the kinetic energy and contain most of the vertical shear in horizontal currents. Subsequent shear instabilities lead to mixing. In the Bay of Bengal, the annual mean wind energy input and near-inertial mixed layer energy is almost as large as in the mid-latitude storm tracks. Also, mixing associated with these waves is known to affect mixed layer heat content, sea surface temperature, and, thus, precipitation in coupled global models. Therefore, the mechanisms leading to the decay of these currents in the mixed layer and below are of considerable importance. Two such decay mechanisms are examined here. One mechanism is the downward propagation of near-inertial internal waves, which is aided by the mesoscale circulation and is observed with a rapidly profiling float. In a few days (faster than at mid-latitudes), the near-inertial wave group propagated from the base of the mixed layer to 250 m depth in the stratified interior. Another decay mechanism is enhanced shear generation at the mixed layer base from periodic alignment of rotating, near-inertial current shear and winds, which is observed with a mooring and analyzed with a simple two-layer model.

Highlights

Importance of Near-Inertial Internal Waves Steady surface winds produce frictional transport to the right of the wind in the Northern Hemisphere, while an impulsive change in wind speed or direction excites oscillations at the local inertial period in the surface mixed layer (ML; Ekman, 1905)
Enhanced Shear Generation at the Mixed Layer Base Figure 2 presents an analysis of the data collected at the BD09 mooring from August to September 2013
SUMMARY Shear at the base of the ML contributes to mixing, which affects the heat content and sea surface temperature of the ML and precipitation. Such effects may be more pronounced with a thin ML as in the Bay of Bengal (BoB)

Summary

Introduction

Importance of Near-Inertial Internal Waves Steady surface winds produce frictional transport to the right of the wind in the Northern Hemisphere, while an impulsive change in wind speed or direction excites oscillations at the local inertial period in the surface mixed layer (ML; Ekman, 1905). Steady winds produce vertical shear in horizontal currents (shear, hereafter) at the base of the ML and subsequently deepen the ML via mechanical mixing. With inertially rotating winds, the rotating currents can propagate into the oceanic interior without much effect on ML depth (MLD; Dohan and Davis, 2011). The inertial currents heave the base of the ML to produce propagating near-inertial internal waves (NIWs) in the stratified ocean (Simmons and Alford, 2012). When global coupled atmosphere-ocean models include NIW-driven mixing, substantial changes occur in the MLD, tropical sea surface temperature, and precipitation (Jochum et al, 2013)

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