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  • Author or Editor: Eric d'Asaro x
  • Air–Sea Interactions from the Diurnal to the Intraseasonal during the PISTON, MISOBOB, and CAMP2Ex Observational Campaigns in the Tropics x
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Dipanjan Chaudhuri
,
Debasis Sengupta
,
Eric D’Asaro
,
R. Venkatesan
, and
M. Ravichandran

Abstract

Cyclone Phailin, which developed over the Bay of Bengal in October 2013, was one of the strongest tropical cyclones to make landfall in India. We study the response of the salinity-stratified north Bay of Bengal to Cyclone Phailin with the help of hourly observations from three open-ocean moorings 200 km from the cyclone track, a mooring close to the cyclone track, daily sea surface salinity (SSS) from Aquarius, and a one-dimensional model. Before the arrival of Phailin, moored observations showed a shallow layer of low-salinity water lying above a deep, warm “barrier” layer. As the winds strengthened, upper-ocean mixing due to enhanced vertical shear of storm-generated currents led to a rapid increase of near-surface salinity. Sea surface temperature (SST) cooled very little, however, because the prestorm subsurface ocean was warm. Aquarius SSS increased by 1.5–3 psu over an area of nearly one million square kilometers in the north Bay of Bengal. A one-dimensional model, with initial conditions and surface forcing based on moored observations, shows that cyclone winds rapidly eroded the shallow, salinity-dominated density stratification and mixed the upper ocean to 40–50-m depth, consistent with observations. Model sensitivity experiments indicate that changes in ocean mixed layer temperature in response to Cyclone Phailin are small. A nearly isothermal, salinity-stratified barrier layer in the prestorm upper ocean has two effects. First, near-surface density stratification reduces the depth of vertical mixing. Second, mixing is confined to the nearly isothermal layer, resulting in little or no SST cooling.

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Dipanjan Chaudhuri
,
Debasis Sengupta
,
Eric D’Asaro
,
J. Thomas Farrar
,
Manikandan Mathur
, and
Sundar Ranganathan

Abstract

We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using six years of hourly observations from four moorings. The mean annual energy input from surface winds to near-inertial mixed-layer currents is 10–20 kJ/m2, occurring mainly in distinct synoptic “events” from April to September. A total of fifteen events are analyzed: Seven when the ocean is capped by a thin layer of low-salinity river water (fresh) and eight when it is not (salty). The average near-inertial energy input from winds is 40% higher in the fresh cases than in the salty cases. During the fresh events, (A) mixed layer near-inertial motions decay about two times faster, and (B) near-inertial kinetic energy below the mixed layer is reduced by at least a factor of three relative to the salty cases. The near-inertial horizontal wavelength was measured for one fresh and one salty event; the fresh was about three times shorter initially. A linear model of near-inertial wave propagation tuned to these data reproduces (B); the thin (10 m) mixed layers during the fresh events excite high modes, which propagate more slowly than the low modes excited by the thicker (40 m) mixed layers in the salty events. The model does not reproduce (A); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible.

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B. Praveen Kumar
,
Eric D’Asaro
,
N. Sureshkumar
,
E. Pattabhi Rama Rao
, and
M. Ravichandran

Abstract

We use profiles from a Lagrangian float in the north Indian Ocean to explore the usefulness of Thorpe analysis methods to measure vertical scales and dissipation rates in the ocean surface boundary layer. An rms Thorpe length scale L T and an energy dissipation rate ε T were computed by resorting the measured density profiles. These are compared to the mixed layer depth (MLD) computed with different density thresholds, the Monin–Obukhov (MO) length L MO computed from the ERA5 reanalysis values of wind stress, and buoyancy flux B 0 and dissipation rates ε from historical microstructure data. The Thorpe length scale L T is found to accurately match MLD for small (<0.005 kg m−3) density thresholds, but not for larger thresholds, because these do not detect the warm diurnal layers. We use ξ = L T /|L MO| to classify the boundary layer turbulence during nighttime convection. In our data, 90% of points from the Bay of Bengal (Arabian Sea) satisfy ξ < 1 (1 < ξ <10), indicating that wind forcing is (both wind forcing and convection are) driving the turbulence. Over the measured range of ξ, ε T decreases with decreasing ξ, i.e., more wind forcing, while ε increases, clearly showing that ε/ε T decreases with increasing ξ. This is explained by a new scaling for ξ ≪ 1, ε T = 1.15B 0 ξ 0.5 compared to the historical scaling ε = 0.64B 0 + 1.76ξ −1. For ξ ≪ 1 we expect ε = ε T . Similar calculations may be possible using routine Argo float and ship data, allowing more detailed global measurements of ε T , thereby providing large-scale tests of turbulence scaling in boundary layers.

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