Ravichandran M.

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Penetrative radiative flux in the Bay of Bengal

2016-06 , Lotliker, Aneesh , Omand, Melissa M. , Lucas, Andrew J. , Laney, Samuel R. , Mahadevan, Amala , Ravichandran, M.

The Bay of Bengal (BoB), a semi-enclosed basin in the northern Indian Ocean, is a complex region with large freshwater inputs and strong vertical stratification that result in a shallow, spatially variable mixed layer. With the exception of shortwave insolation, the air-sea heat exchange occurs at the sea surface and is vertically redistributed by mixing and advection. Strongly stratified, shallow mixed layers inhibit vertical mixing, and the penetration of solar radiation through the base of the mixed layer can lead to redistribution of upper-ocean heat. This paper compiles observations of hyperspectral downwelling irradiance (Ed) from 67 profiles collected during six research cruises in the BoB that span a broad range of regions and seasons between 2009 and 2014. We report attenuation length scales computed using double and single exponential models and quantify the penetration of radiative flux below the mixed layer depth (Qpen). We then evaluate estimates of Qpen obtained from published chlorophyll-based models and compare them to our observations. We find that the largest penetrative heat flux (up to 40% of the incident Ed) occurs near 16°N where the mixed layers are shallow and the water is optically clear.

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ASIRI : an ocean–atmosphere initiative for Bay of Bengal

2016-11-22 , Wijesekera, Hemantha W. , Shroyer, Emily L. , Tandon, Amit , Ravichandran, M. , Sengupta, Debasis , Jinadasa, S. U. P. , Fernando, Harindra J. S. , Agrawal, Neeraj , Arulananthan, India K. , Bhat, G. S. , Baumgartner, Mark F. , Buckley, Jared , Centurioni, Luca R. , Conry, Patrick , Farrar, J. Thomas , Gordon, Arnold L. , Hormann, Verena , Jarosz, Ewa , Jensen, Tommy G. , Johnston, T. M. Shaun , Lankhorst, Matthias , Lee, Craig M. , Leo, Laura S. , Lozovatsky, Iossif , Lucas, Andrew J. , MacKinnon, Jennifer A. , Mahadevan, Amala , Nash, Jonathan D. , Omand, Melissa M. , Pham, Hieu , Pinkel, Robert , Rainville, Luc , Ramachandran, Sanjiv , Rudnick, Daniel L. , Sarkar, Sutanu , Send, Uwe , Sharma, Rashmi , Simmons, Harper L. , Stafford, Kathleen M. , St. Laurent, Louis C. , Venayagamoorthy, Subhas K. , Venkatesan, Ramasamy , Teague, William J. , Wang, David W. , Waterhouse, Amy F. , Weller, Robert A. , Whalen, Caitlin B.

Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.

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A tale of two spicy seas

2016-06 , MacKinnon, Jennifer A. , Nash, Jonathan D. , Alford, Matthew H. , Lucas, Andrew J. , Mickett, John B. , Shroyer, Emily L. , Waterhouse, Amy F. , Tandon, Amit , Sengupta, Debasis , Mahadevan, Amala , Ravichandran, M. , Pinkel, Robert , Rudnick, Daniel L. , Whalen, Caitlin B. , Alberty, Marion S. , Lekha, J. Sree , Fine, Elizabeth C. , Chaudhuri, Dipayan , Wagner, Gregory L.

Upper-ocean turbulent heat fluxes in the Bay of Bengal and the Arctic Ocean drive regional monsoons and sea ice melt, respectively, important issues of societal interest. In both cases, accurate prediction of these heat transports depends on proper representation of the small-scale structure of vertical stratification, which in turn is created by a host of complex submesoscale processes. Though half a world apart and having dramatically different temperatures, there are surprising similarities between the two: both have (1) very fresh surface layers that are largely decoupled from the ocean below by a sharp halocline barrier, (2) evidence of interleaving lateral and vertical gradients that set upper-ocean stratification, and (3) vertical turbulent heat fluxes within the upper ocean that respond sensitively to these structures. However, there are clear differences in each ocean’s horizontal scales of variability, suggesting that despite similar background states, the sharpening and evolution of mesoscale gradients at convergence zones plays out quite differently. Here, we conduct a qualitative and statistical comparison of these two seas, with the goal of bringing to light fundamental underlying dynamics that will hopefully improve the accuracy of forecast models in both parts of the world.

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Quasi-biweekly mode of the Asian summer monsoon revealed in Bay of Bengal surface observations

2020-11-16 , Lekha, J. Sree , Lucas, Andrew J. , Sukhatme, Jai , Joseph, Jossia K. , Ravichandran, M. , Kumar, N. Suresh , Farrar, J. Thomas , Sengupta, Debasis

Asian summer monsoon has a planetary‐scale, westward propagating “quasi‐biweekly” mode of variability with a 10–25 day period. Six years of moored observations at 18°N, 89.5°E in the north Bay of Bengal (BoB) reveal distinct quasi‐biweekly variability in sea surface salinity (SSS) during summer and autumn, with peak‐to‐peak amplitude of 3–8 psu. This large‐amplitude SSS variability is not due to variations of surface freshwater flux or river runoff. We show from the moored data, satellite SSS, and reanalyses that surface winds associated with the quasi‐biweekly monsoon mode and embedded weather‐scale systems, drive SSS and coastal sea level variability in 2015 summer monsoon. When winds are calm, geostrophic currents associated with mesoscale ocean eddies transport Ganga‐Brahmaputra‐Meghna river water southward to the mooring, salinity falls, and the ocean mixed layer shallows to 1–10 m. During active (cloudy, windy) spells of quasi‐biweekly monsoon mode, directly wind‐forced surface currents carry river water away to the east and north, leading to increased salinity at the moorings, and rise of sea level by 0.1–0.5 m along the eastern and northern boundary of the bay. During July–August 2015, a shallow pool of low‐salinity river water lies in the northeastern bay. The amplitude of a 20‐day oscillation of sea surface temperature (SST) is two times larger within the fresh pool than in the saltier ocean to the west, although surface heat flux is nearly identical in the two regions. This is direct evidence that spatial‐temporal variations of BoB salinity influences sub‐seasonal SST variations, and possibly SST‐mediated monsoon air‐sea interaction.

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Adrift upon a salinity-stratified sea : a view of upper-ocean processes in the Bay of Bengal during the southwest monsoon

2016-06 , Lucas, Andrew J. , Nash, Jonathan D. , Pinkel, Robert , MacKinnon, Jennifer A. , Tandon, Amit , Mahadevan, Amala , Omand, Melissa M. , Freilich, Mara , Sengupta, Debasis , Ravichandran, M. , Le Boyer, Arnaud

The structure and variability of upper-ocean properties in the Bay of Bengal (BoB) modulate air-sea interactions, which profoundly influence the pattern and intensity of monsoonal precipitation across the Indian subcontinent. In turn, the bay receives a massive amount of freshwater through river input at its boundaries and from heavy local rainfall, leading to a salinity-stratified surface ocean and shallow mixed layers. Small-scale oceanographic processes that drive variability in near-surface BoB waters complicate the tight coupling between ocean and atmosphere implicit in this seasonal feedback. Unraveling these ocean dynamics and their impact on air-sea interactions is critical to improving the forecasting of intraseasonal variability in the southwest monsoon. To that end, we deployed a wave-powered, rapidly profiling system capable of measuring the structure and variability of the upper 100 m of the BoB. The evolution of upper-ocean structure along the trajectory of the instrument’s roughly two-week drift, along with direct estimates of vertical fluxes of salt and heat, permit assessment of the contributions of various phenomena to temporal and spatial variability in the surface mixed layer depth. Further, these observations suggest that the particular “barrier-layer” stratification found in the BoB may decrease the influence of the wind on mixing processes in the interior, thus isolating the upper ocean from the interior below, and tightening its coupling to the atmosphere above.