Sengupta Debasis

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Sengupta

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Debasis

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Moored observations of the surface meteorology and air-sea fluxes in the Northern Bay of Bengal in 2015

(American Meteorological Society, 2018-12-28) Weller, Robert A. ; Farrar, J. Thomas ; Seo, Hyodae ; Prend, Channing ; Sengupta, Debasis ; Lekha, J. Sree ; Ravichandran, M. ; Venkatesan, Ramasamy

Time series of surface meteorology and air–sea fluxes from the northern Bay of Bengal are analyzed, quantifying annual and seasonal means, variability, and the potential for surface fluxes to contribute significantly to variability in surface temperature and salinity. Strong signals were associated with solar insolation and its modulation by cloud cover, and, in the 5- to 50-day range, with intraseasonal oscillations (ISOs). The northeast (NE) monsoon (DJF) was typically cloud free, with strong latent heat loss and several moderate wind events, and had the only seasonal mean ocean heat loss. The spring intermonsoon (MAM) was cloud free and had light winds and the strongest ocean heating. Strong ISOs and Tropical Cyclone Komen were seen in the southwest (SW) monsoon (JJA), when 65% of the 2.2-m total rain fell, and oceanic mean heating was small. The fall intermonsoon (SON) initially had moderate convective systems and mean ocean heating, with a transition to drier winds and mean ocean heat loss in the last month. Observed surface freshwater flux applied to a layer of the observed thickness produced drops in salinity with timing and magnitude similar to the initial drops in salinity in the summer monsoon, but did not reproduce the salinity variability of the fall intermonsoon. Observed surface heat flux has the potential to cause the temperature trends of the different seasons, but uncertainty in how shortwave radiation is absorbed in the upper ocean limits quantifying the role of surface forcing in the evolution of mixed layer temperature.
A tale of two spicy seas

(The Oceanography Society, 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.
Adrift upon a salinity-stratified sea : a view of upper-ocean processes in the Bay of Bengal during the southwest monsoon

(The Oceanography Society, 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.
Freshwater in the Bay of Bengal : its fate and role in air-sea heat exchange

(The Oceanography Society, 2016-06) Mahadevan, Amala ; Spiro Jaeger, Gualtiero ; Freilich, Mara ; Omand, Melissa M. ; Shroyer, Emily L. ; Sengupta, Debasis

The strong salinity stratification in the upper 50–80 m of the Bay of Bengal affects the response of the upper ocean to surface heat fluxes. Using observations from November to December 2013, we examine the effect of surface cooling on the temperature structure of the ocean in a one-dimensional framework. The presence of freshwater adds gravitational stability to the density stratification and prevents convective overturning, even when the surface becomes cooler than the subsurface. This stable salinity stratification traps heat within subsurface layers. The ocean’s reluctance to release the heat trapped within these subsurface warm layers can contribute to delayed rise in surface temperature and heat loss from the ocean as winter progresses. Understanding the dispersal of freshwater throughout the bay can help scientists assess its potential for generating the anomalous temperature response. We use the Aquarius along-track surface salinity and satellite-derived surface velocities to trace the evolution and modification of salinity in the Lagrangian frame of water parcels as they move through the bay with the mesoscale circulation. This advective tracking of surface salinities provides a Lagrangian interpolation of the monthly salinity fields in 2013 and shows the evolution of the freshwater distribution. The along-trajectory rate of salinification of water as it leaves the northern bay is estimated and interpreted to result from mixing processes that are likely related to the host of submesoscale signatures observed during our field campaigns.
Introduction to the special issue on the Bay of Bengal : from monsoons to mixing

(The Oceanography Society, 2016-06) Mahadevan, Amala ; Paluszkiewicz, Theresa ; Ravichandran, M. ; Sengupta, Debasis ; Tandon, Amit

The Bay of Bengal has a surprisingly large influence on the world. It nurtures the South Asian summer monsoon, a tremendous ocean-atmosphere-land phenomenon that delivers freshwater to more than a third of the human population on this planet. During summer, southwesterly winds gather moisture from the ocean and carry it deep inland over the Indian subcontinent, bringing welcome rains to a parched land. During winter, the winds reverse to northeasterly, and the ocean circulation responds by dispersing the terrestrial freshwater runoff concentrated in the northern part of the bay. This freshwater impacts the ocean’s structure, circulation, and biogeochemistry in numerous ways and, through modification of sea surface temperature, feeds back to influence air-sea fluxes. Because the atmosphere obtains its moisture and heat for convection from the ocean, the interplay between ocean and atmosphere is crucial for the development and sustenance of the monsoon.
Bay of Bengal : 2013 northeast monsoon upper-ocean circulation

(The Oceanography Society, 2016-06) Gordon, Arnold L. ; Shroyer, Emily L. ; Mahadevan, Amala ; Sengupta, Debasis ; Freilich, Mara

The upper 200 m of the two northern Indian Ocean embayments, the Bay of Bengal (BoB) and the Arabian Sea (AS), differ sharply in their salinity stratification, as the Asian monsoon injects massive amounts of freshwater into the BoB while removing freshwater via evaporation from the AS. The ocean circulation transfers salt from the AS to the BoB and exports freshwater from the BoB to mitigate the salinity difference and reach a quasi-steady state, albeit with strong seasonality. An energetic field of mesoscale features and an intrathermocline eddy was observed within the BoB during the R/V Revelle November and December 2013 Air-Sea Interactions Regional Initiative cruises, marking the early northeast monsoon phase. Mesoscale features, which display a surprisingly large thermohaline range within their confines, obscure the regional surface water and thermohaline stratification patterns, as observed by satellite and Argo profilers. Ocean processes blend the fresh and salty features along and across density surfaces, influencing sea surface temperature and air-sea flux. Comparing the Revelle observations to the Argo data reveals a general westward migration of mesoscale features across the BoB.
ASIRI : an ocean–atmosphere initiative for Bay of Bengal

(American Meteorological Society, 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.
Technological advancements in observing the upper ccean in the Bay of Bengal : education and capacity building

(The Oceanography Society, 2016-06) Tandon, Amit ; D'Asaro, Eric A. ; Stafford, Kathleen M. ; Sengupta, Debasis ; Ravichandran, M. ; Baumgartner, Mark F. ; Venkatesan, Ramasamy ; Paluszkiewicz, Theresa

Because the monsoon strongly affects India, there is a clear need for indigenous expertise in advancing the science that underlies monsoon prediction. The safety of marine transport in the tropics relies on accurate atmospheric and ocean environment predictions on weekly and longer time scales in the Indian Ocean. This need to better forecast the monsoon motivates the United States to advance basic research and support training of early career US scientists in tropical oceanography. Earlier Indian field campaigns and modeling studies indicated that an improved understanding of the interactions between the upper ocean and the atmosphere in the Bay of Bengal at finer spatial and temporal scales could lead to improved intraseasonal monsoon forecasts. The joint US Air-Sea Interactions Regional Initiative (ASIRI) and the Indian Ocean Mixing and Monsoon (OMM) program studied these interactions, resulting in scientific advances described by articles in this special issue of Oceanography. In addition to these scientific advances, and while also developing long-lasting collaborations and building indigenous Indian capability, a key component of these programs is training early career scientists from India and the United States. Training has been focusing on fine-scale and mixing studies of the upper ocean, air-sea interactions, and marine mammal research. Advanced methods in instrumentation, autonomous robotic platforms, experimental design, data analysis, and modeling have been emphasized. Students and scientists from India and the United States at all levels have been participating in joint cruises on Indian and US research vessels and in training participants in modern tools and methods at summer schools, at focused research workshops, and during research visits. Such activities are building new indigenous capability in India, training a new cadre of US scientists well versed in monsoon air-sea interaction, and forging strong links between Indian and US oceanographic institutions.
Quasi-biweekly mode of the Asian summer monsoon revealed in Bay of Bengal surface observations

(American Geophysical Union, 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.
Importance of Seasonally Evolving Near‐Surface Salinity Stratification on Mixed Layer Heat Budget During Summer Monsoon Intraseasonal Oscillation in the Northern Bay of Bengal in 2019

(American Geophysical Union, 2023-11-13) Sherin, Vidwil Raju ; Girishkumar, M. S. ; Shivaprasad, S. ; Sureshkumar, N. ; Farrar, J. Thomas ; Athulya, K. ; Ashin, K. ; Rama Rao, E. Pattabhi ; Sengupta, Debasis ; Venkatesan, Ramasamy ; Ravichandran, M.

The discharge of freshwater from major rivers into the northern Bay of Bengal (BoB) increases dramatically during the summer monsoon season, reaching a peak in August–September, and there is a corresponding increase in the vertical salinity gradient in the upper ocean. Here we study the impact of seasonally evolving near-surface salinity stratification on the response of ocean mixed layer temperature (MLT) to Summer Monsoon Intraseasonal Oscillations (MISO), using accurate surface fluxes and high vertical resolution (∼2 m) hydrographic measurements from a mooring in the northern BoB (17.8°N, 89.5°E) during June–September 2019. Prominent MLT warming and cooling with a range of 1.5°C is observed between suppressed (clear skies, calm winds) and active (cloudy, windy) phases of MISO convection. However, the intraseasonal MLT response to the active phase of a late-season MISO event is minimal compared to MISO events in early summer. We infer this is primarily due to the much smaller contribution from oceanic vertical processes (∼6 Wm−2) in late summer 2019, compared to their role in early summer (−15 to −55 Wm−2). During the active phase of the MISO event of late summer 2019, the combined effect of reduced entrainment and weak vertical temperature gradients associated with a barrier layer inhibits near-surface cooling. Conversely, the near-surface salinity stratification and the barrier layer are weak during MISO events in the early summer of 2019—these hydrographic conditions lead to enhanced MLT cooling in response to MISO, apparently through a freer turbulent exchange of cool thermocline water with the surface layer.
Bay of Bengal intraseasonal oscillations and the 2018 monsoon onset

(American Meteorological Society, 2021-10-01) Shroyer, Emily L. ; Tandon, Amit ; Sengupta, Debasis ; Fernando, Harindra J. S. ; Lucas, Andrew J. ; Farrar, J. Thomas ; Chattopadhyay, Rajib ; de Szoeke, Simon P. ; Flatau, Maria ; Rydbeck, Adam ; Wijesekera, Hemantha W. ; McPhaden, Michael J. ; Seo, Hyodae ; Subramanian, Aneesh C. ; Venkatesan, Ramasamy ; Joseph, Jossia K. ; Ramsundaram, S. ; Gordon, Arnold L. ; Bohman, Shannon M. ; Pérez, Jaynise ; Simoes-Sousa, Iury T. ; Jayne, Steven R. ; Todd, Robert E. ; Bhat, G. S. ; Lankhorst, Matthias ; Schlosser, Tamara L. ; Adams, Katherine ; Jinadasa, S. U. P. ; Mathur, Manikandan ; Mohapatra, Mrutyunjay ; Pattabhi Rama Rao, Eluri ; Sahai, Atul Kumar ; Sharma, Rashmi ; Lee, Craig ; Rainville, Luc ; Cherian, Deepak A. ; Cullen, Kerstin ; Centurioni, Luca R. ; Hormann, Verena ; MacKinnon, Jennifer A. ; Send, Uwe ; Anutaliya, Arachaporn ; Waterhouse, Amy F. ; Black, Garrett S. ; Dehart, Jeremy A. ; Woods, Kaitlyn M. ; Creegan, Edward ; Levy, Gad ; Kantha, Lakshmi ; Subrahmanyam, Bulusu

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.
Near-inertial response of a salinity-stratified ocean

(American Meteorological Society, 2024-08-19) Chaudhuri, Dipanjan ; Sengupta, Debasis ; D'Asaro, Eric A. ; Farrar, J. Thomas ; Mathur, Manikandan ; Ranganathan, Sundar

We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using 6 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 m−2, occurring mainly in distinct synoptic “events” from April–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, 1) mixed layer near-inertial motions decay about two times faster and 2) 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 2); 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 1); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by the linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible.

Sengupta Debasis

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