Sharma Rashmi

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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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Bay of Bengal intraseasonal oscillations and the 2018 monsoon onset

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 , Rama Rao, E. Pattabhi , 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.

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Study of the mixed layer depth variations within the north Indian Ocean using a 1-D model

2004-08-24 , Babu, K. N. , Sharma, Rashmi , Agarwal, Neeraj , Agarwal, Vijay K. , Weller, Robert A.

Mixed layer depth (MLD) over the north Indian Ocean (30°S to 30°N and 40°E to 110°E) is computed using the simple one-dimensional model of Price et al. [1986] forced by satellite-derived parameters (winds and chlorophyll). Seasonal chlorophyll observations obtained from the Coastal Zone Color Scanner allow us to examine how biology interacts with physics in the upper ocean by changing the absorption of light and thus the heating by penetrative solar radiation, an effect we refer to as biological heating. Our analysis focus mainly on two aspects: the importance of varying biology in the model simulations relative to runs with constant biology and secondly, the contribution of biology to the seasonal variability of the MLD. The model results are compared with observations from a surface mooring deployed for 1 year (October 1994 to October 1995) in the central Arabian Sea and also with available conductivity-temperature-depth (CTD) observations from the Arabian Sea during the period 1994–1995. The effect of biological heating on the upper ocean thermal structure in central Arabian Sea is found to be greatest in August. In other months it is either the wind, which is the controlling factor in mixed layer variations, or the density variations due to winter cooling and internal dynamics. A large number of CTD observations collected under the Joint Global Ocean Flux study and World Ocean Circulation Experiment have been used to validate model results. We find an overall improvement by approximately 2–3 m in root-mean-square error in MLD estimates when seasonally varying chlorophyll observations are used in the model.