Oceanography The Official Magazine of
The Oceanography Society
Volume 29 Issue 02

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Volume 29, No. 2
Pages 134 - 145

Adrift Upon a Salinity-Stratified Sea: A View of Upper-Ocean Processes in the Bay of Bengal During the Southwest Monsoon

Andrew J. Lucas Jonathan D. NashRobert PinkelJennifer A. MacKinnonAmit TandonAmala Mahadevan Melissa M. Omand Mara Freilich Debasis SenguptaM. RavichandranArnaud Le Boyer
Article Abstract

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. 


Lucas, A.J., J.D. Nash, R. Pinkel, J.A. MacKinnon, A. Tandon, A. Mahadevan, M.M. Omand, M. Freilich, D. Sengupta, M. Ravichandran, and A. Le Boyer. 2016. Adrift upon a salinity-stratified sea: A view of upper-ocean processes in the Bay of Bengal during the southwest monsoon. Oceanography 29(2):134–145, https://doi.org/10.5670/oceanog.2016.46.


Alford, M. 2003. Improved global maps and 54-year history of wind-work on ocean inertial motions. Geophysical Research Letters 30(8):1,424–1,427, https://doi.org/10.1029/2002GL016614.

Akhil, V.P., F. Durand, M. Lengaigne, J. Vialard, M.G. Keerthi, V.V. Gopalakrishna, C. Deitel, F. Papa, and C. de Boyer Montégut. 2014. A modeling study of the processes of surface salinity seasonal cycle in the Bay of Bengal. Journal of Geophysical Research 119:3,926–3,947, https://doi.org/10.1002/2013JC009632.

Bhat, G.S., 2001. Near surface atmospheric characteristics over the North Bay of Bengal during the Indian summer monsoon. Geophysical Research Letters 28:987–990, https://doi.org/10.1029/2000GL012455

Benshila, R., F. Durand, S. Masson, R. Bourdallé-Badie, C. de Boyer Montégut, F. Papa, and G. Madec. 2014. The upper Bay of Bengal salinity structure in a high-resolution model. Ocean Modelling 74:36–52, https://doi.org/10.1016/​j.ocemod.2013.12.001.

Chowdary, J.S., A. Parekh, S. Ojha, C. Gnanaseelan, and R. Kakatkar. 2016. Impact of upper ocean processes and air–sea fluxes on seasonal SST biases over the tropical Indian Ocean in the NCEP Climate Forecasting System. International Journal of Climatology 36:188–207, https://doi.org/10.1002/joc.4336.

Cloern, J.E., C. Grenz, and L. Vidergar-Lucas. 1995. An empirical model of the phytoplankton chlorophyll: Carbon ratio—the conversion factor between productivity and growth rate. Limnology and Oceanography 40(7):1,313–1,321, https://doi.org/10.4319/lo.1995.40.7.1313.

Cullen, J. 1982. The deep chlorophyll maximum: Comparing vertical profiles of chlorophyll a. Canadian Journal of Fisheries and Aquatic Sciences 39(5):791–803, https://doi.org/10.1139/f82-108.

Cullen, J. 2015. Subsurface chlorophyll maximum layers: Enduring enigma or mystery solved? Annual Review of Marine Science 7:207–239, https://doi.org/10.1146/annurev-marine-010213-135111.

Friehe, C.A., W.J. Shaw, D.P. Rogers, K.L. Davidson, W.G. Large, A.A. Stage, G.H. Crescenti, S.J.S. Khalsa, G.K. Greenhut, and F. Li. 1991. Air-sea fluxes and surface layer turbulence around a sea surface temperature front. Journal of Geophysical Research 96(C5):8,593–8,609, https://doi.org/​10.1029/90JC02062.

Gopalakrishna, V.V., V. Murty, D. Sengupta, S. Shenoy, and N. Araligidad. 2002. Upper ocean stratification and circulation in the northern Bay of Bengal during southwest monsoon of 1991. Continental Shelf Research 22(5):791–802, https://doi.org/10.1016/S0278-4343(01)00084-X.

Goswami, B.N., S.A. Rao, D. Sengupta, and S. Chakravorty. 2016. Monsoons to mixing in the Bay of Bengal: Multiscale air-sea interactions and monsoon predictability. Oceanography 29(2):18–27, https://doi.org/10.5670/oceanog.2016.35.

Jensen, T.G., H.W. Wijesekera, E.S. Nyadjro, P.G. Thoppil, J.F. Shriver, K.K. Sandeep, and V. Pant. 2016. Modeling salinity exchanges between the equatorial Indian Ocean and the Bay of Bengal. Oceanography 29(2):92–101, https://doi.org/10.5670/oceanog.2016.42.

Jinadasa, S.U.P., I. Lozovatsky, J. Planella-Morató, J.D. Nash, J.A. MacKinnon, A.J. Lucas, H.W. Wijesekera, and H.J.S. Fernando. 2016. Ocean turbulence and mixing around Sri Lanka and in adjacent waters of the northern Bay of Bengal. Oceanography 29(2):170–179, https://doi.org/10.5670/oceanog.2016.49.

Johnston, T.M.S., D. Chaudhuri, M. Mathur, D.L. Rudnick, D. Sengupta, H.L. Simmons, A. Tandon, and R. Venkatesan. 2016. Decay mechanisms of near-inertial mixed layer oscillations in the Bay of Bengal. Oceanography 29(2):180–191, https://doi.org/10.5670/oceanog.2016.50.

Joseph, P.V., K.P. Sooraj, C.A. Babu, and T.P. Sabin. 2005. A cold pool in the Bay of Bengal and its interaction with the active-break cycle of the monsoon. CLIVAR Exchanges 34(10):3.

Le Bouteiller, A., A. Leynaert, M.R. Landry, R. Le Borgne, J. Neveux, M. Rodier, J. Blanchot, and S.L. Brown. 2003. Primary production, new production, and growth rate in the equatorial Pacific: Changes from mesotrophic to oligotrophic regime. Journal of Geophysical Research 108, 8141, https://doi.org/10.1029/2001JC000914.

Lee, C.M., S.U.P. Jinadasa, A. Anutaliya, L.R. Centurioni, H.J.S. Fernando, V. Hormann, M. Lankhorst, L. Rainville, U. Send, and H.W. Wijesekera. 2016. Collaborative observations of boundary currents, water mass variability, and monsoon response in the southern Bay of Bengal. Oceanography 29(2):102–111, https://doi.org/10.5670/oceanog.2016.43.

MacKinnon, J.A., J.D. Nash, M.H. Alford, A.J. Lucas, J.B. Mickett, E.L. Shroyer, A.F. Waterhouse, A. Tandon, D. Sengupta, A. Mahadevan, and others. 2016. A tale of two spicy seas. Oceanography 29(2):50–61, https://doi.org/​10.5670/oceanog.2016.38.

Mahadevan, A., G. Spiro Jaeger, M. Freilich, M. Omand, E.L. Shroyer, and D. Sengupta. 2016. Freshwater in the Bay of Bengal: Its fate and role in air-sea heat exchange. Oceanography 29(2):72–81, https://doi.org/10.5670/oceanog.2016.40.

Marra, J. 1992. Diurnal variability in chlorophyll fluorescence: Observations and modeling. Conference volume 1750, Ocean Optics XI, July 19, 1992, San Diego, CA, Proceedings of SPIE - The International Society for Optical Engineering, https://doi.org/​10.1117/12.140654

Moum, J.N., and J.D. Nash. 2009. Mixing measurements on an equatorial ocean mooring. Journal of Atmospheric and Oceanic Technology 26(2):317–336, https://doi.org/​10.1175/2008JTECHO617.1.

Pant, V., M.S. Girishkumar, T.V.S. Udaya Bhaskar, M. Ravichandran, F. Papa, and V.P. Thangaprakash. 2015. Observed interannual variability of near-​surface salinity in the Bay of Bengal. Journal of Geophysical Research 120(5):3,315–3,329, https://doi.org/10.1002/2014JC010340.

Papa, F., F. Durand, W.B. Rossow, A. Rahman, and S.K. Bala. 2010. Satellite altimeter-derived monthly discharge of the Ganga-Brahmaputra River and its seasonal to interannual variations from 1993 to 2008. Journal of Geophysical Research 115, C12013, https://doi.org/10.1029/2009JC006075.

Pinkel, R., M.A. Goldin, J.A. Smith, O.M. Sun, A.A. Aja, M.N. Bui, and T. Hughen. 2011. The Wirewalker: A vertically profiling instrument carrier powered by ocean waves. Journal of Atmospheric and Oceanic Technology 28(3):426–435, https://doi.org/10.1175/2010JTECHO805.1.

Rainville, L., and R. Pinkel. 2001. Wirewalker: An autonomous wave-powered vertical profiler. Journal of Atmospheric and Oceanic Technology 18(6):1,048–1,051, https://doi.org/10.1175/​1520-0426(2001)018​<1048:WAAWPV>2.0.CO;2.

Sarkar, S., H.T. Pham, S. Ramachandran, J.D. Nash, A. Tandon, J. Buckley, A.A. Lotliker, and M.M. Omand. 2016. The interplay between submesoscale instabilities and turbulence in the surface layer of the Bay of Bengal. Oceanography 29(2):146–157, https://doi.org/10.5670/oceanog.2016.47.

Sarma, V.V.S.S., G.D. Rao, R. Viswanadham, C.K. Sherin, J. Salisbury, M.M. Omand, A. Mahadevan, V.S.N. Murty, E.L. Shroyer, M. Baumgartner, and K.M. Stafford. 2016. Effects of freshwater stratification on nutrients, dissolved oxygen, and phytoplankton in the Bay of Bengal. Oceanography 29(2):222–231, https://doi.org/10.5670/oceanog.2016.54.

Sengupta, D., S.R. Parampil, G.S. Bhat, V.S.N. Murty, V. Ramesh Babu, T. Sudhakar, K. Premkumar, and Y. Pradhan. 2008. Warm pool thermodynamics from the Arabian Sea Monsoon Experiment (ARMEX). Journal of Geophysical Research 113, C10008, https://doi.org/10.1029/2007JC004623.

Sengupta, D., and M. Ravichandran. 2001. Oscillations of Bay of Bengal sea surface temperature during the 1998 summer monsoon. Geophysical Research Letters 28(10):2,033–2,036, https://doi.org/10.1029/2000GL012548.

Shankar, D., S.R. Shetye, and P.V. Joseph. 2007. Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal. Journal of Earth System Science 116:385–406, https://doi.org/10.1007/s12040-007-0038-y.

Shroyer, E.L., D.L. Rudnick, J.T. Farrar, B. Lim, S.K. Venayagamoorthy, L.C. St. Laurent, A. Garanaik, and J.N. Moum. 2016. Modification of upper-ocean temperature structure by subsurface mixing in the presence of strong salinity stratification. Oceanography 29(2):62–71, https://doi.org/10.5670/oceanog.2016.39.

Smith, J.A., R. Pinkel, M. Goldin, O. Sun, S. Nguyen, T. Hughen, M. Bui, and A. Aja. 2012. Wirewalker dynamics. Journal of Atmospheric and Oceanic Technology 29(1):103–115, https://doi.org/10.1175/JTECH-D-11-00049.1.

Vinayachandran, P.N., V.S.N. Murty, and V. Ramesh Babu. 2002. Observations of barrier layer formation in the Bay of Bengal during summer monsoon. Journal of Geophysical Research 107(C12), 8018, https://doi.org/10.1029/2001JC000831.

Wang, X.J., M. Behrenfeld, R. Le Borgne, R. Murtugudde, and E. Boss. 2009. Regulation of phytoplankton carbon to chlorophyll ratio by light, nutrients and temperature in the Equatorial Pacific Ocean:
A basin-scale model. Biogeosciences 6:391–404, https://doi.org/​10.5194/bg-6-391-2009.

Weller, R.A., J.T. Farrar, J. Buckley, S. Mathew, R. Venkatesan, J. Sree Lekha, D. Chaudhuri, N. Suresh Kumar, and B. Praveen Kumar. 2016. Air-sea interaction in the Bay of Bengal. Oceanography 29(2):28–37, https://doi.org/​10.5670/oceanog.2016.36.