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

View Issue TOC
Volume 29, No. 2
Pages 180 - 191


Decay Mechanisms of Near-Inertial Mixed Layer Oscillations in the Bay of Bengal

By T.M. Shaun Johnston , Dipanjan Chaudhuri , Manikandan Mathur, Daniel L. Rudnick, Debasis Sengupta, Harper L. Simmons, Amit Tandon, and R. Venkatesan 
Jump to
Article Abstract Citation References Copyright & Usage
Article Abstract

Winds generate inertial and near-inertial currents in the upper ocean. These currents dominate the kinetic energy and contain most of the vertical shear in horizontal currents. Subsequent shear instabilities lead to mixing. In the Bay of Bengal, the annual mean wind energy input and near-inertial mixed layer energy is almost as large as in the mid-latitude storm tracks. Also, mixing associated with these waves is known to affect mixed layer heat content, sea surface temperature, and, thus, precipitation in coupled global models. Therefore, the mechanisms leading to the decay of these currents in the mixed layer and below are of considerable importance. Two such decay mechanisms are examined here. One mechanism is the downward propagation of near-inertial internal waves, which is aided by the mesoscale circulation and is observed with a rapidly profiling float. In a few days (faster than at mid-latitudes), the near-inertial wave group propagated from the base of the mixed layer to 250 m depth in the stratified interior. Another decay mechanism is enhanced shear generation at the mixed layer base from periodic alignment of rotating, near-inertial current shear and winds, which is observed with a mooring and analyzed with a simple two-layer model.


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.


Alford, M.H., and M. Gregg. 2001. Near-inertial mixing: Modulation of shear, strain and microstructure at low latitude. Journal of Geophysical Research 106(C8):16, 947–16,968, https://doi.org/10.1029/2000JC000370.

Alford, M.H., J.A. MacKinnon, H.L. Simmons, and J.D. Nash. 2016. Near-inertial internal gravity waves in the ocean. Annual Review of Marine Science 8:95–123, https://doi.org/10.1146/annurev-marine-010814-015746.

Brannigan, L., Y.-D. Lenn, T.P. Rippeth, E. McDonagh, T.K. Chereskin, and J. Sprintall. 2013. Shear at the base of the oceanic mixed layer generated by wind shear alignment. Journal of Physical Oceanography 43(8):1,798–1,810, https://doi.org/​10.1175/JPO-D-12-0104.1.

Chaigneau, A., O. Pizarro, and W. Rojas. 2008. Global climatology of near-inertial current characteristics from Lagrangian observations. Geophysical Research Letters 35, L13603, https://doi.org/​10.1029/2008GL034060.

Chowdary, J.S., A. Parekh, S. Ojha, C. Gnanaseelan, and R. Kakatkar. 2016a. 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(1):188–207, https://doi.org/10.1002/joc.4336.

Chowdary, J.S., G. Srinivas, T.S. Fousiya, A. Parekh, C. Gnanaseelan, H. Seo, and J.A. MacKinnon. 2016. Representation of Bay of Bengal upper-ocean salinity in general circulation models. Oceanography 29(2):38–49, https://doi.org/10.5670/oceanog.2016.37.

Dai, A., and K.E. Trenberth. 2002. Estimates of freshwater discharge from continents: Latitudinal and seasonal variations. Journal of Hydrometeorology 2:660–687, https://doi.org/10.1175/1525-7541(2002)003​<0660:EOFDFC>2.0.CO;2.

D’Asaro, E.A., C.C. Eriksen, M.D. Levine, P. Niiler, C.A. Paulson, and P. Van Meurs. 1995. Upper-ocean inertial currents forced by a strong storm: Part I. Data and comparisons with linear theory. Journal of Physical Oceanography 25:2,909–2,936, https://doi.org/10.1175/1520-0485(1995)025​<2909:UOICFB>2.0.CO;2.

Davis, R.E., J.T. Sherman, and J. Dufour. 2001. Profiling ALACEs and other advances in autonomous subsurface floats. Journal of Atmospheric and Oceanic Technology 18(6):982–993, https://doi.org/10.1175/1520-0426(2001)018<0982:PAAOAI>2.0.CO;2.

Dohan, K., and R. Davis. 2011. Mixing in the transition layer during two storm events. Journal of Physical Oceanography 41:42–66, https://doi.org/10.1175/2010JPO4253.1.

Egbert, G., and S. Erofeeva. 2002. Efficient inverse modeling of barotropic ocean tides. Journal of Atmospheric and Oceanic Technology 19(2):183–204, https://doi.org/10.1175/1520-0426(2002)019​<0183:EIMOBO>2.0.CO;2.

Ekman, V.W. 1905. On the influence of the Earth’s rotation on ocean-currents. Arkiv för matematik, astronomi och fysik 2(11):1–52.

Elipot, S., R. Lumpkin, and G. Prieto. 2010. Modification of inertial oscillations by the mesoscale eddy field. Journal of Geophysical Research 115, C09010, https://doi.org/​10.1029/2009JC005679.

Ferrari, R., and G. Boccaletti. 2004. Eddy-mixed layer interactions in the ocean. Oceanography 17(3):12–21, https://doi.org/​10.5670/oceanog.2004.26.

Fousiya, T., A. Parekh, and C. Gnanaseelan. 2015. Interannual variability of upper ocean stratification in Bay of Bengal: Observational and modeling aspects. Theoretical and Applied Climatology 1–17, https://doi.org/10.1007/s00704-015-1574-z.

Gill, A.E. 1982. Atmosphere-Ocean Dynamics. Academic Press, 662 pp.

Gill, A.E. 1984. On the behavior of internal waves in the wake of a storm. Journal of Physical Oceanography 14:1,129–1,151, https://doi.org/10.1175/​1520-0485(1984)014<1129:OTBOIW>2.0.CO;2.

Gordon, A.L., E.L. Shroyer, A. Mahadevan, D. Sengupta, and M. Freilich. 2016. Bay of Bengal: 2013 northeast monsoon upper-ocean circulation. Oceanography 29(2):82–91, https://doi.org/10.5670/oceanog.2016.41.

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.

Hebert, H., and J. Moum. 1994. Decay of a near-inertial wave. Journal of Physical Oceanography 24(11)2,334–2,351, https://doi.org/10.1175/1520-0485(1994)​024<2334:DOANIW>2.0.CO;2.

Hormann, V., L.R. Centurioni, A. Mahadevan, S. Essink, E.A. D’Asaro, and B. Praveen Kumar. 2016. Variability of near-surface circulation and sea surface salinity observed from Lagrangian drifters in the northern Bay of Bengal during the waning 2015 southwest monsoon. Oceanography 29(2):124–133, https://doi.org/10.5670/oceanog.2016.45.

Jochum, M., B.P. Briegleb, G. Danabasoglu, W.G. Large, N.J. Norton, S.R. Jayne, M.H. Alford, and F.O. Bryan. 2013. The impact of oceanic near-inertial waves on climate. Journal of Climate 26(9):2,833–2,844, https://doi.org/​10.1175/JCLI-D-12-00181.1.

Johnston, T.M.S., and D.L. Rudnick. 2009. Observations of the transition layer. Journal of Physical Oceanography 39:780–797, https://doi.org/10.1175/2008JPO3824.1.

Johnston, T.M.S., D.L. Rudnick, and S.M. Kelly. 2015. Standing internal tides in the Tasman Sea observed by gliders. Journal of Physical Oceanography 45(11):2,715–2,737, https://doi.org/10.1175/JPO-D-15-0038.1.

Johnston, T.M.S., D.L. Rudnick, and E. Pallàs-Sanz. 2011. Elevated mixing at a front. Journal of Geophysical Research 116, C11033, https://doi.org/10.1029/2011JC007192.

Kalnay, E.M., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J. Woollen, and others. 1996. The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77:437–471, https://doi.org/​10.1175/1520-0477(1996)077​<0437:TNYRP>​2.0.CO;2.

Kunze, E. 1985. Near-inertial wave propagation in geostrophic shear. Journal of Physical Oceanography 15:544–565, https://doi.org/10.1175/​1520-0485(1985)015<0544:NIWPIG>2.0.CO;2.

Large, W.G., J.C. McWilliams, and S.C. Doney. 1994. Oceanic vertical mixing: A review and model with a nonlocal boundary layer parameterization. Reviews of Geophysics 32:363–403, https://doi.org/10.1029/94RG01872.

Large, W. G., and S. Pond. 1981. Open ocean momentum flux measurements in moderate to strong winds. Journal of Physical Oceanography 11:324–336.

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.

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.

Majumder, S., A. Tandon, D.L. Rudnick, and J.T. Farrar. 2015. Near-inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the Arabian Sea during the monsoons. Journal of Geophysical Research 120(9):6,492–6,507, https://doi.org/​10.1002/2014JC010198.

Marchesiello, P., J.C. McWilliams, and A. Shchepetkin. 2001. Open boundary conditions for long-term integration of regional oceanic models. Ocean Modelling 3:1–20, https://doi.org/10.1016/S1463-5003(00)00013-5.

Metzger, E.J., O.M. Smedstad, P.G. Thoppil, H.E. Hurlburt, J.A. Cummings, A.J. Wallcraft, L. Zamudio, D.S. Franklin, P.G. Posey, M.W. Phelps, and others. 2014. US Navy Operational Global Ocean and Arctic Ice Prediction Systems. Oceanography 27(3):32–43, https://doi.org/​10.5670/oceanog.2014.66.

Moehlis, J., and S.G. Llewellyn-Smith. 2001. Radiation of mixed layer near-inertial oscillations into the ocean interior. Journal of Physical Oceanography 31:1,550–1,560, https://doi.org/​10.1175/1520-0485(2001)031<1550:ROMLNI>​2.0.CO;2.

Munk, W. 1981. Internal waves and small scale processes. Pp. 264–291 in Evolution of Physical Oceanography. B. Warren, and C. Wunsch, eds, The MIT Press.

Parekh, A., J.S. Chowdary, O. Sayantani, and T.S. Fousiya. 2015. Tropical Indian Ocean surface salinity bias in Climate Forecasting System coupled models and the role of upper ocean processes. Climate Dynamics 46:2,403–2,422, https://doi.org/10.1007/s00382-015-2709-8.

Park, J.J., K. Kim, and R.W. Schmitt. 2009. Global distribution of the decay timescale of mixed layer inertial motions observed by satellite-tracked drifters. Journal of Geophysical Research 114, C11010, https://doi.org/10.1029/2008JC005216.

Plueddemann, A.J., and J.T. Farrar. 2006. Observations and models of the energy flux to mixed-layer inertial currents. Deep Sea Research Part II 53:5–30, https://doi.org/10.1016/​j.dsr2.2005.10.017.

Rudnick, D.L., and S.T. Cole. 2011. On sampling the ocean using underwater gliders. Journal of Geophysical Research 116, C08010, https://doi.org/10.1029/2010JC006849.

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.

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.

Simmons, H.L., and M.H. Alford, 2012. Simulating the long-range swell of internal waves generated by ocean storms. Oceanography 25(2):30–41, https://doi.org/10.5670/oceanog.2012.39.

Tandon, A., E.A. D’Asaro, K.M. Stafford, D. Sengupta, M. Ravichandran, M. Baumgartner, R. Venkatesan, and T. Paluszkiewicz. 2016. Technological advancements in observing the upper ocean in the Bay of Bengal: Education and capacity building. Oceanography 29(2):242–253, https://doi.org/10.5670/oceanog.2016.56.

Thangaprakash, V.P., M.S. Girishkumar, K. Suprit, N. Suresh Kumar, D. Chaudhuri, K. Dinesh, A. Kumar, S. Shivaprasad, M. Ravichandran, J.T. Farrar, and others. 2016. What controls seasonal evolution of sea surface temperature in the Bay of Bengal? Mixed layer heat budget analysis using moored buoy observations along 90°E. Oceanography 29(2):202–213, https://doi.org/10.5670/oceanog.2016.52.

Venkatesan, R., V.R. Shamji, G. Latha, S. Mathew, R. Rao, A. Muthiah, and M. Atmanand. 2013. In situ ocean subsurface time-series measurements from OMNI buoy network in the Bay of Bengal. Current Science 104(9):1,166–1,177.

Weller, R.A. 1982. The relation of near-inertial motions observed in the mixed layer during the JASIN (1978) experiment to the local wind stress and to the quasi-geostrophic flow field. Journal of Physical Oceanography 12:1,122–1,136, https://doi.org/​10.1175/1520-0485(1982)012​<1122:TRONIM>​2.0.CO;2

Weller, R.A., S. Majumder, and A. Tandon. 2014. Diurnal restratification events in the southeast Pacific trade wind regime. Journal of Physical Oceanography 44(9):2,569–2,587, https://doi.org/10.1175/JPO-D-14-0026.1.

Wijesekera, H.W., E. Shroyer, A. Tandon, M. Ravichandran, D. Sengupta, P. Jinadas, H.J.S. Fernando, N. Agrawal, K. Arulanathan, G.S. Bhat, and others. In press. ASIRI: An Ocean-Atmosphere Initiative for Bay of Bengal. Bulletin of the American Meteorological Society, https://doi.org/​10.1175/BAMS-D-14-00197.1.

Young, W.R., and M. Ben Jelloul. 1997. Propagation of near-inertial oscillations through ageostrophic flow. Journal of Marine Research 55:735–766, https://doi.org/10.1357/0022240973224283.

Copyright & Usage

This is an open access article made available under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution, and reproduction in any medium or format as long as users cite the materials appropriately, provide a link to the Creative Commons license, and indicate the changes that were made to the original content. Images, animations, videos, or other third-party material used in articles are included in the Creative Commons license unless indicated otherwise in a credit line to the material. If the material is not included in the article’s Creative Commons license, users will need to obtain permission directly from the license holder to reproduce the material.