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 run off concentrated in the northern part of the

bay

Mahadevan, A

Paluszkiewicz, T

Ravichandran, M

Sengupta, D

Tandon, A

Ministry of Earth Sciences, Government of India

CITATION
Mahadevan, A., T. Paluszkiewicz, M. Ravichandran, D. Sengupta, and A. Tandon. 2016. 
Introduction to the special issue on the Bay of Bengal: From monsoons to mixing. 
Oceanography 29(2):14–17, http://dx.doi.org/10.5670/oceanog.2016.34.
DOI
http://dx.doi.org/10.5670/oceanog.2016.34
COPYRIGHT
This article has been published in Oceanography, Volume 29, Number 2, a quarterly 
journal of The Oceanography Society. Copyright 2016 by The Oceanography Society. 
All rights reserved. 
USAGE
Permission is granted to copy this article for use in teaching and research. 
Republication, systematic reproduction, or collective redistribution of any portion of 
this article by photocopy machine, reposting, or other means is permitted only with the 
approval of The Oceanography Society. Send all correspondence to: info@tos.org or 
The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA.
OceanographyTHE OFFICIAL MAGAZINE OF THE OCEANOGRAPHY SOCIETY
DOWNLOADED FROM HTTP://TOS.ORG/OCEANOGRAPHY
Oceanography |  Vol.29, No.214
Bay of Bengal: 
From Monsoons to Mixing
FROM THE GUEST EDITORS
By Amala Mahadevan, Theresa Paluszkiewicz, M. Ravichandran, 
Debasis Sengupta, and Amit Tandon
Introduction to the Special Issue on the 
The Bay of Bengal has a surprisingly large 
influence on the world. It nurtures the 
South Asian summer monsoon, a tre-
mendous ocean-atmosphere-land phe-
nomenon that delivers freshwater to more 
than a third of the human population 
on this planet. During summer, south-
westerly 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 dis-
persing the terrestrial freshwater runoff 
concentrated in the northern part of the 
bay. This freshwater impacts the ocean’s 
structure, circulation, and biogeochemis-
try in numerous ways and, through mod-
ification 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 suste-
nance of the monsoon. 
The critical dependence of an agrar-
ian economy food production on rainfall 
makes the prediction of seasonal mon-
soonal variability of paramount impor-
tance to many countries in South Asia. 
Global climate models are challenged in 
representing the South Asian monsoon 
and exhibit large biases over the north-
ern Indian Ocean. Although total rain-
fall over India during the summer mon-
soon varies by just about 10% from year 
to year, the spatiotemporal variability in 
summer precipitation is large, difficult 
to predict, and has far-reaching impli-
cations for farmers and water manage-
ment. Intraseasonal wet and dry spells are 
associated with active and break periods 
that develop and propagate north-
ward over the Bay of Bengal (Figure  1). 
Ocean-atmosphere coupled models have 
greater skill in representing and predict-
ing the prominent intraseasonal oscil-
lations (ISOs) than stand-alone atmo-
spheric models. This highlights the 
importance of the feedback between the 
upper-ocean structure and air-sea inter-
action on ISO time scales. 
The introduction of coupled climate 
models has brought the ocean’s role in 
ISOs to light and has led to considerable 
improvement in monsoon prediction. 
But further work is needed for forecasting 
the short- to medium-range variability of 
monsoon rainfall. The skill of ISO predic-
tion is still modest compared to its poten-
tial, giving reason to strive for longer lead 
times to improve forecast accuracy. The 
ocean component of coupled models has 
particular difficulty in representing the 
Bay of Bengal as compared to other ocean 
regions, likely because the bay is differ-
ent from other oceans in many ways. The 
Bay of Bengal and surrounding land areas 
receive the heaviest rainfall in the world, 
and the bay’s surface waters are the fresh-
est of any tropical ocean. The strong, sea-
sonally reversing winds affect the circu-
lation, with cyclones developing over 
the bay’s warm waters during both the 
pre- and post-monsoon seasons. With 
Oceanography  |  June 2016 15
the Indian subcontinent to its north, the 
ocean lacks a place where dense waters 
can form from surface cooling and cause 
overturning. The freshwater at the north-
ern end of the bay offsets the effect of 
cooling by northeasterly winds and sup-
presses vertical mixing. 
Better characterizations of the struc-
ture and processes in the Bay of Bengal 
are needed to improve the representation 
of its role in the monsoons. This was the 
primary motivation for an international 
collaboration between Office of Naval 
Research (ONR)-sponsored scientists 
from the United States and Ministry of 
Earth Sciences (MoES)-sponsored scien-
tists in India, and further, between ONR-
sponsored scientists and colleagues from 
the National Aquatic Research Agency 
(NARA) in Sri  Lanka. These scientific 
teams set out to improve understanding 
of the Bay of Bengal through observa-
tions and process modeling studies. Three 
concurrent programs were launched: 
ONR’s Air-Sea Interactions Regional 
Initiative (ASIRI), MoES’s Ocean Mixing 
and Monsoon (OMM) project under the 
umbrella of India’s Monsoon Mission, 
and NARA’s Coastal Circulation Studies 
in collaboration with US Naval Research 
Laboratory scientists under the Effects of 
Bay of Bengal Freshwater Flux on Indian 
Ocean Monsoon (EBOB) study. This set 
of complementary and coordinated pro-
grams brought an international team 
of scientists together to better under-
stand the Bay of Bengal’s processes 
and role in air-sea interaction, and its 
circulation and transport.
These programs led to several obser-
vational campaigns that have provided a 
wealth of new understanding about the 
bay. Modeling studies were used to syn-
thesize the information collected, to quan-
tify transports, and to extend our under-
standing of processes at work in the bay. 
The observations began in 2013 with two 
US Research Vessel (R/V) Roger Revelle 
cruises to the north and south bay during 
November–December, and a cruise of 
the Indian Ocean Research Vessel (ORV) 
Sagar Nidhi to the north bay (Figure 2). 
Six current moorings were deployed 
from R/V Roger Revelle in the south cen-
tral Bay of Bengal in December 2013 for 
a period of 20 months. In 2014, Revelle 
sailed from Chennai, India, and sam-
pled the central Bay of Bengal during 
June, the early part of the summer mon-
soon; later in the season, Sagar Nidhi 
made observations in the northern Bay of 
Bengal in August–September. The sum-
mer monsoon of 2015 was the intensive 
observational period, when Revelle and 
Sagar Nidhi conducted joint research 
in the northern bay. A highly instru-
mented surface mooring was deployed 
in December 2014 from Sagar Nidhi 
and retrieved in February 2016. Gliders 
surveyed the southern and central bay, 
moorings were used to profile vertically, 
and surface drifters tracked circulation. 
Satellite data from NASA’s Aquarius and 
SMAP (Soil Moisture Active Passive) 
missions and the Indian SARAL (Satellite 
FIGURE 1. Illustration of the atmospheric intraseasonal oscillation (ISO) pat-
tern that propagates over the Bay of Bengal during the southwesterly mon-
soon season. Color in the ocean represents salinity, and was produced using 
the HYbrid Coordinate Ocean Model, or HYCOM (Cummings and Smedstad, 
2013, http://dx.doi.org/10.1007/978-3-642-35088-7_13). Schematic drawing 
by Emily Shroyer and produced by David Reinert, Oregon State University.
Oceanography |  Vol.29, No.216
FIGURE 2. Map of the Bay of Bengal. Colored lines show 2013–2015 tracks of R/V Roger Revelle (RR, solid lines) and ORV Sagar Nidhi 
(SN, dashed lines) during cruises for the Air-Sea Interactions Regional Initiative (ASIRI), the Ocean Mixing and Monsoon (OMM) project, 
and Effects of Bay of Bengal Freshwater Flux on Indian Ocean Monsoon (EBOB) program. Black and white symbols indicate positions 
of various surface and subsurface moorings deployed or utilized as part of the research campaigns. NRL = Naval Research Laboratory. 
OMNI = Ocean Moored Network of buoys for the Northern Indian Ocean. PIES = Pressure Inverted Echo Sounder. RAMA = Research 
Moored Array for African-Asian-Australian Monsoon Analysis and Prediction. WHOI = Woods Hole Oceanographic Institution. White dotted 
lines are tracks of gliders. UW sea = University of Washington Seaglider. SIO spray = Scripps Institution of Oceangraphy spray glider. Major 
riversheds are named along with ports of call. Thin black lines demarcate national Exclusive Economic Zones. Map created by Gualtiero 
Spiro Jaeger, MIT/WHOI Joint Program.
RR 1513
RR 1405
RR 13
17R
R 13
16
RR 1407
RR 15
11
RR 1318
SN
 08
2
SN
 10
0
SN 
088
2013
2014
2015
ORV Sagar Nidhi
R/V Roger Revelle
cr
ui
se
 tr
ac
ks
SI
O
 s
pr
ay
UW sea
gliders
0 500 km
moorings
RAMA
NRL
PIES
OMNIWHOI
Chennai
Colombo
Ganges-Brahm
apu
tra
Trincomalee
Irrawaddy
3000
 m
1000 m2000 m
Oceanography  |  June 2016 17
with ARgos and ALtiKa) and INSAT-3D 
missions, combined with informa-
tion from other satellites, were analyzed 
and used to provide context to the stud-
ies. The Sri Lankan R/V Samudrika sur-
veyed coastal currents, hydrophysical 
fields, and mixing rates off the south 
and east coasts of Sri Lanka. Several cur-
rent moorings and inverted echosound-
ers were also deployed east and south of 
Sri Lanka to examine seasonally varying 
boundary currents. 
The collaborative effort led to sev-
eral technical and observational break-
throughs. The ship surveys, including 
a multi-vessel international operation, 
provided unprecedented horizontal and 
vertical resolution of the Bay of Bengal. 
New technologies for autonomous sam-
pling contributed a detailed view of the 
near-surface structure. A year-long time 
series of surface and upper-ocean vari-
ables, at high vertical resolution, provided 
data that accurately quantifies air-sea 
fluxes of heat, moisture, and momentum. 
A glider deployed from Sagar Nidhi sur-
veyed a small region of the ocean around 
the mooring in the period September to 
December 2015. In addition, moorings 
in the southern bay recorded subsurface 
currents that were hitherto unseen. 
The sharing of scientific methodology, 
findings, and data was achieved through 
several forums—summer schools, joint 
workshops and training activities, and 
special sessions at international confer-
ences. These efforts thrived through col-
laboration between the countries, mul-
tiple institutions, numerous scientists, 
students, technicians, and ships’ crew. 
Some of the findings that have emerged 
from this fruitful endeavor over the past 
five years are presented in this special 
issue of Oceanography. More detailed 
findings will continue to be reported in 
other journals. 
The articles that follow present a new 
understanding of the Bay of Bengal, the 
observed air-sea exchange of heat and 
momentum, the structure of salinity 
and temperature, and rates of mixing, 
transports, and circulation. Sustained 
measurements at the sea surface, as well 
as satellite data, enhance understand-
ing of air-sea interaction, as shown 
here. The biases of climate models in the 
bay suggest the need to represent pro-
cesses crucial for maintaining the fresh-
water stratification. 
Abundant freshwater makes the Bay 
of Bengal unique, and more akin to the 
Arctic than to other tropical regions. 
Strong lateral salinity gradients lead to a 
multitude of fronts that give rise to a host 
of instabilities over a range of spatial and 
temporal scales. Strong vertical gradi-
ents and fine-scale salinity and tempera-
ture structures are reflected in the layered 
density stratification. Freshwater stratifi-
cation thereby influences how the tem-
perature of the upper ocean responds 
to penetrative solar radiation and latent 
and longwave cooling on seasonal and 
diurnal time scales. 
Microstructure measurements reveal 
that rates of diapycnal mixing beneath 
the surface mixed layer are very weak, 
even in the presence of the southwest 
monsoon winds, and even though iner-
tial motions and internal tides pro-
vide a strong source of vertical shear. 
Biogeochemical properties also respond 
to the stratification and have strong verti-
cal gradients, to which the phytoplankton 
and faunal communities respond.
At a larger scale, the water mass char-
acteristics of the Bay of Bengal differ sig-
nificantly from those of the Arabian Sea, 
and the bay’s properties are set by both 
the rate of freshwater input and the cir-
culation that exchanges salinity between 
the Bay of Bengal and the tropical Indian 
Ocean. The circulation and fresh water 
transport are estimated from drift-
ers, moorings, gliders, and models. This 
large-scale circulation affects the salin-
ity gradients and the host of smaller-scale 
instabilities that set the stratification and 
fine-scale structure in the bay.
The studies described in this spe-
cial issue give the world a first high- 
resolution view of the Bay of Bengal’s 
ocean structure. But these initial stud-
ies raise new questions and challenges. 
One that looms in importance is that 
of understanding the coupled ocean- 
atmosphere feedback on the subseasonal, 
10–60 day time scale. This interaction lies 
at the heart of the active and break cycles 
that constitute monsoon variability and 
that beg understanding for improving 
weather predictions. 
ACKNOWLEDGMENT
We are grateful to MIT/WHOI Joint Program graduate 
student Gualtiero Spiro Jaeger for his invaluable edi-
torial and graphical assistance in putting together this 
special issue of Oceanography.
AUTHORS
Amala Mahadevan (amala@whoi.edu) is Senior 
Scientist, Woods Hole Oceanographic Institution, 
Woods Hole, MA, USA. Theresa Paluszkiewicz 
is Program Officer, Office of Naval Research, 
Arlington, VA, USA. M. Ravichandran is Head, 
Observations and Modeling at the Indian 
National Centre for Ocean Information Services, 
Hyderabad, India. Debasis Sengupta is Professor, 
Centre for Atmospheric and Oceanic Sciences, 
Indian Institute of Science, Bangalore, India. 
Amit Tandon is Professor of Mechanical Engineering, 
University of Massachusetts Dartmouth, North 
Dartmouth, MA, USA.
ARTICLE CITATION
Mahadevan, A., T. Paluszkiewicz, M. Ravichandran, 
D. Sengupta, and A. Tandon. 2016. Introduction 
to the special issue on the Bay of Bengal: From 
monsoons to mixing. Oceanography 29(2):14–17, 
http://dx.doi.org/10.5670/oceanog.2016.34.


Bay of Bengal: From monsoons to mixing

http://moeseprints.incois.gov.in/4255/1/Bay%20of%20Bengal%20From%20Monsoons%20to%20Mixing.pdf

Bay of Bengal: From monsoons to mixing

Abstract

Similar works

Full text

Available Versions

Ministry of Earth Sciences, Government of India