S. Prerna, Abhisek Chatterjee, A. Mukherjee, M. Ravichandran and
S. S. C. Shenoi
Source
Journal of Earth System Sciences, Vol 128
Abstract
Direct current measurements observed from Acoustic Doppler Current Profilers in the equatorial Indian Ocean (EIO) and solutions from
an ocean general circulation model are investigated to understand the
dynamics of the Wyrtki jet. These jets are usually described as semi-
annual direct wind forced zonal currents along the central and eastern
equatorial Indian Ocean. We show that both, spring and fall, Wyrtki
jets show predominant semiannual spectral peaks, but significant intraseasonal energy is evident during spring in the central and eastern
EIO. We find that, for the semiannual band, there is strong spectral
coherence between the overlying winds and the currents in the central
EIO, but no coherency is observed in the eastern part of the EIO.
Moreover, for the intraseasonal band, strong coherency between the
winds and currents is evident. During spring, intraseasonal currents
induced by the MJO superimpose constructively with semiannual currents and thus intensify the strength of the spring Wyrtki jet. Also,
the atmospheric intraseasonal variability accounts for the interannual
variabilities observed in spring Wyrtki jets.
Paper No
2
Publication ID : 764 & Year : 2019
Title
Annihilation of the Somali
upwelling system during summer monsoon
Authors
Abhisek Chatterjee, Praveen Kumar B, Satya Prakash and Prerna Singh
Somali upwelling system during northern summer is believed to be the largest upwelling region in the
Indian Ocean and has motivated some of the early studies on the Indian Ocean. Here we present results
from observations and ocean model to show that the upwelling along the Somali coast is limited to
the early phase of the summer monsoon and later primarily limited to the eddy dominated flows in the
northern and some extent in the southern part of the coast. Major part of the Somali coast (~60% of the
entire coastal length) shows prominent downwelling features driven by offshore negative windstress
curl and subsurface entrainment mixing. Further, we show that the surface cooling of coastal waters
are dominantly driven by subsurface entrainment and surface heat fluxes. These findings not only
augment the existing knowledge of the Somali upwelling system, but also have serious implications on
the regional climate. Most importantly, our analysis underscores the use of alongshore winds only to
project future (climate driven) changes in the upwelling intensity along this coast.
Paper No
3
Publication ID : 654 & Year : 2017
Title
Dynamics of Andaman Sea circulation and its role in
connecting the equatorial Indian Ocean to the Bay of
Bengal
Authors
Abhisek Chatterjee, D. Shankar, J. P. McCreary Jr., P. N. Vinayachandran, and A. Mukherjee
Source
Journal of Geophysical Research, doi: 10.1002/2016JC012300
Abstract
Circulation in the Bay of Bengal (BoB) is driven not only by local winds, but are also strongly forced by the reflection of equatorial Kelvin waves (EKWs) from the eastern boundary of the Indian Ocean. The equatorial influence attains its peak during the monsoon transition period when strong eastward currents force the strong EKWs along the equator. The Andaman Sea, lying between the Andaman and Nicobar island chains to its west and Indonesia, Thailand, and Myanmar to the south, east, and north, is connected to the equatorial ocean and the BoB by three primary passages, the southern (6N), middle (10N), and northern (15N) channels. We use ocean circulation models, together with satellite altimeter data, to study the pathways by which equatorial signals pass through the Andaman Sea to the BoB and associated dynamical interactions in the process. The mean coastal circulation within the Andaman Sea and around the islands is primarily driven by equatorial forcing, with the local winds forcing a weak sea-level signal. On the other hand, the current forced by local winds is comparable to that forced remotely from the equator. Our results suggest that the Andaman and Nicobar Islands not only influence the circulation within the Andaman Sea, but also significantly alter the circulation in the interior bay and along the east coast of India, implying that they need to be represented accurately in numerical models of the Indian Ocean.
Paper No
4
Publication ID : 657 & Year : 2017
Title
Numerical simulation of the observed near⿐surface East India
Coastal Current on the continental slope
Authors
A. Mukherjee, D. Shankar, Abhisek Chatterjee, P. N. Vinayachandran
Source
Climate Dynamics
Abstract
We simulate the East India Coastal Current
(EICC) using two numerical models (resolution 0.1 ÿ 0.1 ),
an oceanic general circulation model (OGCM) called Modu-
lar Ocean Model and a simpler, linear, continuously strati-
fied (LCS) model, and compare the simulated current with
observations from moorings equipped with acoustic Doppler
current profilers deployed on the continental slope in the
western Bay of Bengal (BoB). We also carry out numeri-
cal experiments to analyse the processes. Both models
simulate well the annual cycle of the EICC, but the per-
formance degrades for the intra-annual and intraseasonal
components. In a model-resolution experiment, both mod-
els (run at a coarser resolution of 0.25 ÿ 0.25 ) simulate
well the currents in the equatorial Indian Ocean (EIO),
but the performance of the high-resolution LCS model as
well as the coarse-resolution OGCM, which is good in the
EICC regime, degrades in the eastern and northern BoB. An
experiment on forcing mechanisms shows that the annual
EICC is largely forced by the local alongshore winds in the
western BoB and remote forcing due to Ekman pumping
over the BoB, but forcing from the EIO has a strong impact
on the intra-annual EICC. At intraseasonal periods, local
(equatorial) forcing dominates in the south (north) because the Kelvin wave propagates equatorward in the western BoB.
A stratification experiment with the LCS model shows that
changing the background stratification from EIO to BoB
leads to a stronger surface EICC owing to strong coupling
of higher order vertical modes with wind forcing for the
BoB profiles. These high-order modes, which lead to energy
propagating down into the ocean in the form of beams, are
important only for the current and do not contribute signifi-
cantly to the sea level.
Paper No
5
Publication ID : 608 & Year : 2016
Title
Evidence for the existence of Persian Gulf Water and Red Sea Water in the Bay of
Bengal
Authors
V. Jain, D. Shankar, P. N. Vinayachandran, A. Kankonkar, Abhisek Chatterjee,
P. Amol, A. M. Almeida, G. S. Michael, A. Mukherjee, M. Chatterjee, R. Farnan-
des, R. Luis, A. Kamble, A. k. Hegde, S. Chatterjee, U. Das, C. P. Neema
Source
Climate Dynamics, DOI 10.1007/s00382-016-3259-4
Abstract
The high-salinity water masses that originate
in the North Indian Ocean are Arabian Sea High-Salinity
Water (ASHSW), Persian Gulf Water (PGW), and Red Sea
Water (RSW). Among them, only ASHSW has been shown
to exist in the Bay of Bengal. We use CTD data from recent
cruises to show that PGW and RSW also exist in the bay.
The presence of RSW is marked by a deviation of the salin-
ity vertical profile from a fitted curve at depths ranging
from 500 to 1000 m; this deviation, though small (of the
order of ~0.005 psu and therefore comparable to the CTD
accuracy of 0.003 psu), is an order of magnitude larger
than the ~0.0003 psu fluctuations associated with the back-
ground turbulence or instrument noise in this depth regime,
allowing us to infer the existence of RSW throughout the
bay. PGW is marked by the presence of a salinity maximum at 200⿿450 m; in the southwestern bay, PGW can be dis-
tinguished from the salinity maximum due to ASHSW
because of the intervening Arabian Sea Salinity Minimum.
This salinity minimum and the maximum associated with
ASHSW disappear east and north of the south-central bay
(85°E, 8°N) owing to mixing between the fresher surface
waters that are native to the bay (Bay of Bengal Water or
BBW) with the high-salinity ASHSW. Hence, ASHSW is
not seen as a distinct water mass in the northern and eastern
bay and the maximum salinity over most of the bay is asso-
ciated with PGW. The surface water over most of the bay is
therefore a mixture of ASHSW and the low-salinity BBW.
As a corollary, we can also infer that the weak oxygen
peak seen within the oxygen-minimum zone in the bay at
a depth of 250⿿400 m is associated with PGW. The hydro-
graphic data also show that these three high-salinity water
masses are advected into the bay by the Summer Monsoon
Current, which is seen to be a deep current extending to
1000 m. These deep currents extend into the northern bay
as well, providing a mechanism for spreading ASHSW,
PGW, and RSW throughout the bay.
Paper No
6
Publication ID : 575 & Year : 2015
Title
Inhibition of mixed-layer deepening during winter in the northeastern Arabian Sea by the West India Coastal Current, Climate Dynamics
Authors
D. Shankar, R. Remya, P.N. Vinayachandran, Abhisek Chatterjee, and A. Behera
Source
Climate Dynamics, 10.1007/s00382-015-2888-3
Abstract
Though the deep mixed layers (MLs) that form in the northeastern Arabian Sea (NEAS) during the winter monsoon (November⿿February) have been attributed to convective mixing driven by dry, cool northeasterly winds from the Indian subcontinent, data show that the deepest MLs occur in the northern NEAS and the maxima of latent-heat and net heat fluxes in the southern NEAS. We use an oceanic general circulation model to show that the deep MLs in the NEAS extend up to ⿼ 20 N till the end of December, but are restricted poleward of ⿼ 22 N (⿼ 23 N) in January (February). This progressive restriction of the deep mixed layers within the NEAS is due to
poleward advection of water of lower salinity by the West India Coastal Current (WICC). The deep MLs are sustained till February in the northern NEAS because convective mixing deepens the ML before the waters of lower salinity reach this region and the wind stirring and convective overturning generate sufficient turbulent energy for the ML to maintain the depth attained in January. Though the atmospheric fluxes tend to cool the ML in the southern NEAS, this cooling is countered by the warming due to horizontal advection. Likewise, the cooling due to entrainment, which continues in the southern NEAS even as the ML shallows during January⿿February, is almost cancelled by the warming caused by a downwelling vertical velocity field. Therefore, the SST changes very little during December⿿February even as the ML shallows dramatically in the southern NEAS. These deep MLs of the NEAS also preclude a strong intraseasonal response to the intraseasonal variability in the fluxes. This role of horizontal advection implies that the ML depth in the NEAS is determined by an interplay of physical processes that are forced differently. The convective
mixing depends on processes that are local to the region, but the advection is due to the WICC, whose seasonal cycle is primarily forced by remote winds.
Paper No
7
Publication ID : 481 & Year : 2013
Title
Yanai Waves in the Western Equatorial Indian Ocean
Authors
Abhisek Chatterjee, D. Shankar, J. P. McCreary Jr. and P. N. Vinayachandran
Source
Journal of Geophysical Research, 118
, 15561570, doi:10.1002/jgrc.20121
Abstract
Observations and models have shown the presence of intraseasonal fluctuations in 2030-day and 1020-day bands in the equatorial Indian Ocean west of 60E (WEIO). Their spatial and temporal structures characterize them as Yanai waves, which we label low-frequency (LFYW) and high-frequency (HFYW) Yanai waves, respectively. We explore the dynamics of these intraseasonal signals, using an ocean general circulation model (MOM) and a linear, continuously stratified (LCS) model. Yanai waves are forced by the meridional wind τy everywhere in the WEIO most strongly during the monsoon seasons. They are forced both directly in the interior ocean and by reflection of the interior response from the western boundary; interference between the interior and boundary responses results in a complex surface pattern that propagates eastward and has nodes. Yanai waves are also forced by instabilities primarily during June/July in a region offshore from the western boundary (5255E). At that time, eddies, generated by barotropic instability of the Southern Gyre, are advected southward to the equator. There, they generate a westward-propagating, across-equatorial flow field, veq, with a wavenumber/frequency spectrum that fits the dispersion relation of a number of Yanai waves, and these waves are efficiently excited. Typically, Yanai waves associated with several baroclinic modes are excited by both wind and eddy forcing, and typically they superpose to create beams that carry energy vertically and eastward along ray paths. The same processes generate LFYWs and HFYWS, and hence their responses are similar; differences are traceable to the property that HFYWs have longer wavelengths than LFYWs for each baroclinic mode.
Paper No
8
Publication ID : 483 & Year : 2013
Title
Tidal Variations in the Sundarbans Estuarine
System, India
Authors
M. Chatterjee, D. Shankar, G.K. Sen, P. Sanyal, D. Sundar, G.S. Michael, Abhisek Chatterjee, P. Amol, D. Mukherjee, K. Suprit, A. Mukherjee, V. Vijith, S. Chatterjee, A. Basu, M. Das, S. Chakraborti, A. Kalla, S. K. Mishra, S. Mukhopadhyay, G. Mandal, and K. Sarkar
Source
Journal of Earth System Science, Vol-122 (4), 899-933
Abstract
Situated in the eastern coastal state of West Bengal, the Sundarbans Estuarine System (SES) is Indias
largest monsoonal, macro-tidal delta-front estuarine system. It comprises the southernmost part of the
Indian portion of the GangaBrahmaputra delta bordering the Bay of Bengal. The Sundarbans Estuar-
ine Programme (SEP), conducted during 1821 March 2011 (the Equinoctial Spring Phase), was the first
comprehensive observational programme undertaken for the systematic monitoring of the tides within
the SES. The 30 observation stations, spread over more than 3600 km2 , covered the seven inner estuaries
of the SES (the Saptamukhi, Thakuran, Matla, Bidya, Gomdi, Harinbhanga, and Raimangal) and repre-
sented a wide range of estuarine and environmental conditions. At all stations, tidal water levels (every
15 minutes), salinity, water and air temperatures (hourly) were measured over the six tidal cycles. We
report the observed spatio-temporal variations of the tidal water level. The predominantly semi-diurnal
tides were observed to amplify northwards along each estuary, with the highest amplification observed
at Canning, situated about 98 km north of the seaface on the Matla. The first definite sign of decay of
the tide was observed only at Sahebkhali on the Raimangal, 108 km north of the seaface. The degree and
rates of amplification of the tide over the various estuarine stretches were not uniform and followed a
complex pattern. A least-squares harmonic analysis of the data performed with eight constituent bands
showed that the amplitude of the semi-diurnal band was an order of magnitude higher than that of the
other bands and it doubled from mouth to head. The diurnal band showed no such amplification, but
the amplitude of the 6-hourly and 4-hourly bands increased headward by a factor of over 4. Tide curves
for several stations displayed a tendency for the formation of double peaks at both high water (HW)
and low water (LW). One reason for these double-peaks was the HW/LW stands of the tide observed
at these stations. During a stand, the water level changes imperceptibly around high tide and low tide.
The existence of a stand at most locations is a key new finding of the SEP. We present an objective
criterion for identifying if a stand occurs at a station and show that the water level changed impercep-
tibly over durations ranging from 30 minutes to 2 hours during the tidal stands in the SES. The tidal
duration asymmetry observed at all stations was modified by the stand. Flow-dominant asymmetry was
observed at most locations, with ebb-dominant asymmetry being observed at a few locations over some
tidal cycles. The tidal asymmetry and stand have implications for human activity in the Sundarbans.
Paper No
9
Publication ID : 482 & Year : 2013
Title
A summer monsoon pump to keep the Bay of Bengal salty
Authors
P. N. Vinayachandran, D. Shankar, S. Vernekar, K. K. Sandeep, P. Amol, C. P. Neema and Abhisek Chatterjee
Source
Geophysical Research Letters, Vol. 40, 17771782, doi: 10.1002/grl.50274
Abstract
The Bay of Bengal receives a large influx of freshwater from precipitation and river discharge. Outflow of excess freshwater and inflow of saltier water is required to prevent the bay from freshening. Relatively fresh water flows out of the bay along its boundaries and inflow of saltier water occurs via the Summer Monsoon Current (SMC), which flows eastward from the Arabian Sea into the bay. This saltier water, however, slides under the lighter surface water of the bay. Maintaining the salt balance of the bay therefore demands upward mixing of this saltier, subsurface water. Here we show that an efficient mechanism for this mixing is provided by upward pumping of saltier water in several bursts during the summer monsoon along the meandering path of the SMC. Advection by currents can then take this saltier water into the rest of the basin, allowing the bay to stay salty despite a large net freshwater input.
Paper No
10
Publication ID : 480 & Year : 2012
Title
A new atlas of temperature and salinity for
the North Indian Ocean
Authors
Abhisek Chatterjee, D. Shankar, S. S. C. Shenoi, G. V. Reddy, G. S. Michael, M. Ravichandran, V. V.
Gopalkrishna, E. P. R. Rao, T. V. S. U. Bhaskar, V. N. Sanjeevan.
Source
Journal of Earth System Science, 121(3), 559-593, doi:10.1007/s12040-012-0191-9
Abstract
The most used temperature and salinity climatology for the world ocean, including the Indian Ocean, is the World Ocean Atlas (WOA) (Antonov et al 2006, 2010; Locarnini et al 2006, 2010) because of the vast amount of data used in its preparation. The WOA climatology does not, however, include all the available hydrographic data from the Indian Exclusive Economic Zone (EEZ), leading to the potential for improvement if the data from this region are included to prepare a new climatology. We use all the data that went into the preparation of the WOA (Antonov et al 2010; Locarnini et al 2010), but add considerable data from Indian sources, to prepare new annual, seasonal, and monthly climatologies of temperature and salinity for the Indian Ocean. The addition of data improves the climatology considerably in the Indian EEZ, the differences between the new North Indian Ocean Atlas (NIOA) and WOA being most significant in the Bay of Bengal, where the patchiness seen in WOA, an artifact of the sparsity of data, was eliminated in NIOA. The significance of the new climatology is that it presents a more stable climatological value for the temperature and salinity fields in the Indian EEZ.
