ESSO - Indian National Centre for Ocean Information Services
(An Autonomous Body under the Ministry of Earth Sciences, Govt. of India)
E-Publications
| Paper No | 1 | Publication ID : 738 & Year : 2019 |
| Title | Signature of La Niña in interannual variations of the East India Coastal Current during spring | |
| Authors | A. Mukherjee and B. K. Kalita | |
| Source | Climate Dynamics, Doi:10.1007/s00382-018-4601-9 | |
| Abstract | We have investigated interannual variations of the spring (February-April average) East India Coastal Current (EICC) magnitude between 2000 and 2018 using OSCAR (Ocean Surface Current Analysis Real-time) current and a linear, continuously
stratified (LCS) model. Interannual variability of the EICC shows significant decrease in magnitude during spring of 2000,
2008 and 2011, the years when high negative ONI (Oceanic Niño Index for sea surface temperature) value has been observed
due to dominance of strong La Niña events. Our LCS model also successfully simulates these interannual variability of the
spring average EICC between 2000 and 2018. We carried out numerical experiments using LCS model related to local and
remote forcing response on EICC. Dynamics of the EICC during spring are dominated by four different forcing processes;
local wind along east coast of India, remote forcing response from the eastern and northern boundary of the BoB including
islands, interior BoB and the Equatorial Indian Ocean (EIO). During El Niño and normal spring years, strong poleward inter-
annual EICC are due to very weak negligible (order of 0⿿5 cm s ⿿1 ) EICC from EIO remote response and in-phase poleward
EICC formation using other three forcings. However, during La Niña spring years, weak (order of 0⿿10 cm s ⿿1 ) poleward
interannual EICC are formed due to destructive interference between equatorward current (order of 10⿿25 cm s ⿿1 ) from EIO
forcing and in-phase poleward current from other three forcings. We have also found propagation of interannual upwelling
(downwelling) favorable Kelvin wave from EIO via eastern and western boundary of the BoB during spring in the El Niño
(La Niña) years. The interannual variations in the propagation of EIO Kelvin wave are associated with the changes in the
EIO zonal wind direction by climate mode like ENSO (El Niño⿿Southern Oscillation). | |
| Paper No | 2 | Publication ID : 737 & Year : 2018 |
| Title | Wyrtki Jets: Role of Intraseasonal forcing | |
| Authors | P. Singh, A. Chatterjee, A. Mukherjee, M. Ravichandran and S. S. C. Shenoi. | |
| Source | Journal of Earth System Science, 128:21, Doi:10.1007/s12040-018-1042-0 | |
| Abstract | Direct current measurements observed from the 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 semiannual direct wind forced zonal
currents along the central and eastern EIO. 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 a 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 Madden⿿Julian oscillation
(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 | 3 | Publication ID : 736 & Year : 2017 |
| Title | Observations of the sub-inertial, near-surface East India Coastal Current | |
| Authors | S. Mukhopadhyay, D. Shankar, S. G. Aparna and A. Mukherjee | |
| Source | Continental Shelf Research, Doi:http://dx.doi.org/10.1016/j.csr.2017.08.020 | |
| Abstract | We present surface current measurements made using two pairs of HF (high-frequency) radars deployed on the
east coast of India. The radar data, used in conjunction with data from acoustic Doppler current profiler (ADCP)
measurements on the shelf and slope off the Indian east coast, confirm that the East India Coastal Current (EICC)
flows poleward as a deep current during February⿿March. During the summer monsoon, when the EICC flows
poleward, and October⿿December, when the EICC flows equatorward, the current is shallow ( <40 m deep),
except towards the northern end of the coast. Data from Argo floats confirm a shallow mixed layer that leads to a
strong vertical shear off southeast India during October⿿December. A consequence of the strong stratification is
that the upward propagation of phase evident in the ADCP data does not always extend to the surface. Even
within the seasons, however, the poleward and equatorward flows show variability at periods of the order of
20⿿45 days, implying that the EICC direction is the same over the top ⿼100 m for short durations. The high
spatial resolution of the HF radar data brings out features at scales shorter than those resolved by the altimeter
and the high temporal resolution captures short bursts that are not captured in satellite-derived estimates of
surface currents. The radar data show that the EICC, which is a boundary current, leaves a strong imprint on the
current at the coast. Since the EICC is known to be affected significantly by remote forcing, this correlation
between the boundary and nearshore current implies the need to use large-domain models even for simulating
the nearshore current. Comparison with a simulation by a state-of-the-art Ocean General Circulation Model, run
at a resolution of 0.1 ° ÿ 0.1 ° , shows that the model is able to simulate only the low-frequency variability. | |
| Paper No | 4 | Publication ID : 735 & Year : 2017 |
| Title | Numerical simulation of the observed near-surface East India Coastal Current on the continental slope | |
| Authors | A. Mukherjee, D. Shankar, A. Chatterjee and P. N. Vinayachandran | |
| Source | Climate Dynamics , Doi:10.1007/s00382-017-3856-x | |
| Abstract | We simulate the East India Coastal Current (EICC) using two numerical models (resolution 0.1⿿ÿ0.1⿿), an oceanic general circulation model (OGCM) called Modular Ocean Model and a simpler, linear, continuously stratified (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 numerical experiments to analyse the processes. Both models simulate well the annual cycle of the EICC, but the performance degrades for the intra-annual and intraseasonal components. In a model-resolution experiment, both models (run at a coarser resolution of 0.25⿿ÿ0.25⿿) 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 significantly to the sea level. | |
| Paper No | 5 | Publication ID : 734 & Year : 2017 |
| Title | Dynamics of Andaman Sea circulation and its role in connecting the equatorial Indian Ocean to the Bay of Bengal | |
| Authors | A. Chatterjee, D. Shankar, J. P. McCreary, P. N. Vinayachandran and A. Mukherjee | |
| Source | Journal of Geophysical Research, 122, 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⿐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 (6°N), middle (10°N), and northern (15°N) 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⿐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 | 6 | Publication ID : 733 & 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, A. Chatterjee, P. Amol, A. M. Almeida, G. S. Michael, A. Mukherjee, M. Chatterjee, R. Fernandes, R. Luis, A. Kamble, A. K. Hegde, S. Chatterjee, U. Das and C. P. Neema | |
| Source | Climate Dynamics, 48 (9), 3207-3206 | |
| 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 | 7 | Publication ID : 732 & Year : 2014 |
| Title | Observed seasonal and intraseasonal variability of the East India Coastal Current on the continental slope | |
| Authors | A. Mukherjee, D. Shankar, V Fernando, P. Amol , S. G. Aparna, R. Fernandes, G. S. Michael, S. T. Khalap, N. P. Satelkar, Y. Agarvadekar, M. G. Gaonkar, A. P. Tari, A. Kankonkar and S. P. Vernekar. | |
| Source | Journal of Earth System Science, 123 (6), 1197-1232 | |
| Abstract | We present data from three acoustic Doppler current profilers (ADCPs) moored off Cuddalore (12 N),
Kakinada (16.5 N), and Gopalpur (19 N) on the continental slope of the western Bay of Bengal and one
mooring on the slope of the northern bay (89 E, 19 N; referred to as being located at Paradip). The data
were collected during May 2009 to March 2013 and the observations show that the seasonal cycle, which
includes the annual cycle, the semi-annual cycle, and a peak around 120 days, dominates the observed
variability of the East India Coastal Current (EICC). Spectral analysis suggests that the 120-day peak
dominates the seasonal variability at Paradip and is strong at Gopalpur and Kakinada. The annual
cycle is coherent along the western boundary of the bay, i.e., the east coast of India, but with significant
phase differences between moorings. At the semi-annual and 120-day periods, the alongshore coherence is
weaker. Intraseasonal variability is weaker than the seasonal cycle, particularly at Cuddalore and Paradip,
and it exhibits seasonality: the strongest intraseasonal variation is during spring (February⿿April). Peaks
around 12 and 20⿿22 days are also seen at Gopalpur, Kakinada, and Cuddalore. A striking feature of the
currents is the upward phase propagation, but there are also instances when phase propagates downward.
The much lower vertical phase speed in the top ⿼100 m at Cuddalore leads to a distinct undercurrent
at this location; at other locations, the undercurrent, though it exists often, is not as striking. During
spring, however, the EICC tends to flow poleward (eastward) at Cuddalore, Kakinada, and Gopalpur
(Paradip) over the top ⿼300 m, which is the maximum depth to which observations were made. The
cross-shore component of the EICC is much weaker than the alongshore component at Cuddalore and,
except for a few bursts during spring, at Kakinada and Gopalpur. It is only at Paradip, on the slope of
the northern boundary, that significant cross-shore flows are seen during spring and the summer monsoon
(June⿿August) and these flows are seen to be associated with eddy-like circulations in the altimeter
data. We use the ADCP data to validate popular current data products like OSCAR (Ocean Surface
Currents Analyses Real-time), ECCO2 (Estimating the Circulation and Climate of the Ocean, Phase II),
and GODAS (Global Ocean Data Assimilation System). The OSCAR currents at Paradip match the
observed currents well, but the correlation is much weaker at the other three locations. Both ECCO2
and GODAS fair poorly, particularly the latter because its variability in this boundary-current regime
is extremely weak. Though it performs badly at Paradip, ECCO2 does capture the observed variability
on occasions at the other locations. | |
| Paper No | 8 | Publication ID : 731 & Year : 2014 |
| Title | Observed intraseasonal and seasonal variability of the West India Coastal Current on the continental slope | |
| Authors | P. Amol, D. Shankar, V Fernando, A. Mukherjee, S. G. Aparna, R. Fernandes, G. S. Michael, S. T. Khalap, N. P. Satelkar, Y. Agarvadekar, M. G. Gaonkar, A. P. Tari, A. Kankonkar and S. P. Vernekar. | |
| Source | Journal of Earth System Science, 123 (5), 1045-1074 | |
| Abstract | We present current data from acoustic Doppler current profilers (ADCPs) moored on the continental
slope off the west coast of India. The data were collected at four locations (roughly at Kanyakumari,
Kollam, Goa, and Mumbai) extending from ⿼ 7 to ⿼ 20 N during 2008⿿2012. The observations show
that a seasonal cycle, including an annual cycle, is present in the West India Coastal Current (WICC);
this seasonal cycle, which strengthens northward, shows considerable interannual variability and is not
as strongly correlated along the coast as in climatologies based on ship drifts or the altimeter. The
alongshore decorrelation of the WICC is much stronger at intraseasonal periods, which are evident during
the winter monsoon all along the coast. This intraseasonal variability is stronger in the south. A striking
feature of the WICC is upward phase propagation, which implies an undercurrent whose depth becomes
shallower as the season progresses. There are also instances when the phase propagates downward. At
the two southern mooring locations off Kollam and Kanyakumari, the cross-shore current, which is
usually associated with eddy-like circulations, is comparable to the alongshore current on occasions.
A comparison with data from the OSCAR (Ocean Surface Currents Analyses Real-time) data product
shows not only similarities, but also significant differences, particularly in the phase. One possible reason
for this phase mismatch between the ADCP current at 48 m and the OSCAR current, which represents
the current in the 0⿿30 m depth range, is the vertical phase propagation. Current products based on
Ocean General Circulation Models like ECCO2 (Estimating the Circulation and Climate of the Ocean,
Phase II) and GODAS (Global Ocean Data Assimilation System) show a weaker correlation with the
ADCP current, and ECCO2 does capture some of the observed variability. | |
| Paper No | 9 | Publication ID : 730 & 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, A. 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, 122 (4), 899-933 | |
| Abstract | Situated in the eastern coastal state of West Bengal, the Sundarbans Estuarine System (SES) is India⿿s
largest monsoonal, macro-tidal delta-front estuarine system. It comprises the southernmost part of the
Indian portion of the Ganga⿿Brahmaputra delta bordering the Bay of Bengal. The Sundarbans Estuar-
ine Programme (SEP), conducted during 18⿿21 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 km 2 , 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 | 10 | Publication ID : 729 & Year : 2013 |
| Title | Near-inertial currents off the east coast of India. | |
| Authors | A. Mukherjee, D. Shankar, S. G. Aparna, P. Amol, V. Fernando, R. Fernandes, S. Khalap, S. Narayan, Y. Agarvadekar, M. Gaonkar, P. Tari, A. Kankonkar and S. Vernekar | |
| Source | Continental Shelf Research, 55, 29-39 | |
| Abstract | We use data from moorings equipped with Acoustic Doppler Current Profilers (ADCPs) and deployed in
the Bay of Bengal off the east coast of India from May 2009 to February 2012 to study the near-inertial
currents (NICs) on the continental shelf and slope. The data show that the NICs are much weaker at the
shelf break than on the slope. Inertial energy is weak all along the east coast during January⿿April. It is
high during the summer monsoon (May⿿September) in the northern Bay of Bengal and early during the
winter monsoon (October⿿December) in the southern bay; at locations in the central bay, the inertial
energy does not show this seasonality. This difference between the northern and southern bay is due to
the seasonality in the occurrence of storms, which tend to occur in the north (south) during the
summer (winter) monsoon. Variability across years is evident in the three-year record, with the NICs
being weaker during 2010⿿2011 compared to 2009. Upward phase propagation is evident in the data,
indicating downward propagation of energy. During severe cyclones, the data suggest that the strong
NICs extend below the thin surface mixed layer in the bay. A comparison of the NICs amplitude with
that of the detided (residual) current shows that the NICs make a significant contribution to the
observed current on the east-coast slope: the magnitude of the NICs exceeds that of the residual current
on the slope in the northern and southern Bay of Bengal on over 10 days in a year. | |
| Paper No | 11 | Publication ID : 728 & Year : 2012 |
| Title | Observational evidence from direct current measurements for propagation of remotely forced waves on the shelf off the west coast of India | |
| Authors | P. Amol, D. Shankar, S. G. Aparna, S. S. C. Shenoi, V. Fernando, S. R. Shetye, A. Mukherjee, Y. Agarvadekar, S. Khalap, and N. P. Satelkar. | |
| Source | Journal of Geophysical Research, 117, C05017. | |
| Abstract | We use data from six Acoustic Doppler Current Profiler (ADCP) moorings deployed
during March⿿September 2008 on the continental shelf and slope off Bhatkal, Goa,
and Jaigarh on the central west coast of India to present evidence for poleward propagation
of shelf or coastal-trapped waves (CTWs). Wave propagation is seen on the shelf in the
20⿿40-day, 10⿿14-day, and 3⿿5-day-period bands. The lag from south to north indicates
that remote forcing is important even at periods as short as 4 days. Using QuikSCAT wind
data, we show that the contribution of remote forcing to the shelf West Indian Coastal
Current (WICC) is significant even when the local alongshore wind is strong, as during
the summer-monsoon onset during May⿿June, and forces a strong local response that
masks the effect of remote forcing. Forced wave calculations using CTW theory show that
remote forcing of the WICC is present at all times, but is most striking when the local
winds are weak, as during March⿿April. The CTW calculations show that the source
region for the remote forcing may extend beyond the west coast into the Gulf of Mannar
between India and Sri Lanka. On the slope, propagation is seen only at the 4-day period.
At higher periods, the slope WICC decorrelates rapidly along the coast, but upward phase
propagation, implying downward propagation of energy associated with poleward
propagation, is evident even at these higher periods. | |