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Paper No1Publication ID : 738   &   Year : 2019  
TitleSignature 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
AbstractWe 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).

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Paper No2Publication ID : 737   &   Year : 2018  
TitleWyrtki 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
AbstractDirect 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.

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Paper No3Publication ID : 736   &   Year : 2017  
TitleObservations 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
AbstractWe 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.

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Paper No4Publication 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
AbstractWe 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.

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Paper No5Publication ID : 734   &   Year : 2017  
TitleDynamics 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
AbstractCirculation 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.

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Paper No6Publication ID : 733   &   Year : 2016  
TitleEvidence 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
AbstractThe 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.

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Paper No7Publication ID : 732   &   Year : 2014  
TitleObserved 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
AbstractWe 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.

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Paper No8Publication ID : 731   &   Year : 2014  
TitleObserved 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
AbstractWe 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.

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Paper No9Publication ID : 730   &   Year : 2013  
TitleTidal 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
AbstractSituated 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.

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Paper No10Publication ID : 729   &   Year : 2013  
TitleNear-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
AbstractWe 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.

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Paper No11Publication ID : 728   &   Year : 2012  
TitleObservational 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.
AbstractWe 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.

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