The upper oceanic thermal response induced by Tropical Cyclone Phailin (9⿿14 October 2013) under the influence of East India Coastal Current (EICC) and a cyclonic eddy is investigated and contrasted with the response from open ocean region using a high⿐resolution HYbrid Coordinate Ocean Model simulation. There is significant cooling (7° C) inside the cold core eddy and negligible cooling (0.5° C) within the EICC region characterized by the shallow and deeper thermocline, respectively. Our analysis of mixed layer heat budget terms showed that the horizontal advection plays a significant role in determining the temperature tendency for the location within the EICC, in contrary to the general dominance of vertical processes as reported in previous studies during the cyclone period. The analysis for the locations inside eddy and open ocean concurs with the previous studies showing the dominance of vertical processes toward the temperature tendency. Further, near the coast, the surface cooling is minimal compared to the subsurface cooling, dominantly seen between 50⿐ and 100⿐m depth. This disparity indicates that the factors responsible for the surface temperature anomalies are different from those of subsurface. Our analysis of thermal signatures after the passage of cyclone showed that the EICC and cyclonic eddy contribute to the faster advection of cold wake and recovery of sea surface temperature to the pre-storm state.
Paper No
2
Publication ID : 770 & Year : 2018
Title
Shear Flow Instabilities and Unstable Events Over the North
Bay of Bengal
Authors
Venkata Jampana 1
Hasibur Rahaman 1
, M. Ravichandran 2 , Debasis Sengupta 3 , E. A. D⿿Asaro 4 ,
, Sudheer Joseph 1 , J. Sreelekha 3 , and Dipanjan Chaudhuri 3
A year-long mooring data are used to study the upper ocean unstable events and instabilities
at 18 ⿿ N 89 ⿿ E, which is a climatologically important region in the North Bay of Bengal. Near-surface stability
is studied from the context of the buoyancy frequency normalized shear ( V z ⿿N ) and reduced shear ( S 2 ⿿ 4N 2 )
which are convenient measures to quantify flow stability, compared to the more widely used Richardson
number ( Ri ). The analysis is carried out across three contrasting time periods, the monsoon, postmonsoon,
and the winter of year 2012. Although it is well known that the flow stability changes from stable to unstable
at Ri = Ri cr = 0.25, the relative importance of the perturbations of shear and buoyancy frequency in driving
the unstable events is not well studied over the open oceans and more particularly over the Bay of Bengal.
At 18 ⿿ N, 89 ⿿ E both higher than average shear and lower than average buoyancy frequency perturbations
are crucial in driving the unstable events during the summer and premonsoon period. However, at
increasing depths, the influence of shear perturbations becomes more dominant. Invoking the
Miles-Howard criteria for flow instability, it is seen that during the postmonsoon period, the buoyancy
frequency perturbations are more critical than shear perturbations in driving the unstable events. In winter,
the unstable events are influenced by both the buoyancy frequency and shear perturbations.
Paper No
3
Publication ID : 689 & Year : 2017
Title
Ocean atmosphere thermal decoupling in the eastern equatorial Indian ocean
Authors
Sudheer Joseph, M Ravichandran, B. Praveen Kumar, Raju V. Jampana, Weiqing Han
Eastern equatorial Indian ocean (EEIO) is one of the most climatically sensitive regions in the global ocean, which plays a vital role in modulating Indian ocean dipole (IOD) and El Niño southern oscillation (ENSO). Here we present evidences for a paradoxical and perpetual lower co-variability between sea-surface temperature (SST) and air-temperature (Tair) indicating instantaneous thermal decoupling in the same region, where signals of the strongly coupled variability of SST anomalies and zonal winds associated with IOD originate at inter-annual time scale. The correlation minimum between anomalies of Tair and SST occurs in the eastern equatorial Indian ocean warm pool region (⿿70°E⿿100°E, 5°S⿿5°N), associated with lower wind speeds and lower sensible heat fluxes. At sub-monthly and Madden⿿Julian oscillation time scales, correlation of both variables becomes very low. In above frequencies, precipitation positively contributes to the low correlation by dropping Tair considerably while leaving SST without any substantial instant impact. Precipitation is led by positive build up of SST and post-facto drop in it. The strong semi-annual response of SST to mixed layer variability and equatorial waves, with the absence of the same in the Tair, contributes further to the weak correlation at the sub-annual scale. The limited correlation found in the EEIO is mainly related to the annual warming of the region and ENSO which is hard to segregate from the impacts of IOD.
Paper No
4
Publication ID : 690 & Year : 2017
Title
Latent heat flux sensitivity to sea surface temperature - regional perspectives
Authors
B. Praveen Kumar, Meghan F. Cronin, Sudheer Joseph, M. Ravichandran, N. Sureshkumar
A global analysis of latent heat flux (LHF) sensitivity to sea surface temperature (SST) is performed, with focus on the tropics and the north Indian Ocean (NIO). Sensitivity of LHF state variables (surface wind speed Ws and vertical humidity gradients οq) to SST give rise to mutually interacting dynamical (Ws driven) and thermodynamical (οq driven) coupled feedbacks. Generally, LHF sensitivity to SST is pronounced over tropics where SST increase causes Ws (οq) changes, resulting in a maximum decrease (increase) of LHF by ~15 W m⿿2 (°C)⿿1. But the Bay of Bengal (BoB) and north Arabian Sea (NAS) remain an exception that is opposite to the global feedback relationship. This uniqueness is attributed to strong seasonality in monsoon Ws and οq variations, which brings in warm (cold) continental air mass into the BoB and NAS during summer (winter), producing a large seasonal cycle in air⿿sea temperature difference οT (and hence in οq). In other tropical oceans, surface air is mostly of marine origin and blows from colder to warmer waters, resulting in a constant οT ~ 1°C throughout the year, and hence a constant οq. Thus, unlike other basins, when the BoB and NAS are warming, air temperature warms faster than SST. The resultant decrease in οT and οq contributes to decrease the LHF with increased SST, contrary to other basins. This analysis suggests that, in the NIO, LHF variability is largely controlled by thermodynamic processes, which peak during the monsoon period. These observed LHF sensitivities are then used to speculate how the surface energetics and coupled feedbacks may change in a warmer world.
Short term current prediction for operational purposes is commonly carried out with the help of numerical ocean circulation models. The numerical models have advantage that they are based on the physics of the underlying process. However because of their spatial nature they may not be so accurate while making station-specific predictions. In such cases data-driven approaches like artificial neural network (ANN)⿿s trained on the basis of location-specific data may work better. In this paper an attempt is made to do daily predictions of ocean currents by combination of a numerical model and ANNs.
Paper No
6
Publication ID : 409 & Year : 2012
Title
Weakening of Spring Wyrtki Jets in Indian Ocean
during 2006-2011.
Authors
Sudheer Joseph, Alan J. Wallcraft
, Tommy G. Jensen, M. Ravichandran, S. S. C. Shenoi, Shailesh Nayak.
Source
Journal of Geophysical Research(2012)
http://www.agu.org/pubs/crossref/2012/2011JC007581.shtml
Abstract
Beginning in 2006, the Indian Ocean experienced climatologically anomalous conditions due to large scale coupled air-sea interactions that influenced the surface circulation of the equatorial Indian Ocean. Here we present evidence from observations as well as a general circulation model to demonstrate that spring Wyrtki Jets (WJ) were weak during the past six years and were even reversed to westward flow during 2008. We note that this weakening coincided with uniformly high sea-level as well as positive east to west gradient anomalies along the equatorial Indian Ocean during the month of May each year, starting in 2006. The weakened jets occur in conjunction with the latitude of zero zonal wind (LUZ) being close to the equator during these years, resulting in weaker than normal zonal winds along the equator from 2006 and onwards. We find that starting in 2006, the normal tendency of westward propagation of the annual harmonic mode switches to eastward propagation, coherent with the wind forcing. In comparison to the annual harmonic component of the zonal current, the weak WJs are mainly associated with the semi--annual harmonic WJs, as evident from an amplitude reduction of that mode by at least 0.3 m/s during the post 2005 period. Our analysis demonstrates that the variance explained by the semi-annual harmonic is reduced to half (30-40%) at the core of the WJ in 2006 and later years, in comparison with earlier years when it was 70-80 %.
Paper No
7
Publication ID : 408 & Year : 2005
Title
Salinity variability in the Arabian Sea.
Authors
Sudheer Joseph and Howard J. Freeland,
Source
Geophysical Research Letters, Vol. 32, L09607, doi: 10.1029/\\2005GL022972.
Abstract
Argo floats deployed in the Arabian Sea provided an opportunity to look at the temporal variability of the core-depth of Arabian Sea High Salinity Water mass (ASHSW) at three locations in the north, central and southern Arabian Sea. These three locations show distinctly different variability patterns. At the northern location we see a prominent semi-annual cycle, whereas at the central location an annual cycle dominates. Comparison with TOPEX/JASON sea level data shows that this difference can be attributed to the influence of Rossby waves at the central location. In the southern Arabian Sea the variability in core depth is also dominated by an annual-mode, but there are 2 high frequency components near the Madden-Julian Oscillation frequencies. Thus the salinity distribution in the Arabian Sea apparently is influenced by both remote forcing and ocean atmospheric coupling.
Paper No
8
Publication ID : 406 & Year : 2004
Title
First results from the Argo Programme in the Arabian Sea
Authors
Ravichandran M.,
Vinayachandran P.N., Sudheer Joseph, and Radhakrishnan K.
Source
Current Science
Vol. 86, No. 5, 651-659
Abstract
Argo is a revolutionary concept in ocean observation system that envisages real-time sampling of the temperature and salinity profiles of the global oceans at an approximate spatial resolution of 300 km, once in ten days. Argo float is an autonomous drifting profiler that pops up and down in the ocean from the surface up to 2000 m, measuring two most important physical properties of the water column, viz. temperature and salinity as a function of pressure (depth). Moreover, it can provide an estimate of currents both at the surface and at the parking depth in near-real time. India is an active participant in the Argo programme and has already deployed 31 out of 150 proposed floats. In this article, we describe the data received from the first Argo float deployed by India in the southeastern Arabian Sea. The data demonstrate that the temperature and salinity profiles from Argo floats present possibilities for oceanographic studies and spatial and temporal scales that had been hitherto impossible.
Paper No
9
Publication ID : 407 & Year : 2004
Title
Indian Ocean high-productivity Event (10-8 Ma): Linked to Global Cooling or to the
Initiation of the Indian Monsoons?
Authors
Anil K. Gupta, Raj K. Singh, Sudheer Joseph and
Ellen Thomas.
Uplift of the Himalayas and Tibetan Plateau (ca. 108 Ma) has been said to be the main cause of the origin or intensification of the Indian monsoon system, because mountains modulate the land-sea thermal contrast. The intensification of the monsoons, in turn, is seen as the cause of major changes in fauna and flora on land (as a result of changing precipitation patterns) as well as in the Indian Ocean, where the monsoons drive increased upwelling and thus increased productivity. We argue that the interactions between the elevation of the Himalayas and Tibetan Plateau, the onset of the monsoons, and their effects on the Indian Ocean biota remain uncertain. The timing of these events (uplift, monsoons, and biotic change) is not well constrained. Neogene deep-sea benthic foraminiferal faunal and isotope records of the Ninetyeast Ridge combined with published data show that a major increase in biogenic productivity occurred at 108 Ma throughout the Indian Ocean, the equatorial Pacific, and southern Atlantic. We suggest that this Indian Ocean high-productivity event was not simply the result of monsoon-induced upwelling or nutrient delivery from the weathering of newly uplifted mountains, but may have been caused by strengthened wind regimes resulting from global cooling and the increase in volume of the Antarctic ice sheets.
Paper No
10
Publication ID : 405 & Year : 2002
Title
Control and interannual variability of dimethyl sulfide in the Indian Ocean.
Authors
Shenoy, D. M., Joseph S., Dileep Kumar M., and George M.D.
Source
Journal of Geophysical Research Vol. 107, doi: 10.1029/2001JD000371
Abstract
Aqueous dimethyl sulfide (DMS) concentrations and its sea-to-air fluxes were studied in upper layers (200 m) of the Indian Ocean, as a part of the Indian Ocean Experiment (INDOEX) that centered around the movement of the Intertropical Convergence Zone (ITCZ). Observations during the JanuaryMarch period revealed significant interannual differences in water column DMS concentrations between 1998 and 1999. DMS concentrations were about five times higher in 1999 (range: 014.84 nM; average: 3.32 nM) than 1998 (range: 0.054.94 nM; average: 0.70 nM). Similar variation was found with respect to its precursor, dimethylsulfoniopropionate (averages of 3.31 nM in 1998 and 16.36 nM in 1999). DMS exhibited weak relationships with nitrate and chlorophyll but a relatively strong one with mixed layer depths. Our results suggest that physical forcing (wind speed and depth of mixed layer) along with the associated biological processes might have primarily controlled DMS in these waters. The column DMS appears to be a function of mixed layer depth whereas interannual variability in its concentrations might be attributable to changes in wind forcing and plankton composition, Sea-to-air fluxes were higher (seven times) because of higher DMS abundance and wind speeds in 1999 (average: 7.2 μM m−2 d−1) than 1998 (average: 1.0 μM m−2 d−1). Higher DMS fluxes coincided with reported higher aerosol optical depths over the Indian Ocean in 1999.
Paper No
11
Publication ID : 402 & Year : 2000
Title
Increased marine production of N${_2}$O due to intensifying anoxia on the Indian continental shelf.
Authors
Naqvi S.W.A., Jayakumar D.A., Narvekar P.V., Naik H, Sarma V.V.S.S., D'Souza W., Joseph S. and
George M.D.
Eutrophication of surface waters and hypoxia in bottom waters has been increasing in many coastal areas1, 2, 3, 4, leading to very large depletions of marine life in the affected regions4. These areas of high surface productivity and low bottom-water oxygen concentration are caused by increasing runoff of nutrients from land. Although the local ecological and socio-economic effects have received much attention2, 3, 4, the potential contribution of increasing hypoxia to global-change phenomena is unknown. Here we report the intensification of one of the largest low-oxygen zones in the ocean, which develops naturally over the western Indian continental shelf during late summer and autumn. We also report the highest accumulations yet observed of hydrogen sulphide (H2S) and nitrous oxide (N 2O) in open coastal waters. Increased N2O production is probably caused by the addition of anthropogenic nitrate and its subsequent denitrification, which is favoured by hypoxic conditions. We suggest that a global expansion of hypoxic zones may lead to an increase in marine production and emission of N2O, which, as a potent greenhouse gas, could contribute significantly to the accumulation of radiatively active trace gases in the atmosphere5.
Paper No
12
Publication ID : 401 & Year : 1999
Title
New benthic foraminiferal species from Neogene sequence of ODP sites 757B & 758A, Northeastern
Indian Ocean.
