Time filter

Source Type

Girishkumar M.S.,Indian National Center for Ocean Information Services | Ravichandran M.,Indian National Center for Ocean Information Services | McPhaden M.J.,National Oceanic and Atmospheric Administration | Rao R.R.,Andhra University
Journal of Geophysical Research: Oceans | Year: 2011

Time series measurements of temperature and salinity recorded at 8N, 90E in the south central Bay of Bengal from a Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction buoy, along with satellite altimetry and scatterometer data, are utilized to describe the seasonal and intraseasonal variability of barrier layer thickness (BLT) during November 2006 to April 2009. The BLT shows strong seasonality with climatological minima during both March-May and August-September and maxima during December-February. Large-amplitude, intraseasonal fluctuations in BLT are observed during September 2007 to May 2008 and during September 2008 to April 2009. The observed intraseasonal variability in BLT is mainly controlled by the vertical movement of isothermal layer depth (ILD) in the presence of a shallow mixed layer. Further, the analysis shows that both ILD and BLT are modulated by vertical stretching of the upper water column associated with westward propagating intraseasonal Rossby waves in the southern bay. These waves are remotely forced by intraseasonal surface winds in the equatorial Indian Ocean. Copyright 2011 by the American Geophysical Union.


Girishkumar M.S.,Indian National Center for Ocean Information Services | Ravichandran M.,Indian National Center for Ocean Information Services | McPhaden M.J.,National Oceanic and Atmospheric Administration
Journal of Geophysical Research: Oceans | Year: 2013

Time series measurements of temperature, salinity and surface meteorological parameters recorded at 8°N, 90°E in the southern central Bay of Bengal (BoB) from a Research Moored Array for African-Asian-Australian Monsoon Analysis and predication (RAMA) buoy are used to document temperature inversions and their influence on the mixed layer heat budget during the winters, defined as October to March, of 2006-2007 (W67) and 2007-2008 (W78). There is a marked difference in the frequency and amplitude of temperature inversion between these two winters, with variations much stronger in W78 compared to W67. The formation of temperature inversions is favored by the existence of thick barrier layers, which are also more prominent in W78 compared to W67. Inversions occur when heating in the barrier layer below the mixed layer by penetrative shortwave radiation is greater than heating of the mixed layer by net surface heat flux and horizontal advection. Our analysis further demonstrates that intraseasonal and year-to-year variability in the frequency and magnitude of temperature inversions during winter have substantial influence on mixed layer temperature through the modulation of vertical heat flux at the base of mixed layer. ©2013. American Geophysical Union. All Rights Reserved.


Devi E.U.,Indian National Center for Ocean Information Services | Kumar P.,National Geophysical Research Institute Council of Scientific and Industrial Research | Kumar M.R.,National Geophysical Research Institute Council of Scientific and Industrial Research
Geophysical Journal International | Year: 2011

Lithospheric thickness is an important parameter to understand the nature of collision and subduction between the Indian and Asian tectonic plates. In this study, we apply the S receiver function technique to data from a network of broad-band stations in the northeast India and Eastern Himalayan regions and image the geometry of Indian Plate collision. This analysis reveals clear S-to-p conversions from the Moho and Lithosphere--Asthenosphere boundary (LAB) in the various tectonic units of the study region. The Indian lithosphere is found to be only 90 km thick beneath the Shillong plateau deepening to 135 km on either side suggestive of a lithospheric upwarp related to the plateau uplift. The lithosphere thickens northward, with values reaching ∼180 km beneath the Eastern Himalaya. The trend of the LAB north of the foredeep region indicates that the Indian Plate plunges beneath the Eastern Himalaya. The consistent northward-dipping character of the Indian Plate suggests that the Indian Plate is traceable until it gets subducted beneath Tibet just south of Bangong suture zone. The deepening of the LAB and its correlation with the topographic elevation is in agreement with homogeneous thickening of the lithosphere in response to compressive forces due to the continental collision of India with Asia. © 2011 Director, National Geophysical Research Institute Geophysical Journal International © 2011 RAS.


Ravichandran M.,Indian National Center for Ocean Information Services | Girishkumar M.S.,Indian National Center for Ocean Information Services | Riser S.,University of Washington
Deep-Sea Research Part I: Oceanographic Research Papers | Year: 2012

The time series of temperature, salinity, chlorophyll-a, particle scattering at 700nm, and dissolved oxygen obtained from Argo floats deployed in the southeastern Arabian Sea (AS), were used to investigate the variability of these quantities and the mechanisms that modulate them between March 2010 and March 2011. The observations show a persistent occurrence of a subsurface chlorophyll-a maximum (0.75-1mgm -3) near depths of 40-100m throughout the study period, just above the top of permanent thermocline and euphotic depth. The analysis shows that upward and downward movement of thermocline, which is primarily due to westward movement of low-mode baroclinic Rossby waves, significantly influences the depth of the subsurface chlorophyll maxima and its intensity. Further, the vertical movement of the thermocline significantly influences the depth of the oxycline in the AS. The mixed layer deepening associated with wind induced vertical mixing and convective overturning lead to near surface blooms during the summer and winter monsoons. The analysis clearly shows that, in addition to entrainment of nutrients from rich subsurface water in the near surface layer, vertical fluxes from the subsurface chlorophyll maximum also contribute significantly to mixed layer blooms. The availability of light also plays an active role in the mixed layer bloom, particularly during the summer monsoon season. © 2012 Elsevier Ltd.


Shenoi S.S.C.,Indian National Center for Ocean Information Services
Indian Journal of Marine Sciences | Year: 2010

The seasonal cycle of the coastal currents along the Indian coast and their dynamics is well known. Till recently, it was believed that the seasonal signal dominates the coastal currents due the seasonal reversals of monsoonal winds. But the recent studies using satellite based altimeter and in situ measurements using moored instruments provided evidences on the importance of intra and interannual variabilities embedded in them. This article reviews the evidences available from these observations.


Girishkumar M.S.,Indian National Center for Ocean Information Services | Ravichandran M.,Indian National Center for Ocean Information Services
Journal of Geophysical Research: Oceans | Year: 2012

The El Niño-Southern Oscillation (ENSO) influence on tropical cyclone (TC) activity (frequency, genesis location, and intensity) in the Bay of Bengal (BoB) during the primary TC peak season (October-December) are studied for the period of 1993-2010. The study shows that during primary TC peak season, accumulated cyclone energy in the BoB is negatively correlated with Niño3.4 sea surface temperature anomaly. Under La Niña regime number of extreme TC cases (wind speed >64 kt) increases significantly in the BoB during the primary TC peak season. The analysis further shows that negative Indian Ocean dipole year is also favorable for extreme TC activity in the BoB during the primary TC peak season. The existence of low-level cyclonic (anticyclonic) vorticity, enhanced (suppressed) convection, and high (low) tropical cyclonic heat potential (TCHP) in the BoB provides favorable (unfavorable) conditions for the TC activity under La Niña (El Niño) regimes together with weak vertical wind shear and high sea surface temperature (SST). The genesis location of TC shifts to the east (west) of 87°E in the BoB during La Niña (El Niño) regime due to the variability in convective activity. The probable reason for the intense TC during a La Niña regime is likely explained in terms of longer track for TCs over warm SST and high TCHP due to eastward shifting of genesis location together with other favorable conditions. The variability of Madden-Julian Oscillation and its influence on TC activity in the BoB during La Niña and El Niño regime are also examined. Copyright © 2012 by the American Geophysical Union.


Francis P.A.,Indian National Center for Ocean Information Services | Gadgil S.,Indian Institute of Science
Journal of Earth System Science | Year: 2013

It is now well known that there is a strong association of the extremes of the Indian summer monsoon rainfall (ISMR) with the El Niño and southern oscillation (ENSO) and the Equatorial Indian Ocean Oscillation (EQUINOO), later being an east-west oscillation in convection anomaly over the equatorial Indian Ocean. So far, the index used for EQUINOO is EQWIN, which is based on the surface zonal wind over the central equatorial Indian Ocean. Since the most important attribute of EQUINOO is the oscillation in convection/precipitation, we believe that the indices based on convection or precipitation would be more appropriate. Continuous and reliable data on outgoing longwave radiation (OLR), and satellite derived precipitation are now available from 1979 onwards. Hence, in this paper, we introduce new indices for EQUINOO, based on the difference in the anomaly of OLR/precipitation between eastern and western parts of the equatorial Indian Ocean. We show that the strong association of extremes of the Indian summer monsoon with ENSO and EQUINOO is also seen when the new indices are used to represent EQUINOO. © Indian Academy of Sciences.


Girishkumar M.S.,Indian National Center for Ocean Information Services | Thanga Prakash V.P.,Indian National Center for Ocean Information Services | Ravichandran M.,Indian National Center for Ocean Information Services
Climate Dynamics | Year: 2015

The relationship between ENSO and tropical cyclones (TCs) activity in the Bay of Bengal (BoB) during October–December under cold (1950–1974) and warm (1975–2006) phase of Pacific Decadal Oscillation (PDO) is investigated. A statistically significant difference in the formation of total number of TCs and intense TCs (Category-1 and above) between El Niño and La Niña years is observed when the PDO was in warm phase. Our analysis shows that, there is a tendency to form more number of TCs during La Niña years (2.62 TCs per season) than during El Niño years (1.6 TCs per season) under warm phase of PDO. Moreover, the difference is quite high for intense TCs cases, such as, relatively more number of intense TCs forms in the BoB during La Niña years (1.4 TCs per season) compared to El Niño years (0.10 TCs per season) under warm phase of PDO. However, the difference in the formation of total number of TCs and intense TCs between La Niña and El Niño years is not significant under cold phase of PDO. Significant enhancement in low level cyclonic vorticity and mid-troposphere humidity during La Niña years compared to El Niño years when the PDO was in warm phase, rather than the PDO was in cold phase leads to this difference. Our analysis further shows that how the ENSO related teleconnection to the Indian Ocean region differ under warm and cold phase of PDO. © 2014, Springer-Verlag Berlin Heidelberg.


Rahaman H.,Indian National Center for Ocean Information Services | Ravichandran M.,Indian National Center for Ocean Information Services
Journal of Geophysical Research: Oceans | Year: 2013

[1] Daily near-surface air temperature (Ta) and specific humidity (Qa) from three hybrid flux products, namely, Coordinated Ocean-Ice Reference Experiments version II (CORE-II), Objectively Analyzed Air-Sea Fluxes (OAFlux), and Air-Sea Fluxes for the Global Tropical Oceans (TropFlux), are evaluated using in situ data over the North Indian Ocean. The analysis shows that the root-mean-square error (RMSE) value of Ta is ∼0.5°C for all products. TropFlux captures the daily variability of Ta very well, but it has a systematic deviation in Ta. The large drop in Ta observed during intense rainfall events is very well captured by TropFlux. All products overestimate Qa by 0.3-1.5 g/kg; OAFlux has the smallest systematic deviation, whereas TropFlux has the highest correlation with buoy data. The overestimation of Qa by the products is mainly caused by high values of Qa, in the range of 18-22 g/kg. The RMSE of Qa ranges from 0.92 to 1.79 g/kg, with OAFlux having the lowest values. Latent heat flux (LHF) computed from a bulk algorithm is underestimated by all products, which can be primarily attributed to the positive bias in Qa. In the southern Bay of Bengal, LHF decreases with increasing Qa during winter and summer monsoons. In this region, a change in 1 g/kg Qa can cause about 11-15 W/m errors in LHF. The air-sea humidity difference is linearly related to sea surface temperature for values greater than 28°C, similar to findings for the western Pacific Ocean. © 2013 American Geophysical Union. All Rights Reserved.


Francis P.A.,Indian National Center for Ocean Information Services | Gadgil S.,Indian Institute of Science
Journal of Earth System Science | Year: 2010

The Indian summer monsoon season of 2009 commenced with a massive deficit in all-India rainfall of 48% of the average rainfall in June. The all-India rainfall in July was close to the normal but that in August was deficit by 27%. In this paper, we first focus on June 2009, elucidating the special features and attempting to identify the factors that could have led to the large deficit in rainfall. In June 2009, the phase of the two important modes, viz., El Niño and Southern Oscillation (ENSO) and the equatorial Indian Ocean Oscillation (EQUINOO) was unfavourable. Also, the eastern equatorial Indian Ocean (EEIO) was warmer than in other years and much warmer than the Bay. In almost all the years, the opposite is true, i.e., the Bay is warmer than EEIO in June. It appears that this SST gradient gave an edge to the tropical convergence zone over the eastern equatorial Indian Ocean, in competition with the organized convection over the Bay. Thus, convection was not sustained for more than three or four days over the Bay and no northward propagations occurred. We suggest that the reversal of the sea surface temperature (SST) gradient between the Bay of Bengal and EEIO, played a critical role in the rainfall deficit over the Bay and hence the Indian region. We also suggest that suppression of convection over EEIO in association with the El Niño led to a positive phase of EQUINOO in July and hence revival of the monsoon despite the El Niño. It appears that the transition to a negative phase of EQUINOO in August and the associated large deficit in monsoon rainfall can also be attributed to the El Niño. © Indian Academy of Sciences.

Loading Indian National Center for Ocean Information Services collaborators
Loading Indian National Center for Ocean Information Services collaborators