V. S. N. Murty

Sea surface salinity variability during the Indian Ocean Dipole and ENSO events in the tropical Indian Ocean

An ocean reanalysis that covers the period from 1871-2008 is used to analyze interannual variability of Sea Surface Salinity (SSS) in the tropical Indian Ocean. The reanalysis SSS and the SSS anomaly patterns during Indian Ocean Dipole (IOD) and El Nino – Southern Oscillation (ENSO) events are compared with patterns from Argo SSS data. The mean seasonal SSS variation is large in the northern Bay of Bengal compared to variations in the Arabian Sea and Equatorial Indian Ocean. During a positive IOD event positive SSS anomalies are found along the Sumatra coast due to the combination of wind-driven upwelling of subsurface high salinity waters, enhanced evaporation and anomalous surface circulation. The opposite is true, to a lesser extent, during negative IOD events. A Dipole Mode Index for Salinity (DMIS) based on SSS data and a new index based on the average of salinity in a region off the coast of Sumatra is introduced to monitor SSS variability during IOD and ENSO events. The impact of concomitant El Nino events on a positive IOD event is large with freshening (a negative SSS anomaly) in the equatorial Indian Ocean and salting (positive SSS anomaly) off the southern Sumatra coast. The (impact of) intense freshening reaches into the southwestern tropical Indian Ocean. The impact of concomitant La Nina with negative IOD is also large with an intense freshening in the southeastern Arabian Sea and salting off the northern Sumatra coast.

Estimation of the barrier layer thickness in the Indian Ocean using Aquarius Salinity

Monthly barrier layer thickness (BLT) estimates are derived from satellite measurements using a multilinear regression model (MRM) within the Indian Ocean. Sea surface salinity (SSS) from the recently launched Soil Moisture and Ocean Salinity (SMOS) and Aquarius SAC-D salinity missions are utilized to estimate the BLT. The MRM developed relates BLT to sea surface salinity (SSS), sea surface temperature (SST) and sea surface height anomalies (SSHA). Three regions where the BLT variability is most rigorous are selected to evaluate the performance of the MRM for 2012; the Southeast Arabian Sea (SEAS), Bay of Bengal (BoB), and Eastern Equatorial Indian Ocean (EEIO). The MRM derived BLT estimates are compared to gridded Argo and Hybrid Coordinate Ocean Model (HYCOM) BLTs. It is shown that different mechanisms are important for sustaining the BLT variability in each of the selected regions. Sensitivity tests show that SSS is the primary driver of the BLT within the MRM. Results suggest that salinity measurements obtained from Aquarius and SMOS can be useful for tracking and predicting the BLT in the Indian Ocean. Largest MRM errors occur along coastlines and near islands where land contamination skews the satellite SSS retrievals. The BLT evolution during 2012, as well as the advantages and disadvantages of the current model are discussed. BLT estimations using HYCOM simulations display large errors that are related to model layer structure and the selected BLT methodology.

The role of salinity on the dynamics of the Arabian Sea mini warm pool

Warmer (>28°C) Sea Surface Temperature (SST) occurs in the South Eastern Arabian Sea (SEAS, 5°-13°N, 65°-76°E) during March-April, and is known as the Arabian Sea Mini Warm Pool (ASMWP). In this study, we address the role of salinity and the upper layer heat and salt budgets in the formation and collapse of this ASMWP. An assessment of Level 3 Sea Surface Salinity (SSS) data from the Soil Moisture and Ocean Salinity (SMOS) satellite mission for the year 2010 shows that SMOS is able to capture the SSS variability in the SEAS. Analysis of temperature, salinity and currents from the HYbrid Coordinate Ocean Model (HYCOM) during 2003-06, and, in-situ temperature and salinity data from Argo floats during 2003-06 for the SEAS revealed that low salinity waters cap the top 60 m of the SEAS in January-February. This minimum salinity was concurrent with the formation of a barrier layer and with the time when the SEAS gained little net heat flux and the equatorward flowing East India Coastal Current (EICC) fed low saline waters into the SEAS. Subsequently, the net heat flux increased to a peak value under the increased salinity stratification, leading to the formation of the ASMWP in March-April. The ASMWP collapsed by May due to increase in SSS and the associated weakening of the salinity stratification. The monsoon onset vortex in May 2004 could be related to the minimum SSS that occurred in February 2004, followed by higher SST and heat content of the ASMWP in April 2004.

Observational evidence of lower‐frequency Yanai waves in the central equatorial Indian Ocean

The analysis of long time-series current meter data from a mooring at 77 0 E and equator during 20032007 along with mean sea level anomaly (MSLA) data throws light on the occurrence of the lower frequency Yanai wave (24-40 day) in the upper water column of the central equatorial Indian Ocean (EIO) during the positive Indian Ocean Dipole (IOD) years of 2003, 2004, 2006 and 2007 and its absence during the negative IOD year of 2005. This result is in contrast with the earlier studies that observed only the higher frequency (biweekly period) Yanai wave in this region. We propose a new notion for the generation of the lower frequency Yanai wave in the upper central EIO due to the positive IOD phenomenon. The strong meridional current shear created by the northward shifting and strengthening of the westward flowing south equatorial current (SEC) associated with positive IOD and the eastward flowing southwest monsoon current provides energy for the generation of lower frequency Yanai waves. The vertical stratification of the water column appears to be responsible for the trapping of different frequency of Yanai wave with only the higher frequency Yanai wave in the region of lower pycnocline. During positive IOD the strongly stratified upper water column responds to the lower frequency Yanai wave while the deeper ocean (4000m) exhibited longer period (47-day) oscillation. The expected surface signature of Madden-Julian Oscillation (MJO) seems to be suppressed by strong easterlies during the positive IOD years.

On the dynamics of the Sri Lanka Dome in the Bay of Bengal

East of Sri Lanka, in the Northern Indian Ocean, a cold dome, known as the Sri Lanka Dome (SLD), develops during southwest monsoon season (June-September). The SLD first forms around May, matures in July and decays around September, in association with the strong cyclonic wind stress curl. In this study, the structure and dynamics of SLD in response to the climatic events such as the Indian Ocean Dipole (IOD) are examined. Our results reveal that these climatic events modulated the subsurface temperature variability in the thermocline at ∼100 m depth, whose signature was also evident in the satellite derived Sea Surface Height (SSH) anomaly in the SLD region. We find that the mechanisms for the formation of SLD are consistent with previous research, and there is year-to-year variability in the SLD dynamics. This study also reveals that the atmospheric parameters including vertical wind shear and mid-tropospheric relative humidity are high over the SLD region and adjacent southern Bay of Bengal and show an upward (increasing) trend over the decades. This has impacted the atmospheric parameters over the northern Bay of Bengal over the decades and as a consequence the total number of monsoon depressions (June-September) decreased over the decades from 1980 to 2015, as reported by the India Meteorological Department, New Delhi. Thus the new insight emerged from this study is the variability in the cyclogenesis and the occurrence of total number of monsoon depressions over northern Bay of Bengal over the decades are much related to the SLD dynamics.