Bulusu Subrahmanyam

Indian Ocean Rossby waves observed in TOPEX/POSEIDON altimeter data and in model simulations

Analysis of 3 years of TOPEX/POSEIDON (T/P) altimeter data in the Indian Ocean revealed discrepancies between observed and theoretical Rossby wave phase speeds. Rossby waves were observed at all latitudes using Hovmoller diagrams or longitude/time plots; their phase speeds were computed using twodimensional Fast Fourier Transforms (FFTs). Complex Principal Component Analysis (CPCA) was applied to detect travelling waves. A simple wind-driven layer model was implemented for this region and forced with the ERS-1 scatterometer winds, sampled over the same period as the T/P data, to demonstrate that these winds account for much of the observed Rossby wave activity.

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.

ENSO-Modulated Cyclogenesis over the Bay of Bengal*

AbstractThe role of the El Nino–Southern Oscillation (ENSO) on the modulation of tropical cyclone activity over the Bay of Bengal (BoB) for the 1979–2011 period is examined. It is shown that Nino-3.4 sea surface temperature (SST) anomalies are negatively correlated with the BoB tropical cyclone activity to a statistically significant percentage by a lead time of 5 months. Composites of 10-m zonal winds exhibit greater variance during La Nina events, favoring the development of low-level cyclonic vorticity. Low vertical wind shear over the central and northern BoB also aids in the development of tropical cyclones during La Nina events. Increased relative humidity is the result of enhanced moisture transport and higher precipitable water under La Nina conditions. Furthermore, storm-relative composites of relative humidity show stronger moisture pulses over the BoB during La Nina. The enhanced moisture associated with tropical cyclogenesis likely aids in the development and strengthening of the systems. ENSO...

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.

Preliminary SMOS Salinity Measurements and Validation in the Indian Ocean

Global sea surface salinity (SSS) measurements retrieved from the European Space Agencys Soil Moisture and Ocean Salinity (SMOS) mission are the first highest resolution salinity data available from space. There are many challenges to measuring salinity from space and obtaining a targeted accuracy of 0.1 psu. Comparisons of Level 2 (L2) SMOS SSS data with the 1/12° high resolution HYbrid Coordinate Ocean Model (HYCOM) simulations of SSS reveal large differences. These differences are minimized for an extent during the creation of Level 3 (L3) SMOS data through spatial and temporal averaging. Depending on the retrieval algorithm used, there are differences between ascending and descending passes with data collected during the descending pass exhibiting a bias toward lower SSS. It is challenging to process SMOS SSS data in the northern Indian Ocean due to radio frequency interference and large seasonal variability due to monsoonal circulation. Comparisons of SMOS L3 data with Argo float SSS and HYCOM SSS indicate the lowest discrepancies in SSS for these data sets occur in the southern tropical Indian Ocean and the largest differences between the compared salinity products are noticed in the Arabian Sea and Bay of Bengal with an erratic root mean square error in the latter region. Higher errors in SSS occurred in coastal areas compared to the open ocean. The accuracy of SMOS salinity measurements is increasing with the maturity of the data and new algorithms.

Sea Surface Height Variability in the Indian Ocean from TOPEX/POSEIDON Altimetry and Model Simulations

Sea Surface Height (SSH) variability in the Indian Ocean during 1993-1995 is studied using TOPEX/POSEIDON (T/P) altimetry data. Strong interannual variability is seen in the surface circulation of the western Arabian Sea, especially in the Somali eddy structure. During the Southwest (SW) monsoon, a weak monsoon year is characterized by a single eddy system off Somalia, a strong or normal monsoon year by several energetic eddies. The Laccadive High (LH) and Laccadive Low (LL) systems off southwest India are observed in the altimetric SSH record. The variability of the East India Coastal Current (EICC), the western boundary current in the Bay of Bengal, is also detected. Evidence is found for the propagation of Kelvin and Rossby waves across the northern Indian Ocean; these are examined in the context of energy transfer to the western boundary currents, and associated eddies. A simple wind-driven isopycnal model having three active layers is implemented to simulate the seasonal changes of surface and subsur...Sea Surface Height (SSH) variability in the Indian Ocean during 1993-1995 is studied using TOPEX/POSEIDON (T/P) altimetry data. Strong interannual variability is seen in the surface circulation of the western Arabian Sea, especially in the Somali eddy structure. During the Southwest (SW) monsoon, a weak monsoon year is characterized by a single eddy system off Somalia, a strong or normal monsoon year by several energetic eddies. The Laccadive High (LH) and Laccadive Low (LL) systems off southwest India are observed in the altimetric SSH record. The variability of the East India Coastal Current (EICC), the western boundary current in the Bay of Bengal, is also detected. Evidence is found for the propagation of Kelvin and Rossby waves across the northern Indian Ocean; these are examined in the context of energy transfer to the western boundary currents, and associated eddies. A simple wind-driven isopycnal model having three active layers is implemented to simulate the seasonal changes of surface and subsurface circulation in the North Indian Ocean and to examine the response to different wind forcing. The wind forcing is derived from the ERS-1 scatterometer wind stress for the same period as the T/P altimeter data, enabling the model response in different (active/weak) monsoon conditions to be tested. The model output is derived in 10-day snapshots to match the time period of the T/P altimeter cycles. Complex Principal Component Analysis (CPCA) is applied to both altimetric and model SSH data. This confirms that long Rossby waves are excited by the remotely forced Kelvin waves off the southwest coast of India and contribute substantially to the variability of the seasonal circulation in the Arabian Sea.

Investigating decadal changes in sea surface salinity in oceanic subtropical gyres

Sea surface salinity trends over the past six decades were analyzed, as salinity can be a potential diagnostic of the acceleration pattern of the global water cycle. Global salinity data from Simple Ocean Data Assimilation reanalysis highlight surface ocean salinity trends in evaporative-dominated subtropical gyre systems over the period of 1950–2010. Global salinity observations from NASAs Aquarius and European Space Agencys Soil Moisture and Ocean Salinity missions were used with relation to more recent trends and future implications on global water cycle studies. Results indicated an average salinity increase of 0.12 practical salinity unit (psu) in the subtropical gyres over the 61 year study, with the greatest increase occurring in the southern hemisphere gyres. Lateral gyre drift was also inferred through salinity, as three of five gyres showed significant drift over 60 years within their respective basins. There is evidence of periodicity related to these migrations on multidecadal time scales. A comparison of satellite, in situ, and model simulations was conducted in an effort to resolve the near-surface salinity stratification as it pertains specifically to the subtropical gyre regions and also to show the growing relevance of satellite data in global water cycle studies.

SMOS Mission Reveals the Salinity Structure of the Indian Ocean Dipole

This letter reports the European Space Agencys Soil Moisture and Ocean Salinity (SMOS) observations of the sea surface salinity (SSS) structure during an Indian Ocean Dipole (IOD) event. Comparisons with Argo data show that the SMOS satellite is able to resolve the observed SSS pattern in the Indian Ocean despite some challenges in the northern Indian Ocean. Results of box averages for the Java Sumatra Coast (JSC) and South Central Indian Ocean (SCIO) regions show low SSS anomalies in the former and high SSS anomalies in the latter during the 2010 negative IOD event. Analyses of salt flux and salt budget terms suggest that, in the JSC region, salt tendency is an interplay between freshwater forcing and horizontal advection terms, with increased precipitation having a higher impact in driving SSS anomalies than advection. In the SCIO region, advection seems to be more important than the freshwater forcing term.

Heat transports in the Indian Ocean estimated from TOPEX/POSEIDON altimetry and model simulations

Estimates of the heat budget of the Indian Ocean computed using TOPEX/Poseidon (T/P) sea-level anomalies and the Miami Isopycnal Coordinate Ocean Model are compared to study the redistribution of heat in the Indian Ocean. In particular, the horizontal heat transport and heat storage are used because they typically change on time scales of months or years or longer, and are therefore a predictable element of the climate system. The results show that T/P-derived heat storage is weaker than that derived from the model but has similar spatial structure and temporal evolution. Complex principal component analysis shows that there are two main modes of heat content redistribution in the Indian Ocean. The most dominant mode has an annual signal peaking in the boreal summer, and depicts the response to strong southwest monsoon winds. This involves offshore propagation of heat in the northern Indian Ocean and southward propagation of heat across the equator. The other main mode of heat content redistribution in the Indian Ocean results from westward propagating equatorial Rossby waves. This process is prominent in the boreal fall to spring, and represents the dynamic readjustment of the Indian Ocean to near-equatorial wind forcing. This mode indirectly relates to the dipole mode index in the Indian Ocean. The minima of this time series coincide with the occurrence of the anomalous dipole structure in the equatorial Indian Ocean.