Ebenezer S. Nyadjro

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.

Seasonal Variability of Salinity and Salt Transport in the Northern Indian Ocean

AbstractAnalyses using a suite of observational datasets (Aquarius and Argo) and model simulations are carried out to examine the seasonal variability of salinity in the northern Indian Ocean (NIO). The model simulations include Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), the European Centre for Medium-Range Weather Forecasts–Ocean Reanalysis System 4 (ECMWF–ORAS4), Simple Ocean Data Assimilation (SODA) reanalysis, and the Hybrid Coordinate Ocean Model (HYCOM). The analyses of salinity at the surface and at depths up to 200 m, surface salt transport in the top 5-m layer, and depth-integrated salt transports revealed different salinity processes in the NIO that are dominantly related to the semiannual monsoons. Aquarius proves a useful tool for observing this dynamic region and reveals some aspects of sea surface salinity (SSS) variability that Argo cannot resolve. The study revealed large disagreement between surface salt transports derived from observed- and analysis-derived sa...

Intraseasonal sea surface warming in the western Indian Ocean by oceanic equatorial Rossby waves

A novel process is identified whereby equatorial Rossby (ER) waves maintain warm sea surface temperature (SST) anomalies against cooling by processes related to atmospheric convection in the western Indian Ocean. As downwelling ER waves enter the western Indian Ocean, SST anomalies of +0.15 °C develop near 60°E. These SST anomalies are hypothesized to stimulate convective onset of the Madden-Julian oscillation (MJO). The upper ocean warming that manifests in response to downwelling ER waves is examined in a mixed layer heat budget using observational and reanalysis products, respectively. In the heat budget, horizontal advection is the leading contributor to warming, in part due to an equatorial westward jet of 80 cm s-1 associated with downwelling ER waves. When anomalous currents associated with ER waves are removed in the budget, the warm intraseasonal temperature anomaly in the western Indian Ocean is eliminated in observations and reduced by 55% in reanalysis.

On the Relationship Between Wind, SST, and the Thermocline in the Seychelles–Chagos Thermocline Ridge

This letter investigates the relationship between wind, sea surface temperature (SST), and thermocline in the Seychelles–Chagos Thermocline Ridge (SCTR, 5°S–10°S, 50°E–80°E) using a combination of satellite data and a reanalysis version of the HYbrid Coordinate Ocean Model from 1993 to 2012. The asymmetry of this relationship during positive and negative Indian Ocean Dipole (IOD) events and the impacts on the SST—thermocline depth (represented by the 20 °C isotherm depth, D20) relationship—are examined. On interannual timescales, an asymmetric relation between SST and zonal wind stress causes a strengthening of easterlies that enhances anticyclonic wind stress curl and local Ekman downwelling, which in turn deepens the D20 and increases the heat content during positive IOD (PIOD) events. In contrast, during negative IOD (NIOD) events, the winds reverse to be westerlies and cause a three times greater impact on remotely generated upwelling Rossby waves. Subsequently, these asymmetric relations cause an asymmetric D20–SST feedback in the SCTR such that a shoaling D20 is ~2.5 times more effective at lowering SST during NIOD events than a deepening D20 is at raising the SST during PIOD events. The changes to D20 are observed to extend into the year following IOD events, persisting into the end of the year following a PIOD event (due to stronger asymmetric reinforcing effects of warm SST anomalies on zonal wind anomalies) but only to May–June of the year following NIOD events.