S. S. C. Shenoi

Spatiotemporal structure of the East India Coastal Current from satellite altimetry

We use a newly processed altimeter data set to present a hitherto unprecedented description of the spatiotemporal structure of the East India Coastal Current (EICC): the data set resolves timescales ranging from a few months to a few years, and the high along-track resolution yields the first description of the cross-shore structure of the current. The seasonal cycle dominates the variability, but the nonannual timescales have similar energy levels all along the EICC path. There are short-lived, intense intraseasonal bursts. In contrast to the seasonal cycle, the interannual and intraseasonal components are decorrelated along the coast, and possible mechanisms for the decorrelation (discontinuity in the flow) are discussed. In the cross-shore direction, the current is highly correlated at all timescales: the EICC is trapped against the shelf with the current offshore flowing in the opposite direction at most locations. The EICC appears as an inherently discontinuous flow, taking the form of a few recirculating loops along the EICC path, with a typical cross-shore spatial scale of 150-200 km. The loops are highly variable in direction at all timescales from intraseasonal to interannual. This discontinuity of the EICC in space and time implies that the basic pathways and advective timescales for the interbasin exchange of water masses between the Bay of Bengal and the Arabian Sea are not robust when the full spatiotemporal variability of the EICC is considered.

A new atlas of temperature and salinity for the North Indian Ocean

The most used temperature and salinity climatology for the world ocean, including the Indian Ocean, is the World Ocean Atlas (WOA) (Antonov et al 2006, 2010; Locarnini et al 2006, 2010) because of the vast amount of data used in its preparation. The WOA climatology does not, however, include all the available hydrographic data from the Indian Exclusive Economic Zone (EEZ), leading to the potential for improvement if the data from this region are included to prepare a new climatology. We use all the data that went into the preparation of the WOA (Antonov et al 2010; Locarnini et al 2010), but add considerable data from Indian sources, to prepare new annual, seasonal, and monthly climatologies of temperature and salinity for the Indian Ocean. The addition of data improves the climatology considerably in the Indian EEZ, the differences between the new North Indian Ocean Atlas (NIOA) and WOA being most significant in the Bay of Bengal, where the patchiness seen in WOA, an artifact of the sparsity of data, was eliminated in NIOA. The significance of the new climatology is that it presents a more stable climatological value for the temperature and salinity fields in the Indian EEZ.

Hydrography and water masses in the southeastern Arabian Sea during March–June 2003

This paper describes the hydrographic observations in the southeastern Arabian Sea (SEAS) during two cruises carried out in March–June 2003 as part of the Arabian Sea Monsoon Experiment. The surface hydrography during March–April was dominated by the intrusion of low-salinity waters from the south; during May–June, the low-salinity waters were beginning to be replaced by the highsalinity waters from the north. There was considerable mixing at the bottom of the surface mixed layer, leading to interleaving of low-salinity and high-salinity layers. The flow paths constructed following the spatial patterns of salinity along the sections mimic those inferred from numerical models. Time-series measurements showed the presence of Persian Gulf and Red Sea Waters in the SEAS to be intermittent during both cruises: they appeared and disappeared during both the fortnight-long time series.

Observational evidence from direct current measurements for propagation of remotely forced waves on the shelf off the west coast of India

[1]xa0We use data from six Acoustic Doppler Current Profiler (ADCP) moorings deployed during March–September 2008 on the continental shelf and slope off Bhatkal, Goa, and Jaigarh on the central west coast of India to present evidence for poleward propagation of shelf or coastal-trapped waves (CTWs). Wave propagation is seen on the shelf in the 20–40-day, 10–14-day, and 3–5-day-period bands. The lag from south to north indicates that remote forcing is important even at periods as short as 4xa0days. Using QuikSCAT wind data, we show that the contribution of remote forcing to the shelf West Indian Coastal Current (WICC) is significant even when the local alongshore wind is strong, as during the summer-monsoon onset during May–June, and forces a strong local response that masks the effect of remote forcing. Forced wave calculations using CTW theory show that remote forcing of the WICC is present at all times, but is most striking when the local winds are weak, as during March–April. The CTW calculations show that the source region for the remote forcing may extend beyond the west coast into the Gulf of Mannar between India and Sri Lanka. On the slope, propagation is seen only at the 4-day period. At higher periods, the slope WICC decorrelates rapidly along the coast, but upward phase propagation, implying downward propagation of energy associated with poleward propagation, is evident even at these higher periods.

Weakening of spring Wyrtki jets in the Indian Ocean during 2006–2011

[1]xa0Beginning 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 6xa0years 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 onward. 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 semiannual harmonic WJs, as evident from an amplitude reduction of that mode by at least 0.3xa0m s−1 during the post-2005 period. Our analysis demonstrates that the variance explained by the semiannual 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%.

Hydrography of the eastern Arabian Sea during summer monsoon 2002

Hydrographic observations in the eastern Arabian Sea (EAS) during summer monsoon 2002 (during the first phase of the Arabian Sea Monsoon Experiment (ARMEX)) include two approximately fortnight-long CTD time series. A barrier layer was observed occasionally during the two time series. These ephemeral barrier layers were caused byin situ rainfall, and by advection of low-salinity (high-salinity) waters at the surface (below the surface mixed layer). These barrier layers were advected away from the source region by the West India Coastal Current and had no discernible effect on the sea surface temperature. The three high-salinity water masses, the Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW), and the Arabian Sea Salinity Minimum also exhibited intermittency: they appeared and disappeared during the time series. The concentration of the ASHSW, PGW, and RSW decreased equatorward, and that of the RSW also decreased offshore. The observations suggest that the RSW is advected equatorward along the continental slope off the Indian west coast.

Seasonal cycle of hydrography in the Bab el Mandab region, southern Red Sea

The seasonal cycle of temperature—salinity variations in the Bab el Mandab region (southern Red Sea) is described using CTD data collected during four cruises spread over the period May 1995—August 1997. A two layer system exists during early summer, winter and spring while a three layer system exists during summer. During summer, a large amount of the Gulf of Aden water intrudes into the Bab el Mandab region; up to the northern limit (14.5‡N). The quantity of Red Sea water that flows into the Gulf of Aden is maximum during the winter and minimum during the summer

Impact of convection over the equatorial trough on summer monsoon activity over India

Causes of disruption of rainfall (break in monsoon conditions) over the Indian subcontinent during the monsoon months for the period 1979–1998 are investigated using pentad rainfall data from the Global Precipitation Climatology Project (GPCP). Most (about 73%) of the break in monsoon (BM) events were associated with convective activity (rainfall more than 30 mm/pentad) over the equatorial trough (ET) region. The association between these events and the convective activity over the western (WET) and eastern equatorial trough (EET) regions of the tropical Indian Ocean were further explored. These relationships were tested for different (deficit, normal and excess) monsoon conditions over the Indian subcontinent and the El Niño conditions in the Pacific Ocean. There appears to be a negative and significant correlation between the Central Indian Region (CIR) rainfall and EET during deficit and non‐El Niño years. During deficit and El Niño years (1982 and 1987), both CIR and all India rainfall (AIR) exhibited a negative correlation with WET. In the case of years with no breaks, EET was negatively (positively) correlated during the years 1982 and 1992 (1994 and 1997) with AIR. The convective activity was more intense over EET than WET during prolonged BM and also in a deficit and non‐El Niño year (1979).