Study of Dynamics of Upper High Salinity Water Mass in the Arabian Sea
By: Naeem Ahmed
National Institute of Oceanography, ST 47, Block 1 Clifton, Karachi-75600, Pakistan.
The water masses of specific characteristics are formed in different regions of the oceans by the processes occurring at the surface of the oceans due to the ocean-atmosphere interactions at specific locations and sink from the surface to subsurface depths. The water mass distribution is controlled by temperature and salinity. The Arabian Sea is an active region of air-sea interaction processes and plays a major role in driving the monsoon system. The semi-annual reversal in wind stresses associated with the monsoon, water mass intrusions from marginal seas and other oceans, geographical features give the upper waters of the Arabian Sea a unique thermohaline structure and circulation. In the Arabian Sea due to air – sea interaction processes, water mass forms at the surface and sinks to just below the mixed layer depth.
At different location of Arabian Sea high salinity water masses are formed. Rochford (1964) reported a water mass and termed it as ‘D’; Wyrtki (1971) reported this water mass as Arabian Sea water (ASW) mass, Kumar and Prasad (1997) reported an other high salinity water mass as the Arabian Sea High Salinity Water (ASHSW) mass, and Banse (1984) announced the presence of a water mass in the northern Arabian Sea and called it as the North Arabian Sea High Salinity Water (NASHSW) mass due to its formation in the northern Arabian Sea. Whereas, Morrison (1997) referred NASHSW as a denser fraction of Arabian Sea Water (ASW). In view of the above, the present study has been planned with the prime objective of understanding the dynamical processes that govern the formation of upper salinity maximum in the north Arabian Sea by analysing original data collected under the North Arabian Sea Environment and Ecosystem Research (NASEER) Programme (1992 – 1995) and Argo floats launched in the Arabian Sea under the pilot programme of the Global Ocean Observing System (GOOS). For the identification of water masses T – S diagrammes were prepared using CTD data of NASEER and Argo floats. Heat budget of the Arabian Sea was computed using the algorithm for the standard bulk parameters at the 25 hours times series stations of NASEER cruises. The results inferred were validated with the simulation of one-dimensional turbulent model of Mellor and Durbin (1975) that was further modified by Miller (1976).
The heat budget of the Arabian Sea which includes short-wave flux Qi, long-wave flux Qb, latent heat fluxes Qe and sensible heat fluxes Qs was computed using meteorological data. The January short-wave flux Qi as the day time averages at the time series stations decrease from east to west which is similar even in December data. Whereas, during March, solar radiation increased from south to north, however in the May data it is inversed and showing increase from northeast to southwest. The long-wave flux Qb showed that except in January 1992, the long-wave radiation decreased from south to north. While, in January it decreased from east to west.
In spite of some constraints in availability of ideal data which required extensive coverage of CTD data synchronized with meteorological data, it is concluded that in the northern Arabian Sea there is only one subsurface salinity maximum which exits just underneath subsurface salinity minimum near the mixed layers. At all the sampling stations of NASEER or Argo profiling sites in the northern Arabian Sea, it was 25 kg m-3 density water which is described as North Arabian Sea High Salinity Water (NASHSW) by Banse (1990). Since a water mass of 24 kg m-3 density could not be found in the sub-surfaces of northern Arabian Sea, so it can be inferred that the conclusion drawn by the Morrison (1997, 2003) that 25 kg m-3 water is a denser part of the ASW appears to be correct.