Abstract
Background: Several authors have pointed out the relationship between seasonal changes of atmospheric winds, phytoplankton distribution and productivity in the Arabian Sea. The best example of such interaction is the western Arabian Sea region, which presents a large seasonal variation in temperature and chlorophyll because of the monsoons throughout the year. The main feature in the area is the strong western boundary current along the Somali and Arabian Peninsula coasts, which, with favorable winds, result in two important upwellings during the SW Monsoon. This work presents a characterization of the surface of the mixed layer along the Somali and Arabian coasts, based on temperature/salinity time diagram (TS-time diagram), temperature/chlorophyll time diagram (TC-time diagrams) and harmonic analysis techniques.
Materials and Methods: Geographic distribution of amplitudes and phases of harmonic waves are generated for the chlorophyll, temperature, and salinity data, which are computed for an average year over a 5-degree grid along the Somali and Arabian coasts. They are based on the World Ocean Atlas Data Set and the AVHRR MCSST and CZCS phytoplankton pigment concentration data set. This classification includes the analysis of the annual and semi-annual amplitudes, plus multiple regression analysis between temperature, salinity, chlorophyll, Ekman pumping, components of the pseudo-stress wind and net-down- fresh-water-flow (ndff). To clarify the processes currently involved with the surface water properties, sea surface temperature (SST), sea surface salinity (SSS) and sea surface chlorophyll (SSC) are treated as pseudo-harmonic terms.
Results: TC-time diagrams show the upwelling along the Arabian and Somali coasts. Two peaks of low temperature are observed during the Northeast and Southwest Monsoons. The highest chlorophyll concentration and low temperature are related to the upwelling that occurs in these areas during the SW Monsoon. The chlorophyll bloom starts in May and declines in October. The maximum chlorophyll values occur during August, when temperatures are close to 24 °C. Comparing the TC-time and TS-time diagrams for the same area, it can be observed that the highest chlorophyll concentration from the TC-time diagrams, during the SW Monsoon, is not only related to a low temperature, but also to low salinity values. The meridional component of pseudostress wind and wind magnitude play a major role on temperature variability along the Arabian coast, during the SW Monsoon.
Conclusion: The application of time diagram technique has successfully identified the upwelling and the Arabian Sea Surface Water (ASSW) along Arabian and Somali coasts, as well as the geographical distribution of the seasonal cycles of temperature, salinity, and chlorophyll.
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