Temporal and spatial variation of particulate matter and chlorophyll in the Arabian Sea

Abstract

The Arabian Sea is of special interest because of reversals in the extreme atmospheric forcing that lead to the greatest seasonal variability among the world's oceans. An intensive series of cruises was conducted in the Northern Arabian Sea as part of the 1995 U.S. Joint Global Ocean Flux Study. Temporal and spatial variations of particulate matter (PM) and chlorophyll were determined via transrnissometers and fluorometers during a monsoonal cycle. Seasonal variations of the standing stock of PM and chlorophyll were on the same order of magnitude as spatial variations. The abundance and distribution of PM and chlorophyll varied throughout the monsoonal cycle. During the Spring Intermonsoonal period standing stocks of PM and chlorophyll in the upper 100m were -35% less than the SW Monsoon. The standing stocks during the SW Monsoon were only-1 0% greater than during the NE Monsoon. This difference was much smaller than expected. During the Spring Intermonsoon period conditions in the surface waters throughout the Arabian Sea became characteristically oligotrophic. Subsurface maxima of both PM and chlorophyll dominated during that time. Variations in the PM and chlorophyll were directly related to the biology and were greatly affected by nutrient concentrations and mixed layer depths. Convective mixing was prevalent during the NE Monsoon producing deep mixed layers (-70m) with large diel variations (as large as 90m). Wind-mixing and upwelling dominated the SW Monsoon producing deep nutrient-rich mixed layers (-50m) with little diel variation. Light winds during the Spring Intermonsoon allowed shallow mixed layers (10-20m) to persist with small diel variations. Spacial distribution throughout the year was high near the coast and decreased offshore. Horizontal frequent presence of mesoscale features.