Effects of monsoons on the seasonal and spatial distributions of POC and chlorophyll in the Arabian Sea

The Arabian Sea is of special interest because of annual reversals in the atmospheric forcing that lead to large seasonal variations in biogeochemical cycles. A series of cruises spanning an annual monsoonal cycle was conducted in the Northern Arabian Sea as part of the 1995 U.S. Joint Global Ocean Flux Study. Distributions of particulate organic carbon (POC) and chlorophyll a were determined via transmissometers and fluorometers. Throughout the year concentrations were generally high near the coast and decreased offshore. Spatial distributions were patchy and were influenced strongly by mesoscale features. Seasonal variations of the standing stock of POC and chlorophyll a in the upper 100 m at some locations were of the same order of magnitude as spatial variations across the basin and were greatly affected by nutrient concentrations and mixed layer depths. Integrations of chlorophyll a and POC to 100 m revealed much less uniform seasonal and spatial variations than expected. In fact, in many locations the standing stock of POC and chlorophyll a to 100 m unexpectedly showed higher values during the Spring Intermonsoon than during the Southwest Monsoon because a ubiquitous subsurface layer of POC and chlorophyll a existed deeper than could be detected by satellite color sensors.