Oceanic surface layer properties in the central Arabian Sea at the height of the northeast 1986, 1987 and southwest 1987 monsoons are described. Data from two research cruises on the R.R.S.
Charles Darwin are combined with Comprehensive Ocean—Atmosphere Data Set (COADS) winds and a simple model to consider the influence of the monsoons on the surface layers of the Arabian Sea. In December 1986 (NE monsoon) the top of the thermocline was fairly uniform with a mean depth of approximately 60 m. In contrast, this depth varied with latitude in the SW monsoon, with the shallowest depths occurring under the region of maximum winds and the maximum depths to the south of the monsoon jet. This situation is tied to the Ekman flow which leads to open ocean upwelling to the north of the wind maximum, thus suppressing the development of the mixed layer. The thermocline to the south is deepened by a combination of Ekman pumping and advection of dense fluid from the north. A simple two-dimensional model incorporating a non-penetrative mixed layer and a slab Ekman layer flow is used to examine the factors involved in the evolution of the surface layers during the monsoons. The combination of upwelling and surface layer deepening leads to latitudinal gradients in phytoplankton biomass. The major effect in the SW monsoon is the upwelling induced by Ekman dynamics and a consequent shoaling of the nitracline in the northern Arabian Sea. Vertical mixing and the associated deepening of the surface layer dominate phytoplankton distributions in the NE monsoons
