Offshore limit of coastal ocean variability identified from hydrography and altimeter data in the eastern Arabian Sea

Abstract

In this communication, we describe a hitherto-unknown offshore limit to the coastal ocean variability signatures away from the continental shelf in the eastern Arabian Sea, based on hydrographic observations and satellite altimeter (TOPEX/Poseidon) information. Vertical sections of temperature from two contrasting seasons—south-west (SW) and north-east (NE) monsoons—when projected together are found to intersect offshore at 350–400 km from the coast along a transect in the eastern Arabian Sea during 1996. Formation of such intersections along transects from south to north in the entire eastern Arabian Sea is noticed in the hydrographic data of 1987–88 also. Existence of these intersections has further been authenticated by the altimeter-derived sea level anomalies during 1992–97. It is proposed that these intersections arise due to the westward propagation of Rossby waves from the coastal Kelvin wave guide in the eastern Arabian Sea where alternate upwelling and downwelling processes occur during the contrasting seasons of SW and NE monsoons. We interpret these intersections as the offshore limit of variability of the water properties in the coastal region. Indications are that areas of high productivity in the coastal regions overlap a major portion of the width of intersection implying extension of the upwelling features towards offshore requiring revised estimates of primary productivity and carbon budgeting.