Upwelling Off the Southwest Coast of India

Various factors r esponsible in inducing upwe lling are discussed. A distinction is m a de between the process of upwelling: and its etlects On the e nviro nme ntal cOnditions. Upwelling oft' the southwest coast of India is discussed in relation °to the time variation of density structure ( CSt), horizontal divergence of surface CUrrent vectors, wind stress components - parallel and perpendicular to the COast, thermal structure and sea level. U pwelling In this r egion com­ m ences in the deeper layers of about 90 m in March a n d the upwelled water reaches the surface by May. The process continues to occur vigorously till June. From then, with the increase in strength o f southwest winds, inte nsity of upwe lling reduces and finally ceases to be pre sent by July/August when the onshore winds of the southwest monsoon have maximum strength. Th~ r everse process, sinking, sets in by September. Ve rtical velocity, i.e. upwelling, calculate d (1) from the ve r tical shift of the isopycnal with time and (ii) from the a verage horizontal divergence, is 1·5 X 10-s and 1·8 x 10-s em sec-I respectively. V ERTICAL motions are an integral part of the oceanic circulation. Of these, ascending motion known as upwelling is one of the several oceanic processes which are considered to have a significant effec t on the distri bution of pelagic fishes. Upwelling not only affects the en­ vironmental condition in which the fishes live, but also increases the nutrient content in the euphotic zone, thereby increasing the productivity of the region. Upwelling off the west coast of India and its influence on the fishery is evident from various earlier reportsl - 6 . However, some controversy exists regarding the actual period of upwelling and its duration. This is mainly because, the term up­ welling is commonly used to describe a wide variety of conditions involving upward transport of water in the sea and often is misused in describing the effect of the process rather than the process itself. Various explicit and implicit definitions have bee n published. Smith'" surveyed the literature on upwelling and suggested the following defini tion the most acceptable, HAn .ascending motion, of some minimum duration and extent, by which water from subsurface layers is brought into the surface layer and is removed from the area of upwellin g by horizontal flow". This use of the t erm upwelling foll ows Sverdrup' and Wyrtki'O In this paper, apart from hydrographic conditions that show the sequence of water movements in a t ime series, various factors responsible for such a ver:ti ca l circulation and also the effects of other processes that may lead to misi nterpretation of the observations, are discussed. Upwel1in~