Watermass modification in the upper layers of the Arabian Sea during ISMEX·73 …metnet.imd.gov.in/mausamdocs/541417.pdf · 2019. 2. 27. · equator and anot her statio n (18' N,
10
Mausam (1990), 4, 551.465 .4 Watermass modification in the upper layers of the Arabian Sea du ring ISMEX·73 R. R. RAO. P. v. HAREESH K MAR and BASIL MATHEW Navul P;,y siL' a{ turd Oceanograph ic Laboratory, Cochin ( Reed .. ed 18 January /988) " fl: _ '1ffT"f Jl "lf<fll'J To a.; p q!1=f I1IOflJ."'f , 'U9' %: m- 'f .... "ir Ji"Jr.f1"l lin J;f lltiAf'r jf;:nn oit .rf :itT'tlT7.tT 'l ' u.::fr 11-" . '!':P'FI.:t I iii"' 1iF q 1fT'll!! 'f if. if ir ;sf Pi '.: ,,"r " Hrqr IfTT I 3fT1f."f( 'fol !;IHt-'I f'fltliOf"fr itrhur <iT tri a & {m if'l1l vP" lH ( 611° 'X e ) q\: :;.q'T i{ jf{;;r ( ISO ", IJ. e ) 'P: :it '.: '1"" 1fTiPi "' itrrif fil( ot"""t 'for f... f ... flf:l :frq'fr zrro1 IJ. <f'r <rl fi iif j{f "f lrllR .r.r if """ of "" '" '!f.ltrf"" '" fHOT r ..r 01'11 I ABSTRACT. The intra-seasonal variability norice.I in the therm ohaline characteristics in the upper layer, of the Arabi an Sea during the summer monsoo n 'CJSOI1 of 1973 descr ibed wnh the aid of transects made by USSR and Indian ships along some typical zona l and meridional sections covered during the pre and post un set regimes of Indo-Soviet Monsoon Exreriment (IS :\1E.'(-7J). An IS mad e 10 this o bserved variab ility during the summer monsoon Sl:;lSO nS o f 1971, 1971 anJ 1973 wit h the limited available data sets. The observed short-term variability during: the prc and past on set rcgr.n r- at oae slal ion (60 'c ) on the equator and anot her statio n (18' N, E) inme Arabian . .1 is describe.I and di sc-ussed to highl igh t the uneven response of the Arabian Sea In Us spatial dom ain to the variable seasonal mon- soonalforcing. • 1. Int rod uction The summer monsoon produces large scale varia- tions in the waterrnass cha racter [sties of the upper \.1yen of the Arabian Sea. Most of the descrip uon on tbe observed var iability in the ther mal regime of the uppe r layers of the Ar ab ia n Sea documented by severa l wor ker s (Wooster et al. 1967, Wyrtki 1971, Colborn 1975, Rob inson et al. 1979, Duing and Lcctrna 1980, Levitus 1982, Molinari "I al. 1986, Rao 1988, Rao et al. 1989) provide information on climato logical annual cycle fro m multi-year averaged data sets . Rela- t ively few stu dies (Sastry & D'Souza 1970, Rao ('/ "I . 1976, Swall ow et ul. 1983, Sastry & Ramesh Babu 1985) were carried o ut to highl i ght the obse rved var ia- bility in the thermal regime of the up per layers of the Arabian Sea during summer monsoon seaso n. These investigations were, however, based on observations made from resear ch ships which usually do n ot have space-time continuity. In these studies considerable smoothing in space-time could have smeared out some interesting feature .. Further, su ffi cient information is not available on these aspects for any single year due to lack of adequate measurements. The cor res- p onding inf ormation on the variability in the salinity field is still mea gre due to lack of salinity measure- ments in the Arabia n Sea (Sast ry a nd D'Souza 1971, Levitu s 1986, Rao and Sa nil Ku ma r 1989). The transects repeated during May (pre-onset regime) and Ju ly! August (post-o nset regime) were fcw even during Int ernat ional Indian Ocean Exp ed itio n (1l0E) to discern the t emp oral changes in the thermal and sal init y H:lds ion the Ar abi an Sea (Duing 1970, Wyr tki 1971). 1 he hold expe ri me nts co nd ucte d du ring the summer mo nsoon Seasons of 1973, 1977 and 1979 pro vided very useful da ta on the obse rved intra-se aso nal varia - bility in t he waterrnass structur e at some statio nary positions and along some typical zonal and meridional transects in t he . Ar abian Sea. .Differ ent physical pro - ccsscs '\0ith varying Importance In the space-lime domain appear to play. significant role in producing the se obse rved vana uo ns. Coa stal upwellmg, latcral advecti on from boundary areas into interior of the basin, surface heat excha nge processes, entramrnent across the mixed layer base are all identified as important mechanisms responsible for the observed changes. Un der the mon- soo nal drive, the clockwise gyral circulatio n advects high saline waters from the n orthern Arabian Sea southward. These high saline waters form a tongue shaped traje ctory willI a d ownward slope t owards south while maintaining its ill situ equilibrium. This high saline sub-surface core (» 36 ppt) below the mixed layer is usually observed in tho east cen tra l Arabian Sea before the onset of the monso on. With the onset and progress of the m onso on the vertical mixing pene- tratesd own ward s and destroys this sub-surface salinity (611 I
