Deep-Sea Research II 49 (2002) 1173–1195 Winter and summer monsoon water mass, heat and freshwater transport changes in the Arabian Sea near 81N Lothar Stramma a, *, Peter Brandt a , Friedrich Schott a , Detlef Quadfasel b , J . urgen Fischer a a Institut f . ur Meereskunde an der Universit . at Kiel, D . usternbrooker Weg 20, 24105 Kiel, Germany b University of Copenhagen, Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark Abstract The differences in the water mass distributions and transports in the Arabian Sea between the summer monsoon of August 1993 and the winter monsoon of January 1998 are investigated, based on two hydrographic sections along approximately 81N. At the western end the sections were closed by a northward leg towards the African continent at about 551E. In the central basin along 81N the monsoon anomalies of the temperature and density below the surface- mixed layer were dominated by annual Rossby waves propagating westward across the Arabian Sea. In the northwesternpartofthebasintheannualRossbywaveshavemuchsmallerimpact,andthedensityanomaliesobserved there were mostly associated with the Socotra Gyre. Salinity and oxygen differences along the section reflect local processessuchasthespreadingofwatermassesoriginatingintheBayofBengal,northwardtransportofIndianCentral Water, or slightly stronger southward spreading of Red Sea Water in August than in January. The anomalous wind conditions of 1997/98 influenced only the upper 50–100m with warmer surface waters in January 1998, and Bay of Bengal Water covered the surface layer of the section in the eastern Arabian Sea. Estimates of the overturning circulationoftheArabianSeawerecarriedoutdespitethefactthatmanyuncertaintiesareinvolved.Forbothcruisesa verticaloverturningcellofabout4–6Svwasdetermined,withinflowbelow2500mandoutflowbetweenabout300and 2500m.Intheupper300–450maseasonallyreversingshallowmeridionaloverturningcellappearstoexistinwhichthe Ekman transport is balanced by a geostrophic transport. The heat flux across 81N is dominated by the Ekman transport, yielding about –0.6PW for August 1993, and 0.24PW for January 1998. These values are comparable to climatological and model derived heat flux estimates. Freshwater fluxes across 81N also were computed, yielding northward freshwater fluxes of 0.07Sv in January 1998 and 0.43Sv in August 1993. From climatological salinities the strongerfreshwaterfluxinAugustwasfoundtobecausedbytheseasonalchangeofsalinitystorageintheArabianSea north of 81N. The near-surface circulation follows complex pathways, with generally cyclonic-circulation in January 1998 affected at the eastern side by the Laccadive High, and anticyclonic circulation in August 1993. r 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction The Arabian Sea undergoes large changes in response to the seasonal reversal of the monsoon winds, leading to a reversal of the Arabian Sea circulation and the Somali Current during the *Corresponding author. Tel.: +49-431-600-4103; fax: +49- 431-600-174101. E-mail address: lstramma@ifm.uni-kiel.de (L. Stramma). 0967-0645/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0967-0645(01)00169-2