Seasonal variability of upper-layer geostrophic transport in the tropical Indian Ocean during 1992-1996 along TOGA-I XBT tracklines

Upper layer (0-400 m) geostrophic volume transport associated with the major current systems of the tropical Indian Ocean along the near-meridional TOGA-India XBT tracklines from Mumbai to Mauritius during 1992-1996; is discussed. The transport of the eastward flowing Indian Monsoon Current (IMC) varies between 9 and 14 Sv (1 Sv = 10(6) m(3) s(-1)) during southwest monsoon. The westward flowing North Equatorial Current (NEC) transports about 8 Sv during boreal winter. The westward flowing south equatorial current (SEC) has its lowest transport (10-12 Sv) during austral fall and spring and highest transport (18-21 Sv) during austral winter and summer. The northern boundary of the SEC extends equatorward as far north as for 4-5 degrees S during southwest monsoon from its usual location of 8-10 degrees S. The eastward flowing South Equatorial Counter Current (SECC) exhibits highest transport (37 Sv) during peak austral summer (January) and between 5 and 20 Sv during the rest of the year. In March and May, eastward undercurrents are noticed near the equator. In June and July, the undercurrents are present south of Sri Lanka away from the equator. The equatorial flow in the zonal belts 1-2 degrees N and 1-2 degrees S exhibits westward transport of 6-20 Sv during January-February. This broad westward flow and the SECC constitute an anticlockwise equatorial gyre during boreal winter. In addition to the above current systems, signatures of a warm-core anticyclonic eddy centered at 8 degrees N, 72.5 degrees E and a cold-core cyclonic eddy centered at 10.5 degrees N, 72.5 degrees E are also noticed in February and October, respectively, off the southwest coast of India. The volume transport associated with the warm-core eddy is about 10 Sv directed equatorward and that with the cold-core eddy is 2.5 Sv directed poleward. (C) 2000 Elsevier Science Ltd. All rights reserved.