IUGG 2003 Abstract

P05-Posters The Physical Oceanography of the Indian Ocean Monday, June 30 PM Location: Site D

TIME [ 1645-262 ] [ P05/30P/D-006 ] [ Poster ]

VARIABILITY OF EQUATORIAL INDIAN OCEAN ZONAL CURRENTS DURING 1999-2002

Retish SENAN(Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India.)

Debasis SENGUPTA(Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India.) V. S. N. MURTY ( Physical Oceanography Division, National Institute of Oceanography, Goa, India. ) B. N. GOSWAMI ( Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India. ) M. S. S. SARMA ( Physical Oceanography Division, National Institute of Oceanography, Goa, India. )

A subsurface current meter mooring, with six current meters at depths from 100m to 4000m, was placed at 93E on the equator as a part of the Indian Ocean Observing System Programme in February 2000. The currents from these instruments show rich variability on intraseasonal to interannual time scales. We interpret these observations with the help of an ocean general circulation model (OGCM) forced by three day scatterometer winds from the QuikSCAT satellite during September 1999-December 2002. The OGCM zonal currents in the upper 200m are in reasonable agreement with currents from the above moorings, as well as a JAMSTEC ADCP mooring at 90E. Zonal Pressure gradient (ZPG) anomalies from the model agree well with those estimated from TOPEX/Poseidon sea surface height anomalies. This is not true of the model forced by daily NCEP reanalysis winds. Therefore the good performance of the model can be attributed to the quality of the winds. We find that near-surface zonal flow in the equatorial Indian Ocean (EqIO), including the Wyrtki jets (WJ), is mainly a response to the sum of surface wind stress and ZPG on all time scales from intraseasonal to interannual. The WJ can consist of more than one episode with intraseasonal time scales. The fall jet is generally stronger and longer lived than the spring jet, in agreement with past observations. A new finding is that cumulus heating in the eastern EqIO gives rise to strong intraseasonal zonal wind stress during the summer monsoon; these winds drive intraseasonal ″monsoon jets″ in the region 80E-95E. The WJ deepen the thermocline in the eastern EqIO by advection. The resultant enhanced positive ZPG in the upper ocean, moving westward at equatorial Rossby wave speeds (50-60 cm/s), decelerates the jets although the surface wind stress continues to supply eastward momentum. Subsurface (120m-200m) ZPG associated with thermocline displacements give rise to eastward or westward equatorial undercurrents (EUC). The dominant balance at these depths is between zonal acceleration and ZPG, which is a simpler dynamical balance than that in the Atlantic or Pacific. The zonal wind stress in the EqIO is always eastward, except in winter 2001-2002. This is the only period when the surface ZPG in the EqIO is westward, and there is a prominent eastward EUC with upwelling above the core. The presence of a favourable ZPG gives rise to an anomalously strong spring EJ in 2002.