IUGG 2003 Abstract

P05 The Physical Oceanography of the Indian Ocean Monday, June 30 PM Location: Site B, Room 23 Presiding Chairs:W. Han, J.S. Godfrey

TIME [ 1710 ] [ P05/30P/B23-008 ]

BIWEEKLY VARIABILITY OF MERIDIONAL CURRENTS AT THE EQUATOR IN THE EASTERN INDIAN OCEAN

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

Retish SENAN(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. ) Vijayan FERNANDO ( 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 observations show subseasonal reversals of meridional velocity (v) at all instrument depths from 100 m to 4000 m, with 10-15 day period in the upper ocean. The 8-21 day filtered v from the model matches the filtered v from the observations almost exactly at all depths down to 1000m. 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 from the model that the fluctuations of equatorial v are associated with equatorially trapped mixed Rossby-gravity (MRG) or Yanai waves with periods of 10-15 days. These vertically propagating MRG waves have a zonal wavelength of about 4000 km and propagate westward at about 4 m/s. They are associated with off-equatorial maxima of zonal velocity (u) which is antisymmetric about the equator. Strong upwelling and downwelling, with amplitude of a few meters per day in the upper ocean, accompanies the observed biweekly oscillations of v throughout the year. The biweekly mode in the ocean is forced mainly by intraseasonal fluctuations of surface wind stress curl. The wind forced biweekly MRG mode also explains past observations of 10-15 day oscillations of equatorial v and off-equatorial u in the central Indian Ocean. The continual deep upwelling in the equatorial region is likely to be important in the physics and biogeochemistry of the Indian Ocean.