Inter-Annual Variability of Currents, Temperature and Salinity during 1998-2012 in a High-Resolution Ocean Model of the Western Indian Ocean

Abstract:

A simulation of the 3-dimensional ocean temperature, salinity and currents of from 1998 through 2012 was done using the Navy Coastal Ocean Model (NCOM) with a 6-km resolution and 40 vertical sigma-z levels. The forcing is 3-hourly atmospheric output from the Navy’s Operational Global Prediction System (NOGAPS). The boundary conditions are provided by the 1/8^o Global NCOM model. In addition, 8 semi-diurnal and diurnal tidal components and monthly climatological discharges of 37 rivers are included. The model assimilates satellite altimeter data, AVHRR SST and synthetic temperature and salinity profiles from the Modular Ocean Data Assimilation System (MODAS). The model output is filtered to remove tides and saved as daily averages. From out analysis we find large inter-annual variability in the strength of the South Equatorial Counter Current and in the subsurface temperature cold anomaly associated with Seyshelles-Chagos thermocline ridge. The cold anomaly was significantly reduced in 2007 during which had a rare co-occurrence of La Nina and positive Indian Ocean Dipole conditions. Year to year variability in the equatorial currents affects the Somali Current and the time of development and strength of the Great Whirl during the southwest monsoon.