Seasonal and Interannual Variability of eddy field and surface circulation in the Gulf of Aden.
Mohammed Al saafani(1) and Sateesh Shenoi(1)
National Institute of Oceanography,
National Institute of Oceanography, Dona Paula,,
Panaji, Goa 403004,
The circulation in the Gulf of Aden is inferred from three different data sets: historical ship drifts, hydrography, and satellite altimeter derived sea level (Topex/Poseidon, Jason and ERS). The circulation in this semi-enclosed basin is marked with strong seasonality with reversals in the direction of flows twice a year following the reversal in monsoonal winds. During the winter monsoon (November - February) there is an inflow from Arabian Sea; an extension of Arabian Coastal Current (ACC). During southwest monsoon (June - August) the flow is generally towards east especially along the northern coast of Gulf of Aden. The geostrophic currents also show that the circulation in the gulf is embedded with mesoscale eddies. These westward propagating eddies appear to enter the Gulf of Aden from the western Arabian Sea in winter. The relative contributions of mesoscale eddies, annual signal and interannual signal to the circulation in the gulf were estimated using altimeter derived SLA for the years 1993 to 2003. In the absence of interannual variability, the mesoscale eddies and the annual variability caused by the monsoonal winds appears to be the only contributions to the circulation in the gulf. The contribution from the mesoscale eddies is equal to that from the annual signal. The effect of these mesoscale eddies extend over the entire water column. The vertical structure of density and the geostrophic currents derived from the hydrographic data confirms the presence of eddies at deeper levels (~400 m). The propagation speeds, of these eddies, estimated using weekly spaced altimeter for winter season (2002 - 2003) is ~ 4 cm/s. The propagation speeds of Rossby waves due to first and second baroclinic modes (estimated using the Brunt-Vaisala frequency profiles derived from the hydrographic data for the winter season) works out to be 8 cm/s and 2.5 cm/s respectively and the speed of westward flowing current during winter was 1.3 cm/s. The vector sum of the speeds of second mode Rossby wave and the mean current (3.8 cm/s) matches with the propagation speeds of eddies estimated from the altimeter data (4 cm/s). Hence, second mode baroclinic Rossby waves are responsible for the westward propagation of eddies in the Gulf of Aden. The presence of these eddies in the temperature-salinity climatology confirms that they are not transient features.