The seasonal cycle of the South Indian Ocean from model and altimeter data.
Ricardo Matano(1) and Ted Strub(1)
Oregon State University,
Ocean Admn. Bldg. 104,
The seasonal cycle of the South Indian Ocean has been investigated using the results of numerical simulations and altimeter information. In the first portion of our research we investigated the seasonal response of the South Indian Ocean circulation to the wind stress forcing from a series of process-oriented numerical experiments. According to these experiments the topographic ridges that run south of Madagascar regulate the variability in the south Indian Ocean. These topographic features isolate the western boundary region from the subtropical gyre seasonal timescales. To confirm the conclusions from our process studies we also analyzed the results of an eddy-permitting global simulation (POCM_4C) and altimeter data. The conclusions drawn from the analysis of POCM were consistent with those obtained from our regional model and further revealed that seasonal changes of the Agulhas Current transport are linked to the large-scale circulation in the tropical region (not the subtropics). According to this model, the Agulhas Current transport has a seasonal variation with a maximum at the transition between the austral winter and the austral spring and a minimum between the austral summer and the austral autumn. Regional and basin-scale mass balances indicate that although the mean flow of the Agulhas Current has a substantial contribution from the Indonesian throughflow, there appears to be no dynamical linkage between the seasonal oscillations of these two currents. Instead, we found evidence that the seasonal cycle of the western Indian Ocean is the result of the oscillation of barotropic modes forced directly by the wind. The analysis of altimeter data confirmed that the influence of the Indonesian throughflow on the seasonal cycle appears to be confined to the easternmost portion of the basin, while the influence of the wind stress forcing is important everywhere. Our analysis indicates that seasonal variations of tropical origin propagate to the subtropics in the central portion of the basin and in the western region. In the central region, there is strong evidence of an annual wave that propagates southwestward between ~105˚E and 75˚E in a lapse of ~4 months. The connection between the tropics and subtropics in the western region is less robust. Preliminary calculations using Pathfinder data, however, appears to confirm the propagation of seasonal variations of tropical origin through the Mozambique Channel.