Agulhas Current Seasonality: sensitivity to Rossby wave propagation speed, and the influence of local versus remote winds
The Agulhas Current plays a critical role in both local and global ocean circulation and climate regulation, yet the mechanisms that determine the seasonal cycle of the current remain poorly understood. Model studies predict an austral winter-spring (July-October) maximum in volume transport, whilst observations reveal an austral summertime (January- March) maximum. Here, the seasonality of the Agulhas Current is investigated using shallow water models forced with climatological winds. A reduced gravity first baroclinic mode model is shown to successfully reproduce the seasonal phasing of the current. This seasonal variability is found to be highly sensitive to the propagation speed of Rossby waves. By matching Rossby wave speeds to those observed using altimetry, an Agulhas Current with a maximum flow in January-February-March and a minimum flow in July is simulated, agreeing well with observations. Near-field winds, to the west of 35E, dominate this seasonality, as signals from more remote wind forcing dissipate due to destructive interference while crossing the basin. A barotropic single layer model is unable to correctly capture the magnitude, position and seasonal cycle of the Agulhas Current, predicting a wintertime maximum in transport. Results from the barotropic simulations are similar to previous model studies, where seasonality is dominated by a southward propagation of signals via the Mozambique Channel and along the Mozambique Ridge, suggesting that these models are too barotopic in their response to the winds. Findings from this study elucidate the role of near-field winds and baroclinic processes in determining the seasonality of the Agulhas Current.