The influence of air-sea coupling on the path of the Gulf Stream

The Gulf Stream (GS) is known to have a strong influence on the climate and on transports of heat from the tropics to middle and high latitudes. Although the GS strength presents a clear interannual variability, satellite observations reveal its mean path is stable and does not present a strong inter-annual variability. Uncoupled ocean models, forced by an atmospheric product, are capable of realistically simulating the mean GS separation and its post-separation path. However, recent studies point out significant numerical sensitivities remain. Small changes in sub-grid scale parametrization, topography, and surface forcing, can lead to significant changes in the GS path. The reason for this sensitivity is net yet clear, although a strong link between GS separation and the strength and depth of the southward flowing deep western boundary current has been shown and a minimum resolution of 1/10˚ is deemed to be required for proper separation dynamics. We investigate a source of sensitivity that results from the decoupling of the wind stress curl patterns over the North West Atlantic. In an ocean-only model, the change in sign of the wind-stress curl is not related to the position of the GS north wall temperature gradient. In reality, the wind stress curl is strongly coupled through thermal, and ocean surface current feedbacks. The path of the GS will shape the local wind stress. In this presentation, using uncoupled and coupled ocean-atmosphere simulations, we will address to what extent the ocean-atmosphere coupling can reduce the simulated GS path sensitivity and hence improve the realism of the oceanic simulations.