Thermohaline Circulation Enhanced by Wind Forcing

Abstract

A thermohaline circulation enhanced by wind forcing is demonstrated in an idealized basin model, and a mechanism that provides a connection between wind forcing and a thermohaline circulation is clarified. A rectangular ocean that extends over the Northern and Southern Hemispheres is driven by differential heating and wind stress at the sea surface. The differential heating is so distributed that the deep water is formed at the southern end of the model ocean. The wind stress is so distributed that there are three wind-driven gyres in the Northern Hemisphere, and it is not imposed in the Southern Hemisphere. Comparison is made between the cases with and without the wind stress. When the wind forcing is imposed, the basin-scale meridional circulation increases in intensity. This is due to the enhanced surface heating in the cyclonic wind-driven gyre with the Ekman upwelling and the accompanying enhanced surface cooling in the deep-water formation region. In the cyclonic wind-driven gyre, the Ekman upwelling brings up the thermocline to the subsurface depths to enhance the surface heating and also the downward heat conduction from the sea surface to the deep layer, which leads to warming of the deep water. Thus, the enhanced surface heating in the cyclonic gyre is balanced with the enhanced surface cooling in the deep-water formation region due to the warmed deep water. In this way, the wind forcing enhances a thermohaline circulation that connects the deep-water formation region to the cyclonic wind-driven gyre with the Ekman upwelling.