Meridional Heat Transport by the Subtropical Cell

The wind-driven circulation adds a significant contribution to poleward meridional heat transport. Numerical models indicate that equatorward of ϕ0, the zero wind stress latitude (30° lat), most of the wind-induced heat transport is due to the meridional overturning circulation known as the subtropical cell. The volume transport of this overturning is approximately given by the surface Ekman transport. By combining this fact with the assumption that Ekman-downwelled water approximately follows isotherms except near the equator, the authors derive an expression for the meridional heat transport that depends only on wind stress and surface temperature. The expression is confirmed in numerical models with simplified geometry and forcing. Numerical results indicate that peak heat transport due to the subtropical cell is about 0.1 × 1015 W for the North Atlantic and 0.3 × 1015 W for the North Pacific.