The role of an atmospehric shallow meridional circulation in tropical air-sea interaction

Recently analyses of in situ observations, global reanlayses, and GCM simulations have revealed that there is an atmospheric shallow meridional overturning circulation in the tropical eastern Pacific and Atlantic. The southerly cross-equatorial flow of this shallow circulation is the same as the deep, Hadley-type circulation. But the northerly returning flow is in the lower troposphere, sometimes immediately atop the atmospheric boundary layer, in contrast to that of the Hadley-type circulation in the upper troposphere. This low-level return flow enhances the vertical wind shear cross the top of the boundary layer. It is hypothesized that this enhanced shear may increase momentum entrainment there and act as a brake to the boundary layer southerly flow. It is also hypothesized that the shallow meridional circulation exists only in a absence of deep convection in the ITCZ. The effect of deep convection is to reduce or eliminate this shallow circulation. In consequence, the “braking effect” of the low-level return flow is released. This offers a new mechanisms for air-sea interaction in the tropical region where the ITCZ and cold tongue are prominent, as in the eastern Pacific and Atlantic Oceans.