The ITCZ is deemed to play an active role in air-sea interaction in the eastern Pacific by modulating surface southerly inflow across the equatorial cold tongue and SST front. In one commonly accepted mechanism for this role, an increase in deep convection in the ITCZ tends to enhance the local meridional circulation including its lower branch, the southerly inflow in the boundary layer. This mechanism of thermally direct response (TDR) is, however, inconsistent to theories that predict deep convective heating can efficiently induce atmospheric response only in the free troposphere, not near the surface. Here, we propose an alternative mechanism based on the existence of a shallow meridional circulation (SMC).
Observations from EPIC2001 and other recently studies have shown: (i) There often is a shallow meridional circulation associated with the ITCZ in the eastern Pacific, with its southerly branch in the boundary layer and its northerly branch immediately atop the boundary layer as a low-level return flow. (ii) The extraordinary vertical shear associated with the low-level return flow induces strong turbulent entrainment at the top of the boundary layer; the entrainment is a major component of the meridional momentum balance of the boundary layer and it constrains the surface southerly flow in the presence of strong meridional pressure gradient due to SST.
In the proposed mechanism, the SMC is strong in the absence of deep convection in the ITCZ but becomes weaker as deep convection increases. Deep convection in the ITCZ may enhance the surface southerly flow by weakening the SMC and momentum entrainment at the top of the boundary layer associated with its low-level return flow. This mechanism may coexist with the TDR mechanism. Their relative importance to air-sea interaction has yet to be quantified.