Adjustment of the Remote Tropical Climate to El Niño Conditions

Abstract

The adjustment of the tropical climate outside the Pacific (the ?remote Tropics?) to the abrupt onset of El Niño conditions is examined in a tropical atmosphere model that assumes simplified vertical structure and quasi-equilibrium (QE) convective closure. The El Niño signal is rapidly (?1 week) communicated to the remote Tropics via an eastward-propagating Kelvin-like wave that induces both anomalous subsidence and tropospheric warming. Widespread reductions in convective precipitation occur in conjunction with the spreading of the temperature and subsidence anomalies. The remote rainfall suppression peaks roughly 5?15 days after the initiation of El Niño conditions, after which the anomalous remote rainfall field recovers to a state characterized by a smaller remote areal mean rainfall deficit and the appearance of localized positive rainfall anomalies. The initial remote precipitation reduction after El Niño onset is tied to both tropospheric warming (i.e., stabilization of the troposphere to deep convection) and the suppression of remote humidity levels; recovery of the initial deficits occurs as feedbacks modulate the subsequent evolution of humidity anomalies in the tropospheric column. Apart from the short-term response, there is a longer-term adjustment of the remote climate related to the thermal inertia of the underlying surface: surface thermal disequilibrium, which is related to the depth of the ocean mixed layer, maintains larger precipitation deficits than would be expected for equilibrated conditions. This result supports a previous prediction by one of the authors for a significant disequilibrium mechanism in the precipitation teleconnection to El Niño resulting from the local vertical coupling of the troposphere to the surface through moist convection.