Adjustment of the remote tropical climate system to El Niño conditions

The adjustment of tropical climate outside of the Pacific (the “remote tropics”) to the abrupt onset of El Niño conditions is examined in an intermediate level complexity model of the tropical atmosphere. 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 related tropospheric warming. Widespread reductions in convective precipitation occur in conjunction with the propagation of the Kelvin-wave influence. The remote rainfall suppression peaks roughly 5-15 days after the initiation of El Niño conditions, after which the anomalous remote rainfall field evolves toward an equilibrium state characterized by a smaller aggregate remote rainfall deficit and the appearance of localized pockets of positive rainfall anomalies. The temporal behavior indicates both “fast” and “slow” contributions to the remote tropical rainfall response to El Niño. The fast response arises from tropospheric warming (through thermal stabilization of the troposphere to deep convection) as well as an initial suppression of remote humidity levels, both associated with subsidence induced over the remote tropics. The slow response is tied to the climate system feedbacks that modulate the evolution of humidity anomalies in the atmospheric column (and thus the convective reference temperature profile of the model's Betts-Miller convective parameterization), and these in turn are tied to the precipitation response itself. The transition to equilibrium occurs substantially more rapidly over remote land regions relative to remote ocean regions (treated as thermodynamic mixed layer of constant depth). The lengthier adjustment timescale for the remote ocean is associated with more pronounced disequilibrium between the atmosphere and ocean thermal conditions induced by the substantially larger thermal inertia of the remote ocean. This result supports a previous prediction by one of the authors for to the operation of a disequilibrum effect in the precipitation teleconnection to El Niño.