A diabatic mechanism for decadal variability in the tropics

Abstract

Can atmospheric forcing of the ocean in high latitudes induce decadal variability in low latitudes? Most theoretical studies that have considered this question assign a critical role to adiabatic, advective, subsurface oceanic links between the tropics and extra-tropics. Observational evidence of such links is proving elusive. This study posits that given the constraint of a balanced heat budget for the ocean in a state of equilibrium, atmospheric forcing over a broad spectrum of frequencies in high latitudes can force decadal variability in low latitudes without any explicit evidence of oceanic links. The oceanic response to an abrupt change in diabatic forcing, a sudden increase in heat loss in high latitudes say, is characterized by two time-scales. The one, τ w, is relatively short and is associated with planetary and coastal waves that propagate from the disturbed region to the equator (and then back to higher latitudes.) The other, τ d, is on the order of a few years and depends on diabatic processes responsible for increasing the oceanic heat gain in low latitudes. Through these processes the system is driven towards a new balanced heat budget in which the heat gain, mainly in the equatorial upwelling zones, equals the heat loss in high latitudes. When the forcing, rather than abrupt, is sinusoidal with period P, then the amplitude of the response depends on the ratio P/ τ d. The response is modest when that ratio is small because the period P is too short for the ocean to adjust. As P gets larger compared to τ d, the amplitude increases, but explicit evidence of the waves that connect high and low latitudes is very hard to detect. The ocean acts as a low pass filter to the forcing with characteristic timescale τ d.