A Study of the Interaction of the North Atlantic Oscillation with Ocean Circulation

Abstract

Observed patterns of wind stress curl and air-sea heat flux associated with the North Atlantic oscillation (NAO) are used to discuss the response of ocean gyres and thermohaline circulation to NAO forcing and their possible feedback on the NAO. The observations motivate, and are interpreted in the framework of, a simple mathematical model that couples Ekman layers, ocean gyres, and thermohaline circulation to the atmospheric jet stream. Meridional shifts in the zero wind stress curl line are invoked to drive anomalies in ocean gyres, and north-south dipoles in air-sea flux drive anomalous thermohaline circulation. Both gyres and thermohaline circulation play a role in modulating sea surface temperature anomalies and hence, through air-sea interaction, the overlying jet stream. The model, which can be expressed in the form of a delayed oscillator with ocean gyres and/or thermohaline circulation providing the delay, identifies key nondimensional parameters that control whether the ocean responds passively to NAO forcing or actively couples. It suggests that both thermohaline circulation and ocean gyres can play a role in coupled interactions on decadal timescales.

A mechanistic model of mid-latitude decadal climate variability

“…The present conceptual model (4)- (7), in its simplified form (14, 15), is formally similar to delayed oscillators formulated elsewhere (Marshall et al 2000;Dewar 2001); the dynamics it represents, though, is fundamentally different.…”

“…In contrast, the linear models summarized by Marshall et al (2000) can exhibit spectral peaks in an uncoupled setting, in which ocean-state-independent atmospheric noise excites a damped oceanic standing-wave oscillation. Coupling merely enhances such spectral peaks, since a delayed feedback due to planetary wave propagation tends to overcome damping.…”

“…Extending the linear-model analysis of Marshall et al (2000), Dewar (2001) studied effects of oceanic turbulence on coupled mid-latitude variability. The primary effect of coupling in Dewar (2001) was to arrest the inter-gyre heat flux due to the ocean's intrinsic variability on decadal and longer time scales; the atmospheric decadal variability was thus completely controled by the oceanic processes.…”

“…The latter is formally identical to a classical delayed oscillator equation (Bhattacharya et al 1982;Suarez and Schopf 1988;Battisti and Hirst 1989;Bar-Eli and Field 1998;Marshall et al 2000), except that x is a red-noise, rather than a white-noise process. More importantly, however, the possibility of active coupling between x and y stems from the atmospheric model's nonlinear sensitivity to the oceanic state; the latter sensitivity is expressed via non-Gaussian changes to the atmospheric-flow PDF, namely the atmospheric mean state changes as in (14); see also Neelin and Weng (1999).…”

“…The proposed delays have been related to a variety of ocean processes; examples include mean ocean advection (Saravanan and McWilliams 1998) or planetary wave propagation (Jin 1997;Neelin and Weng 1999;Czaja and Marshall 2001). Coupling can amplify the linear modes associated with the above physical processes and imprint the slow time scales onto the NAO variability (Marshall et al 2000).…”