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BEHAVIOR OF MIDLATITUDE DECADAL OSCILLATIONS IN A SIMPLE ATMOSPHERE-OCEAN SYSTEM

 

Masahiro Watanabe (Center for Climate System Research, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan)

e-mail: hiro@ccsr.u-tokyo.ac.jp

Masahide Kimoto (Center for Climate System Research, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan)

 

ABSTRACT

 

Toward a physical understanding of the decadal oscillation found in midlatitude sea surface temperatures (SSTs), a numerical study was carried out using a simple atmosphere-ocean coupled model. The ocean part consists of a linearized model for the constant-depth upper layer, while the atmospheric part is an empirical model adopting a statistical relationship between SST and surface wind anomalies. The coupled system was integrated to investigate the behavior of numerical solutions depending on three parameters of the mean current strength (λc), dynamical and thermodynamic coupling coefficients (λt and λh, respectively).

Numerical solution with standard parameters yields decadal-scale oscillations in the North Atlanitc and the North Pacific, respectively. Temporal evolutions of the temperature anomalies are similar to observed decadal cycles, except for the amplitude much smaller than observations. Behavior of solutions in the parameter space of λc and λt shows that the advective effect by the mean currents, as well as the Rossby wave adjustment due to wind stress curl anomalies, is of importance for the oscillatory mechanism. It is noted that the oscillation often occurs for extreme cases of λt = 0 or λc = 0.

Stochastic weather noise introduced into the system increases the variance of decadal oscil-lation if the noise is distinctly larger than the coupled anomalies. However, the noise forcing without coupled components only reddens the temperature spectrum without distinct decadal peak, indicating the significant contribution of the air-sea coupling to the decadal variability even if the coupling is weak and/or the thermal feedback is negative.

 

INTRODUCTION

 

It is known that decadal-scale climate variations are found in the extratropical Pacific and Atlantic Oceans. Observational and numerical studies reported in the last decade indicate that they are primarily regarded as fluctuations of the atmosphere-ocean system. On the other hand, it is relevant to consider that the memory of such a slowly-varying change resides in the ocean having larger inertia than the atmosphere.

Spatial and temporal features of decadal/interdecadal variations have been documented for observed sea surface temperature (SST) and associated surface atmospheric conditions over the North Pacific (Tanimoto et al. 1993; Trenberth and Hurrell 1994) and over the North Atlantic (Deser and Blackmon 1993). On the other hand, some insights for the causal mechanism of the decadal-scale variations in the atmosphere-ocean system are obtained from a series of numerical simulations using coupled general circulation models (CGCMs). Latif and Barnett (1994, 1996) found the North Pacific interdecadal variations similar to observations in their CGCM and suggested the dominant role of the ocean dynamics. It is argued that the slow dynamic adjustment of the upper-ocean circulation associated with long Rossby waves is responsible for the phase reversal in the simulated variability (Robertson 1996; Yukimoto et al. 1996). The ocean dynamic response seems to have a certain role in the observed variations as in the simulations in the North Pacific (Zhang and Levitus 1997). On the other hand, it is suggested that the advective effect by the mean subtropical gyre is as important as the ocean adjustment in the decadal variations in the North Atlantic (Sutton and Allen 1997; Watanabe et al. 1998).

 

 

 

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