The Stationary Wave Response to a Midlatitude SST Anomaly in an Idealized GCM

Abstract

The atmospheric stationary wave response to a midlatitude sea surface temperature (SST) anomaly is examined with an idealized general circulation model (GCM) as well as steady linear model, in a similar way as Ting and Held, for a tropical SST anomaly. The control climate of the GCM is zonally symmetric; this symmetric climate is then perturbed by a monopole SST anomaly centered at 40°N. Two experiments, with SST anomalies of opposite sign, have been conducted. The stationary response is roughly linear in the sign of the SST anomaly, despite the fact that precipitation shows strong nonlinearity. The linear model, which is an exact linearization of the GCM equations in use, when forced by anomalous heating and transients, reproduces the GCM's stationary response excellently. The low-level transient eddy heat fluxes act to damp the lower level temperature signal. When this damping effect is mimicked by a horizontal thermal diffusion in the linear model, the response to the diabatic heating alone gives a reasonably good simulation of the GCM's anomaly; the effect of the anomalous transient momentum fluxes is relatively small. A crude latent heat parameterization scheme, using an evaporation anomaly that is proportional to the mean air?sea surface moisture difference and including the effects of mean moisture advection, is developed. When the perturbation mixing ratio is approximated by assuming fixed relative humidity and by linearizing the Clausius?Clapeyron equation, the linear model's response, utilizing this latent heat parameterization scheme, gives a useful fit to the GCM's anomalous flow.