The extratropical signal generated by a midlatitude SST anomaly. Part I: Sensitivity at equilibrium

A simple GCM (SGCM) is constructed by adding empirically derived time-indep endent forcing terms to a dry primitive equation model. This yields a model with realistic time-mean jets and storm tracks. The SGCM is then used to s tudy the equilibrium response to an imposed heating anomaly in the midlatit ude Pacific, meant to represent an anomaly in the sea surface temperature. Using the SGCM's own climatology as a basic state, the same model is then u sed to find the time-independent linear response to the same heating anomal y. The difference between the two responses is clearly attributed to the fo rcing due to anomalous transient eddies. The sensitivity of the response to the strength and vertical profile of the heating, and to the presence of the wind speed in the surface flux paramet erization, is explored. It is found that for a reasonable range of heating amplitude the transient eddy forcing is proportional to the heating and the responses to heating and cooling are almost antisymmetric. The antisymmetr y breaks down at large amplitude. The vertical profile of heating has a sma ll but systematic effect on the response: deeper heating leads to stronger equivalent barotropic features. The inclusion of wind speed in the surface flux parameterization alters the response mainly by virtue of altering the basic model climatology, rather than by any local effect on the heating. The position of the heating anomaly is varied in both latitude and longitud e to gain insight into the possible effects of systematic errors in GCMs. T he time-independent linear response tends to move with the heating, but the eddy-driven nonlinear part remains relatively fixed and varies only in amp litude. The heating perturbation slightly modifies the first empirical orth ogonal function of the model's internal low frequency variability. The resp onse projects strongly onto this pattern and the probability distribution f unction of the projection is significantly skewed.