A dry primitive equation model is used to investigate the remote response t
o a fixed tropical heat source. The basic forcing for the model takes the f
orm of time-independent terms added to the prognostic equations in two conf
igurations. One produces a perturbation model, in which anomalies grow on a
fixed basic state. The other gives a simple GCM, which can be integrated f
or a long time and delivers a realistic climate simulation with realistic s
torm tracks. A series of experiments is performed, including 15-day perturb
ation runs, ensemble experiments, and long equilibrium runs, to isolate dif
ferent dynamical influences on the fully developed Pacific-North American (
PNA) type response to an equatorial heating anomaly centered on the date li
ne.
The direct linear response is found to be very sensitive to changes in the
basic state of the same order as the atmosphere's natural variability, and
to the natural progression of the basic state over the time period required
to set up the response. However, interactions with synoptic-scale noise in
the ambient flow are found to have very little systematic effect on the li
near response. Nonlinear interactions with a fixed basic state lead to chan
ges in the position, but not the amplitude, of the response. Feedback with
finite-amplitude transient eddies leads to downstream amplification of the
PNA pattern, both within the setup time for the response and in a fully adj
usted equilibrium situation.
Nonlinearity of the midlatitude dynamics gives rise to considerable asymmet
ry between the response to tropical heating and the response to an equal an
d opposite cooling.