The response of the atmospheric winter circulation in both hemispheres to changes in the meridional gradient of sea surface temperature (SST) is examined in an atmospheric general circulation model. Climatological SSTs are employed for the control run. The other runs differ in that a zonally symmetric component is added to or subtracted from the climatological SST field. The meridional structure of the variation in SST gradient is based on the observed change in zonally averaged SST over the last century. The SST trend has maxima of about 1 K at high latitudes of both hemispheres. Elsewhere, the increase in SST over the last century is fairly uniform at about 0.5 K.

In both hemispheres the response to decreased SST gradients is decreased baroclinity in the lower troposphere and increased baroclinity in the upper troposphere, with the reverse response when the SST gradient is increased. Because the cases with decreased SST gradients correspond to warmer SSTs everywhere, they are accompanied by an increase in moisture and a general expansion of the troposphere. The warming cases in the Northern Hemisphere (NH) winter are marked by greatly increased tropical convection, a stronger subtropical jet that is shifted upward and equatorward, and a robust stationary-wave response. Many aspects of the response are remarkably consistent among the different warming experiments, both in pattern and amplitude. The storm-track response is weaker but still consistent among the different warming experiments. Despite general decrease in storm-track activity, there is a tendency for the upper-level NH storm tracks to strengthen at their downstream end and to weaken at their upstream and northward end. When the zonally symmetric SST anomaly field is subtracted from the climatological SST (resulting in lower SST with increased latitudinal gradient), the response is different in many fields and is considerably weaker.

In the winter Southern Hemisphere the change in baroclinity of the low-level flow plays a greater role in the response than in the winter NH. The response in the storm track is zonal with a decrease in midlatitude storm-track activity in the warming cases and an increase in the case that has an increased SST gradient (and cooler SST). There is close correspondence between the pattern of response in all the experiments, irrespective of the sign of the SST anomaly field.

The nonlinear behavior of planetary waves excited by midlatitude topography is considered in an atmospheric GCM. The GCM is run at standard resolution (T42) and includes all of the complexity normally associated with a GCM. Only two simplifications are made to the model. First, it is run in perpetual January mode, so that the solar radiation takes the diurnally varying value associated with 15 January. Second, the lower boundary is simplified so that it is entirely ocean with zonally symmetric SSTs. Planetary waves are excited by Gaussian-shaped topography centered at 45°N, 90°W. As in earlier studies, the excited wave train propagates toward low latitudes where, for sufficiently large forcing amplitude (i.e., height of topography), the wave will break. Several different experiments are run with different mountain heights. Each experiment is run for a total of 4015 days.

The response of the model depends on the height of the mountain. For the small-amplitude mountain (500 m), the wave is dissipated at low latitudes near its critical latitude. For large-amplitude mountains (2000, 3000, and 4000 m), wave breaking and nonlinear reflection out of the wave breaking region is observed. The spatial character of the reflected wave train is similar to that detected in earlier studies with more idealized models.

The nonlinear behavior of quasi-stationary planetary waves excited by midlatitude orographic forcing is considered in a three-dimensional primitive equation model that includes a representation of the Hadley circulation. The Hadley circulation is forced by Newtonian cooling to a zonally symmetric reference temperature and vertical diffusion on the zonally symmetric component of the flow. To quantify the effect of the Hadley circulation on wave propagation, breaking, and nonlinear reflection, an initial state with no meridional flow, but with the same zonal flow as the Hadley state, is also considered. In order to allow the propagation of large-scale waves over extended periods, Rayleigh friction is applied at low levels to delay the onset of baroclinic instability.

As in the absence of a Hadley circulation, the waves in the Hadley state propagate toward low latitudes where the background flow is weak and the waves are therefore likely to break. Potential vorticity fields on isentropic surfaces are used to diagnose wave breaking. Nonlinear pseudomomentum conservation relations are used to quantify the absorption–reflection behavior of the wave breaking region. In the presence of a Hadley circulation representative of winter conditions, the nonlinear reflection requires more forcing to get established, but a reflected wave train is still present in the numerical simulations, both for a longitudinally symmetric forcing and for the more realistic case of an isolated forcing. The effect of the thermal damping on the waves is more severe in the current three-dimensional simulations than in the shallow water case considered in an earlier study. Both the directly forced wave train and the reflected wave train are quite barotropic in character; however, in the shallow water case one is essentially assuming an infinite vertical scale.