Baroclinic Adjustment: A Comparison between Theory, Observations, and Models.

Isentropic slopes calculated from Northern Hemisphere analyses of the zonal mean state of the atmosphere are compared with isentropic slopes calculated from baroclinic adjustment theory. In midlatitudes, the isentropic slopes are relatively close to the adjusted values in the layer from 2 to 7 km (800 to 400 mb). They also have very little variation in latitude and season, which implies that baroclinic eddy fluxes supply a strong negative feedback to changes in isentropic slopes. The vertical structure of the temperature field in the `adjusted' layers and the location of the layers suggest that vertical eddy heat fluxes play a significant role in this feedback process.Isentropic slopes are also calculated from simulations of the current climate by the GISS Model II GCM and the NCAR CCM2. Both models have regions of apparent baroclinic adjustment similar to that in the Northern Hemisphere atmosphere. However, compared to the observations, the agreement of the simulations with baroclinic adjustment theory is not as good, and the isentropic slopes show stronger latitudinal and seasonal variations. The discrepancies are associated primarily with errors in the models' meridional temperature gradients in the lower troposphere in midlatitudes. The seasonal changes in these gradients are much larger than in the observations, particularly in the CCM2, even though the model simulations were constrained by climatological sea surface temperatures.