1990 Articles

The tropical circulation: Simple model versus general circulation model

Robertson, Andrew W.; Frankignoul, Claude

An attempt is made to simulate two general-circulation-model-produced February climatologies in the tropics (from the GISS and the LMD GCMs), using a linear steady-state multi-layer primitive equations model which is forced thermally. A hypothesis-testing strategy is used to quantify the simulations of the target climatologies taking into account the natural variability of the GCM fields. When the zonal-mean flow and the diabatic heating field are prescribed from the GCM, the linear model is found to be generally consistent with both GCMs at the 5% level of statistical significance. The linear model simulation of the GCM is most successful for the vertical velocity. The horizontal wind field is moderately well simulated at low and high levels, but poorly in the mid troposphere. More than 40% of the spatial variance at 850 and 950mb between 30°S and 15°N is explained in the LMD GCM climatology whereas the corresponding figure for the GISS climatology is 34% at 850mb, falling off toward the surface. the linear model's skill at simulating the LMD climatology near the ground is little diminished when the forcing is restricted to the trade-cumulus layer only. However, midtropospheric heating is found to be important for the zonal component of the flow at low levels, especially close to the equator. Sensitivity, especially at upper levels, was found to the inclusion of the zonal component of the mean wind in the linearization, but little to the mean meridional circulation. Sensitivity to the linear model dissipation is also discussed.