AbstractA recent version of the GLA fourth‐order GCM is used to investigate the short‐term relationship between tropical heating and upper tropospheric subtropical westerly maxima in the summer hemisphere. Emphasis is focused on a 15‐day forecast of the circulation corresponding to 6‐20 January 1979. During this time, observational data from the FGGE suggested that strong convection over the South Pacific was helping to maintain a nearby subtropical jet. the dominant term in the zonal momentum budget was the Coriolis force applied to the diabatically driven meridional circulation, although the mean advection of zonal momentum and transient eddy momentum and sensible‐heat transports made important contributions. A ‘full‐physics’ forecast made with the GCM is able to simulate these processes to a reasonable degree of accuracy.To examine the impact of direct meridional overturning on the rotational flow in the South Pacific, the latent energy supplied to the model atmosphere is reduced in two ways. First, negative sea‐surface‐temperature anomalies are imposed in the area of maximum convection. Second, temperature changes resulting from precipitation processes are ignored in the model thermodynamic equation. Results from both types of experiments are consistent, and they substantiate our earlier observational findings that, at least for this case study, summertime subtropical westerly maxima are largely forced by divergent outfl