In boreal winter, subtropical westerlies in the upper troposphere reach maxima in speed over the eastern edges of the Asian and North American continents. The zonal variations in the westerlies are generally attributed to large-scale orography and thermal forcing, but the latter mechanism remains largely unsubstantiated. Here we conduct general circulation model (GCM) experiments without orography to identify the most important thermal forcing for generating zonal asymmetries in subtropical westerlies. By changing sea surface temperature (SST) distribution in the GCM, we find that the tropical SST distribution plays a decisive role in producing a subtropical jet core to the north of the tropical warm water pool, while the effects of extratropical continent-ocean heating contrast on upper-level zonal wind speed distribution are secondary. The results from Aqua Planet runs further support this conclusion. ¿ 2000 American Geophysical Union