Formation of subtropical westerly jet core in an idealized GCM without mountains

In boreal winter, subtropical westerlies in the upper troposphere reach max ima in speed over the eastern edges of the Asian and North American contine nts. The zonal variations in the westerlies are generally attributed to lar ge-scale orography and thermal forcing, but the latter mechanism remains la rgely unsubstantiated. Here we conduct general circulation model (GCM) expe riments without orography to identify the most important thermal forcing fo r generating zonal asymmetries in subtropical westerlies. By changing sea s urface 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 extratr opical continent-ocean heating contrast on upper-level zonal wind speed dis tribution are secondary. The results from Aqua:Planet runs further support this conclusion.