| Summary: | The causes of the seasonal cycles of the subtropical anticyclones, and the associated zonal asymmetries of sea surface temperature (SST) across the subtropical oceans, are examined. In all basins the cool waters in the east and warm waters in the west are sustained by a mix of atmosphere and ocean processes. When the anticyclones are best developed, during local summer, subsidence and equatorward advection on the eastern flanks of the anticyclones cool SSTs, while poleward flow on the western flanks warms SSTs. During local winter the SST asymmetry across the subtropical North Atlantic and North Pacific is maintained by warm water advection in the western boundary currents that offsets the large extraction of heat by advection of cold, dry air of the continents and by transient eddies. In the Southern Hemisphere ocean processes are equally important in cooling the eastern oceans by upwelling and advection during local winter. Ocean dynamics are important in amplifying the SST asymmetry, as experiments with general circulation models show. This amplification has little impact on the seasonal cycle of the anticyclones in the Northern Hemisphere, strengthens the anticyclones in the Southern Hemisphere, and helps position the anticyclones over the eastern basins in both hemispheres. Experiments with an idealized model are used to suggest that the subtropical anticyclones arise fundamentally as a response to monsoonal heating over land but need further amplification to bring them up to observed strength. The amplification is provided by local air–sea interaction. The SST asymmetry, generated through local air–sea interaction by the weak anticyclones forced by heating over land, stabilizes the atmosphere to deep convection in the east and destabilizes it in the west. Convection spreads from the land regions to the adjacent regions of the western subtropical oceans, and the enhanced zonal asymmetry of atmospheric heating strengthens the subtropical anticyclones.
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