| dc.description.abstract | During the boreal warm season (May through September), the circulation in the upper troposphere and lower stratosphere is dominated by two large anticyclones: the Asian monsoon anticyclone (AMA) and North American monsoon anticyclone (NAMA). Due to variations in the diabatic heating, interactions with Rossby waves propagating along the subtropical jet, and internal dynamics within the anticyclone, the circulation of the AMA is unsteady. Here we use the ERA-Interim Reanalysis and trajectories computed with ERA-Interim winds to show that a single circulation center is present only 23% of the time, while two or three sub-vortices are simultaneously present 69% of the time. More than three simultaneous sub-vortices are uncommon. Observed behaviors of the regional sub-vortices include (1) splitting of a single vortex into two vortices; (2) merger of two vortices into a single vortex; (3) vortex shedding in the eastward direction; (4) vortex shedding in the westward direction; and (5) formation, movement, and dissipation of a vortex. The transitions between different states of sub-vortices are associated with the appearance and disappearance of hyperbolic regions between the sub-vortices.
The existence of the AMA has long been linked to Asian monsoon precipitation using the Matsuno–Gill framework, but the origin of the NAMA has not been clearly understood. Here the forcing mechanisms of the NAMA are investigated using a simplified dry general circulation model. The simulated anticyclones are in good agreement with observations when the model is forced by a zonally-symmetric meridional temperature gradient plus a realistic geographical distribution of heating based on observed tropical and subtropical precipitation in the Northern Hemisphere. Model experiments show that the AMA and NAMA are largely independent of one another, and the NAMA is not a downstream response to the Asian monsoon. The primary forcing of the NAMA is precipitation in the longitude sector between 60° and 120°W, with the largest contribution coming from the subtropical latitudes within that sector. Experiments with idealized regional heating distributions reveal that the extratropical response to tropical and subtropical precipitation depends approximately linearly on the magnitude of the forcing, but nonlinearly on its latitude. The AMA is stronger than the NAMA primarily because precipitation in the subtropics over Asia is much heavier than at similar latitudes in the Western Hemisphere. | en |
