Origin of the Upper-Tropospheric North American Monsoon Anticyclone

During the boreal warm season, the circulation of the upper troposphere and lower stratosphere is dominated by the Asian monsoon anticyclone (AMA) and North American monsoon anticyclone (NAMA). The existence of the AMA has long been linked to the neighboring Asian monsoon precipitation using the Gill–Matsuno framework. Satellite observations show that the western-hemisphere precipitation is dominated by the Intertropical Convergence Zone (ITCZ). The origin and dynamics of the NAMA are examined using a simplified dry General Circulation Model. The simulated anticyclones are in good agreement with observations when the model is thermally forced by realistic heating distributions. By partitioning the western-hemisphere heating, the tropical and subtropical parts of 120–60W longitude sector are found to contribute to most of the NAMA response. The dynamics of the responses driven by the western-hemisphere heating is investigated using idealized heating distributions. For the most part of the ITCZ, the low latitude location and the narrow shape are not effective to excite the planetary wave response because of the small Coriolis parameter therein. On the contrary, the zonally asymmetric precipitation near the eastern Pacific ITCZ and the North American monsoon region is responsible for driving the NAMA response. The heavier precipitation and more effective vortex stretching mechanism at higher latitudes over Asia are the major causes of the AMA being stronger than the NAMA.