Intraseasonal Modulation of Summer Precipitation over North America

Abstract The intraseasonal rainfall variability over North America is examined using singular spectrum analysis (SSA) and composites of outgoing longwave radiation anomalies (OLRAs), 200-hPa divergence and a gridded rainfall dataset over the United States. The evolution of the Arizona and New Mexico (AZNM) monsoon based on composites indicates that rainfall anomalies propagate eastward from the North Pacific through AZNM, the Great Plains, to the eastern United States. During summer, the wet and dry periods of the AZNM monsoon are modulated by an oscillatory mode with a period of 22–25 days (22-day mode). This is also the dominant mode associated with rainfall events over the Great Plains. The influence of the Madden–Julian Oscillation (MJO) on the AZNM monsoon is secondary. The strongest impact of the MJO is on precipitation over Mexico. SSA performed on the 200-hPa divergence and OLRAs averaged over Mexico show only one oscillatory mode with a period of about 36–40 days. The 22–25-day mode also exists i...

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