Convective Forcing of the North American Monsoon Anticyclone at Intraseasonal and Interannual Time Scales

During boreal summer, the North American Monsoon Anticyclone (NAMA) plays an important role in the large-scale transport and mixing of air in the upper troposphere and lower stratosphere. Due to the confining nature of the anticyclonic flow, the anticyclone interior is associated with higher moisture. Since the strength, size, and dynamics of the anticyclone will modulate the transport of water vapor from the North American region to the global stratosphere, it is critical to understand the dynamics of the NAMA and how it relates to its forcing. Precipitation associated with the North American Monsoon (NAM) is regarded as the forcing for the NAMA, and this has been verified in theoretical and modeling studies. However, the connection between NAM precipitation and the NAMA is not well understood and so far sparsely explored using observations. In this study, we utilized the ERA5 reanalysis in conjunction with satellite-derived outgoing longwave radiation (OLR) and precipitation rates to examine the connection between monsoonal precipitation and the NAMA. Lag correlations at the intraseasonal time scale were assessed by correlating pentad time series of OLR and geopotential height. We show that decreased OLR (increased precipitation) over Mexico leads the increase of geopotential heights over the U.S. and East Pacific in the 300100 hPa layer, consistent with the theory that latent heating from the NAM forces the NAMA. In the lower troposphere (800650 hPa), heights are lower following periods of strong precipitation, showing that the vertical structure of the response is baroclinic as described by Gill (1980). Temperatures also exhibit significant correlations with OLR, suggesting the formation of a cold (warm) anomaly above (below) 150 hPa. Correlations using seasonal-mean time series show that the spatial and vertical pattern of correlations between OLR and heights/temperature are similar to those at the intraseasonal time scale, demonstrating that precipitation is related to the NAMA at both time scales in a similar manner.