Abstract. We quantify the connection between deep convective occurrence and summertime 100hPa water vapor anomaly over the North American(NA) region and find substantial consistency between their interannual variations and also that the water vapor mixing ratio over the NAregion is up to 1ppmv higher when deep convection occurs. We use a Lagrangian trajectory model to demonstrate that the structure and the location of the NAanticyclone, as well as the tropical upper tropospheric temperature, mediate the moistening impact of convection. The deep convection mainly occurs over the Central Plains region. Most of the convectively moistened air is then transported to the center of the NA anticyclone, and the anticyclonic structure helps maintain high water vapor content there. This explains both the summer seasonal cycle and interannual variability of the convective moistening efficiency in the NAregion and can provide valuable insight into modeling stratospheric water vapor.