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Abstract
Abstract. We quantify the connection between deep convective occurrence and summertime 100 hPa 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 NA region is up to ∼1 ppmv higher when deep convection occurs. We use a Lagrangian trajectory model to demonstrate that the structure and the location of the NA anticyclone, 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 NA region and can provide valuable insight into modeling stratospheric water vapor.
Highlights
We investigated the contribution of convection to the stratospheric water vapor in the North American (NA) monsoon region, including the seasonal cycle and interannual variation of convective contributions during the Northern Hemisphere summer
We have shown that the deep convection moistens the lower stratosphere, adding up to ∼ 1 ppmv to the summertime NA water vapor at 100 hPa based on the observations from Microwave Limb Sounder (MLS)
We have shown that it is not the amount of convection alone that determines the impact on water vapor: NA monsoon dynamics play a role
Summary
Stratospheric water vapor influences both the climate (Forster and Shine, 1997, 2002; Smith et al, 2001; Solomon et al, 2010; Dessler et al, 2013) and chemistry of the atmosphere (Ramaswamy et al, 1996; Evans et al, 1998; Dvortsov and Solomon, 2001; Shindell, 2001; Stenke and Grewe, 2005). Over the Asian monsoon and North American monsoon, higher water vapor mixing ratios, sometimes exceeding 12 ppmv, are observed This value is much higher than the water vapor mixing ratio in the tropics, indicating that the air did not go through the TTL or is moistened further after leaving the TTL (Anderson et al, 2012; Schwartz et al, 2013; Randel et al, 2015; Smith et al, 2017). Previous case studies have shown that deep convection over North America (NA) can reach the lowermost stratosphere (between the local tropopause and the 380 K isentropic surface) and can even enter the stratospheric overworld (above 380 K), thereby bringing a high water vapor content to the stratosphere (Hanisco et al, 2007; Herman et al, 2017; Smith et al, 2017). We use a back trajectory model to illustrate the processes that influence where deep convection moistens the NA stratosphere, from the perspectives of spatial distribution, seasonal cycle, and interannual variability
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