Identifying Outflow Regions of North American Monsoon Anticyclone-Mediated Meridional Transport of Convectively Influenced Air Masses in the Lower Stratosphere

We analyzed the effect of the North American monsoon anticyclone (NAMA) on the meridional transport of summertime cross-tropopause convective outflow by applying a trajectory analysis to a climatology of convective overshooting tops (OTs) identified in GOES satellite images, which covers the domain from 29°S to 68°N and from 205°W to 1.25°W for the time period of May to September, 2013. From this analysis, we identify seasonal development of geographically distinct outflow regions of convectively influenced air masses (CIAMs) from the NAMA circulation to the global stratosphere and quantify the associated meridional displacement of CIAMs. We find that prior to the development of the NAMA, the majority of CIAMs exit the study area in a southeastern region between 5°N and 35°N at 45°W (75.5% in May). During July and August, when the NAMA is strongest, two additional outflow regions develop that constitute the majority of outflow: 68.1% in a northeastern region between 35°N and 60°N at 45°W and 13.4% in a southwestern region between 5°N and 35°N at 145°W. The shift in the location of most CIAM outflow from the pre-NAMA southeastern region to NAMA-dependent northeastern and southwestern regions corresponds to a change in average meridional displacement of CIAMs from 3.3° northward in May to 24.5° northward in July and August. Meridional transport of CIAMs through persistent outflow regions from the NAMA circulation to the global stratosphere has the potential to impact global stratospheric composition beyond convective source regions. Plain Language Summary We analyze the seasonal and regional variability of the transport of convection that has reached the lower stratosphere (an exceptional height) over North America from May to September within the large-scale but transient summertime anticyclone over North America. Overshooting convection that has reached the lower stratosphere is identified using infrared and visible satellite imagery. We simulate the circulation of the convectively influenced air to track its motion throughout the season from its initial position until it exits the study region and enters the global stratosphere. We find that the development of the anticyclone dictates through which of three outflow regions the majority of convectively influence air masses exit. Prior to the anticyclone's development most air masses exit through southeastern region over the Atlantic Ocean below 35°N. When the anticyclone is strongest, during July and August, most air masses exit through a northeastern region over the Atlantic Ocean above 35°N and through a southwestern region over the Pacific Ocean. During July and August the average air mass is transported 24.5° northward from its initial position. This transport may influence the global stratospheric water budget, as well as transport pollutants from this region into the global stratosphere.