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.

Identifying Source Regions and the Distribution of Cross-Tropopause Convective Outflow Over North America During the Warm Season.

We analyzed the interaction between the North American monsoon anticyclone (NAMA) and 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 to 1.25°W for the time period of May through September 2013. With this analysis we identified seasonally, geographically, and altitude-dependent variability in NAMA strength and in cross-tropopause convection that control their interaction. We find that the NAMA has the strongest impact on the circulation of convectively influenced air masses in August. Over the entire time period examined the intertropical convergence zone contributes the majority of OTs with a larger fraction of total OTs at 370 K (on average 70%) than at 400 K (on average 52%). During August at 370 K, the convectively influenced air masses within the NAMA circulation, as determined by the trajectory analysis, are primarily sourced from the intertropical convergence zone (monthly average of 66.1%), while at 400 K the Sierra Madres and the Central United States combined constitute the dominant source region (monthly average of 44.1%, compared to 36.6% of the combined Intertropical Convergence Zone regions). When evaluating the impact of cross-tropopause convection on the composition and chemistry of the upper troposphere and lower stratosphere, the effects of the NAMA on both the distribution of convective outflow and the residence time of convectively influenced air masses within the NAMA region must be considered.

Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution.

Satellite observations have shown that the Asian Summer Monsoon strongly influences the upper troposphere and lower stratosphere (UTLS) aerosol morphology through its role in the formation of the Asian Tropopause Aerosol Layer (ATAL). Stratospheric Aerosol and Gas Experiment II solar occultation and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar observations show that summertime UTLS Aerosol Optical Depth (AOD) between 13 and 18 km over Asia has increased by three times since the late 1990s. Here we present the first in situ balloon measurements of aerosol backscatter in the UTLS from Western China, which confirm high aerosol levels observed by CALIPSO since 2006. Aircraft in situ measurements suggest that aerosols at lower altitudes of the ATAL are largely composed of carbonaceous and sulfate materials (carbon/sulfur elemental ratio ranging from 2 to 10). Back trajectory analysis from Cloud-Aerosol Lidar with Orthogonal Polarization observations indicates that deep convection over the Indian subcontinent supplies the ATAL through the transport of pollution into the UTLS. Time series of deep convection occurrence, carbon monoxide, aerosol, temperature, and relative humidity suggest that secondary aerosol formation and growth in a cold, moist convective environment could play an important role in the formation of ATAL. Finally, radiative calculations show that the ATAL layer has exerted a short-term regional forcing at the top of the atmosphere of -0.1 W/m2 in the past 18 years. KEY POINTS: Increase of summertime upper tropospheric aerosol levels over Asia since the 1990s Upper tropospheric enhancement also observed by in situ backscatter measurements Significant regional radiative forcing of -0.1 W/m2.

Comment on "Large volcanic aerosol load in the stratosphere linked to Asian monsoon transport".

Bourassa et al. (Reports, 6 July 2012, p. 78) have suggested that deep convection associated with the Asian monsoon played a critical role in transporting sulfur dioxide associated with the Nabro volcanic eruption (13 June 2011) from the upper troposphere (9 to 14 kilometers) into the lower stratosphere. An analysis of the CALIPSO lidar data indicates, however, that the main part of the Nabro volcanic plume was injected directly into the lower stratosphere during the initial eruption well before reaching the Asian monsoon deep convective region.