Infrequent, Rapid Transport Pathways to the Summer North American Upper Troposphere and Lower Stratosphere

Transport pathways from the Northern Hemisphere surface into the North American upper troposphere-lower stratosphere (NA UTLS) during summertime are diagnosed from Boundary Impulse Response idealized tracers implemented at the Northern Hemisphere surface. In ensemble average, air masses enter the NA UTLS region via deep convection above Central America, and then slowly mix into the higher latitudes. However, fast transport pathways with a modal age around two weeks are evident in some tracer ensembles. For these rapid transport pathways, the tracers first reach the UTLS region over the eastern Pacific and the Gulf of Mexico as a result of enhanced deep convection and vertical advection, followed by horizontal transport over the United States by a stronger than normal UTLS anticyclone circulation. Plain Language Summary Efficient pathways connecting surface air to the North American upper troposphere and lower stratosphere (NA UTLS) are important as short-lived tropospheric chemical species could survive transit into the lower stratosphere and hence affect ozone depletion. Our modeling study shows that the fast transport into the NA UTLS region can occur in a small fraction of cases with modal age <20 days. For these fast cases, the origin is enhanced deep convection over the eastern Pacific and the Gulf of Mexico, rather than locally over the contiguous United States. Deep convection controls transport from the boundary layer to 200 hPa, and then enhanced large-scale circulation, as a balanced response to the enhanced convection, dominates transport upwards into the UTLS region and toward southern United States following the NA monsoon anticyclonic flow. Key Points . Fast pathways connecting surface air to the North American UTLS occur over the eastern Pacific and the Gulf of Mexico A small fraction (10%) of tracers reaches the North American UTLS with ensemble-mean modal age <20 days Fast transport is associated with interaction between deep convection and resolved dynamics