Abstract
Air mass transport within the summertime Asian monsoon circulation provides a major source of anthropogenic pollution for the upper troposphere and lower stratosphere (UTLS). Here, we investigate the quasi-horizontal transport of air masses from the Asian summer monsoon anticyclone (ASMA) into the extratropical lower stratosphere and their chemical evolution. For that reason, we developed a method to identify and track the air masses exported from the monsoon. This method is based on the anomalously low potential vorticity (PV) of these air masses (tropospheric low–PV cutoffs) compared to the lower-stratosphere, and uses trajectory calculations and chemical fields from the Chemical Lagrangian Model of the Stratosphere (CLaMS). The results show evidence for frequent summertime transport from the monsoon anticyclone to mid-latitudes over the North Pacific, even reaching high latitude regions of Siberia and Alaska. Most of the low–PV cutoffs related to air masses exported from the ASMA have lifetimes shorter than one week (about 90 %) and sizes smaller than 1 percent of the northern hemisphere (NH) area. The chemical composition of these air masses is characterised by carbon monoxide, ozone and water vapour mixing ratios at an intermediate range between values typical for the monsoon anticyclone and the lower-stratosphere. The chemical evolution during transport within these low–PV cutoffs shows a gradual change from characteristics of the monsoon anticyclone to characteristics of the lower stratospheric background during about one week, indicating continuous mixing with the background atmosphere.
Highlights
The Asian summer monsoon anticyclone (ASMA) is the dominant circulation pattern in the summertime upper troposphere and lower-stratosphere upper troposphere and lower stratosphere (UTLS) (Randel and Jensen, 2013)
We extend the method of Kunz et al (2015) and further investigate the pathways of air masses from the ASMA 65 into the extratropical lower-stratosphere, motivated by the two questions: (i) What are the main pathways of isentropic, quasihorizontal air mass transport from the monsoon anticyclone into the extratropical UTLS? (ii) What is the chemical composition of the air masses exported from the ASMA and how does the composition evolve during transport to the extratropics? For that reason, we carry out complementary potential vorticity (PV)-cutoff detection calculations for different PV values corresponding to the ASMA edge, the annual mean dynamical PV-gradient-based extratropical tropopause, and an even larger PV value characterizing the 70 summertime PV-gradient-based tropopause
We investigated low-PV cutoffs that transport air masses from the ASMA into the extratropical lower-stratosphere
Summary
The Asian summer monsoon anticyclone (ASMA) is the dominant circulation pattern in the summertime upper troposphere and lower-stratosphere UTLS (Randel and Jensen, 2013). Subsequent upward transport across the monsoon tropopause is related to slow upwelling and positive diabatic heating rates of around 1K day−1 (von Hobe et al, 2021), and is in the horizontal plane characterized by an anticyclonic, spiralling motion (Vogel et al, 2019) In this vertical range, anomalous trace gas distributions indicate confinement in the ASMA (Park et al, 2009), with tracer anomalies correlating well with low PV anomalies both in the time-mean climatology and with regard to day-to-day variability (Garny 35 and Randel, 2013). The fact that the air masses exported from the ASMA are characterized by anomalously low PV values and that PV is to first order materially conserved on isentropic levels, offers an opportunity to identify these air masses and to investigate their 50 pathways into the stratosphere In this sense, air masses with anomalously low PV in the lowermost stratosphere (low–PV cutoffs) indicate air transported from the troposphere into the stratosphere.
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