Abstract
Abstract. The Asian summer monsoon provides an important pathway of tropospheric source gases and pollution into the lower stratosphere. This transport is characterized by deep convection and steady upwelling, combined with confinement inside a large-scale anticyclonic circulation in the upper troposphere and lower stratosphere (UTLS). In this paper, we show that a barrier to horizontal transport along the 380 K isentrope in the monsoon anticyclone can be determined from a local maximum in the gradient of potential vorticity (PV), following methods developed for the polar vortex (e.g., Nash et al., 1996). The monsoon anticyclone is dynamically highly variable and the maximum in the PV gradient is weak, such that additional constraints are needed (e.g., time averaging). Nevertheless, PV contours in the monsoon anticyclone agree well with contours of trace gas mixing ratios (CO, O3) and mean age from model simulations with a Lagrangian chemistry transport model (CLaMS) and satellite observations from the Microwave Limb Sounder (MLS) instrument. Hence, the PV-based transport barrier reflects the separation between air inside the core of the anticyclone and the background atmosphere well. For the summer season 2011 we find an average PV value of 3.6 PVU for the transport barrier in the anticyclone on the 380 K isentrope.
Highlights
An efficient pathway for anthropogenic pollution and tropospheric source gases into the stratosphere is linked to the Asian summer monsoon, as has been shown from satellite observations of HCN (Randel et al, 2010)
We investigated to what extent meteorological fields and trace gas distributions reflect the existence of a barrier to horizontal transport along isentropic surfaces in the Asian monsoon anticyclone
We showed that the potential vorticity field reflects the existence of a barrier to horizontal transport between the anticyclone and its surroundings
Summary
An efficient pathway for anthropogenic pollution and tropospheric source gases into the stratosphere is linked to the Asian summer monsoon, as has been shown from satellite observations of HCN (Randel et al, 2010). Strong diabatic heating processes play a role in the monsoon and, potential vorticity (PV) is not well conserved (e.g., Holton, 1992) For these reasons, the confinement of air inside the Asian monsoon anticyclone appears to be much weaker than in the polar vortex, and it turns out to be very challenging to locate a barrier to horizontal transport (Garny and Randel, 2013). The confinement of air inside the Asian monsoon anticyclone appears to be much weaker than in the polar vortex, and it turns out to be very challenging to locate a barrier to horizontal transport (Garny and Randel, 2013) That such a transport barrier exists, at least to some degree, is reflected in the observed trace gas anomalies within the anticyclone
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