Air‐mass origin in the tropical lower stratosphere: The influence of Asian boundary layer air

Abstract

A climatology of air‐mass origin in the tropical lower stratosphere is presented for the Goddard Earth Observing System Chemistry Climate Model. During late boreal summer and fall, air‐mass fractions reveal that as much as 20% of the air in the tropical lower stratosphere last contacted the planetary boundary layer (PBL) over Asia; by comparison, the air‐mass fractions corresponding to last PBL contact over North America and over Europe are negligible. Asian air reaches the extratropical tropopause within a few days of leaving the boundary layer and is quasi‐horizontally transported into the tropical lower stratosphere, where it persists until January. The rapid injection of Asian air into the lower stratosphere—and its persistence in the deep tropics through late (boreal) winter—is important as industrial emissions over East Asia continue to increase. Hence, the Asian monsoon may play an increasingly important role in shaping stratospheric composition.

Quantifying pollution transport from the Asian monsoon anticyclone into the lower stratosphere

“…The vertical nature of cross-tropopause transport is consistent with the findings of Garny and Randel (2016), but with the addition that above the tropopause a substantial amount of anticyclone air is mixed into the NH extratropics. This strong horizontal transport is, on the other hand, consistent with Orbe et al (2015) and Pan et al (2016), but with the difference that horizontal transport (either isentropic advection or mixing) in our case occurs mainly above the tropopause. It is important to note that we defined vertical and horizontal transport with respect to potential temperature as the vertical coordinate.…”

“…Hence, in summary we refine the findings of Orbe et al (2015), Garny and Randel (2016) and Pan et al (2016) by describing transport from the Asian monsoon anticyclone into the stratosphere as a "blowing chimney", using the terminology of Pan et al (2016). This characterization emphasizes the vertical "chimney-like" nature of cross-tropopause transport (with respect to potential temperature as vertical coordinate), but with the pollutants transported away quasi-horizontally along isentropes above the tropopause (see Fig.…”

“…Garny and Randel (2016) concluded from 60-day backward trajectory ensembles that the preferred pathway of air masses is to travel from within the upper-tropospheric anticyclone region to the tropical lower stratosphere, but they did not further investigate where (relative to the tropopause) horizontal mixing from the monsoon region to low and high latitudes occurs. Orbe et al (2015) analyzed air mass origin tracers in a climate model. They found that Asian surface air is transported upwards in the monsoon, reaches the extratropical tropopause within a few days, and is first transported quasi-horizontally into the extratropical lower stratosphere before eventually being transported subsequently into the tropics.…”

“…On 1 July of the year thereafter, the tracer is set to zero everywhere and is then reinitialized for the following monsoon season. By definition, the tracer mixing ratio at any location in the stratosphere equals the fraction of air which has left the monsoon anticyclone during the previous monsoon season (see Orbe et al, 2013). Initializing the air mass origin tracer in the UTLS part of the Asian monsoon avoids our results being affected by smallscale transport processes in the troposphere (e.g., convection), whose representation in global reanalysis data is uncertain (e.g., Russo et al, 2011).…”

“…On the one hand, the back trajectory study of Garny and Randel (2016) shows strongest transport from the anticyclone directly into the tropical stratosphere. On the other hand, climate model simulations by Orbe et al (2015) show the tropopause crossing of air masses from the anticyclone largely in the extratropics and subsequent transport into the extratropical lower stratosphere.…”