Transport pathways from the Asian monsoon anticyclone to the stratosphere

Abstract

Abstract. Transport pathways of air originating in the uppertropospheric Asian monsoon anticyclone are investigated based on three-dimensional trajectories. The Asian monsoon anticyclone emerges in response to persistent deep convection over India and southeast Asia in northern summer, and this convection is associated with rapid transport from the surface to the upper troposphere and possibly into the stratosphere. Here, we investigate the fate of air that originates within the upper-tropospheric anticyclone from the outflow of deep convection, using trajectories driven by ERA-interim reanalysis data. Calculations include isentropic estimates, plus fully three-dimensional results based on kinematic and diabatic transport calculations. Isentropic calculations show that air parcels are typically confined within the anticyclone for 10-20 days and spread over the tropical belt within a month of their initialization. However, only few parcels (3 % at 360 K, 8 % at 380 K) reach the extratropical stratosphere by isentropic transport. When considering vertical transport we find that 31 % or 48 % of the trajectories reach the stratosphere within 60 days when using vertical velocities or diabatic heating rates to calculate vertical transport, respectively. In both cases, most parcels that reach the stratosphere are transported upward within the anticyclone and enter the stratosphere in the tropics, typically 10-20 days after their initialization at 360 K. This suggests that trace gases, including pollutants, that are transported into the stratosphere via the Asian monsoon system are in a position to enter the tropical pipe and thus be transported into the deep stratosphere. Sensitivity calculations with respect to the initial altitude of the trajectories showed that air needs to be transported to levels of 360 K or above by deep convection to likely ( 50 %) reach the stratosphere through transport by the large-scale circulation.

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

“…This quasi-horizontal transport pathway from the monsoon into the UTLS is supported by recent in situ measurements (Mueller et al, 2016). At lower levels below the tropical tropopause (about 380 K) horizontal transport from the anticyclone core to the NH extratropics is very weak due to strong gradients in PV, in agreement with the findings of Garny and Randel (2016).…”

“…5. 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.…”

“…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.…”

“…However, recently even tracer-independent model diagnostics have yielded inconclu-sive results. 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.…”