Upward transport into and within the Asian monsoon anticyclone as inferred from StratoClim trace gas observations

Abstract

Abstract. Every year during the Asian summer monsoon season from about mid-June to early September, a stable anticyclonic circulation system forms over the Himalayans. This Asian summer monsoon (ASM) anticyclone has been shown to promote transport of air into the stratosphere from the Asian troposphere, which contains large amounts of anthropogenic pollutants. Essential details of Asian monsoon transport, such as the exact time scales of vertical transport, the role of convection in cross-tropopause exchange, and the main location and level of export from the confined anticyclone to the stratosphere are still not fully resolved. Recent airborne observations from campaigns near the ASM anticyclone edge and centre in 2016 and 2017 respectively show a steady decrease in carbon monoxide (CO) and increase in ozone (O3) with height starting from tropospheric values of 80–100 ppb CO and 30–50 ppb O3 at about 365 K potential temperature. CO mixing ratios reach stratospheric background values of ~ 20 ppb at about 420 K and do not show a significant vertical gradient at higher levels, while ozone continues to increase throughout the altitude range of the aircraft measurements. Nitrous oxide (N2O) remains at or only marginally below its 2017 tropospheric mixing ratio of 326 ppb up to about 400 K, which is above the local tropopause. A decline in N2O mixing ratios that indicates a significant contribution of stratospheric air is only visible above this level. Based on our observations, we draw the following picture of vertical transport and confinement in the ASM anticyclone: rapid convective uplift transports air to near 16 km in altitude, corresponding to potential temperatures up to about 370 K. Although this main convective outflow layer extends above the level of zero radiative heating (LZRH), our observations of CO concentration show little to no evidence of convection actually penetrating the tropopause. Rather, further ascent occurs more slowly, consistent with isentropic vertical velocities of 0.3–0.8 K day−1. For gases not subject to microphysical processes, neither the lapse rate tropopause (LRT) around 380 K nor the cold point tropopause (CPT) around 390 K marks the strong discontinuity of the key tracers (CO, O3, and N2O). Up to about 10 to 20 K above the CPT, isolation of air inside the ASM anticyclone prevents significant in-mixing of stratospheric air. The observed changes in CO and O3 likely result from in-situ chemical processing. Above about 420 K, mixing processes become more significant and the air inside the anticyclone is exported vertically and horizontally into the surrounding stratosphere.

The ATAL within the 2017 Asian Monsoon Anticyclone: Microphysical aerosol properties derived from aircraft-borne in situ measurements

“…This is also consistent with the observations, that during the StratoClim 2017 campaign period enhanced aerosol particle SR values could only be observed within the confinement of the AMA at a EQLAT larger than 63 degree and below its top of confinement (at about 420 K potential temperature). This regional limitation of the ATAL with respect to the dynamics of the AMA is in good agreement with the horizontal (Ploeger et al (2015)) and vertical (von Hobe et al (2020)) limitations of the AMA-caused confinement.…”

Sensitivity of Total Column Ozone to Stratospheric Sulfur Injection Strategies

“…Both Low and EQ experiments further show larger increases in stratospheric water vapor than the other cases (not shown), which is expected to lead to more tropospheric warming to be countered by additional injections, as discussed in Kravitz et al (2019) and . The lowest injection rates are needed for the 110L case, possibly because of the reduced stratosphere-troposphere exchange as a result of a better resolved gradient of chemical tracers, like ozone, across the tropopause in the 110L model (von Hobe et al, 2021). Furthermore, the 110L control simulation shows less increase in surface temperature, which may require less sulfur injection to maintain 2020 surface temperature conditions.…”