Constraining large-scale mixing in the subtropical southern hemisphere free troposphere with stable isotopologues of water vapor: Results from the Chajnantor Plateau, Chile

Several recent studies have pointed out the importance of large-scale mixing in setting the humidity of the subtropics, but the balance between isentropic mixing of dry air from the extratropics and cross-isentropic vertical mixing from the boundary layer remains enigmatic, and the way that such balances may change as the climate warms is unknown. In July and August of 2010, we collected three weeks of data on water vapor concentration and isotopologue abundance from the Chajnantor Plateau in northern Chile. At a latitude of 23 degrees S and an elevation of 5500 meters, the Chajnantor Plateau is in a prime location for monitoring atmospheric water vapor in the southern hemisphere subtropics. We have extended the large-scale mixing theory of Galewsky and Hurley (2010) and applied it to the Chajnantor dataset. Temperatures and locations of last saturation were computed using the Advanced Research Weather Research and Forecast (WRF-ARW) model in a nudged channel configuration that avoids numerical stability problems at the poles while still covering the important extratropical regions. In line with existing theories, the time series of water vapor concentration data correlates well with the average temperature of last saturation. Water vapor δ D values are consistent with large-scale mixing between air parcels of diverse origin, and WRF last-saturation analysis shows a correlation between the changing positions of last saturation during the field campaign and the water vapor isotopic ratios, but deviations from large-scale mixing theory provide additional insight into processes that control the humidity of the subtropical southern hemisphere. When combined with emerging datasets from the northern hemisphere subtropics, long-term monitoring of water vapor isotopologues from the Chajnantor Plateau will help constrain the processes that control projected subtropical moistening and provide insight into interhemispheric asymmetries in circulation and humidity.