Horizontal water vapor transport in the lower stratosphere from subtropics to high latitudes during boreal summer

Abstract

We compare global water vapor observations from Microwave Limb Sounder (MLS) and simulations with the Lagrangian chemical transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) to investigate the pathways of water vapor into the lower stratosphere during Northern Hemisphere (NH) summer. We find good agreement between the simulation and observations, with an effect of the satellite averaging kernel especially at high latitudes. The Asian and American monsoons emerge as regions of particularly high water vapor mixing ratios in the lower stratosphere during boreal summer. In NH midlatitudes and high latitudes, a clear anticorrelation between water vapor and ozone daily tendencies reveals a large region influenced by frequent horizontal transport from low latitudes, extending up to about 450K during summer and fall. Analysis of the zonal mean tracer continuity equation shows that close to the subtropics, this horizontal transport is mainly caused by the residual circulation. In contrast, at higher latitudes, poleward of about 50°N, eddy mixing dominates the horizontal water vapor transport. Model simulations with transport barriers confirm that almost the entire annual cycle of water vapor in NH midlatitudes above about 360K, with maximum mixing ratios during summer and fall, is caused by horizontal transport from low latitudes. In the model, highest water vapor mixing ratios in this region are clearly linked to horizontal transport from the subtropics.

Variability of stratospheric mean age of air and of the local effects of residual circulation and eddy mixing

“…This poleward transport is caused by advection by the residual circulation equatorward of about 40 • and by eddy mixing poleward. Recently, Ploeger et al [2013] found a similar latitudinal separation for the local effects of residual circulation and eddy mixing on horizontal water vapor transport. The enhanced vertical gradient in the local residual circulation effect around 500 K (∼20 km) and the strong horizontal gradient around 40 • N/S indicate the separation of a shallow circulation branch from the deep branch above [compare Birner and Bönisch, 2011].…”

“…The Lagrangian transport model CLaMS has been shown to simulate the global stratospheric trace gas distributions well and even the small-scale structures and steep gradients therein [e.g., Konopka et al, 2004]. In particular, the multiyear simulation analyzed here has been shown to realistically represent transport of ozone and water vapor in the lower stratosphere [e.g., Konopka et al, 2010;Ploeger et al, 2013]. The mean age of air in the model is calculated from a "clock tracer" [e.g., Hall and Plumb, 1994;Waugh and Hall, 2002], a synthetic species with linearly increasing mixing ratio in the lowest model layer (orography following) and without any sources or sinks above.…”

Satellite observations of cirrus clouds in the Northern Hemisphere lowermost stratosphere

“…Comprehensive analyses of transport processes from the troposphere into the stratosphere and tracer proportions of the extratropical UTLS region have been published (e.g. Hegglin et al, 2009;Hoor et al, 2010;Ploeger et al, 2013). Rossby wave breaking (Ploeger et al, 2013) and midlatitude overshoot convection (Dessler, 2009) result in the transport and mixing of air masses into the extratropical UTLS on short timescales (weeks).…”

“…Hegglin et al, 2009;Hoor et al, 2010;Ploeger et al, 2013). Rossby wave breaking (Ploeger et al, 2013) and midlatitude overshoot convection (Dessler, 2009) result in the transport and mixing of air masses into the extratropical UTLS on short timescales (weeks). On seasonal timescales, downwelling by the deep Brewer-Dobson circulation branch moistens the extratropical UTLS at altitudes above 450 K (Ploeger et al, 2013).…”

“…Rossby wave breaking (Ploeger et al, 2013) and midlatitude overshoot convection (Dessler, 2009) result in the transport and mixing of air masses into the extratropical UTLS on short timescales (weeks). On seasonal timescales, downwelling by the deep Brewer-Dobson circulation branch moistens the extratropical UTLS at altitudes above 450 K (Ploeger et al, 2013). Aged air masses transported into the extratropical lower stratosphere from above are moistened by methane oxidation in the upper and middle stratosphere and represent an important source of water vapour in the middle stratosphere (e.g.…”