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.

[1] Using isentropic trace gas gradients of O 3 and CO, the discontinuity in the chemical composition of the upper troposphere (UT) and lower stratosphere (LS) is examined on middle world isentropes from 300 to 380 K. The analysis is a follow-up study of the dynamical discontinuity as represented by the potential vorticity (PV) gradient-based tropopause, which is based on the product of isentropic PV gradients and wind speed. Overall, there is fairly good consistency between the chemical discontinuity in trace gas distributions and the PV gradient-based tropopause. Trace gas gradients at the PV gradient-based tropopause are stronger in winter than in summer, revealing the seasonal cycle of the tropopause transport barrier. The analysis of the trace gas gradients also identifies atmospheric transport pathways in the upper troposphere-lower stratosphere (UTLS). Several regions where trace gas gradients are found to be decoupled from the dynamical field indicate preferred transport pathways between the UT and LS. In particular, anomalous CO and O 3 gradients above eastern Africa, eastern Asia, and the West Pacific are likely related to convective transport, and anomalous O 3 gradients over the North Atlantic and North Pacific are related to isentropic transport connected to frequent wave breaking. The results indicate that the PV gradient-based tropopause definition provides a good identification of the dynamical and chemical discontinuity and is therefore effective in locating the physical boundary in the UTLS.

[1] Since its inception, the dynamical tropopause based on potential vorticity (PV) is identified by the PV gradient on isentropes. Conceptually, significant isentropic gradients shown on the middle world PV maps reflect the underlying transport barrier associated with the tropopause, formed by jet streams that separate tropospheric air masses at low latitudes and stratospheric air masses at high latitudes. Largely owing to the lack of a general method, the dynamical tropopause has often been represented by a PV value chosen ad hoc without any temporal or spatial differentiation. In this work, we present a method for determining the PV isoline of the dynamical tropopause based on the isentropic PV gradients. Using 1 year of data from the European Centre for Medium-Range Weather Forecasts, the spatial and temporal variability of this PV gradient-based dynamical tropopause is examined. The results show that in general there is a broad distribution of PV values at the dynamical tropopause, ranging from 1.5 to 5 potential vorticity units. Therefore, a fixed PV surface for all isentropes and seasons does not accurately represent the location of the "tropopause barrier." The PV at the dynamical tropopause increases with increasing potential temperature. This increase is more pronounced in the Southern Hemisphere than in the Northern Hemisphere. The seasonal cycle shows higher PV values at the dynamical tropopause during summer than during winter. This seasonal cycle is larger on higher isentropes. The dispersion of the PV at the dynamical tropopause about its mean is twofold larger during summer and autumn than during winter and spring in both hemispheres.

[1] The relationship between the static stability N 2 and the mixing in the tropopause inversion layer (TIL) is investigated using in situ aircraft observations during SPURT (trace gas transport in the tropopause region). With a new simple measure of mixing degree based on O 3 -CO tracer correlations, high N 2 related to an enhanced mixing in the extratropical mixing layer is found. This relation becomes even more pronounced if fresh mixing events are excluded, indicating that mixing within the TIL occurs on a larger than synoptic timescale. A temporal variance analysis of N 2 suggests that processes responsible for the composition of the TIL take place on seasonal timescales. Using radiative transfer calculations, we simulate the influence of a change in O 3 and H 2 O vertical gradients on the temperature gradient and thus on the static stability above the tropopause, which are contrasted in an idealized nonmixed atmosphere and in a reference mixed atmosphere. The results show that N 2 increases with enhanced mixing degree near the tropopause. At the same time, the temperature above the tropopause decreases together with the development of an inversion and the TIL. In the idealized case of nonmixed profiles the TIL vanishes. Furthermore, the results suggest that H 2 O plays a major role in maintaining the temperature inversion and the TIL structure compared to O 3 . The results substantiate the link between the extratropical mixing layer and the TIL.