Mengchu Tao

Hemispheric asymmetries and seasonality of mean age of air in the lower stratosphere: Deep versus shallow branch of the Brewer‐Dobson circulation

Based on multiannual simulations with the Chemical Lagrangian Model of the Stratosphere, (CLaMS) driven by ECMWF ERA-Interim reanalysis, we discuss hemispheric asymmetries and the seasonality of the mean age of air (AoA) in the lower stratosphere. First, the planetary wave forcing of the Brewer-Dobson circulation is quantified in terms of Eliassen Palm flux divergence calculated by using the isentropic coordinate θ. While the forcing of the deep branch at θ = 1000 K (around 10 hPa) has a clear maximum in each hemisphere during the respective winter, the shallow branch of the Brewer-Dobson circulation, i.e., between 100 and 70 hPa (380 < θ < 420 K), shows almost opposite seasonality in both hemispheres with a pronounced minimum between June and September in the Southern Hemisphere. Second, we decompose the time-tendency of AoA into the contributions of the residual circulation and of eddy mixing by analyzing the zonally averaged tracer continuity equation. In the tropical lower stratosphere between ±30°, the air becomes younger during boreal winter and older during boreal summer. During boreal winter, the decrease of AoA due to tropical upwelling outweighs aging by isentropic mixing. In contrast, weaker isentropic mixing outweighs an even weaker upwelling in boreal summer and fall making the air older during these seasons. Poleward of 60°, the deep branch locally increases AoA and eddy mixing locally decreases AoA with the strongest net decrease during spring. Eddy mixing in the Northern Hemisphere outweighs that in the Southern Hemisphere throughout the year.

Impact of stratospheric major warmings and the quasi‐biennial oscillation on the variability of stratospheric water vapor

Based on simulations with the Chemical Lagrangian Model of the Stratosphere for the 1979–2013 period, driven by the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis, we analyze the impact of the quasi-biennial oscillation (QBO) and of Major Stratospheric Warmings (MWs) on the amount of water vapor entering the stratosphere during boreal winter. The amplitude of H2O variation related to the QBO amounts to 0.5 ppmv. The additional effect of MWs reaches its maximum about 2–4 weeks after the central date of the MW and strongly depends on the QBO phase. Whereas during the easterly QBO phase there is a pronounced drying of about 0.3 ppmv about 3 weeks after the MW, the impact of the MW during the westerly QBO phase is smaller (about 0.2 ppmv) and more diffusely spread over time. We suggest that the MW-associated enhanced dehydration combined with a higher frequency of MWs after the year 2000 may have contributed to the lower stratospheric water vapor after 2000.

A Lagrangian Model Diagnosis of Stratospheric Contributions to Tropical Midtropospheric Air

Airborne in situ observations during the Convective Transport of Active Species in the Tropics campaign in January–February 2014 revealed a large region over the tropical western Pacific where the midtroposphere had a layered structure with a distinct chemical signature of high ozone and low water vapor (HOLW). The observed anticorrelation between ozone and water vapor is a strong indication of transport from the midlatitude upper troposphere and lower stratosphere. This work presents a diagnosis of stratospheric air in the tropical western Pacific midtroposphere through isentropic transport and mixing. Using the Chemical Lagrangian Model of the Stratosphere, we characterize and quantify the contribution of transported stratospheric air to the observed HOLW layers. The result indicates that the isentropic transport is an effective process for stratospheric air to mix into the tropical midtroposphere. Using the modeled stratospheric tracer and 3-D back trajectories, we identified that 60% of the observed HOLW air masses contain significant stratospheric influence. We have also examined possible contribution to the HOLW layer from ozone production related to biomass burning emissions. Clear chemical signature of this process is found in ∼8% of the HOLW air masses, identified by positive correlations among O3, HCN, and CO. This analysis provides the first quantitative diagnosis of the contribution from the stratosphere-to-troposphere transport, highlights the importance of mixing in chemical transport, and demonstrates the limitations of pure Lagrangian trajectory calculations in quantifying transport.

Regionally Resolved Diagnostic of Transport: A Simplified Forward Model for CO2

AbstractSimply diagnostic tools are useful to understand transport processes in complex chemistry transport models (CTMs). For this purpose, a combined use of the air-mass origin fractions (AOFs) and regionally resolved mean ages (RMAs) is presented. This approach merges the concept of the origin of air with the well-known theory of the mean age of air (AoA) for different regions covering the whole Earth. We show how the AoA calculated relative to the Earth’s surface can be decomposed into regionally resolved components (i.e. into RMAs). Using both AOFs and RMAs, we discuss differences in the seasonality of transport from the northern and southern hemispheres into the tropical tropopause layer (TTL), the asymmetries of the interhemispheric exchange as well as differences in relation to the continental or oceanic origin of air. Furthermore, a simplified transport model for a chemically passive species (tracer) is formulated, which has some potential to approximate the full transport within a CTM. This anal...