Felix Ploeger

Impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer

[1] To assess the impact of the vertical velocity scheme on modeling transport in the tropical tropopause layer (TTL), 3 month backward trajectories are initialized in the TTL for boreal winter and summer 2002. The calculations are done in either a kinematic scenario with pressure tendency as the vertical velocity or in a diabatic scenario with cross-isentropic velocity deduced from various diabatic heating rates due to radiation (clear sky, all sky) and latent, diffusive and turbulent heating. This work provides a guideline for assessing the sensitivity of trajectory and chemical transport model (CTM) results on the choice of the vertical velocity scheme. We find that many transport characteristics, such as time scales, pathways and dispersion, crucially depend on the vertical velocity scheme. The strongest tropical upwelling results from the operational European Centre for Medium-Range Weather Forecasts kinematic scenario with the time scale for ascending from 340 to 400 K of 1 month. For the ERA-Interim kinematic and total diabatic scenarios, this time scale is about 2 months, and for the all-sky scenario it is as long as 2.5 months. In a diabatic scenario, the whole TTL exhibits mean upward motion, whereas in a kinematic scenario, regions of subsidence occur in the upper TTL. However, some transport characteristics robustly emerge from the different scenarios, such as an enhancement of residence times between 350 and 380 K and a strong impact of meridional in-mixing from the extratropics on the composition of the TTL. Moreover, an increase of meridionally transported air from the summer hemisphere into the TTL (maximum for boreal summer) is found as an invariant feature among all the scenarios.

Interaction of gravity waves with the QBO: A satellite perspective

One of the most important dynamical processes in the tropical stratosphere is the quasi-biennial oscillation (QBO) of the zonal wind. Still, the QBO is not well represented in weather and climate models. To improve the representation of the QBO in the models, a better understanding of the driving of the QBO by atmospheric waves is required. In particular, the contribution of gravity waves is highly uncertain because of the small horizontal scales involved, and there is still no direct estimation based on global observations. We derive gravity wave momentum fluxes from temperature observations of the satellite instruments HIRDLS and SABER. Momentum flux spectra observed show that particularly gravity waves with intrinsic phase speeds <30m/s (vertical wavelengths <10km) interact with the QBO. Gravity wave drag is estimated from vertical gradients of observed momentum fluxes and compared to the missing drag in the tropical momentum budget of ERA-Interim. We find reasonably good agreement between their variations with time and in their approximate magnitudes. Absolute values of observed and ERA-Interim missing drag are about equal during QBO eastward wind shear. During westward wind shear, however, observations are about 2 times lower than ERA-Interim missing drag. This could hint at uncertainties in the advection terms in ERA-Interim. The strong intermittency of gravity waves we find in the tropics might play an important role for the formation of the QBO and may have important implications for the parameterization of gravity waves in global models.

Evaluating the advective Brewer‐Dobson circulation in three reanalyses for the period 1979–2012

Most chemistry-climate models show an intensification of the Brewer-Dobson circulation (BDC) in the stratosphere associated with increasing greenhouse gas emissions and ozone depletion in the last decades, but this trend remains to be confirmed in observational data. In this work the evolution of the advective BDC for the period 1979–2012 is evaluated and compared in three modern reanalyses (ERA-Interim, MERRA, and JRA-55). Three different estimates of the BDC are computed for each reanalysis, one based on the definition of the residual circulation and two indirect estimates derived from momentum and thermodynamic balances. The comparison among the nine estimates shows substantial uncertainty in the mean magnitude (∼40%) but significant common variability. The tropical upwelling series show variability linked to the stratospheric quasi-biennial oscillation and to El Nino–Southern Oscillation (ENSO) and also reflect extreme events such as major sudden stratospheric warmings and volcanic eruptions. The trend analysis suggests a strengthening of tropical upwelling of around 2–5%/decade throughout the layer 100–10 hPa. The global spatial structure of the BDC trends provides evidence of an overall acceleration of the circulation in both hemispheres, with qualitative agreement among the estimates. The global BDC trends are mainly linked to changes in the boreal winter season and can be tracked to long-term increases in the resolved wave drag in both hemispheres.

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

We analyze the variability of mean age of air (AoA) and of the local effects of the stratospheric residual circulation and eddy mixing on AoA within the framework of the isentropic zonal mean continuity equation. AoA for the period 1988–2013 has been simulated with the Lagrangian chemistry transport model CLaMS driven by ERA-Interim winds and diabatic heating rates. Model simulated AoA in the lower stratosphere shows good agreement with both in situ observations and satellite observations from Michelson Interferometer for Passive Atmospheric Sounding, even regarding interannual variability and changes during the last decade. The interannual variability throughout the lower stratosphere is largely affected by the quasi-biennial-oscillation-induced circulation and mixing anomalies, with year-to-year AoA changes of about 0.5 years. The decadal 2002–2012 change shows decreasing AoA in the lowest stratosphere, below about 450 K. Above, AoA increases in the Northern Hemisphere and decreases in the Southern Hemisphere. Mixing appears to be crucial for understanding AoA variability, with local AoA changes resulting from a close balance between residual circulation and mixing effects. Locally, mixing increases AoA at low latitudes (40°S–40°N) and decreases AoA at higher latitudes. Strongest mixing occurs below about 500 K, consistent with the separation between shallow and deep circulation branches. The effect of mixing integrated along the air parcel path, however, significantly increases AoA globally, except in the polar lower stratosphere. Changes of local effects of residual circulation and mixing during the last decade are supportive of a strengthening shallow circulation branch in the lowest stratosphere and a southward shifting circulation pattern above.

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.

Departure from Clausius‐Clapeyron scaling of water entering the stratosphere in response to changes in tropical upwelling

Water entering the stratosphere ([H2O]entry) is strongly constrained by temperatures in the tropical tropopause layer (TTL). Temperatures at tropical tropopause levels are 15–20 K below radiative equilibrium. A strengthening of the residual circulation as suggested by general circulation models in response to increasing greenhouse gases is, based on radiative transfer calculations, estimated to lead to a temperature decrease of about 2 K per 10% change in upwelling (with some sensitivity to vertical scale length). For a uniform temperature change in the inner tropics, [H2O]entry may be expected to change as predicted by the temperature dependence of the vapor pressure, referred here as “Clausius-Clapeyron (CC) scaling.” Under CC scaling, this corresponds to ∼1 ppmv change in [H2O]entry per 10% change in upwelling. However, the change in upwelling also changes the residence time of air in the TTL. We show with trajectory calculations that this affects [H2O]entry, such that [H2O]entry changes ∼10 % less than expected from CC scaling. This residence time effect for water vapor is a consequence of the spatiotemporal variance in the temperature field. We show that for the present-day TTL, a little more than half of the effect is due to the systematic relation between flow and temperature field. The remainder can be understood from the perspective of a random walk problem, with slower ascent (longer path) increasing each air parcels probability to encounter anomalously low temperatures. Our results show that atmospheric water vapor may depart from CC scaling with mean temperatures even when all physical processes of dehydration remain unchanged.

Influence of enhanced Asian NOx emissions on ozone in the upper troposphere and lower stratosphere in chemistry-climate model simulations

8 Asian summer monsoon convection plays an important role in efficient vertical transport from the 9 surface to the anticyclone. In this paper we investigate the potential impact of convectively transported 10 anthropogenic nitrogen oxides (NOx) on the distribution of ozone in the Upper Troposphere and Lower 11 Stratosphere (UTLS) from simulations with the fully-coupled aerosol chemistry climate model, 12 ECHAM5-HAMMOZ. We performed anthropogenic NOx emission sensitivity experiments over India 13 and China. In these simulations, anthropogenic NOx emissions for the period 2000-2010 have been 14 increased by 38% over India and by 73% over China in accordance with satellite observed trends over 15 India of 3.8 % per year and China of 7.3% per year. These NOx emission sensitivity simulations show 16 that strong convection over the Bay of Bengal and the Southern slopes of the Himalayas transports 17 Indian emissions into the UTLS. Convective transport from the South China Sea injects Chinese 18 emissions into the lower stratosphere. Indian and Chinese emissions are partially transported over the 19 Arabian Sea and west Asia by the tropical easterly jet. Enhanced NOx emissions over India and China 20 increase the ozone radiative forcing over India by 0.112 W/m 2 and 0.121 W/m 2 respectively. These 21 elevated emissions produces significant warming over the Tibetan Plateau and increase precipitation 22 over India due to a strengthening of the monsoon Hadley circulation. 23 However doubling of NOx emissions over India (73%); equal to China, produced high ozone in the 24 Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-582, 2016 Manuscript under review for journal Atmos. Chem. Phys. Published: 20 July 2016 c

Significant contributions of volcanic aerosols to decadal changes in the stratospheric circulation

The stratospheric circulation is an important element of climate as it determines the concentration of radiatively active species like water vapor and aerosol above the tropopause. Climate models predict that increasing greenhouse gas levels speed up the stratospheric circulation. However, these results have been challenged by observational estimates of the circulation strength, constituting an uncertainty in current climate simulations. Here, we quantify the effect of volcanic aerosol on the stratospheric circulation focusing on the Mount Pinatubo eruption and discussing further the minor extratropical volcanic eruptions after 2008. We show that the observed pattern of decadal circulation change over the past decades is substantially driven by volcanic aerosol injections. Thus, climate model simulations need to realistically take into account the effect of volcanic eruptions, including the minor eruptions after 2008, for a reliable reproduction of observed stratospheric circulation changes.

Sensitivities of modelled water vapour in the lower stratosphere: temperature uncertainty, effects of horizontal transport and small-scale mixing

Water vapour (H2O) in the upper troposphere and lower stratosphere (UTLS) has a significant role for global radiation. A realistic representation of H2O is therefore critical for accurate climate model predictions of future climate change. In this paper we investigate the effects of current uncertainties in tropopause temperature, horizontal transport and small-scale mixing on simulated H2O in the lower stratosphere (LS). To assess the sensitivities of simulated H2O, we use the Chemical Lagrangian Model of the Stratosphere (CLaMS). First, we examine CLaMS, which is driven by two reanalyses, from the European Centre of Medium-Range Weather Forecasts (ECMWF) ERA-Interim and the Japanese 55-year Reanalysis (JRA-55), to investigate the robustness with respect to the meteorological dataset. Second, we carry out CLaMS simulations with transport barriers along latitude circles (at the Equator, 15 and 35 N/S) to assess the effects of horizontal transport. Third, we vary the strength of parametrized small-scale mixing in CLaMS. Our results show significant differences (about 0.5 ppmv) in simulated stratospheric H2O due to uncertainties in the tropical tropopause temperatures between the two reanalysis datasets, JRA-55 and ERA-Interim. The JRA-55 based simulation is significantly moister when compared to ERAInterim, due to a warmer tropical tropopause (approximately 2 K). The transport barrier experiments demonstrate that the Northern Hemisphere (NH) subtropics have a strong moistening effect on global stratospheric H2O. The comparison of tropical entry H2O from the sensitivity 15 N/S barrier simulation and the reference case shows differences of up to around 1 ppmv. Interhemispheric exchange shows only a very weak effect on stratospheric H2O. Small-scale mixing mainly increases troposphere–stratosphere exchange, causing an enhancement of stratospheric H2O, particularly along the subtropical jets in the summer hemisphere and in the NH monsoon regions. In particular, the Asian and American monsoon systems during a boreal summer appear to be regions especially sensitive to changes in small-scale mixing, which appears crucial for controlling the moisture anomalies in the monsoon UTLS. For the sensitivity simulation with varied mixing strength, differences in tropical entry H2O between the weak and strong mixing cases amount to about 1 ppmv, with small-scale mixing enhancing H2O in the LS. The sensitivity studies presented here provide new insights into the leading processes that control stratospheric H2O, which are important for assessing and improving climate model projections.