The Influence of Ozone Changes on the Stratospheric Dynamics in 4xCO2 Climate Simulations

An increase in atmospheric CO2 concentration changes the temperatures and dynamics of the stratosphere and thus also the ozone concentrations. Since ozone is sensitive to longwave as well as shortwave radiation, the altered ozone distribution in turn affects temperature and thus also modifies the CO2-induced change in stratospheric dynamics. To study this in detail, we performed model simulations using the EMAC climate-chemistry model in which CO2 concentrations were quadrupled compared to pre-industrial times. For the 4xCO2 simulations, this was done by prescribing once an unchanged (pre-industrial) and once a changed ozone distribution from a previous 4xCO2 simulation. It is shown here, that the change in ozone leads to a strengthening of the stratospheric easterly winds in summer, as well as a weakening of the polar vortex in both hemispheres. While the high variability in the northern hemisphere does not lead to clear results, they are statistically significant in the southern hemisphere. In addition, the duration of the polar vortex westerlies is shorter due to the CO2-induced change in the ozone field. Furthermore, the acceleration of the Brewer-Dobson circulation, which is caused by an increase in CO2, is damped in the summer hemisphere by the ozone influence. This in turn affects the transport from the tropics to the extra-tropics. The ozone-induced changes in the stratospheric circulation also affect the tropospheric circulation and have an effect on the tropospheric polar front jet. It experiences a systematically weaker shift toward the pole due to a changed ozone distribution in the southern hemisphere than with a constant ozone distribution. We discuss the results, presented here, and place them in the context of the influence of stratospheric ozone on climate change dynamics

Disentangling the Advective Brewer-Dobson Circulation Change

Climate models robustly project acceleration of the Brewer-Dobson circulation (BDC) in response to climate change. However, the BDC trends simulated by comprehensive models are poorly constrained by observations, which cannot even determine the sign of potential trends. Additionally, the changing structure of the troposphere and stratosphere has received increasing attention in recent years. The extent to which vertical shifts of the circulation are driving the acceleration is under debate. In this study, we present a novel method that enables the attribution of advective BDC changes to structural changes of the circulation and of the stratosphere itself. Using this method allows studying the advective BDC trends in unprecedented detail and sheds new light into discrepancies between different data sets (reanalyses and models) at the tropopause and in the lower stratosphere. Our findings provide insights into the reliability of model projections of BDC changes and offer new possibilities for observational constraints

Stratospheric Ozone Changes Damp the CO2-Induced Acceleration of the Brewer-Dobson Circulation

The increase of atmospheric CO2 concentrations changes the atmospheric temperature distribution, which in turn affects the circulation. A robust circulation response to CO2 forcing is the strengthening of the stratospheric Brewer–Dobson circulation (BDC), with associated consequences for transport of trace gases such as ozone. Ozone is further affected by the CO2-induced stratospheric cooling via the temperature dependency of ozone chemistry. These ozone changes in turn influence stratospheric temperatures and thereby modify the CO2-induced circulation changes. In this study, we perform dedicated model simulations to quantify the modification of the circulation response to CO2 forcing by stratospheric ozone. Specifically, we compare simulations of the atmosphere with preindustrial and with quadrupled CO2 climate conditions, in which stratospheric ozone is held fixed or is adapted to the new climate state. The results of the residual circulation and mean age of air show that ozone changes damp the CO2-induced BDC increase by up to 20%. This damping of the BDC strengthening is linked to an ozone-induced relative enhancement of the meridional temperature gradient in the lower stratosphere in summer, thereby leading to stronger stratospheric easterlies that suppress wave propagation. Additionally, we find a systematic weakening of the polar vortices in winter and spring. In the Southern Hemisphere, ozone reduces the CO2-induced delay of the final warming date by 50%

The Climatology of Elevated Stratopause Events in the UA-ICON Model and the Contribution of Gravity Waves

The climatologies of the stratopause height and temperature in the UA-ICON model are examined by comparing them to 17-years (2005–2021) of Microwave Limb Sounder (MLS) observations. In addition, the elevated stratopause (ES) event occurrence, their main characteristics, and driving mechanisms in the UA-ICON model are examined using three 30-year time-slice experiments. While UA-ICON reasonably simulates the large-scale stratopause properties similar to MLS observations, at polar latitudes in the Southern Hemisphere the stratopause is ∼8 K warmer and ∼3 km higher than observed. A time lag of about two months also exists in the occurrence of the tropical semiannual oscillation of the stratopause compared to the observations. ES events occur in ∼20% of the boreal winters, after major sudden stratospheric warmings (SSWs). Compared to the SSWs not followed by ES events (SSW-only), the ES events are associated with the persistent tropospheric forcing and prolonged anomalies of the stratospheric jet. Our modeling results suggest that the contributions of both gravity waves (GW)s and resolved waves are important in explaining the enhanced residual circulation following ES events compared to the SSW-only events but their contributions vary through the lifetime of ES events. We emphasize the role of the resolved wave drag in the ES formation as in the sensitivity test when the non-orographic GW drag is absent, the anomalously enhanced resolved wave forcing in the mesosphere gives rise to the formation of the elevated stratopause at about 85 km

Emulating lateral gravity wave propagation in a global chemistry-climate model (EMAC v2.55.2) through horizontal flux redistribution

The columnar approach of gravity wave (GW) parameterisations in weather and climate models has been identified as a potential reason for dynamical biases in middle-atmospheric dynamics. For example, GW momentum flux (GWMF) discrepancies between models and observations at 60°S arising through the lack of horizontal orographic GW propagation are suspected to cause deficiencies in representing the Antarctic polar vortex. However, due to the decomposition of the model domains onto different computing tasks for parallelisation, communication between horizontal grid boxes is computationally extremely expensive, making horizontal propagation of GWs unfeasible for global chemistry-climate simulations. To overcome this issue, we present a simplified solution to approximate horizontal GW propagation through redistribution of the GWMF at one single altitude by means of tailor-made redistribution maps. To generate the global redistribution maps averaged for each grid box, we use a parameterisation describing orography as a set of mountain ridges with specified location, orientation and height combined with a ray-tracing model describing lateral propagation of so-generated mountain waves. In the global chemistry-climate model (CCM) EMAC (ECHAM MESSy Atmospheric Chemistry), these maps then allow us to redistribute the GW momentum flux horizontally at one level, obtaining an affordable overhead of computing resources. The results of our simulations show GWMF and drag patterns that are horizontally more spread out than with the purely columnar approach; GWs are now also present above the ocean and regions without mountains. In this paper, we provide a detailed description of how the redistribution maps are computed and how the GWMF redistribution is implemented in the CCM. Moreover, an analysis shows why 15 km is the ideal altitude for the redistribution. First results with the redistributed orographic GWMF provide clear evidence that the redistributed GW drag in the Southern Hemisphere has the potential to modify and improve Antarctic polar vortex dynamics, thereby paving the way for enhanced credibility of CCM simulations and projections of polar stratospheric ozone

Interhemispheric comparison of mesosphere / lower thermosphere winds from GAIA, WACCM-X and ICON-UA simulations and meteor radar observations at mid- and polar latitudes

In this study, we cross-compare the nudged models Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) and Whole Atmosphere Community Climate Model Extended Version (Specified dynamics) ( WACCM-X(SD)), a free-running version of Upper Atmosphere ICOsahedral Non-hydrostatic (ICON-UA) with six meteor radars located at conjugate polar and mid-latitudes. Mean winds, diurnal and semidiurnal tidal amplitudes and phases were obtained from the radar observations at the mesosphere and lower thermosphere (MLT) and compared to the GAIA, WACCM-X(SD), and ICON-UA data for similar locations applying a harmonized diagnostic. Our results indicate that GAIA zonal and meridional winds show a good agreement to the meteor radars during the winter season on both hemispheres, whereas WACCM-X(SD) and ICON-UA seem to reproduce better the summer zonal wind reversal. However, the mean winds also exhibit some deviation in the seasonal characteristic concerning the meteor radar measurements, which are attributed to the gravity wave parameterizations implemented in the models. All three models tend to reflect the seasonality of diurnal tidal amplitudes, but show some dissimilarities in tidal phases. We also found systematic interhemispheric differences in the seasonal characteristic of semidiurnal amplitudes and phases. The free-running ICON-UA apparently shows most of these interhemispheric differences, whereas WACCM-X(SD) and GAIA trend to have better agreement of the semidiurnal tidal variability in the northern hemisphere

Stratospheric contraction caused by increasing greenhouse gases

Rising emissions of anthropogenic greenhouse gases (GHG) have led to tropospheric warming and stratospheric cooling over recent decades. As a thermodynamic consequence, the troposphere has expanded and the rise of the tropopause, the boundary between the troposphere and stratosphere, has been suggested as one of the most robust fingerprints of anthropogenic climate change. Conversely, at altitudes above ∼55 km (in the mesosphere and thermosphere) observational and modeling evidence indicates a downward shift of the height of pressure levels or decreasing density at fixed altitudes. The layer in between, the stratosphere, has not been studied extensively with respect to changes of its global structure. Here we show that this atmospheric layer has contracted substantially over the last decades, and that the main driver for this are increasing concentrations of GHG. Using data from coupled chemistry-climate models we show that this trend will continue and the mean climatological thickness of the stratosphere will decrease by 1.3 km following representative concentration pathway 6.0 by 2080. We also demonstrate that the stratospheric contraction is not only a response to cooling, as changes in both tropopause and stratopause pressure contribute. Moreover, its short emergence time (less than 15 years) makes it a novel and independent indicator of GHG induced climate change

Comparing interhemispheric differences of mesosphere/lower thermosphere dynamics from ground-based observations and three general circulation models

Meteor radars have been proven to be valuable assets in investigating and monitoring mesosphere/lower thermosphere winds for the last two decades. In this study we present a comparison of almost continuous meteor radar measurements obtained from six meteor radars located at mid- and polar conjugate latitudes in both hemispheres. For this purpose we havecompiled harmonized data sets for the Sodankylä (67.9°N, 21.1°E), Esrange (67.4°N, 26.6°E), Davis (68.6°S, 78.0°E), Collm (51.3°N, 13.0°E), Tierra del Fuego meteor radar (53.7°S, 67.7°W) and the Canadian Meteor Orbit Radar (CMOR) (43.3°N, 80.8°W). The analysis revealed characteristic differences between the northern and southern hemisphere in the mean winds, in the strength of the mesospheric jets as well as in the tidal climatologies. In particular, semidiurnal tides show significant and distinct interhemispheric differences, notably a strong seasonal asymmetry in amplitude and phase, most prominent during the hemispheric fall transition from September to November. We also compared the observational climatologies with predictions from the three general circulation models GAIA, WACCM-X(SD) and ICON-UA. The model data were analyzed by simulating the radar in the model domain and applying an identical diagnostic to extract mean winds, tides and gravity wave activity. Our comparison reveals substantial differences between model and observational mean winds and tides that vary seasonally, by model and hemisphere. GAIA indicates similar winds during the hemispheric winter conditions compared to the observations, whereas WACCM-X(SD) showed a better agreement to the observations for the summer zonal wind reversal. The models are only partially able to capture interhemispheric differences, with the free-running ICON-UA model best reproducing the interhemispheric difference of the semidiurnal tide in reasonable agreement to observations