ALBERT

Description: Springtime stratospheric final warming (SFW) variability has been suggested to be linked to the tropospheric circulation, particularly over the North Atlantic sector. These findings, however, are based on reanalysis data that cover a rather short period of time (1979 to present). The present work aims to improve the understanding of drivers, trends and surface impact of dynamical variability of boreal SFWs using chemistry-climate models. We use multidecadal integrations of the fully coupled chemistry-climate models Community Earth System Model version 1 (Whole Atmosphere Community Climate Model) and ECHAM/Modular Earth Submodel System Atmospheric Chemistry-O. Four sensitivity experiments are analyzed to assess the impact of external factors; namely, the quasi-biennial oscillation, sea surface temperature (SST) variability, and anthropogenic emissions. SFWs are classified into two types with respect to their vertical development; that is, events which occur first in the midstratosphere (10-hPa first SFWs) or first in the upper stratosphere (1-hPa first SFWs). Our results confirm previous reanalysis results regarding the differences in the time evolution of stratospheric conditions and near-surface circulation between 10 and 1-hPa first SFWs. Additionally, a tripolar SST pattern is, for the first time, identified over the North Atlantic in spring months related to the SFW variability. Our analysis of the influence of remote modulators on SFWs revealed that the occurrence of major warmings in the previous winter favors the occurrence of 10-hPa first SFWs later on. We further found that quasi-biennial oscillation and SST variability significantly affect the ratio between 1-hPa first and 10-hPa first SFWs. Finally, our results suggest that ozone recovery may impact the timing of the occurrence of 1-hPa first SFWs. ©2019. American Geophysical Union. All Rights Reserved.

Description: The effect of the winter Brewer-Dobson circulation (BDC) on the seasonal and decadal evolution of total ozone in both hemispheres is investigated using satellite total ozone data from the merged GOME/SCIAMACHY/GOME-2 (GSG) data set (1995–2010) and outputs from two chemistry-climate models (CCM), the FUB-EMAC and DLR-E39C-A models. Combining data from both hemispheres a linear relationship between the winter average extratropical 100 hPa eddy heat flux and the ozone ratio with respect to fall ozone levels exists and is statistically significant for tropical as well as polar ozone. The high correlation at high latitudes persists well into the summer months until the onset of the next winter season. The anti-correlation of the cumulative eddy heat flux with tropical ozone ratios, however, breaks down in spring as the polar vortex erodes and changes to a weak positive correlation similar to that observed at high latitudes. The inter-annual variability and decadal evolution of ozone in each hemisphere in winter, spring, and summer are therefore driven by the cumulative effect of the previous winter's meridional circulation. This compact linear relationship is also found in both CCMs used in this study indicating that current models realistically describe the variability in stratospheric circulation and its effect on total ozone. Both models show a positive trend in the winter mean eddy heat flux (and winter BDC strength) in both hemispheres until year 2050, however the inter-annual variability (peak-to-peak) is two to three times larger than the mean change between 1960 and 2050. It is, nevertheless, possible to detect a shift in this compact linear relationship related to past and future changes in the stratospheric halogen load. Using the SBUV/TOMS/OMI (MOD V8) merged data set (1980–2010), it can be shown that from the decade 1990–1999 to 2000–2010 this linear relationship remained unchanged (before and after the turnaround in the stratospheric halogen load), while a shift is evident between 1980–1989 (upward trend in stratospheric halogen) and the 1990s, which is a clear sign that an onset of recovery is detectable despite the large variability in polar ozone. Because of the large variability from year to year in the BDC circulation substantial polar ozone depletion may still occur in coming decades in selected winters with weak BDC and very low polar stratospheric temperatures.

Description: In the recent past, the evolution of stratospheric ozone (O3) was affected by both increasing ozone depleting substances (ODSs) and greenhouse gases (GHGs). The impact of the single forcings on O3 is well known. Interactions between the simultaneously increased GHG and ODS concentrations, however, can occur and lead to nonlinear O3 changes. In this study, we investigate if nonlinear processes have affected O3 changes between 1960 and 2000. This is done with an idealized set of timeslice simulations with the chemistry–climate model (CCM) EMAC. Nonlinearity leads to a net reduction of ozone decrease throughout the stratosphere, with a maximum of 1.2% at 3 hPa. The total ozone column loss between 1960 and 2000 that is mainly attributed to the ODS increase is mitigated in the extra-polar regions by up to 1.1% due to nonlinear processes. A separation of the O3 changes into the contribution from chemistry and transport shows that nonlinear interactions occur in both. In the upper stratosphere a reduced efficiency of the ClOx-catalysed O3 loss chiefly causes the nonlinear O3 increase. An enhanced formation of halogen reservoir species through the reaction with methane (CH4) reduces the abundance of halogen radicals significantly. The temperature induced deceleration of the O3 loss reaction rate in the Chapman cycle is reduced, which leads to a nonlinear O3 decrease and counteracts the increase due to ClOx. Nonlinear effects on the NOx abundance cause hemispheric asymmetric nonlinear changes of the O3 loss. Nonlinear changes in O3 transport occur in particular in the Southern Hemisphere (SH) during the months September to November. Here, the residual circulation is weakened in the lower stratosphere, which goes along with a reduced O3 transport from the tropics to high latitudes. Thus, O3 decreases in the SH polar region, but increases in the SH midlatitudes.

Description: MIPAS-Envisat is a satellite-borne sensor which was measuring vertical profiles of a wide range of trace gases from 2002 to 2012 using IR emission spectroscopy. We present geophysical validation for the operational retrieval (version 6.0) of N2O, CH4, CFC-12 and CFC-11 by the European Space Agency (ESA) of MIPAS-Envisat. The geophysical validation data are derived from measurements of samples collected by a cryogenic whole air sampler flown to altitudes of up to 34 km by means of large scientific balloons. In order to increase the number of coincidences between the satellite and the balloon observations we applied a trajectory matching technique. The results are presented for different time periods due to a change in the spectroscopic resolution of MIPAS as of early 2005. Retrieval results for N2O, CH4 and CFC-12 show partly good agreement for some altitude regions, which differs for the periods with different spectroscopic resolution. However, significant differences to the balloon data are also observed for some altitude regions, which depend on species and spectroscopic resolution. These differences need to be considered when using these data. The CFC-11 results from the operation retrieval version 6 cannot be recommended for scientific studies due to a systematic overestimation of the CFC-11 mixing ratios.

Description: The effect of the winter Brewer-Dobson circulation (BDC) on the seasonal and decadal evolution of total ozone in both hemispheres is investigated using satellite total ozone data and outputs from two chemistry-climate models (CCM). Combining data from both hemispheres a linear relationship between the winter cumulative extratropical 100 hPa eddy heat flux and the ozone ratio with respect to fall ozone levels exists and is statistically significant for tropical as well as polar ozone. The high correlation at high latitudes persists well into the summer months until the onset of the next winter season. The anti-correlation of the cumulative eddy heat flux with tropical ozone ratios, however, breaks down in spring as the polar vortex erodes and changes to a weak positive correlation similar to that observed at high latitudes. The inter-annual variability and decadal evolution of ozone in each hemisphere in winter, spring, and summer are therefore driven by the cumulative effect of the previous winter's meridional circulation. This compact linear relationship is also found in two different CCMs (EMAC-FUB, DLR-E39C-A) indicating that current models realistically describe the variability in stratospheric circulation and its climate effect on total ozone. Both models show a positive trend in the winter mean eddy heat flux (and winter BDC strength) in both hemispheres until year 2050, however the inter-annual variability (peak-to-peak) is two to three times larger than the mean change between 1960 and 2050. It is, therefore, possible to detect a shift in this compact linear relationship related to past and future changes in the stratospheric halogen load. A similar shift is difficult to derive from observational data since the satellite era now spanning more than thirty years is still fairly short.

Description: In the recent past, the evolution of stratospheric ozone (O3) was affected by both increasing ozone depleting substances (ODSs) and greenhouse gases (GHGs). The impact of the single forcings on O3 is well known. Interactions between the simultaneously increased GHG and ODS concentrations, however, can occur and lead to nonlinear O3 changes. In this study, we investigate if nonlinear processes have affected O3 changes between 1960 and 2000. This is done with an idealised set of time slice simulations with the chemistry-climate model EMAC. Due to nonlinearity the past ozone loss is diminished throughout the stratosphere, with a maximum reduction of 1.2 % at 3 hPa. The total ozone column loss between 1960 and 2000 that is mainly attributed to the ODS increase is mitigated in the extra-polar regions by up to 1.1 % due to nonlinear processes. A separation of the O3 changes into the contribution from chemistry and transport shows that nonlinear interactions occur in both. In the upper stratosphere a reduced efficiency of the ClOx-catalysed O3 loss chiefly causes the nonlinear O3 increase. An enhanced formation of halogen reservoir species through the reaction with methane (CH4) reduces the abundance of halogen radicals significantly. The temperature-induced deceleration of the O3 loss reaction rate in the Chapman cycle is reduced, which leads to a nonlinear O3 decrease and counteracts the increase due to ClOx. Nonlinear effects on the NOx abundance cause hemispheric asymmetric nonlinear changes of the O3 loss. Nonlinear changes in O3 transport occur in particular in the Southern Hemisphere (SH) during the months September to November. Here, the residual circulation is weakened in the lower stratosphere, which goes along with a reduced O3 transport from the tropics to high latitudes. Thus, O3 decreases in the SH polar region but increases in the SH midlatitudes. The existence of nonlinearities implies that future ozone change due to ODS decline slightly depends on the prevailing GHG concentrations. Therefore the future ozone evolution will not simply be a reversal of the past.

Description: Major stratospheric warmings (MSWs) are one of the most important phenomena of wintertime Arctic stratospheric variability. They consist of a warming of the Arctic stratosphere and a deceleration of the polar night jet, triggered by an anomalously high injection of tropospheric wave activity into the stratosphere. Due to the relevance and the impact of MSWs on the tropospheric circulation, several model studies have investigated their potential responses to climate change. However, a wide range of results has been obtained, extending from a future increase in the frequency of MSWs to a decrease. These discrepancies might be explained by different factors such as a competition of radiative and dynamical contributors with opposite effects on the Arctic polar vortex, biases of models to reproduce the related processes, or the metric chosen for the identification of MSWs. In this study, future changes in wintertime Arctic stratospheric variability are examined in order to obtaina more precise picture of future changes in the occurrence of MSWs. In particular, transient REFC2 simulations of different CCMs involved in the Chemistry Climate Model Initiative (CCMI) are used. These simulations extend from 1960 to 2100 and include forcings by halogens and greenhouse gases following the specifications of the CCMI-REF-C2 scenario. Sea surface temperatures (SSTs) and sea-ice distributions are either prescribed from coupled climate model integrations or calculated internally in the case of fully coupled atmosphere-ocean CCMs. Potential changes in the frequency and main characteristics of MSWs in the future are investigated with special focus on the dependence of the results on the criterion for the identification of MSWs and the tropospheric forcing of these phenomena.

Description: Springtime stratospheric final warming (SFW) variability has been suggested to be linked to the tropospheric circulation, particularly over the North Atlantic sector. These findings, however, are based on reanalysis data that cover a rather short period of time (1979 to present). The present work aims to improve the understanding of drivers, trends and surface impact of dynamical variability of boreal SFWs using chemistry‐climate models. We use multidecadal integrations of the fully coupled chemistry‐climate models Community Earth System Model version 1 (Whole Atmosphere Community Climate Model) and ECHAM/Modular Earth Submodel System Atmospheric Chemistry‐O. Four sensitivity experiments are analyzed to assess the impact of external factors; namely, the quasi‐biennial oscillation, sea surface temperature (SST) variability, and anthropogenic emissions. SFWs are classified into two types with respect to their vertical development; that is, events which occur first in the midstratosphere (10‐hPa first SFWs) or first in the upper stratosphere (1‐hPa first SFWs). Our results confirm previous reanalysis results regarding the differences in the time evolution of stratospheric conditions and near‐surface circulation between 10 and 1‐hPa first SFWs. Additionally, a tripolar SST pattern is, for the first time, identified over the North Atlantic in spring months related to the SFW variability. Our analysis of the influence of remote modulators on SFWs revealed that the occurrence of major warmings in the previous winter favors the occurrence of 10‐hPa first SFWs later on. We further found that quasi‐biennial oscillation and SST variability significantly affect the ratio between 1‐hPa first and 10‐hPa first SFWs. Finally, our results suggest that ozone recovery may impact the timing of the occurrence of 1‐hPa first SFWs.