Extratropical Lower Stratosphere Research Articles

Updating the radiation infrastructure in MESSy (based on MESSy version 2.55)

Abstract. The calculation of the radiative transfer is a key component of global circulation models. In this article, we describe the most recent updates of the radiation infrastructure in the Modular Earth Submodel System (MESSy). These updates include the implementation of the PSrad radiation scheme within the RAD submodel. Furthermore, the radiation-related submodels CLOUDOPT (for the calculation of cloud optical properties) and AEROPT (for the calculation of aerosol optical properties) have been updated and are now more flexible in order to deal with different sets of shortwave and longwave bands of radiation schemes. In the wake of these updates, a new submodel (ALBEDO), which features solar-zenith-angle-dependent albedos and a new satellite-based background (white sky) albedo, was created. All of these developments are backward compatible, and previous features of the MESSy radiation infrastructure remain available. Moreover, these developments mark an important step in the use of the ECHAM/MESSy Atmospheric Chemistry (EMAC) model, as the update of the radiation scheme was a key aspect in the development of the sixth generation of the European Centre for Medium-Range Weather Forecasts – HAMburg (ECHAM6) model from ECHAM5. The developments presented here are also aimed towards using the MESSy infrastructure with the ICOsahedral Non-hydrostatic (ICON) model as a base model. The improved infrastructure will also aid in the implementation of additional radiation schemes once this should be needed. We have optimized the set of free parameters for two general circulation model-type (GCM-type) setups for pre-industrial and present-day conditions: one with the radiation scheme that was used to date (i.e. the radiation scheme of ECHAM5) and one with the newly implemented PSrad radiation scheme. After this parameter optimization, we performed four model simulations and evaluated the corresponding model results using reanalysis and observational data. The most apparent improvements related to the updated radiation scheme are the reduced cold biases in the tropical upper troposphere and lower stratosphere and the extratropical lower stratosphere and a strengthened polar vortex. The former is also related to improved stratospheric humidity and its variability if the new radiation scheme is employed. Using the multiple radiation call capability of MESSy, we have applied the two model configurations to calculate instantaneous and stratospheric-adjusted radiative forcings related to changes in greenhouse gases. Overall, we find that for many forcing experiments the simulations with the new radiation scheme show improved radiative forcing values. This is in particular the case for methane radiative forcings, which are considerably higher when assessed with the new radiation scheme and thus in better agreement with reference values.

Intra-Seasonal Variations and Frequency of Major Sudden Stratospheric Warmings for Northern Winter in Multi-System Seasonal Hindcast Data

This study investigates intra-seasonal variations and frequency of major sudden stratospheric warmings (MSSWs) in Northern winter seasonal hindcasts of six systems from 1993/1994 to 2016/2017, in comparison to the Japanese 55-year Reanalysis data. Results show that, over all, all systems reproduce precursory signals to the MSSWs well, such as the increase in the planetary wave heat flux in the extratropical lower stratosphere and the anomalous planetary wave patterns in the troposphere. Some systems are suggested to underestimate or overestimate the mean MSSW frequency. Such differences in the frequency of the MSSWs among the systems are related to those in the mean strength of the stratospheric polar vortex, and also may be partly contributed by those in the frequency of notable heat flux events. The hindcast data exhibit a weaker mean vortex and an increased MSSW frequency for a warm phase than for a cold phase of El Niño/Southern Oscillation, and for an easterly phase than for a westerly phase of the Quasi-Biennial Oscillation. These are qualitatively consistent with reanalysis results, except for a lower MSSW frequency for the warm phase in the reanalysis data. The reanalysis teleconnection results are larger in magnitude than the hindcast results for most ensemble members, although they are included near the edge of the distributions of the ensemble members. The changes in the MSSW frequency with the two external factors are correlated to those in the mean vortex strength among the ensemble members and also the ensemble means for some systems.

Characterization of transport from the Asian summer monsoon anticyclone into the UTLS via shedding of low potential vorticity cutoffs

Abstract. Air mass transport within the summertime Asian monsoon circulation provides a major source of anthropogenic pollution for the upper troposphere and lower stratosphere (UTLS). Here, we investigate the quasi-horizontal transport of air masses from the Asian summer monsoon anticyclone (ASMA) into the extratropical lower stratosphere and their chemical evolution. For that reason, we developed a method to identify and track the air masses exported from the monsoon. This method is based on the anomalously low potential vorticity (PV) of these air masses (tropospheric low PV cutoffs) compared to the lower stratosphere and uses trajectory calculations and chemical fields from the Chemical Lagrangian Model of the Stratosphere (CLaMS). The results show evidence of frequent summertime transport from the monsoon anticyclone to midlatitudes over the North Pacific, even reaching the high-latitude regions of Siberia and Alaska. Most of the low PV cutoffs related to air masses exported from the ASMA have lifetimes shorter than 1 week (about 90 %) and sizes smaller than 1 % of the Northern Hemisphere (NH) area. The chemical composition of these air masses is characterized by carbon monoxide, ozone, and water vapour mixing ratios at an intermediate range between values typical for the monsoon anticyclone and the lower stratosphere. The chemical evolution during transport within these low PV cutoffs shows a gradual change from the characteristics of the monsoon anticyclone to characteristics of the lower stratospheric background during about 1 week, indicating continuous mixing with the background atmosphere.

In situ observations of CH<sub>2</sub>Cl<sub>2</sub> and CHCl<sub>3</sub> show efficient transport pathways for very short-lived species into the lower stratosphere via the Asian and the North American summer monsoon

Abstract. Efficient transport pathways for ozone-depleting very short-lived substances (VSLSs) from their source regions into the stratosphere are a matter of current scientific debate; however they have yet to be fully identified on an observational basis. Understanding the increasing impact of chlorine-containing VSLSs (Cl-VSLSs) on stratospheric ozone depletion is important in order to validate and improve model simulations and future predictions. We report on a transport study using airborne in situ measurements of the Cl-VSLSs dichloromethane (CH2Cl2) and trichloromethane (chloroform, CHCl3) to derive a detailed description of two transport pathways from (sub)tropical source regions into the extratropical upper troposphere and lower stratosphere (Ex-UTLS) in the Northern Hemisphere (NH) late summer. The Cl-VSLS measurements were obtained in the upper troposphere and lower stratosphere (UTLS) above western Europe and the midlatitude Atlantic Ocean in the frame of the WISE (Wave-driven ISentropic Exchange) aircraft campaign in autumn 2017 and are combined with the results from a three-dimensional simulation of a Lagrangian transport model as well as back-trajectory calculations. Compared to background measurements of similar age we find up to 150 % enhanced CH2Cl2 and up to 100 % enhanced CHCl3 mixing ratios in the extratropical lower stratosphere (Ex-LS). We link the measurements of enhanced CH2Cl2 and CHCl3 mixing ratios to emissions in the region of southern and eastern Asia. Transport from this area to the Ex-LS at potential temperatures in the range of 370–400 K takes about 6–11 weeks via the Asian summer monsoon anticyclone (ASMA). Our measurements suggest anthropogenic sources to be the cause of these strongly elevated Cl-VSLS concentrations observed at the top of the lowermost stratosphere (LMS). A faster transport pathway into the Ex-LS is derived from particularly low CH2Cl2 and CHCl3 mixing ratios in the UTLS. These low mixing ratios reflect weak emissions and a local seasonal minimum of both species in the boundary layer of Central America and the tropical Atlantic. We show that air masses uplifted by hurricanes, the North American monsoon, and general convection above Central America into the tropical tropopause layer to potential temperatures of about 360–370 K are transported isentropically within 5–9 weeks from the boundary layer into the Ex-LS. This transport pathway linked to the North American monsoon mainly impacts the middle and lower part of the LMS with particularly low CH2Cl2 and CHCl3 mixing ratios. In a case study, we specifically analyze air samples directly linked to the uplift by the Category 5 Hurricane Maria that occurred during October 2017 above the Atlantic Ocean. CH2Cl2 and CHCl3 have similar atmospheric sinks and lifetimes, but the fraction of biogenic emissions is clearly higher for CHCl3 than for the mainly anthropogenically emitted CH2Cl2; consequently lower CHCl3 : CH2Cl2 ratios are expected in air parcels showing a higher impact of anthropogenic emissions. The observed CHCl3 : CH2Cl2 ratio suggests clearly stronger anthropogenic emissions in the region of southern and eastern Asia compared to those in the region of Central America and the tropical Atlantic. Overall, the transport of strongly enhanced CH2Cl2 and CHCl3 mixing ratios from southern and eastern Asia via the ASMA is the main factor in increasing the chlorine loading from the analyzed VSLSs in the Ex-LS during the NH late summer. Thus, further increases in Asian CH2Cl2 and CHCl3 emissions, as frequently reported in recent years, will further increase the impact of Cl-VSLSs on stratospheric ozone depletion.

Variation in the Holton–Tan effect by longitude

AbstractThe teleconnection between the Quasi‐Biennial Oscillation (QBO) and the boreal winter polar vortex, the Holton–Tan effect, is analyzed in the Whole Atmosphere Community Climate Model (WACCM) with a focus on how stationary wave propagation varies by QBO phase. These signals are difficult to isolate in reanalyses because of large internal variability in short observational records, especially when decomposing the data by QBO phase. A 1,500‐year ensemble is leveraged by defining the QBO index at five different isobars between 10 and 70 hPa. The Holton–Tan effect is a robust part of the atmospheric response to the QBO in WACCM with warming of the polar stratosphere during easterly QBO (QBOE). A nudging technique is used to reduce polar stratospheric variability in one simulation. This enables isolation of the impact of the QBO on the atmosphere in the absence of a polar stratospheric response to the QBO: referred to as the “direct effect” and the polar stratospheric response, “indirect effect.” This simulation reveals that the polar stratospheric warming during QBOE pushes the tropospheric jet equatorward, opposing the poleward shift of the jet by the QBOE, especially over the North Pacific. The Holton–Tan effect varies over longitude. The QBO induces stronger planetary wave forcing to the mean flow in the extratropical lower stratosphere between Indonesia and Alaska. The North Pacific polar stratosphere responds to this before other longitudes. What follows is a shift in the position of the polar vortex toward Eurasia (North America) during easterly (westerly) QBO. This initiates downstream planetary wave responses over North America, the North Atlantic, and Siberia. This spatiotemporal evolution is found in transient simulations in which QBO nudging is “switched on.” The North Pacific lower stratosphere seems more intrinsically linked to the QBO while other longitudes appear more dependent on the mutual interaction between the QBO and polar stratosphere.

Recent decline in extratropical lower stratospheric ozone attributed to circulation changes.

1998-2016 ozone trends in the lower stratosphere (LS) are examined using the Modern-Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) and related NASA products. After removing biases resulting from step-changes in the MERRA-2 ozone observations, a discernible negative trend of -1.67±0.54 Dobson units per decade (DU/decade) is found in the 10-km layer above the tropopause between 20°N and 60°N. A weaker but statistically significant trend of -1.17±0.33 DU/decade exists between 50°S and 20°S. In the Tropics, a positive trend is seen in a 5-km layer above the tropopause. Analysis of an idealized tracer in a model simulation constrained by MERRA-2 meteorological fields provides strong evidence that these trends are driven by enhanced isentropic transport between the tropical (20°S-20°N) and extratropical LS in the past two decades. This is the first time that a reanalysis dataset has been used to detect and attribute trends in lower stratospheric ozone.

Quantifying pollution transport from the Asian monsoon anticyclone into the lower stratosphere

Abstract. Pollution transport from the surface to the stratosphere within the Asian monsoon circulation may cause harmful effects on stratospheric chemistry and climate. Here, we investigate air mass transport from the monsoon anticyclone into the stratosphere using a Lagrangian chemistry transport model. We show how two main transport pathways from the anticyclone emerge: (i) into the tropical stratosphere (tropical pipe), and (ii) into the Northern Hemisphere (NH) extratropical lower stratosphere. Maximum anticyclone air mass fractions reach around 5 % in the tropical pipe and 15 % in the extratropical lowermost stratosphere over the course of a year. The anticyclone air mass fraction correlates well with satellite hydrogen cyanide (HCN) and carbon monoxide (CO) observations, confirming that pollution is transported deep into the tropical stratosphere from the Asian monsoon anticyclone. Cross-tropopause transport occurs in a vertical chimney, but with the pollutants transported quasi-horizontally along isentropes above the tropopause into the tropics and NH.

Impact of the Asian monsoon on the extratropical lower stratosphere: trace gas observations during TACTS over Europe 2012

Abstract. The transport of air masses originating from the Asian monsoon anticyclone into the extratropical upper troposphere and lower stratosphere (Ex-UTLS) above potential temperatures Θ = 380 K was identified during the HALO aircraft mission TACTS in August and September 2012. In situ measurements of CO, O3 and N2O during TACTS flight 2 on 30 August 2012 show the irreversible mixing of aged stratospheric air masses with younger (recently transported from the troposphere) ones within the Ex-UTLS. Backward trajectories calculated with the trajectory module of CLaMS indicate that these tropospherically affected air masses originate from the Asian monsoon anticyclone. These air masses are subsequently transported above potential temperatures Θ = 380 K from the monsoon circulation region into the Ex-UTLS, where they subsequently mix with stratospheric air masses. The overall trace gas distribution measured during TACTS shows that this transport pathway had affected the chemical composition of the Ex-UTLS during boreal summer and autumn 2012. This leads to an intensification of the tropospheric influence on the extratropical lower stratosphere with PV > 8 pvu within 3 weeks during the TACTS mission. During the same time period a weakening of the tropospheric influence on the lowermost stratosphere (LMS) is determined. The study shows that the transport of air masses originating from the Asian summer monsoon region within the lower stratosphere affects the change in the chemical composition of the Ex-UTLS over Europe and thus contributes to the flushing of the LMS during summer 2012.

Water vapour stratification and dynamical warming behind the sharpness of the Earth's midlatitude tropopause

Recent studies have highlighted the fact that the extratropical lower stratosphere, up to ∼10 km from the thermal tropopause, is far from being isothermal. The tropopause is sharper than once thought and the vertical temperature profile in the next layers undergoes significant seasonal variations. Here, an analytical model is formulated for studying the seasonal temperature structure of that region of the Earth's atmosphere. A model fitting to over 10 years of stratospheric balloon observations at two midlatitude locations in opposite hemispheres captures the observed temperature profiles, including the tropopause inversion layer. Several sensitivity tests clarify the radiative role of ozone (through solar radiation) and water vapour (through thermal radiation) on the thermal stratification of the midlatitude tropopause region, and further demonstrate the role of heating from atmospheric motions. In spite of the major role played by ozone in upper layers, ozone heating only accounts for a small part (∼1 K) of the temperature maximum of the tropopause inversion layer, and its effect is negligible in the first kilometre right above the tropopause. In contrast, the vertical structure of water vapour between the tropopause and the hygropause combined with dynamical warming is found to explain the sharp change in lapse rate that marks the midlatitude tropopause, and most of its immediate inversion layer. This reveals how both radiation – chiefly thermal absorption and emission by stratified water vapour – and dynamics are behind the mysterious sharpness of the tropopause in midlatitudes.

Sensitivity analysis of the potential impact of discrepancies in stratosphere–troposphere exchange on inferred sources and sinks of CO<sub>2</sub>

Abstract. The upper troposphere and lower stratosphere (UTLS) represents a transition region between the more dynamically active troposphere and more stably stratified stratosphere. The region is characterized by strong gradients in the distribution of long-lived tracers, whose representation in models is sensitive to discrepancies in transport. We evaluate the GEOS-Chem model in the UTLS using carbon dioxide (CO2) and ozone (O3) observations from the HIAPER (The High-Performance Instrumented Airborne Platform for Environmental Research) Pole-to-Pole Observations (HIPPO) campaign in March 2010. GEOS-Chem CO2/O3 correlation suggests that there is a discrepancy in mixing across the tropopause in the model, which results in an overestimate of CO2 and an underestimate of O3 in the Arctic lower stratosphere. We assimilate stratospheric O3 data from the Optical Spectrograph and InfraRed Imager System (OSIRIS) and use the assimilated O3 fields together with the HIPPO CO2/O3 correlations to obtain an adjustment to the modeled CO2 profile in the Arctic UTLS (primarily between the 320 and 360 K isentropic surfaces). The HIPPO-derived adjustment corresponds to a sink of 0.60 Pg C for March–August 2010 in the Arctic. Imposing this adjustment results in a reduction in the CO2 sinks inferred from GOSAT observations for temperate North America, Europe, and tropical Asia of 19, 13, and 49 %, respectively. Conversely, the inversion increased the source of CO2 from tropical South America by 23 %. We find that the model also underestimates CO2 in the upper tropical and subtropical troposphere. Correcting for the underestimate in the model relative to HIPPO in the tropical upper troposphere leads to a reduction in the source from tropical South America by 77 %, and produces an estimated sink for tropical Asia that is only 19 % larger than the standard inversion (without the imposed source and sink). Globally, the inversion with the Arctic and tropical adjustment produces a sink of −6.64 Pg C, which is consistent with the estimate of −6.65 Pg C in the standard inversion. However, the standard inversion produces a stronger northern land sink by 0.98 Pg C to account for the CO2 overestimate in the high-latitude UTLS, suggesting that this UTLS discrepancy can impact the latitudinal distribution of the inferred sources and sinks. We find that doubling the model resolution from 4° × 5° to 2° × 2.5° enhances the CO2 vertical gradient in the high-latitude UTLS, and reduces the overestimate in CO2 in the extratropical lower stratosphere. Our results illustrate that discrepancies in the CO2 distribution in the UTLS can affect CO2 flux inversions and suggest the need for more careful evaluation of model errors in the UTLS.

Intercomparison of daytime stratospheric NO<sub>2</sub> satellite retrievals and model simulations

Abstract. This paper evaluates the agreement between stratospheric NO2 retrievals from infrared limb sounders (Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and High Resolution Dynamics Limb Sounder (HIRDLS)) and solar UV/VIS backscatter sensors (Ozone Monitoring Instrument (OMI), Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) limb and nadir) over the 2005–2007 period and across the seasons. The observational agreement is contrasted with the representation of NO2 profiles in 3-D chemical transport models such as the Whole Atmosphere Community Climate Model (WACCM) and TM4. A conclusion central to this work is that the definition of a reference for stratospheric NO2 columns formed by consistent agreement among SCIAMACHY, MIPAS and HIRDLS limb records (all of which agree to within 0.25 × 1015 molecules cm−2 or better than 10%) allows us to draw attention to relative errors in other data sets, e.g., (1) WACCM overestimates NO2 densities in the extratropical lower stratosphere, particularly in the springtime and over northern latitudes by up to 35% relative to limb observations, and (2) there are remarkable discrepancies between stratospheric NO2 column estimates from limb and nadir techniques, with a characteristic seasonally and latitudinally dependent pattern. We find that SCIAMACHY nadir and OMI stratospheric columns show overall biases of −0.5 × 1015 molecules cm−2 (−20%) and +0.6 × 1015 molecules cm−2 (+20%) relative to limb observations, respectively. It is argued that additive biases in nadir stratospheric columns are not expected to affect tropospheric retrievals significantly, and that they can be attributed to errors in the total slant column density, related either to algorithmic or instrumental effects. In order to obtain accurate and long-term time series of stratospheric NO2, an effort towards the harmonization of currently used differential optical absorption spectroscopy (DOAS) approaches to nadir retrievals becomes essential, as well as their agreement to limb and ground-based observations, particularly now that limb techniques are giving way to nadir observations as the next generation of climate and air quality monitoring instruments pushes forth.

Chemical contribution to future tropical ozone change in the lower stratosphere

Abstract. The future evolution of tropical ozone in a changing climate is investigated by analysing time slice simulations made with the chemistry–climate model EMAC. Between the present and the end of the 21st century a significant increase in ozone is found globally for the upper stratosphere and the extratropical lower stratosphere, while in the tropical lower stratosphere ozone decreases significantly by up to 30%. Previous studies have shown that this decrease is connected to changes in tropical upwelling. Here the dominant role of transport for the future ozone decrease is confirmed, but it is found that in addition changes in chemical ozone production and destruction do contribute to the ozone changes in the tropical lower stratosphere. Between 50 and 30 hPa the dynamically induced ozone decrease of up to 22% is amplified by 11–19% due to a reduced ozone production. This is counteracted by a decrease in the ozone loss causing an ozone increase by 15–28%. At 70 hPa the large ozone decrease due to transport (−52%) is reduced by an enhanced photochemical ozone production (+28%) but slightly increased (−5%) due to an enhanced ozone loss. It is found that the increase in the ozone production in the lowermost stratosphere is mainly due to a transport induced decrease in the overlying ozone column while at higher altitudes the ozone production decreases as a consequence of a chemically induced increase in the overlying ozone column. The ozone increase that is attributed to changes in ozone loss between 50 and 30 hPa is mainly caused by a slowing of the ClOx and NOx loss cycles. The enhanced ozone destruction below 70 hPa can be attributed to an increased efficiency of the HOx loss cycle. The role of ozone transport in determining the ozone trend in this region is found to depend on the changes in the net production as a reduced net production also reduces the amount of ozone that can be transported within an air parcel.

Transport from convective overshooting of the extratropical tropopause and the role of large‐scale lower stratosphere stability

Simulations of observed convective systems with the Advanced Research Weather Research and Forecasting (ARW‐WRF) model are used to test the influence of the large‐scale lower stratosphere stability environment on the vertical extent of convective overshooting and transport above the extratropical tropopause. Three unique environments are identified (double tropopause, stratospheric intrusion, and single tropopause), and representative cases with comparable magnitudes of convective available potential energy are selected for simulation. Convective injection into the extratropical lower stratosphere is found to be deepest for the double‐tropopause case (up to 4 km above the lapse‐rate tropopause) and at comparable altitudes for the remaining cases (up to 2 km above the lapse‐rate tropopause). All simulations show evidence of gravity wave breaking near the overshooting convective top, consistent with the identification of its role as a transport mechanism in previous studies. Simulations for the double‐tropopause case, however, also show evidence of direct mixing of the overshooting top into the lower stratosphere, which is responsible for the highest levels of injection in that case. In addition, the choice of bulk microphysical parameterization for ARW‐WRF simulations is found to have little impact on the transport characteristics for each case.

Rossby Wave Breaking and Transport between the Tropics and Extratropics above the Subtropical Jet

AbstractRossby wave breaking is an important mechanism for the two-way exchange of air between the tropical upper troposphere and lower stratosphere and the extratropical lower stratosphere. The authors present a 30-yr climatology (1981–2010) of anticyclonically and cyclonically sheared wave-breaking events along the boundary of the tropics in the 350–500-K potential temperature range from ECMWF Interim Re-Analysis (ERA-Interim). Lagrangian transport analyses show net equatorward transport from wave breaking near 380 K and poleward transport at altitudes below and above the 370–390-K layer. The finding of poleward transport at lower levels is in disagreement with previous studies and is shown to largely depend on the choice of tropical boundary. In addition, three distinct modes of transport for anticyclonic wave-breaking events are found near the tropical tropopause (380 K): poleward, equatorward, and symmetric. Transport associated with cyclonic wave-breaking events, however, is predominantly poleward. The three transport modes for anticyclonic wave breaking are associated with specific characteristics of the geometry of the mean flow. In particular, composite averages show that poleward transport is associated with a “split” subtropical jet where the jet on the upstream side of the breaking wave extends eastward and lies poleward and at lower altitudes of the subtropical jet on the downstream side, producing a substantial longitudinal overlap between the two jets. Equatorward transport is not associated with a split subtropical jet and is found immediately downstream of stationary anticyclones in the tropics, often associated with monsoon circulations. It is further shown that, in general, the transport direction of breaking waves is determined primarily by the relative positions of the jets.

The Response of the Tropospheric Circulation to Water Vapor–Like Forcings in the Stratosphere

Abstract An idealized, dry general circulation model is used to examine the response of the tropospheric circulation to thermal forcings that mimic changes in stratospheric water vapor (SWV). It is found that SWV-like cooling in the stratosphere produces a poleward-shifted, strengthened jet and an expanded, weakened Hadley cell. This response is shown to be almost entirely driven by cooling located in the extratropical lower stratosphere; when cooling is limited to the tropical stratosphere, it generates a much weaker and qualitatively opposite response. It is demonstrated that these circulation changes arise independently of any changes in tropopause height, are insensitive to the detailed structure of the forcing function, and are robust to model resolution. The responses are quantitatively of the same order as those due to well-mixed greenhouse gases, suggesting a potentially significant contribution of SWV to past and future changes in the tropospheric circulation.

Quantification of isentropic water-vapour transport into the lower stratosphere

The contour advection technique is used to quantify the quasi-horizontal isentropic transport of water vapour across the tropopause between the tropical upper troposphere and the extratropical lower stratosphere. The calculations are based on European Centre for Medium-Range Weather Forecasts analyses for 1997 and 1998. It is found that the isentropic transport of water vapour into the extratropical lower stratosphere has a similar magnitude to the upward (cross-isentropic) transport of water vapour into the stratosphere in the tropics. The isentropic flux is largest during summer, and is more than enough to account for the observed summertime increase in specific humidity in the lowermost stratosphere. The annual isentropic moisture transport into the extratropical lower stratosphere in the northern hemisphere is about one order of magnitude larger than that in the southern hemisphere. This difference in transport explains why the extratropical lower stratosphere is moister in the northern hemisphere during northern summer than it is in the southern hemisphere during southern summer. The results further show that there was more moisture transport into the lower stratosphere of both hemispheres during 1998, under El Nino conditions, than during 1997.

Atmospheric Chemistry Experiment (ACE) observations of aerosol in the upper troposphere and lower stratosphere from the Kasatochi volcanic eruption

Near‐infrared (NIR) atmospheric extinction profile observations from the Atmospheric Chemistry Experiment (ACE) Imager and from Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) are presented, illustrating the impact of the Kasatochi volcanic eruption in August 2008 on the aerosol loading of the upper troposphere and lower stratosphere in the subsequent months. In September 2008, profiles of NIR extinction show a significant increase relative to each of the previous four Septembers (2004–2007). The aerosol enhancement is observable up to 18.5 km in Northern Hemisphere NIR extinction data, and peaks at 9.5 km in the extratropics, where the extinction is ∼8 times the normal September value. The particulate matter in the troposphere is quickly dispersed over the Northern Hemisphere during September 2008 and vanishes by the end of November 2008. An upper layer, initially in the extratropical lower stratosphere, persists through March 2009, descending with time into the troposphere where, by coagulation of sulphate aerosol, the size of the particles increases with time.

Solar modulation of the El‐Niño/Southern Oscillation impact on the Northern Hemisphere annular mode

A statistical study reveals that the stratospheric circulation response to the El‐Niño/Southern Oscillation (ENSO) significantly alters between opposite phases of the 11‐year solar cycle. During solar cycle maxima, the ENSO impact is insignificant throughout the whole extratropical lower stratosphere. During solar cycle minima, the ENSO strongly impacts the lower stratosphere, with the response resembling the Northern Hemisphere annular mode (NAM) pattern in geopotential height fields and in zonal mean zonal flow. It explains the low statistical significance of the relationships between the NAM and ENSO when the data are not stratified according to solar cycle maximum and minimum.

Isentropic Cross-Tropopause Ozone Transport in the Northern Hemisphere

This paper investigates isentropic ozone exchange between the extratropical lower stratosphere and the subtropical upper troposphere in the Northern Hemisphere. The quantification method is based on the potential vorticity (PV) mapping of Stratospheric Aerosol and Gas Experiment (SAGE)-II ozone measurements and contour advection calculations using the NASA Goddard Space Center Data Assimilation Office (DAO) analysis for the year 1990. The magnitude of the annual isentropic stratosphere-to-troposphere ozone flux is calculated to be approximately twice the flux that is directed from the troposphere into the stratosphere. The net effect is that approx.46 x 10(exp 9) kg/yr of ozone are transferred quasi horizontally from the extratropical lower stratosphere into the subtropical upper troposphere between the isentropic surfaces of 330 and 370 K. The estimated monthly ozone fluxes show that the isentropic cross-tropopause ozone transport is stronger in summer/fall than in winter/ spring, and this seasonality is more obvious at the upper three levels (i.e., 345, 355, and 365 K) than at 335 K. The distributions of the estimated monthly ozone fluxes indicate that the isentropic stratosphere-to-troposphere ozone exchange is associated with wave breaking and occurs preferentially over the eastern Atlantic Ocean and northwest Africa in winter and over the Atlantic and Pacific Oceans in summer.

Reactive organic species in the northern extratropical lowermost stratosphere: Seasonal variability and implications for OH

We present C2–C6 nonmethane hydrocarbon (NMHC) measurements from canister samples obtained in the extratropical lower stratosphere during the fall (November/December 1995), winter (March 1997), and summer seasons (July 1998) as part of the stratosphere‐troposphere experiments by aircraft measurements campaign. The flights were carried out from Amsterdam (Netherlands, 52°N, 4.5°E) during fall, from Kiruna (Sweden, 68°N, 20°E) during winter, and from Timmins (Canada, 48.2°N, 70.3°W) during summer. The NMHC measurements have been evaluated along with concurrent in situ measurements of acetone (CH3COCH3), CO, O3, N2O, and CFC‐12 (CCl2F2). The vertical distributions of NMHC and acetone as a function of O3 and potential temperature in the lowermost stratosphere show a strong seasonality. Enhanced concentrations of NMHC + CH3COCH3 were found during July up to potential temperatures of Θ = 370 K, whereas during March this was limited to Θ = 340 K, in agreement with stronger isentropic cross‐tropopause transport during summer. Increasing methyl chloride (CH3Cl) concentrations with altitude were measured during July, pointing to mixing at the subtropical tropopause. During summer and fall, mean NMHC + acetone concentrations were more than a factor of 2 higher than that during winter. Box model calculations indicate that the observed acetone levels of 0.5–1 ppbv can explain 30–50% of the enhanced OH radical concentrations in the summertime lowermost stratosphere. Using mass balance calculations, we show that a significant tropospheric fraction (≤30%) was present up to Θ = 370 K in the summertime lowermost stratosphere. During winter, the tropospheric fraction approached zero at about Θ = 350 K. The time between selected troposphere‐to‐stratosphere mixing events and the aircraft measurements has been estimated at 3–14 days. Our results emphasize that isentropic cross‐tropopause transport can be a fast process occurring on timescales of days to weeks.