Tropical Middle Stratosphere Research Articles

Role of Southern Hemisphere sudden stratospheric warming in altering the intensity of Brewer-Dobson circulation and its impact on stratospheric ozone and water vapor

The changes in the intensity of Brewer-Dobson circulation (BDC) in the record-breaking Southern Hemisphere sudden stratospheric warming (SH SSW) event in 2019 are investigated in the present study. Wave driving was observed to be at its highest in 2019 SH SSW, despite the event being classified as a minor warming event. The Eliassen-Palm flux divergence, indicative of wave driving, was observed to be increasing before the SSW, and subsiding afterwards. The stratospheric circulation followed the wave driving and intensified along the warming episode. The resulting stratospheric distribution from Aura Microwave Limb Sounder showed a negative ozone anomaly (∼20%) in the tropical lower stratosphere and a downward propagating positive anomaly in the Southern Hemisphere polar stratosphere around the central dates. The water vapor mixing ratio showed an increase of more than 25% in the polar lower stratosphere and 10% in the tropical upper stratosphere. A cooling of about 5 K was observed in the tropical middle stratosphere. The results indicate that SH SSW can change the intensity of the stratospheric circulation which in turn affects the ozone and water vapor transport from the tropics to the pole.

Stratospheric ozone: down and up through the anthropocene

The stratospheric ozone layer protects life on Earth by absorbing harmful ultraviolet (UV) radiation from the sun. The spatial and temporal distribution of stratospheric ozone is determined by chemical and dynamical processes. Maximum ozone mixing ratios are found in the tropical middle stratosphere as the result of photochemical processes involving oxygen. From its tropical production region ozone is then transported to higher latitudes by the meridional circulation. In addition, ozone is destroyed in catalytic chemical cycles involving reactive, so-called ozone depleting substances (ODSs). These ODSs include chlorine, bromine, hydrogen and nitrogen compounds from source gases emitted at Earth’s surface by industry and transported into the stratosphere. With increasing production and consumption of ODSs since the 1970s stratospheric ozone began to decline globally. Due to a combination of cold meteorological conditions and specific chemical reactions, the ozone hole developed over Antarctica each springtime since the early 1980s. In a tremendous effort, scientists, politicians and industry managers responded to this threat to the ozone layer. In 1987, the Montreal Protocol was accepted by the member states of the United Nations. The regulations of ODSs defined by the Montreal Protocol and its Amendments and adjustments ultimately led to a turning point of ozone depletion and a slow recovery of stratospheric ozone since the 2000s. This article provides the background of ozone chemistry and dynamics and reviews anthropogenic ozone depletion in the past as well as recent model projections of future ozone recovery and its interaction with climate change.

Merging of ozone profiles from SCIAMACHY, OMPS and SAGE II observations to study stratospheric ozone changes

Abstract. This paper presents vertically and zonally resolved merged ozone time series from limb measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) and the Ozone Mapping and Profiler Suite (OMPS) Limb Profiler (LP). In addition, we present the merging of the latter two data sets with zonally averaged profiles from Stratospheric Aerosol and Gas Experiment (SAGE) II. The retrieval of ozone profiles from SCIAMACHY and OMPS-LP is performed using an inversion algorithm developed at the University of Bremen. To optimize the merging of these two time series, we use data from the Microwave Limb Sounder (MLS) as a transfer function and we follow two approaches: (1) a conventional method involving the calculation of deseasonalized anomalies and (2) a “plain-debiasing” approach, generally not considered in previous similar studies, which preserves the seasonal cycles of each instrument. We find a good correlation and no significant drifts between the merged and MLS time series. Using the merged data set from both approaches, we apply a multivariate regression analysis to study ozone changes in the 20–50 km range over the 2003–2018 period. Exploiting the dense horizontal sampling of the instruments, we investigate not only the zonally averaged field, but also the longitudinally resolved long-term ozone variations, finding an unexpected and large variability, especially at mid and high latitudes, with variations of up to 3 %–5 % per decade at altitudes around 40 km. Significant positive linear trends of about 2 %–4 % per decade were identified in the upper stratosphere between altitudes of 38 and 45 km at mid latitudes. This is in agreement with the predicted recovery of upper stratospheric ozone, which is attributed to both the adoption of measures to limit the release of halogen-containing ozone-depleting substances (Montreal Protocol) and the decrease in stratospheric temperature resulting from the increasing concentration of greenhouse gases. In the tropical stratosphere below 25 km negative but non-significant trends were found. We compare our results with previous studies and with short-term trends calculated over the SCIAMACHY period (2002–2012). While generally a good agreement is found, some discrepancies are seen in the tropical mid stratosphere. Regarding the merging of SAGE II with SCIAMACHY and OMPS-LP, zonal mean anomalies are taken into consideration and ozone trends before and after 1997 are calculated. Negative trends above 30 km are found for the 1985–1997 period, with a peak of −6 % per decade at mid latitudes, in agreement with previous studies. The increase in ozone concentration in the upper stratosphere is confirmed over the 1998–2018 period. Trends in the tropical stratosphere at 30–35 km show an interesting behavior: over the 1998–2018 period a negligible trend is found. However, between 2004 and 2011 a negative long-term change is detected followed by a positive change between 2012 and 2018. We attribute this behavior to dynamical changes in the tropical middle stratosphere.

Long-term trends in stratospheric ozone, temperature, and water vapor over the Indian region

Abstract. We have investigated the long-term trends in and variabilities of stratospheric ozone, water vapor and temperature over the Indian monsoon region using the long-term data constructed from multi-satellite (Upper Atmosphere Research Satellite (UARS MLS and HALOE, 1993–2005), Aura Microwave Limb Sounder (MLS, 2004–2015), Sounding of the Atmosphere using Broadband Emission Radiometry (SABER, 2002–2015) on board TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics)) observations covering the period 1993–2015. We have selected two locations, namely, Trivandrum (8.4∘ N, 76.9∘ E) and New Delhi (28∘ N, 77∘ E), covering northern and southern parts of the Indian region. We also used observations from another station, Gadanki (13.5∘ N, 79.2∘ E), for comparison. A decreasing trend in ozone associated with NOx chemistry in the tropical middle stratosphere is found, and the trend turned to positive in the upper stratosphere. Temperature shows a cooling trend in the stratosphere, with a maximum around 37 km over Trivandrum (−1.71 ± 0.49 K decade−1) and New Delhi (−1.15 ± 0.55 K decade−1). The observed cooling trend in the stratosphere over Trivandrum and New Delhi is consistent with Gadanki lidar observations during 1998–2011. The water vapor shows a decreasing trend in the lower stratosphere and an increasing trend in the middle and upper stratosphere. A good correlation between N2O and O3 is found in the middle stratosphere (∼ 10 hPa) and poor correlation in the lower stratosphere. There is not much regional difference in the water vapor and temperature trends. However, upper stratospheric ozone trends over Trivandrum and New Delhi are different. The trend analysis carried out by varying the initial year has shown significant changes in the estimated trend. Keywords. Atmospheric composition and structure (middle atmosphere – composition and chemistry; troposphere – composition and chemistry) – meteorology and atmospheric dynamics (climatology)

Vertical distributions of N<sub>2</sub>O isotopocules in the equatorial stratosphere

Abstract. Vertical profiles of nitrous oxide (N2O) and its isotopocules, isotopically substituted molecules, were obtained over the Equator at altitudes of 16–30 km. Whole air samples were collected using newly developed balloon-borne compact cryogenic samplers over the eastern equatorial Pacific in 2012 and Biak Island, Indonesia, in 2015. They were examined in the laboratory using gas chromatography and mass spectrometry. The mixing ratio and isotopocule ratios of N2O in the equatorial stratosphere showed a weaker vertical gradient than the previously reported profiles in the subtropical and mid-latitude and high-latitude stratosphere. From the relation between the mixing ratio and isotopocule ratios, further distinct characteristics were found over the Equator: (1) observed isotopocule fractionations (ε values) in the middle stratosphere (25–30 km or [N2O] < ca. 260 nmol mol−1) are almost equal to ε values reported from broadband photolysis experiments conducted in the laboratory; (2) ε values in the lower stratosphere (< ca. 25 km or [N2O] > ca. 260 nmol mol−1) are about half of the experimentally obtained values, being slightly larger than those observed in the mid-latitude and high-latitude lower stratosphere ([N2O] > ca. 170 nmol mol−1). These results from the deep tropics suggest the following. (i) The timescale for quasi-horizontal mixing between tropical and mid-latitude air in the tropical middle stratosphere is sufficiently slow relative to the tropical upwelling rate that isotope fractionation approaches the Rayleigh limit for N2O photolysis. (ii) The air in the tropical lower stratosphere is exchanged with extratropical air on a timescale that is shorter than that of photochemical decomposition of N2O. Previously observed ε values, which are invariably smaller than those of photolysis, can be explained qualitatively using a three-dimensional chemical transport model and using a simple model that assumes mixing of “aged” tropical air and extratropical air during residual circulation. Results show that isotopocule ratios are useful to examine the stratospheric transport scheme deduced from tracer–tracer relations.

Stratospheric O<sub>3</sub> changes during 2001–2010: the small role of solar flux variations in a chemical transport model

Abstract. Solar spectral fluxes (or irradiance) measured by the SOlar Radiation and Climate Experiment (SORCE) show different variability at ultraviolet (UV) wavelengths compared to other irradiance measurements and models (e.g. NRL-SSI, SATIRE-S). Some modelling studies have suggested that stratospheric/lower mesospheric O3 changes during solar cycle 23 (1996–2008) can only be reproduced if SORCE solar fluxes are used. We have used a 3-D chemical transport model (CTM), forced by meteorology from the European Centre for Medium-Range Weather Forecasts (ECMWF), to simulate middle atmospheric O3 using three different solar flux data sets (SORCE, NRL-SSI and SATIRE-S). Simulated O3 changes are compared with Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite data. Modelled O3 anomalies from all solar flux data sets show good agreement with the observations, despite the different flux variations. The off-line CTM reproduces these changes through dynamical information contained in the analyses. A notable feature during this period is a robust positive solar signal in the tropical middle stratosphere, which is due to realistic dynamical changes in our simulations. Ozone changes in the lower mesosphere cannot be used to discriminate between solar flux data sets due to large uncertainties and the short time span of the observations. Overall this study suggests that, in a CTM, the UV variations detected by SORCE are not necessary to reproduce observed stratospheric O3 changes during 2001–2010.

Decadal variability of the tropical stratosphere: Secondary influence of the El Niño–Southern Oscillation

A decadal variation of tropical lower stratospheric ozone and temperature has previously been identified that correlates positively with the 11 year solar activity cycle. However, the El Niño–Southern Oscillation (ENSO) also influences lower stratospheric ozone and temperature. It is therefore legitimate to ask whether quasi‐decadal ENSO variability can contribute to this apparent solar cycle variation, either accidentally because of the short measurement record or physically because solar variability affects ENSO. Here we present multiple regression analyses of available data records to compare differences in results obtained with and without including an ENSO term in the statistical model. In addition, simulations are performed using the NRL NOGAPS‐ALPHA GCM for warm/cold ENSO conditions to test for consistency with the ENSO regression results. We find only very minor changes in annual mean solar regression coefficients when an ENSO term is included. However, the observed tropical ENSO response provides useful insights into the origin of the unexpected vertical structure of the tropical solar cycle ozone response. In particular, the ENSO ozone response is negative in the lower stratosphere due to increased upwelling but changes sign, becoming positive in the middle stratosphere (5–10 hPa) due mainly to advective decreases of temperature and NOx, which photochemically increase ozone. A similar mechanism may explain the observed lower stratospheric solar cycle ozone and temperature response and the absence of a significant response in the tropical middle stratosphere.

Is there change over time in the season-of-birth effect for schizophrenia?: Data from the Southern hemisphere

Data from the draft COSPAR reference model for stratospheric nitric acid (HNO3) are analysed. Eight months of LIMS HNO3 measurements allow the analysis of dynamics of regimes associated with the annual HNO3 maximum followed by the HNO3 decrease in the Northern Hemisphere and the annual HNO3 minimum followed by the HNO3 increase in the Southern Hemisphere. The HNO3 minimum is noted earlier (in November) in the Southern Hemisphere subtropical upper stratosphere, from where the regime of minimum HNO3 values propagates to the southern high-latitude middle stratosphere, and then (in Austral summer) the equatorward propagation of the regime is observed, with a persistent downward component. The regime of the HNO3 annual maximum in the Northern Hemisphere propagates from the Arctic lower stratosphere (in autumn) and from the tropical middle stratosphere (in late summer), so that in the mid-latitude middle stratosphere the downward propagation of the regime is observed. Evolution of areas with HNO3 increase and decrease by 1 ppbv against the January HNO3 distribution quantifies intensity of the HNO3 decrease in winter-spring in the Northern Hemisphere and the HNO3 increase in Austral summer-autumn in the Southern Hemisphere.

Correction to “Laminae in the tropical middle stratosphere: Origin and age estimation”

Correction to “Laminae in the tropical middle stratosphere: Origin and age estimation”

Space‐time patterns of trends in stratospheric constituents derived from UARS measurements

The spatial and temporal behavior of low‐frequency changes (trends) in stratospheric constituents measured by instruments on the Upper Atmosphere Research Satellite (UARS) during 1991–98 is investigated. The data include CH4, H2O, HF, HCl, O3, and NO2 from the Halogen Occultation Experiment (HALOE), and O3, ClO, and HNO3 from the Microwave Limb Sounder (MLS). Time series of global anomalies are analyzed by linear regression and empirical orthogonal function analysis. Each of the constituents show significant linear trends over at least some region of the stratosphere, and the spatial patterns exhibit coupling between the different species. Several of the constituents (namely CH4, H2O, HF, HCl, O3, and NO2) exhibit a temporal change in trend rates, with strong changes prior to 1996 and weaker (or reversed) trends thereafter. Positive trends are observed in upper stratospheric ClO, with a percentage rate during 1993–97 consistent with stratospheric HCl increases and with tropospheric chlorine emission rates. Significant negative trends in ozone in the tropical middle stratosphere are found in both HALOE and MLS data during 1993–97, together with positive trends in the tropics near 25 km. These trends are very different from the decadal‐scale ozone trends observed since 1979, and this demonstrates the variability of trends calculated over short time periods. Positive trends in NO2 are found in the tropical middle stratosphere, and spatial coincidence to the observed ozone decreases suggests the ozone is responding to the NO2 increase. Significant negative trends in HNO3 are found in the lower stratosphere of both hemispheres. These coupled signatures offer a fingerprint of chemical evolution in the stratosphere for the UARS time frame.

Laminae in the tropical middle stratosphere: Origin and age estimation

In situ measurements of N2O and O3 aboard the Observations from the Middle Stratosphere (OMS) balloon payload in the tropics are presented. In November 1997 at 7°S very distinct laminated structures in the N2O profile were observed in contrast to earlier studies based solely on O3 profiles. The laminae probably are an observation of Rossby wave‐breaking events that lead to transport of mid‐latitude air into the tropics. Using a photochemical model of the ozone response within those air parcels, their residence time in the tropics is estimated to be 4 to 6 weeks. In February 1997 at 7°S a similar vertical profile of N2O revealed no mid‐latitude laminae. We present dynamical arguments to support the existence of such laminae in November, but not in February.

Correlations of stratospheric abundances of NOy, O3, N2O, and CH4 derived from ATMOS measurements

Correlations are presented for [NOy] relative to [N2O] and [O3] derived from measurements from the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument from a wide range of altitudes and latitudes, including the tropics for which previous analyses have not extended above ∼20 km. Relationships for [O3] versus [N2O] are also given. The results are shown to be in good agreement with aircraft‐ and balloon‐based observations. Distinct correlations are observed for the tropics, the springtime polar vortex, and the extratropics‐extravortex regions. These correlations demonstrate rapid production of NOy and O3 in the tropical middle stratosphere and episodic export of air from this region to higher latitudes. Isolation of air within the developing polar vortices in the fall is also shown. Arctic vortex data from April 1993 appear to indicate denitrification of 25–30%, which is evident as a 3.0–4.5 ppb deficit in [NOy] when the vortex [NOy]:[N2O] correlation is compared with the extravortex correlation. A mixture of air descended from above 40 km with air from lower altitudes can fully account for this deficit in [NOy], in addition to approximately half of an apparent Arctic ozone loss of 50–60%, as inferred by comparison of the vortex and extravortex [O3]:[N2O] correlations. Comparison of Antarctic vortex and extravortex correlations from November 1994 similarly show a 60–80% deficit in [NOy] and 80–100% deficit in [O3]; at least half of this apparent denitrification and ozone loss can be attributed to mixing of air descended from higher altitudes with air from lower altitudes.

Effects of an Imposed Quasi-Biennial Oscillation in a Comprehensive Troposphere–Stratosphere–Mesosphere General Circulation Model

A 48-yr integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI troposphere‐ stratosphere‐mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly twothirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonalmean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model. The QBO was also found to have a profound influence on stratospheric stationary waves at low latitudes. Near and above 10 mb the NH stationary waves were found to penetrate across the equator during the westerly QBO phase, but to be restricted to latitudes poleward of ;108N during the easterly phase. This means that the equatorial QBO in prevailing wind near and above 10 mb has a significant zonally asymmetric component. If this is also true in the real atmosphere, there are important implications for the adequacy of the current observational rawindsonde network near the equator. Analysis of the zonal-mean zonal momentum budget in the tropical stratosphere reveals that the resolved waves in the model are strong enough to force the realized accelerations through much of the QBO cycle. The exception appears to be the easterly acceleration phase below about 20 mb. The implications of this for the generation of a self-consistent QBO by GCMs will be considered. The effects of the imposed QBO on the troposphere were found to be very modest. There does appear to be a statistically significant weakening (by ; 1ms 21) of the high-latitude winter vortex in the middle and upper troposphere. Given the very high predictability of the stratospheric QBO itself, this effect could possibly be used to enhance the skill of seasonal weather forecasts. No significant QBO influence was found in the model precipitation field.

Analysis of seasonal variation of stratospheric nitric acid

Data from the draft COSPAR reference model for stratospheric nitric acid (HNO 3) are analysed. Eight months of LIMS HNO 3 measurements allow the analysis of dynamics of regimes associated with the annual HNO 3 maximum followed by the HNO 3 decrease in the Northern Hemisphere and the annual HNO 3 minimum followed by the HNO 3 increase in the Southern Hemisphere. The HNO 3 minimum is noted earlier (in November) in the Southern Hemisphere subtropical upper stratosphere, from where the regime of minimum HNO 3 values propagates to the southern high-latitude middle stratosphere, and then (in Austral summer) the equatorward propagation of the regime is observed, with a persistent downward component. The regime of the HNO 3 annual maximum in the Northern Hemisphere propagates from the Arctic lower stratosphere (in autumn) and from the tropical middle stratosphere (in late summer), so that in the mid-latitude middle stratosphere the downward propagation of the regime is observed. Evolution of areas with HNO 3 increase and decrease by 1 ppbv against the January HNO 3 distribution quantifies intensity of the HNO 3 decrease in winter-spring in the Northern Hemisphere and the HNO 3 increase in Austral summer-autumn in the Southern Hemisphere.

Ozone trends deduced from combined Nimbus 7 SBUV and NOAA 11 SBUV/2 data

The long‐term time series of global ozone from the Nimbus‐7 SBUV (Nov. 1978–June 1990) are extended through June 1994 by using measurements from the NOAA‐11 SBUV/2. The data sets are merged based upon comparisons during the 18‐month overlap period in which both instruments were operational. During this period, the average offset between the two time series is less than 2% in total ozone, and less than 6% in Umkehr layers 1–10. A linear‐regression trend model is applied to the extended time series to calculate updated trends as a function of latitude and altitude. Trends through June 1994 are 1.5‐2% per decade less negative than through June 1990 in the tropical middle stratosphere (35–40 km) and in the upper stratosphere (45–50 km) at mid‐latitudes. In the lower stratosphere, the trends are nearly 1.5% per decade more negative in the southern hemisphere tropical regions to 25°S, but remain relatively unchanged elsewhere. The seasonal structure of the total ozone trends is similar to past trend study results, but the magnitude of the seasonal trend can vary by 2% per decade depending on the length of the time series. Both TOMS (through April 1993) and SBUV total ozone time series show small negative trends in the equatorial region, though they are not statistically significant at the 2‐σ level.

The Quasi-biennial Oscillation of Ozone in the Tropical Middle Stratosphere: A One-Dimensional Model

A one-dimensional model of the quasi-biennial oscillation (QBO) of ozone in the tropical middle stratosphere is derived based on assumed (observed) zonal wind QBO in a coupled dynamic, radiative/photochemical system. It is found that the derived vertical variation of the ozone QBO amplitude has two maxima, one at 32 km and the other at 22 km, and a minimum at 28 km. These are in qualitative agreement with observations. In the height interval 30-35 km, the ozone QBO is closely related to temperature dependent photochemistry, and the ozone and temperature variations are out of phase. Below 28 km, where vertical ozone and thermal transports are important, ozone and temperature oscillations are in phase, but both are approximately 270 deg out of phase with the vertical wind variation.