H. Struthers

Assessment of temperature, trace species, and ozone in chemistry-climate model simulations of the recent past

Simulations of the stratosphere from thirteen coupled chemistry-climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long-term trends are compared for an extended period (1960–2004). Comparisons of polar high-latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long-term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.

Methane carbon isotope effects caused by atomic chlorine in the marine boundary layer: Global model results compared with Southern Hemisphere measurements

[1]xa0Recent measurements of the apparent kinetic isotope effect (KIE) of the methane (CH4) atmospheric sink in the extratropical Southern Hemisphere (ETSH) have shown the apparent KIE to be larger in magnitude than expected if the sink were the hydroxyl radical (OH•) alone. We present results from simulations using the U.K. Met Offices Unified Model (UM) to evaluate whether atomic chlorine (Cl•) in the marine boundary layer (MBL) could give this effect. We modify the UM to include sources of 12CH4 and 13CH4, soil and stratospheric sinks, and a tropospheric OH• sink. Also included is a Cl• sink in the MBL with a large seasonal cycle and a constant mean value (Cl•mean) in latitude. We show that analysis of the simulated seasonal cycles in CH4 mixing ratio and δ13C give an accurate estimate of the OH• KIE at ETSH midlatitudes. The apparent KIE of the combined OH• and Cl• sink increases in magnitude as Cl•mean increases. The experimentally measured values of apparent KIE in the ETSH midlatitudes of −15‰ in 1994–1996 and −7‰ in 1998–2000 are attained with MBL Cl•mean values of 28 × 103 atoms cm−3 and 9×103 atoms cm−3, respectively (although we consider the latter to be a lower bound). We suggest that 18×103 atoms cm−3 is a reasonable midrange estimate of Cl•mean in the MBL. This value results in a Cl• sink strength of 25 Tg y−1 (range 13–37 Tg y−1) and an enrichment in δ13C of atmospheric CH4 by 2.6‰ (range 1.4–3.8‰). This sink strength is significant but has not yet been included in global CH4 budgets.

Interannual variability in Antarctic ozone depletion controlled by planetary waves and polar temperature

[1]xa0The dependence of Antarctic ozone depletion on midlatitude planetary wave activity and South Pole temperatures was examined from 1979–2003 using NCEP/NCAR reanalyses and column ozone data. The annual severity of Antarctic ozone depletion was quantified using the seasonal mean of daily ozone mass deficit (OMD). The dependence of annual mean OMD on effective equivalent stratospheric chlorine (EESC) was removed to produce an anomaly time series (OMD′). Similar anomaly time series for 100 hPa South Pole temperatures (T′) and 20 hPa, 60°S midlatitude planetary wave activity (PWA′) were calculated. Regression of OMD′ against T′ and PWA′ shows that most of the interannual variability in Antarctic ozone depletion can be explained by variability in midlatitude planetary wave activity and South Pole temperatures. To estimate how future changes in South Pole temperatures, midlatitude wave activity and EESC will affect Antarctic ozone depletion, the regression model was applied to T′ and PWA′ values from a chemistry-climate model run (1975–2019).

Interannual variation of 13C in tropospheric methane: Implications for a possible atomic chlorine sink in the marine boundary layer

[1]xa0We present methane mixing ratio and δ13C time series measured at Baring Head, New Zealand, and Scott Base, Antarctica, over the years 1991–2003. These data demonstrate that the apparent kinetic isotope effect (KIE) of the methane atmospheric sink (derived from the amplitudes of the mixing ratio and δ13C seasonal cycles) is generally much larger than would be expected if the sink were the hydroxyl radical alone and has changed significantly during the observation period on a timescale of ∼3 years. We show using a global transport model that this technique for deriving the KIE should be quite accurate for a single atmospheric sink and that the change with time is unlikely to arise from El Nino–Southern Oscillation transport effects. We infer that a sink in addition to hydroxyl is required. A strong candidate for this extra sink is atomic chlorine in the marine boundary layer (MBL). We derive the amplitude of the chlorine concentration seasonal cycle that would fully account for the apparent KIE. This amplitude ranges from ∼104 atom cm−3 in 1994–1996 to about 3 × 103 atom cm−3 in 1998–2000. If the KIE is enhanced throughout the free troposphere, the seasonal mean concentrations of atomic chlorine required in the MBL would be about 3 × 104 atom cm−3 in 1994–1996 and ∼104 atom cm−3 in 1998–2000.

Assimilation of ozone profiles and total column measurements into a global general circulation model

[1] Ozone profiles from the Microwave Limb Sounder (MLS) instrument flown on board the Upper Atmosphere Research Satellite (UARS) and total ozone columns measured by the Global Ozone Monitoring Experiment (GOME) on board the Second European Remote Sensing Satellite (ERS-2) have been assimilated using a troposphere-stratosphere data assimilation system. The analysis system is based on the global analysis system used for operational analysis of the stratosphere at the Meteorological Office from 1991 to 2000. Three assimilation runs have been completed for a three-week period in April 1997 to test the advantage of using a combination of MLS and GOME observations, compared with the assimilation of each observation data set separately. The statistical information produced by the assimilation system shows that the combination of MLS and GOME observations via the assimilation process produces ozone fields that show improvement compared with analysis fields produced by the assimilation of either MLS or GOME separately. Comparison of the analyzed ozone fields with independent observations (ozonesondes, Halogen Occultation Experiment (HALOE) profiles and Total Ozone Mapping Spectrometer (TOMS) total ozone column measurements) corroborates these results and shows that the combined MLS and GOME ozone analyses provide a realistic representation of the atmospheric ozone distribution. The global root-mean-square residual (difference between the analyses and independent observations) against HALOE and TOMS observations is comparable to the quoted errors in the HALOE and TOMS instruments (5% in each case).

Dynamical containment of Antarctic ozone depletion

[1]xa0Over the past two decades both the severity of Antarctic ozone depletion and the size of the Antarctic ozone hole have increased. A satellite-based, assimilated total column ozone data set and NCEP/NCAR meteorological fields from 1979 to 2000 have been used to examine the long-term evolution of the ozone hole and its dependence on the size of the dynamical vortex and the size of the region of low temperatures. Equivalent latitude zonal means of these data have been calculated to show more directly the expansion of the Antarctic ozone hole and its encroachment on the vortex edge. While the size of the dynamical vortex and the vortex region with temperatures below 195 K (where polar stratospheric clouds are expected to form) has changed little, the area with ozone below 220 DU (the contour defining the ozone hole) has steadily increased. Over the 20 year period, the severity of the ozone depletion within the core of the vortex has increased. This, combined with the nature of the mixing regimes within the polar vortex may explain the increase in the area of the ozone hole.

Coupled chemistry climate model simulations of stratospheric temperatures and their trends for the recent past

Temperature results from multi-decadal simulations of coupled chemistry climate models for the recent past are analyzed using multi-linear regression including a trend, solar cycle, lower stratospheric tropical wind, and volcanic aerosol terms. The climatology of the models for recent years is in good agreement with observations for the troposphere but the model results diverge from each other and from observations in the stratosphere. Overall, the models agree better with observations than in previous assessments, primarily because of corrections in the observed temperatures. The annually averaged global and polar temperature trends simulated by the models are generally in agreement with revised satellite observations and radiosonde data over much of their altitude range. In the global average, the model trends underpredict the radiosonde data slightly at the top of the observed range. Over the Antarctic some models underpredict the temperature trend in the lower stratosphere, while others overpredict the trends.

Seasonal cycles of mixing ratio and 13C in atmospheric methane at Suva, Fiji

[1]xa0A series of clean air samples has been collected at a coastal site near Suva, Fiji (18°08′S, 178°26′E) by researchers at the University of the South Pacific. These samples, covering the period 1994 to mid-2002, have been analyzed for methane mixing ratio and δ13C and provide the first ever time series of these species reported for this part of the tropical South Pacific. The data show large variability when compared to similar time series of the same species measured farther south in the extratropical Pacific. In particular, summer variability at the Fiji site is high, especially through La Nina conditions. A modeling study was carried out using a modified version of the UK Meteorological Offices Unified Model (a general circulation model) and TM2 (a chemical transport model driven by stored meteorological fields). These showed that a large amount of the variability in the methane mixing ratio and its δ13C can be attributed to complex tropical meteorology in the region changing the rate of transport of methane from the Northern into the Southern Hemispheres. Enhanced interhemispheric transport occurred during the summer months, especially during La Nina conditions which lead to the suppression of expected minima in the methane mixing ratio caused by OH oxidation. Although enriched signals in δ13C were expected at the site caused by intrusions of methane emitted from tropical biomass burning in Indonesia, relatively few of these events could be identified in the time series.

A comparison of the transport of long-lived atmospheric trace gas species from two advection schemes incorporated into an atmospheric general circulation model

Two tracer advection schemes have been compared using an atmospheric chemical transport model based on the U.K. Met Office’s Unified Model (UM). Two experiments were carried out. The first used an inert calibration tracer (SF6) whilst the second experiment modelled two methane (CH4) carbon isotopic species. In both experiments, tracer emissions were prescribed at the Earth’s surface. Results from both experiments suggest that the global distribution of long lived trace species in the troposphere is not strongly dependent on the choice of the advection scheme. This is attributed to the fact that turbulent boundary layer mixing and vertical transport by convection, in addition to advection play a role in determining the large-scale tracer distribution in the troposphere. For the SF6 experiment, results from the UM were comparable to results from other transport models using the same experimental protocol. The UM’s default, total variation diminishing (TVD) advection scheme produced low values of marine boundary layer mixing ratios compared with other models and measurements. This was attributed to the TVD scheme being too diffusive resulting in unrealistically fast vertical transport. Comparisons between measured and modelled CH4 profiles in the stratosphere clearly shows that the vertical transport in the TVD scheme is too fast.