Joan E. Rosenfield

Two‐dimensional model simulations of the QBO in ozone and tracers in the tropical stratosphere

[1] Meteorological data from the United Kingdom Meteorological Office (UKMO) and constituent data from the Upper Atmospheric Research Satellite (UARS) are used to construct yearly zonal mean dynamical fields for the 1990s for use in the NASA/Goddard Space Flight Center (GSFC) two-dimensional (2-D) chemistry and transport model. This allows for interannual dynamical variability to be included in the model constituent simulations. In this study, we focus on the tropical stratosphere. We find that the phase of quasi-biennial oscillation (QBO) signals in equatorial CH4 and profile and total column O3 data are resolved quite well using this empirically based 2-D model transport framework. However, the QBO amplitudes in the model constituents are systematically underestimated relative to the observations at most levels. This deficiency is probably due in part to the limited vertical resolutions of the 2-D model and the UKMO and UARS input data sets. We find that using different heating rate calculations in the model affects the interannual and QBO amplitudes in the constituent fields, but has little impact on the phase. Sensitivity tests reveal that the QBO in transport dominates the ozone interannual variability in the lower stratosphere, with the effect of the temperature QBO being dominant in the upper stratosphere via the strong temperature dependence of the ozone loss reaction rates. We also find that the QBO in odd nitrogen radicals, which is caused by the QBO modulated transport of NOy, plays a significant but not dominant role in determining the ozone QBO variability in the middle stratosphere. The model mean age of air is in good overall agreement with that determined from tropical lower-middle stratospheric OMS balloon observations of SF6 and CO2. The interannual variability of the equatorial mean age in the model increases with altitude and maximizes near 40 km, with a range of 4–5 years over the 1993–2000 time period. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334); 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 3319 Meteorology and Atmospheric Dynamics: General circulation; KEYWORDS: interannual variability, stratospheric circulation, ozone

Version 8 SBUV ozone profile trends compared with trends from a zonally averaged chemical model

[1]xa0A statistical time series analysis was applied to the new version 8 merged Solar Backscatter Ultraviolet (SBUV) data set of ozone profiles for the years 1979–2003. Linear trends for the 1979–1997 time period are reported and are compared to trends computed using ozone profiles from the Goddard Space Flight Center (GSFC) zonally averaged coupled model. Observed and modeled annual trends between 50°N and 50°S were a maximum in the higher latitudes of the upper stratosphere, with Southern Hemisphere (SH) trends greater than Northern Hemisphere (NH) trends. The observed upper stratospheric maximum annual trend is −7.0 ± 2.0%/decade (2σ) at 47.5°S and −4.7 ± 1.3%/decade at 47.5°N, to be compared with the modeled trends of −5.8 ± 0.3%/decade in the SH and −5.2 ± 0.3%/decade in the NH. Both observed and modeled trends are most negative in winter and least negative in summer, although the modeled seasonal difference is less than observed. Model trends are shown to be greatest in winter because of a repartitioning of chlorine species and the increasing abundance of chlorine with time. The model results illustrate the trend differences that can occur at 3 hPa depending on whether ozone profiles are in mixing ratio or number density coordinates and on whether they are recorded on pressure or altitude levels.

Potential impact of subsonic and supersonic aircraft exhaust on water vapor in the lower stratosphere assessed via a trajectory model

[1]xa0We employ a trajectory model to assess the impact on the stratosphere of water vapor present in the exhaust of subsonic and a proposed fleet of supersonic aircraft. Air parcels into which water vapor from aircraft exhaust has been injected are run through a 6-year simulation in the trajectory model using meteorological data from the UKMO analyses with emissions dictated by the standard 2015 emissions scenario. For the subsonic aircraft, our results suggest maximum enhancements of ∼150 ppbv just above the Northern Hemisphere tropopause and of much less than 50 ppbv in most other regions. Inserting the perturbed water vapor profiles into a radiative transfer model, but not considering the impact of additional cirrus formation resulting from emissions by subsonic aircraft, we find that the impact of subsonic water vapor emissions on the radiative balance is negligible. For the supersonic case, our results show maximum enhancements of ∼1.5 ppmv in the tropical stratosphere near 20 km. Much of the remaining stratosphere between 12 and 25 km sees enhancements of greater than 0.1 ppmv, although enhancements above 35 km are generally less than 50 ppbv, in contrast to previous 2-D and 3-D model studies. Radiative calculations based upon these projected water vapor perturbations indicate they may cause a nonnegligible impact on tropical temperature profiles. Since our trajectory model includes no chemistry and our radiative calculations use the most extreme water vapor perturbations, our results should be viewed as upper limits on the potential impacts.

Recovery of the tropical lower stratospheric ozone layer

[1]xa0Long-term time dependent simulations out to the year 2050 using the Goddard Space Flight Center interactive 2D chemistry-radiation-dynamics model are used to predict tropical lower stratospheric ozone. The model results show that when chlorine levels are enhanced, heterogeneous chemistry on the surfaces of middle and high latitude sulfate aerosols and polar stratospheric clouds induces circulation changes which lead to ozone depletion in the tropical lower stratosphere. We find that despite a return to background chlorine levels, the tropical lower stratospheric ozone layer will not recover because of a reduction in penetrating ultraviolet radiation. This reduction is due to the faster upper stratospheric ozone recovery with increasing carbon dioxide. With background aerosol amounts, tropical ozone at 50 hPa is reduced by ∼2% between 1980 and 1997 and remains at that level of depletion out to 2050. The simulations also suggest that tropical tropopause temperatures will be lowered by a maximum of 0.1 K between 1980 and 2015, resulting in a maximum reduction of tropical lower stratospheric water vapor of 2% between 1980 and 2018. These results may have important implications for recovery detection strategies.

Effects of Volcanic Eruptions on Stratospheric Ozone Recovery

The effects of the stratospheric sulfate aerosol layer associated with the Mt. Pinatubo volcano and future volcanic eruptions on the recovery of the ozone layer is studied with an interactive two-dimensional photochemical model. The time varying chlorine loading and the stratospheric cooling due to increasing carbon dioxide have been taken into account. The computed ozone and temperature changes associated with the Mt. Pinatubo eruption in 1991 agree well with observations. Long model runs out to the year 2050 have been carried out, in which volcanoes having the characteristics of the Mount Pinatubo volcano were erupted in the model at 10-year intervals starting in the year 2010. Compared to a non-volcanic run using background aerosol loading, transient reductions of globally averaged column ozone of 2-3 percent were computed as a result of each of these eruptions, with the ozone recovering to that computed for the non-volcanic case in about 5 years after the eruption. Computed springtime Arctic column ozone losses of from 10 to 18 percent also recovered to the non-volcanic case within 5 years. These results suggest that the long-term recovery of ozone would not be strongly affected by infrequent volcanic eruptions with a sulfur loading approximating Mt. Pinatubo. Sensitivity studies in which the Arctic lower stratosphere was forced to be 4 K and 10 K colder resulted in transient ozone losses of which also recovered to the non-volcanic case in 5 years. A case in which a volcano five times Mt. Pinatubo was erupted in the year 2010 led to maximum springtime column ozone losses of 45 percent which took 10 years to recover to the background case. Finally, in order to simulate a situation in which frequent smaller volcanic eruptions result in increasing the background sulfate loading, a simulation was made in which the background aerosol was increased by 10 percent per year. This resulted in a delay of the recovery of column ozone to 1980 values of more than 10 years.

The impact of increasing carbon dioxide on ozone recovery: IMPACT OF INCREASING CARBON DIOXIDE

[1]xa0We have used the Goddard Space Flight Center coupled two-dimensional model to study the impact of increasing carbon dioxide from 1980 to 2050 on the recovery of ozone to its pre-1980 amounts. We find that the changes in temperature and circulation arising from increasing CO2 affect ozone recovery in a manner which varies greatly with latitude, altitude, and time of year. Middle and upper stratospheric ozone recovers faster at all latitudes due to a slowing of the ozone catalytic loss cycles. In the lower stratosphere the recovery of tropical ozone is delayed due to a decrease in production and a speed up in the overturning circulation. The recovery of high northern latitude lower stratospheric ozone is delayed in spring and summer due to an increase in springtime heterogeneous chemical loss, and is speeded up in fall and winter due to increased downwelling. The net effect on the higher northern latitude column ozone is to slow down the recovery from late March to late July, while making it faster at other times. In the high southern latitudes the impact of CO2 cooling is negligible. Annual mean column ozone is predicted to recover faster at all latitudes, and globally averaged ozone is predicted to recover approximately 10 years faster as a result of increasing CO2.