Multi-annual simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) were conducted to study the seasonality of O3 within the stratospheric part of the tropical tropopause layer ...(TTL), i.e. above θ=360 K potential temperature level. In agreement with satellite (HALOE) and in-situ observations (SHADOZ), CLaMS simulations show a pronounced annual cycle in O3, at and above θ=380 K, with the highest mixing ratios in the late boreal summer. Within the model, this cycle is driven by the seasonality of both upwelling and in-mixing. The latter process occurs through enhanced horizontal transport from the extratropics into the TTL that is mainly driven by the meridional, isentropic winds. The strongest in-mixing occurs during the late boreal summer from the Northern Hemisphere in the potential temperature range between 370 and 420 K. Complementary, the strongest upwelling occurs in winter reducing O3 to the lowest values in early spring. Both CLaMS simulations and Aura MLS O3 observations consistently show that enhanced in-mixing in summer is mainly driven by the Asian monsoon anticyclone.
Variations in the mixing ratio of trace gases of tropospheric origin entering the stratosphere in the tropics are of interest for assessing both troposphere to stratosphere transport fluxes in the ...tropics and the impact of these transport fluxes on the composition of the tropical lower stratosphere. Anomaly patterns of carbon monoxide (CO) and long-lived tracers in the lower tropical stratosphere allow conclusions about the rate and the variability of tropical upwelling to be drawn. Here, we present a simplified chemistry scheme for the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the simulation, at comparatively low numerical cost, of CO, ozone, and long-lived trace substances (CH4, N2O, CCl3F (CFC-11), CCl2F2 (CFC-12), and CO2) in the lower tropical stratosphere. For the long-lived trace substances, the boundary conditions at the surface are prescribed based on ground-based measurements in the lowest model level. The boundary condition for CO in the lower troposphere (below about 4 km) is deduced from MOPITT measurements. Due to the lack of a specific representation of mixing and convective uplift in the troposphere in this model version, enhanced CO values, in particular those resulting from convective outflow are underestimated. However, in the tropical tropopause layer and the lower tropical stratosphere, there is relatively good agreement of simulated CO with in situ measurements (with the exception of the TROCCINOX campaign, where CO in the simulation is biased low 10–15 ppbv). Further, the model results (and therefore also the ERA-Interim winds, on which the transport in the model is based) are of sufficient quality to describe large scale anomaly patterns of CO in the lower stratosphere. In particular, the zonally averaged tropical CO anomaly patterns (the so called "tape recorder" patterns) simulated by this model version of CLaMS are in good agreement with observations, although the simulations show a too rapid upwelling compared to observations as a consequence of the overestimated vertical velocities in the ERA-Interim reanalysis data set. Moreover, the simulated tropical anomaly patterns of N2O are in good agreement with observations. In the simulations, anomaly patterns of CH4 and CFC-11 were found to be very similar to those of N2O; for all long-lived tracers, positive anomalies are simulated because of the enhanced tropical upwelling in the easterly shear phase of the quasi-biennial oscillation.
Balloon-borne observations of ozone from the South Pole Station have been reported to reach ozone mixing ratios below the detection limit of about 10 ppbv at the 70 hPa level by late September. After ...reaching a minimum, ozone mixing ratios increase to above 1 ppmv on the 70 hPa level by late December. While the basic mechanisms causing the ozone hole have been known for more than 20 yr, the detailed chemical processes determining how low the local concentration can fall, and how it recovers from the minimum have not been explored so far. Both of these aspects are investigated here by analysing results from the Chemical Lagrangian Model of the Stratosphere (CLaMS). As ozone falls below about 0.5 ppmv, a balance is maintained by gas phase production of both HCl and HOCl followed by heterogeneous reaction between these two compounds in these simulations. Thereafter, a very rapid, irreversible chlorine deactivation into HCl can occur, either when ozone drops to values low enough for gas phase HCl production to exceed chlorine activation processes or when temperatures increase above the polar stratospheric cloud (PSC) threshold. As a consequence, the timing and mixing ratio of the minimum ozone depends sensitively on model parameters, including the ozone initialisation. The subsequent ozone increase between October and December is linked mainly to photochemical ozone production, caused by oxygen photolysis and by the oxidation of carbon monoxide and methane.
Variations in the mixing ratio of long‐lived trace gases entering the stratosphere in the tropics are carried upward with the rising air with the signal being observable throughout the tropical lower ...stratosphere. This phenomenon, referred to as “atmospheric tape recorder” has previously been observed for water vapor, CO2, and CO which exhibit an annual cycle. Recently, based on Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) satellite measurements, the tape recorder signal has been observed for hydrogen cyanide (HCN) but with an approximately two‐year period. Here we report on a model simulation of the HCN tape recorder for the time period 2002–2008 using the Chemical Lagrangian Model of the Stratosphere (CLaMS). The model can reproduce the observed pattern of the HCN tape recorder signal if time‐resolved emissions from fires in Indonesia are used as lower boundary condition. This finding indicates that inter‐annual variations in biomass burning in Indonesia, which are strongly influenced by El Niño events, control the HCN tape recorder signal. A longer time series of tropical HCN data will probably exhibit an irregular cycle rather than a regular biannual cycle.
Multi-annual simulations with the Chemical Model of the Stratosphere (CLaMS) are used to study the seasonality of O sub(3) and of the mean age within the stratospheric part of the tropical tropopause ...layer (TTL) In agreement with satellite (HALOE) and in-situ observations (SHADOZ), CLaMS simulations show above ≈ 360 K potential temperature, a pronounced annual cycle in O sub(3) and in the mean age of air with highest values in the late boreal summer. Within the model, this seasonality is driven by the seasonality of both upwelling and in-mixing. The latter process describes enhanced meridional transport from the extratropics into the TTL. The strongest in-mixing occurs from the Northern Hemisphere during the boreal summer in the potential temperature range between 380 and 420 K. Contrary, an increase of upwelling with highest values in winter reduces O sub(3) up to the lowest values in early spring. Both, CLaMS simulations and Aura MLS O sub(3) observations show that this enhanced equatorward transport in summer is mainly driven by the Asian monsoon anticyclone.
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