A12A-01 INVITED 10:20h
The impact of boreal summer monsoon circulations on dynamics and transport in the Upper Troposphere and Lower Stratosphere
Simulations from a Global Chemical Transport Model and observations from in-situ aircraft are used to understand the transport patterns in the Upper Troposphere and Lower Stratosphere (UT/LS). Model simulations are able to reproduce many of the climatological features of the UT/LS ozone and water vapor distributions, as well as chemical observations from the CRYSTAL-FACE experiment in July 2002. The boreal summer monsoons over South Asia and North America dominate UT/LS transport in the July-September period. Simulations indicate that up to 75% of net vertical water vapor transport during this period occurs in the monsoon region. Some of this air may by-pass the tropical tropopause. The implications for long term trends in trace species are discussed.
A12A-02 10:35h
A Mass Balance for the Lowermost Stratosphere: Extratropical Versus Tropical Origin
During the SPURT-project between 2001 and 2003 airborne in-situ measurements of trace gases in particular of CO, N2O and CO2 were performed in the tropopause region over Europe. A major objective was to get an overview on the distribution of these trace gases in the tropopause region to identify transport and mixing from the troposphere into the lowermost stratosphere and seasonal variations of the underlying processes. Mixing across the extratropical tropopause was found to affect mainly a layer which follows the local tropopause. This mixing layer exhibits the photochemical characteristic of both, the troposphere and the stratosphere. At large distances from the local tropopause the high precision measurements of CO2 with its well known tropospheric seasonal cycle facilitated to identify the tropical tropopause region as a source region for tropospheric air in the extratropical lowermost stratosphere, using the propagation of the CO2-seasonal cycle. The estimated transport times from the tropics are only on the order of weeks. On the basis of the in-situ measurements of CO we establish a mass balance for the lowermost stratosphere including transport from the extratropics as well as from the tropical tropopause region. We estimate that at large distances from the local tropopause the tropical tropospheric fraction dominates over tropospheric input from the extratropics, highlighting the importance of the tropical troposphere even for the lowermost stratosphere over Europe.
A12A-03 INVITED 10:50h
Mixing in the Extratropical Tropopause Region: Observations and Modeling
Stratosphere-troposphere exchange (STE) in the extratropics is a key factor that controls the chemical composition of the upper troposphere and lower stratosphere. Improving our ability to quantify STE is an important step for forecasting future change in chemical forcing to the climate change. To better understand the controlling mechanisms of STE in the extratropics, we investigate the role of dynamical processes at various scales in producing exchange and mixing. In particular, we focus on the transport characteristics in the region of the subtropical jet. While the jet core exhibits a barrier to isentropic mixing, the synoptic scale and mesoscale dynamics in the region surrounding the jet, in conjunction with large-scale wave breaking, produce irreversible exchange. As an initial step, we investigate the transport and mixing driven by large-scale winds, using both models and observations. Airborne observations from LIDAR and in situ tracers are used to identify the region of irreversible STE. A 3-D trajectory model is used to diagnose the region of mixing due to large-scale wind shear. A Lagrangian CTM (CLaMS) is used to investigate the aggregated effect of the wind shear to mixing of the stratospheric and tropospheric air. The results show that by computing the Lagrangian deformation of the atmospheric flow, it is possible to identify regions where mixing is occurring and to map out volumes of fluids exchanged in the mixing process. Potential contributions of turbulence and gravity wave breaking will be discussed.
A12A-04 11:05h
Measurements of HCl in the TTL
We have measured HCl and HNO$_{3}$ abundances in the tropical tropopause layer (TTL) by chemical ionization mass spectrometry (CIMS). The measurements were made from the NASA WB-57F high alitiude aircraft flying from Costa Rica over the eastern Pacific Ocean in January 2004. The observed HCl and HNO$_{3}$ increase sharply above the thermal tropopause, as expected. Average HCl values range from 10 to 50 ppt in the TTL and are near zero at lower altitudes. This, along with the presence of increased HNO3 below the TTL, suggests that the HCl in the TTL does not come from below, in the free troposphere. HCl in the TTL indicates either transport of stratospheric air into the TTL or possible in situ production of HCl from short-lived organic species. Concentrations of other tracers, such as H$_{2}$O and CO indicate that HCl containing air in the TTL also has substantial tropospheric character. These measurements constrain the HCl budget in the TTL and, with use of the correlations of HCl with the simultaneously measured ozone, provide an upper bound for the fraction of the ozone in the TTL that has also been transported from the stratosphere.
A12A-05 11:20h
Changes in the strength of the Brewer-Dobson circulation in a simple AGCM
Recent modelling work using the UK Met Office's Unified Model has shown that the strength of the Brewer-Dobson circulation increases in an double CO2 climate. A dynamical explanation for this strengthening is not yet well established. In order to find such an explanation, we use a simple general circulation model to investigate changes to the Brewer-Dobson circulation under different climate change scenarios. The model is a dry, primitive-equation model on a sphere which is forced via Newtonian relaxation of the temperature field towards radiative equilibrium values. As a first proxy of a double CO2 climate, we simply increase the radiative relaxation timescale in the stratosphere. Additional climate change simulations are constructed by changing the radiative equilibrium temperature field to values consistent with a double CO2 climate. Results from these model runs will be compared against a control case which is representative of current January mean conditions. The role of planetary waves will be discussed, as well as the importance of the subtropical jet structure.
A12A-06 INVITED 11:35h
Extratropical Stratosphere-Troposphere Dynamical Coupling: Perspectives from a Simple AGCM
As evidence grows that stratospheric conditions might signficantly influence the tropospheric circulation, many questions remain about the dynamics of this influence. A useful tool for beginning to answer these questions is a class of relatively simple atmospheric general circulation models (AGCMs) that use highly idealized model "physics" and are capable of simulating realistic extratropical stratosphere-tropospohere eddy-mean-flow interactions. In this talk, two sets of simple-AGCM calculations are described that lend insight into the time dependence of strat-trop coupling on 10-100 day and on climate timescales. In the first set, transient stratosphere-troposphere coupling events are stimulated using wave-activity pulses that are initiated in the troposphere, propagate into the stratosphere, and return with some delay into the troposphere. These calculations show that the initial stratospheric state can exert significant control on the delay of the return signal. In the second set of calculations, a long-term tropospheric circulation response is stimulated by stratospheric cooling. When the model is run without a seasonal cycle, the troposphere responds strongly but takes several hundred days to fully equilibrate. When the model is run with a seasonal cycle, the troposphere responds more weakly, but so does the stratosphere, so that the ratio of the stratospheric response to the tropospheric response remains approximately constant with and without a seasonal cycle. These and other results demonstrate that the eddy-drag responses in the stratosphere and troposphere are highly coupled and serve to dampen, not amplify, the imposed thermal perturbation. Taken together, both sets of calculations leave open several questions, including the issue of what ultimately controls the timescale of the downward propagation of stratosphere-to-troposphere signals and of the long-term tropospheric adjustment.
http://individual.utoronto.ca/paul_kushner
A12A-07 11:50h
On recent trends in the stratospheric circulation
The temperature of the stratosphere has decreased substantially over the past few decades. As noted in previous literature, the cooling of the lower stratosphere is driven largely by stratospheric ozone depletion, while the cooling in the upper stratosphere reflects contributions from both ozone depletion and increased greenhouse gases. Increasing water vapor may have also contributed to the global-mean cooling, particularly in the lower stratosphere. Numerous studies have noted that the amplitude of recent stratospheric cooling is not globally uniform, but is largest at polar latitudes during the spring season. However, with the exception of the springtime maxima in polar stratospheric cooling, previous studies have not identified a distinct spatial structure in global stratospheric temperature trends. In this talk, I will document the existence of such a structure and also highlight the striking degree of hemispheric symmetry in the seasonality of recent trends in the polar stratosphere.
A12A-08 12:05h
Sensitivity of Ozone to Bromine in the Lower Stratosphere
Measurements of BrO suggest that inorganic bromine (Bry) at and above the tropopause is 4 to 8 ppt greater than assumed in most ozone trend assessment models. This additional bromine is likely carried to the stratosphere by short-lived biogenic compounds and their decomposition products, including tropospheric BrO. Inclusion of this additional bromine in an ozone trend simulation increases the computed ozone depletion over the past ~25 years, leading to better agreement between measured and modeled ozone trends. Ozone loss increases because BrO associated with enhanced Bry provides a reaction partner for ClO, supplied by anthropogenic chlorine. Enhanced Bry causes photochemical loss of ozone below 16 km to switch from being controlled by pure HOx catalytic cycles (primarily HO2+O3) to a regime where loss by the BrO+HO2 cycle is nearly comparable to loss by HOx.