Stratospheric Influence on Tropospheric Climate Change in Climate Models
Research since the 1980's has shown that the simulation of tropospheric climate in comprehensive climate
models is significantly influenced by how the stratosphere is represented in the models. This influence
appears most clearly in the extratropics, where the stratosphere can influence the troposphere dynamically via
eddy mean-flow interactions. This talk will discuss how stratospheric representation also affects the simulated
tropospheric circulation response to climate change in climate models. Two distinct roles for the stratosphere
under climate change will be discussed. First, there is the stratosphere's direct influence on the tropospheric
response, which arises from stratospheric radiative forcing from stratospheric ozone and greenhouse gas
changes. Second, there is the stratosphere's indirect influence on the tropospheric response, which arises
from climate changes, such as changes to ocean surface temperatures, that do not involve stratospheric
radiative forcing. It will be argued that the first type of influence is better characterized than the second type, and
that systematic comparison of climate models is required to quantify the second type of influence.
Modelling the Response of Northern Hemisphere Sudden Stratospheric Warmings to Changes in CO2
Dynamical responses in the stratosphere to increased CO2 loading are investigated in simulations with four times pre-industrial CO2 concentrations using the HadSM3-L64 Met Office model. Consistent with radiative considerations the troposphere warms and the stratosphere undergoes a cooling. This contrasts however with the Arctic lower stratosphere, which undergoes a significant dynamical warming during NH winter. We find a large increase in early winter variability, and attribute the warming response to a strong frequency modulation of stratospheric sudden warming (SSW) events. To investigate the SSW response we have performed two additional experiments which aim to separate the influences of tropospheric and stratospheric climate change. We show that changes in both the tropospheric wave driving and the stratospheric mean state contribute to the resulting NH polar warming, but the increase in tropospheric wave driving is primarily responsible for the warming extending deep into the lower stratosphere, where it may influence tropospheric dynamics. The impact of SSW frequency modulation on the surface response is further investigated in an intermediate complexity GCM.
Influence of the Stratosphere on the Northern Annular Mode Response to Increasing Greenhouse Gases
Obtaining credible climate change projections in NH extratropical winter is challenging as the current generation of coupled atmosphere-ocean models shows a wide range in the Northern Annular Mode (NAM) response to increasing greenhouse gases. Previous studies have suggested that the NAM response critically depends on the stratospheric representation in climate models. In this study, we assess the influence of the stratosphere on the tropospheric circulation response to increasing greenhouse gases by comparing the response in various versions of a comprehensive atmospheric general circulation model (AGCM) without a well-resolved stratosphere ('low-top' model), to the response in a version of the same AGCM with a well-resolved stratosphere ('high-top' model). We show that the circulation response is more sensitive to orographic gravity wave drag (OGWD) parameter settings than to the model lid height. The causal relationship between OGWD and changes in NH wintertime circulation response is further investigated by introducing a methodology that allows OGWD forcing fixed to its 1×CO2 value when CO2 is doubled. Such experiments show that the changes in GWD forcing due to CO2 doubling have essentially no impact on the NAM response. The primary conclusion is that the OGWD influence is limited to its impact on the 1×CO2 basic-state climatology, which defines the propagation characteristics of resolved planetary waves. It is shown that the action of planetary waves explains essentially all of the NH wintertime circulation sensitivity. Finally, we will show preliminary results of an investigation of the sensitivity of the NH wintertime circulation response between IPCC models and its possible relation to the basic state climate.
The Ozone Hole and Southern Hemisphere Climate Change
Climate change in the Southern Hemisphere (SH) has been robustly documented in the last several years. It has altered the atmospheric circulation in a surprising number of ways: a rising global tropopause, a poleward intensification of the westerly jet, a poleward shift in storm tracks, a poleward expansion of the Hadley cell, and many others. While these changes have been extensively related with anthropogenic warming resulting from the increase in greenhouse gases, their potential link to stratospheric cooling resulting from ozone depletion remains tentative, and a comprehensive picture the polar ozone's role in the SH climate system is still lacking. Examining model output from the coupled climate models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment (AR4), and grouping them depending on the stratospheric ozone forcing used, we here show that stratospheric ozone affects the entire atmospheric circulation in the SH, from the polar regions to the subtropics, and from the stratosphere to the surface. Furthermore, model projections suggest that the anticipated ozone recovery, resulting from the implementation of the Montreal Protocol, will likely decelerate future climate change resulting from the increased greenhouse gases, although it might accelerate surface warming over Antarctica.
21st Century Trends in the Potential for Ozone Depletion
We find robust trends in the area where Antarctic stratospheric temperatures are below the threshold for polar stratospheric cloud (PSC) formation in Goddard Earth Observing System (GEOS) chemistry-climate model (CCM) simulations of the 21st century. In late winter (September-October-November), cold area trends are consistent with the respective trends in equivalent effective stratospheric chlorine (EESC), i.e. negative cold area trends in 'realistic future' simulations where EESC decreases and the ozone layer recovers. In the early winter (April through June), regardless of EESC scenario, we find an increasing cold area trend in all simulations; multiple linear regression analysis shows that this early winter cooling trend is associated with the predicted increase in greenhouse gas concentrations in the future. We compare the seasonality of the potential for Antarctic ozone depletion in two versions of the GEOS CCM and assess the impact of the above-mentioned cold area trends on polar stratospheric chemistry.
What can Idealized GCMs Tell us About the Stratosphere and Climate Change?
The potential role for idealized GCMs -- models that resolve the full 3-D dynamics of the stratosphere-troposphere coupled system, but with idealized physics and forcings -- in determining how the stratosphere may shape future climate change will be discussed. Idealized models can help one interpret the response in more complex systems, as demonstrated by a survey of existing studies which have implications for the tropospheric response to ozone loss and recovery, changes in the Brewer-Dobson Circulation, and coupling between the stratosphere and troposphere on intraseasonal time scales. Idealized models also provide complementary information to comprehensive GCMs, allowing one to explore a wider range of climatologies and numerical sensitivities, experiments too costly or complicated to run in comprehensive GCMs. Here, a case study of the sensitivity of the polar vortex and Brewer-Dobson circulations to both resolution and tropospheric wave forcing is presented, with a focus on the cold pole and age of air biases of most chemistry climate models.
Modes of Annular Variability in the Atmosphere, Eddy-Zonal Flow Interactions and the State of the Stratosphere
Idealised-forcing experiments have been performed previously using a simplified, Newtonian forced, global circulation model. In each of these experiments, changes to the stratospheric equilibrium temperature distribution lead to changes in the strength and position of the tropospheric mid-latitude jets and storm-tracks and to the extent of the Hadley cells and mean meridional circulation. The work presented here investigates how such shifts in the tropospheric jet can be understood by examining combined fluctuations of the first two modes of annular variability. Attention is paid to the evolution of the flow on different timescales as defined by empirical mode decomposition and related to the autocorrelation timescale for the mode of variability. At low frequencies the zonal flow and baroclinic eddies are in quasi-equilibrium and anomalies propagate poleward. The eddies are shown primarily to reinforce the anomalous state and are closely balanced by the linear damping, leaving slow evolution as a residual. At high frequencies the flow is strongly evolving and anomalies are initiated on the poleward side of the tropospheric jet and propagate equatorward. The eddies are shown to drive the evolution more strongly. Eddy amplitudes reflect the past baroclinicity and their feedback on the mean flow can be understood in accordance with traditional ideas of baroclinic lifecycle events. The state of the stratosphere determines the background position and strength of the jet upon which the variability is superimposed, and also the timescale at which the behaviour switches from low-frequency to high-frequency (as defined above). When the temperature structure of the stratosphere is such that the jet is positioned more equatorward then high frequency behaviour is dominant to much longer timescales.