A13C-01 13:40h
Saturation of poleward atmospheric heat transport in warm climates and the low-gradient paradox.
The equable climates of the deep past featured higher atmospheric greenhouse gas concentrations, greater global-mean surface temperatures and much weaker equator-to-pole temperature contrasts than today. Climate models readily reproduce the higher mean temperatures, given sufficient increases in greenhouse gases, but they have proved incapable of matching the low meridional gradients indicated by proxy data. A crucial step in resolving this 'low-gradient paradox' is uderstanding why climate models fail to reproduce the correct feedback between global mean temperature and its meridional gradient. Though models do achieve some reduction in temperature gradients, mostly through snow and sea-ice albedo feedback, the remaining discrepancy must be accounted for by either more exotic forms of radiative forcing feedback, which are not represented in current models, or by more efficient oceanic and/or atmospheric poleward heat transports, which the models for some reason do not capture. This latter feature is especially puzzling for the atmosphere, since there are plausible reasons to expect atmospheric energy transport to be be considerably more efficient in a warmer climate. We explore this issue by systematically studying the response of atmospheric heat transpor in a GCM to a very broad range of global mean temperatures and meridional gradients. We find that heat transport increases with global mean temperature when the latter is less than about 15C; above this value, heat transport saturates, becoming insensitive to surface temperature. This behavior has a dynamical origin traceble to changes in the structure of the atmosphere's general circulation. Mean tropospheric static stability increases with surface temperature, reducing baroclinicity and suppressing storm-track eddy activity. Furthermore, as temperature increases the storm-tracks as a whole migrate poleward over cooler waters, and thus do not experience the full global-mean surface temperature increase. These results appear to rely on physically robust mechanisms not obviously connected with questionable parameterizations in the GCM. This suggests that the low-gradient paradox arises not from deficient representation of known processes in GCMs, but from unknown or as yet unimplemented physics.
A13C-02 13:55h
The Role of ENSO in Regulating the Stability of the Tropical Pacific Climatology
Increasing evidence indicates that the time-mean climate state of the tropical Pacific hovers closely around the neutral point. What enables the climatology to triumph over the destabilizing effect of the radiative heating? The answer may be ENSO. To investigate the role of ENSO in regulating the stability of the tropical Pacific climatology, perturbation experiments are conducted in pairs with a coupled model. Perturbations are introduced through either enhanced tropical heating or subtropical cooling such that the coupled system is subject to increased thermal "stress". In one experiment, the ENSO is turned off while in another experiment the ENSO is turned on. Whether the perturbation comes from enhanced tropical heating or enhanced subtropical cooling, the response of the time mean value of Tw-Tc (the temperature difference between the warm-pool SST and the characteristic temperature of the equatorial thermocline water) is much reduced when ENSO is on than when ENSO is off. In the enhanced tropical heating case, ENSO enables surface heat to be pumped to the depths of the equatorial thermocline and effectively "mix" the heat downward against a stable stratification. In the enhanced subtropical cooling case, the same effect of ENSO enables heat pumped to the depths of the equatorial subsurface to diminish the cooling to the equatorial thermocline water caused by the enhanced subtropical cooling. To the degree that the value of Tw-Tc controls the coupled stability of the tropical Pacific climatology, the numerical experiments suggest a fundamental role of ENSO in regulating the tropical Pacific climatology. The study raises the question whether models with poor simulations of ENSO can give reliable predictions of changes in the mean climate.
http://www.cdc.noaa.gov/people/dezheng.sun
A13C-03 14:10h
Towards Defining Probability Forecasts of Likely Climate Change
There is strong desire for probabilistic forecasts of climate change, both for policy making and risk management, as well as scientific interest. The extent to which this desire can be satisfied scientifically is unclear. The aim of this paper is to explore (i) current methods for extracting probability forecasts and (ii) alternative deliverables which have a firm scientific basis given the current limitations to the state of the art. Even ``physics-based" models contain empirically determined paramters and parameterizations. While it is straightforward to make `ensembles' over initial conditions, parameter values and even several model structures, the interpretation of the resulting ensemble of simulations requires some care. Methods for extracting probability forecasts from ensembles of model simulations will be discussed in terms of their relevance and internal consistency, a particular example being provided by Murphy et al (Nature, 2004). This approach will be compared and contrasted with one proposed by climateprediction.net (Stainforth et al, Nature, in review), which strives to produce policy relevant information when no coherent probability forecast can be extracted from the limited ensembles available in practice. The role of the Bayesian paradigm will be considered in both cases. Extracting probability distributions in the context of climate change requires consideration of a discrete sample drawn from a high dimensional space. The analysis of small ensembles requires further assumptions of linearity and smoothness which must be verified explicitly; even when ``large" ensembles are to hand, the analysis of the collection of simulations requires combining mutually exclusive runs, sampling a restricted region of the parameter space, under a set of models with related shortcoming. Historical observations serve to lift some of these difficulties, but attempts to fold observations into the analysis (say, in terms of weighting sets of simulations differently) are complicated by the fact that we are trying to model climate (a distribution) while we have only one realization of the earth system, and that we are extrapolating into regions where we have no empirical data (in CO2 concentration, for example). We conclude that while new ensembles of model simulations provide significant insights into the future of the earth system, current attempts to extract physically relevant, objective probability distributions are not coherent. Nevertheless, policy and risk management decisions can be better informed through a coherent interpretation of these simulations, which also provide useful guidance toward the design of improved climate studies.
A13C-04 14:25h
Optimum Conditions for Terrestrial Vegetation and Associated Vegetation Feedbacks to the Climate System
Terrestrial vegetation plays an important role in moderating the energy- and water fluxes at the land surface, thereby affecting climate. One of the fundamental features of terrestrial vegetation is its ability to adapt to its climatic environment, thereby maximizing its capacity to perform photosynthesis. Here I apply this concept of optimum adaptation to the parameterization of vegetation in a coupled dynamic vegetation-climate system model of intermediate complexity. I show that there are optimum values for key land surface characteristics, such as stomatal conductance, rooting zone depth, and surface roughness, for which the climatic conditions to do photosynthesis are optimal. These optima result from the close coupling of photosynthesis and transpiration in terrestrial vegetation that links the availability of carbon dioxide within the leaves to the rate of water loss by transpiration. Different rates of transpiration lead to a fundamental trade-off: increasing rates of transpiration increases the availability of carbon dioxide, but at the same time results in more clouds, thereby reducing the amount of incoming solar radiation. Threfore, there is an optimum at which the rate of photosynthesis is at a maximum. Sensitivity simulations with the coupled model show (a) that the resulting optimum values for these land surface characteristics are realistic; (b) that the associated climate is close to the present-day control climate; and (c) that the climate is unique in that the rate of continental evapotranspiration is maximized, leading to the most active hydrological cycle over land. Because realistic optima exist to which vegetation would adapt to, it is concluded that the resulting feedbacks of terrestrial vegetation to climatic perturbations are primarily negative, bringing climatic conditions back to the optimum.
A13C-05 14:40h
Stratification-Turbulence Feedbacks Limit Nonlinear Interactions Between Large-Scale Eddies in the Atmosphere
It is generally held that atmospheric macroturbulence can be strongly nonlinear. Yet weakly nonlinear models successfully account for the length scales ($\sim$4000~km), time scales ($\sim$2~days), and for aspects of the structure of the energy-containing baroclinic eddies in the extratropics of the Earth atmosphere. Here we present theoretical arguments and simulations that suggest that the historic successes of weakly nonlinear models of atmospheric macroturbulence are not a coincidence but a result of self-organization of atmospheric macroturbulence into critical states of weak nonlinearity. A negative feedback between the extra\-tro\-pi\-cal thermal stratification and atmospheric macroturbulence limits nonlinear eddy--eddy interactions and the concomitant inverse cascade of eddy energy from the length scales of baroclinic instability to larger scales. The theory and simulations point to fundamental constraints on the climate of Earth and other planets.
A13C-06 INVITED 15:00h
An hypothesis for tropical climate stability and extratropical instability
A curious feature of climate change over the last several hundred millions years is the relative stability of tropical climate. While high latitude temperature fluctuates over a range of 10o C or more, tropical surface temperatures change by only a few degrees. For example, during the early Eocene, polar temperatures are thought to have been about 15 o C warmer than present, while tropical sea surface temperatures are estimated to have been only a few degrees warmer. In this talk, I will present recent research results that suggests that the comparative stability of tropical climate can be explained by a feedback involving the ocean's thermohaline circulation. The strength of this circulation is rate-limited by vertical mixing in the Tropics, and yet the source of this mixing is controversial. In current climate models, the mixing coefficient is largely independent of climate. I will show that much of the mixing is due to global tropical cyclone activity, which itself is a strong function of tropical climate. Increasing tropical temperature leads to greatly increased tropical storm activity, increasing vertical mixing and accelerating the thermohaline circulation. This in turn transports heat poleward, cooling the Tropics but accelerating the warming of high latitudes.
A13C-07 INVITED 15:20h
Control Theory and Analysis of Feedback Systems
Control theory has developed a collection of tools that can be used to better understand the behavior of complex, interconnected systems. This talk provides an introduction to some of the control theory tools relevant to climate dynamics. Specific concepts that will be described include input/output modeling, modeling reduction, and analysis in the presence of unmodeled dynamics.