Abstract:
A fundamental question about the climate system is why its state is always changing, and yet has so far avoided a runaway instability that could destroy all life. It is also against this background that the importance of understanding human impact on the state of the climate system better emerges. With our industrial activity changing the chemical composition of the atmosphere, are we prodding a "lamb" who poses no harm, or a "beast" who may react to devour all of us?
This course attempts to shed some light on the above questions through guided reading and discussion of key papers in the area of climate dynamics, and through lectures by leading climate experts in the Boulder climate community. The course will start with convection over the Indo--Pacific warm-pool and its planetary-scale organization - the Madden-Julian Oscillation, and proceed to variability on longer time-scales and modes of variability in the extratropics: the El Nino-Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation, the Antarctic Oscillation, and abrupt climate changes. After examining the processes that give rise to these natural variability--the "contained instability"--in the climate system, the course then proceeds to deal with more directly the impact of anthropogenic forcing on the state of the climate system by having lectures on climate forcing and feedbacks, the water and energy cycle, paleoclimate and projected future climate change, and the impact of global warming on the statistics of weather events.
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Abstract:
Modern computers enable convective dynamics on scales of ~10 km upward to be simulated explicitly, even on global scales. While cumulus convection per se is not resolved in such simulations, the effects of fields of cumulus organized into dynamically coherent structures ("mesoscale convective organization") on large-scale dynamics are approximated reasonably well. The MJO is an outstanding example of multiscale convective organization in the tropics. After briefly introducing the MJO from an observational perspective and summarizing problems in contemporary prediction models (where convection is parameterized), progress in the numerical simulation and dynamical modeling of the MJO will be described. Finally, a selection of key research issues facing the future will be discussed.
References:
(1) Moncrieff, 2004: Analytic representation of the large-scale organization of tropical convection. J. Atmos. Sci., 61, 1521-1538. (download pdf file)
(2) Zhang, C. 2005: Madden-Julian Oscillation, Rev. Geophys, 43 # RG2003 (download pdf file)
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Abstract:
Much of the variability in tropical weather is related to the passage of cells of deep convection. It is well-known that a portion of this activity is organized into large scale waves, ranging from mesoscale squall lines to planetary scale features such as the Madden-Julian Oscillation (MJO). Observations show that larger scale features tend to be composed of smaller scale equatorial waves, so that for example the "envelope" of the MJO is often comprised of Kelvin, westward-inertio gravity, and mixed Rossby gravity waves, and these in turn are comprised of a broad spectrum of mesoscale features on down to individual cumulus clouds. This suggests a dominance of both upscale and downscale interactions in the organization of tropical convection. A potential aid to the understanding of scale interactions within the tropical atmosphere is the fact that there is a certain degree of "self-similarity" in observed gross features of the dynamical structures of organized tropical convection. In general these disturbances display a warm lower troposphere ahead of the wave, with cooling behind, and a warm mid-troposphere within the convective region. These dynamical signals are consistent with the observation that the waves show a progression from a dominance of shallow to deep convection, and then stratiform precipitation, regardless of scale or propagation direction. It is a remarkable fact that the temporal and spatial evolution of mesoscale convective complexes, which can be traced back to microphysical arguments, also exists at a certain level on the scale of the MJO.
References:
(1) Wheeler, M., and G.N. Kiladis, 1999: Convectively-coupled equatorial waves: Analysis of clouds in the wavenumber-frequency domain. J. Atmos. Sci., 56, 374-399. (download pdf file)
(2) Wheeler, M., G.N. Kiladis, and P.J. Webster, 2000: Large-scale dynamical fields associated with convectively-coupled equatorial waves. J. Atmos. Sci., 57, 613-640. (download pdf file)
(3) Straub, K.H., and G.N. Kiladis, 2002: Observations of a convectively-coupled Kelvin wave in the eastern Pacific ITCZ. J. Atmos. Sci. 59, 30-53. (download pdf file)
(4) Kiladis, G.N., K.H. Straub, and P.T. Haertel, 2005: Zonal and vertical structure of the Madden-Julian Oscillation. J. Atmos. Sci., 62, 2790-2809. (download pdf file)
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Abstract:
I will begin with a synopsis of some El Niño observations, including the life cycle of an event. After that, we shall review some considerations of El Niño models, particularly efforts to relate the observations to each of the three classes of dynamical attractors: the chaotic class, the limit cycle class, and the fixed points.
References:
Click here for introductory material of El Niño
Penland, C., and P. D. Sardeshmukh, 1995: "The optimal growth of tropical sea surface temperature anomalies," J. Climate, 8, 1999-2024. (download pdf file)
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Abstract:
ENSO have been traditionally explained as Linear Oscillators or Forced Varibility by weather noise. The validity of these traditional views hinges on the assumption that ENSO is an anomaly about an independently given background state--a fixed climatology. At a more fundamental level, I will argue that ENSO may be viewed as a Heat-Pump that regulates the time-mean upper ocean thermal stratification and more generally the stability of the tropical Pacific climatology. I will present results from perturbation experiments with a coupled model. The perturbation experiments are conducted in pairs. 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. The Heat-Pump picture of ENSO has implications for a number of key climate issues and these implications will be discussed in the class.
References:
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Abstract:
In this talk we will start by reviewing the major observed atmospheric and oceanic changes associated with ENSO, and we will introduce some simple paradigms proposed to explain the nature of the ENSO cycle. We will then examine how ENSO is represented in a suite of state-of-the-art climate models. The major modeling centers around the world have recently completed a set of simulations to assess the consequences of increasing greenhouse gas concentration. Models are the only tool we have to predict the climate evolution in the future. Since sea surface temperature variations in the tropical Pacific can impact temperatures and precipitation over large areas of the globe, a realistic representation of ENSO in the different models is central to the models' performance. Although the models' ability to reproduce ENSO is considerably improved in the last years, we will see that present climate models still show deficiencies in their simulation of tropical Pacific interannual variability. One of these deficiencies involves the temporal structure of ENSO. In the models, ENSO events tent to occur too frequently, and too regularly than in nature. Based on the dynamical frameworks outlined in the first part of the talk, we will discuss possible reasons for the erroneous ENSO evolution in the models.
References:
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Abstract:
Pronounced decadal fluctuations occurred over the North Pacific during the 20th century, which Mantua et al. (1997) termed the Pacific Decadal Oscillation (PDO) based on transitions between relatively stable states of the dominant pattern of North Pacific sea surface temperature (SST) anomalies. The decadal SST transitions were accompanied by widespread changes in the physical and biological state of the North Pacific Ocean and in the atmosphere and ecosystems downstream over the adjacent continents. Here we explore the processes that impact the PDO and Pacific Decadal Variability (PDV) in general, including: i) stochastic atmospheric variability, e.g. random fluctuations of the Aleutian Low; ii) extratropical ocean dynamics with/without feedback on the atmosphere, iii) atmospheric signal originating in the tropics; e.g. ENSO and lower-frequency changes in the Indo-Pacific basin and iv) tropical-extratropical interactions through the atmosphere and ocean. Based on our understanding of these mechanisms we will examine the predictability of the PDO on interannual to decadal time scales. Finally, we will investigate the extent to which PDO-related SSTs influence the atmospheric circulation over the Pacific North America (PNA) region.
References:
Miller, A. J. and N. Schneider, 2000: Interdecadal climate regime dynamics in the North Pacific Ocean: theories, observations, and ecosystem impacts. Progress in Oceanogr., 47, 355-379.
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Abstract:
The leading modes of variability of the circulation in the Northern and Southern Hemisphere sea level pressure exhibit a pressure seasaw between mid-and high latitudes. In my lecture, I will talk about definition of the annular modes, the relationship to other climate variables like temperature and precipitation, the connection to the stratosphere,trends, possible effects of natural and anthropogenic climate forcings on these modes. More information about annular modes can be found on the following web page: http://www.atmos.colostate.edu/ao/
References:
1. Wallace, JM; Thompson, DWJ Annular modes and climate prediction PHYSICS TODAY, 55 (2): 28-33 FEB 2002
2. Thompson, DWJ; Wallace, JM Annular modes in the extratropical circulation. Part I: Month-to-month variability JOURNAL OF CLIMATE, 13 (5): 1000-1016 MAR 1 2000
3. ,Thompson, DWJ; Wallace, JM; Hegerl, GC Annular modes in the extratropical circulation. Part II: Trends JOURNAL OF CLIMATE, 13 (5): 1018-1036 MAR 1 2000
4. Hartmann, DL; Wallace, JM; Limpasuvan, V; et al. Can ozone depletion and global warming interact to produce rapid climate change? PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 97 (4): 1412-1417 FEB 15 2000
5. Thompson, DWJ; Solomon, S. Interpretation of recent Southern Hemisphere climate change SCIENCE, 296 (5569): 895-899 MAY 3 2002
6. Rind, D; Perlwitz, J; Lonergan, P. AO/NAO response to climate change: Respective influences of stratospheric and tropospheric climate changes JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 110 (D12): Art. No. D12107 JUN 21 2005
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Abstract:
Arctic sea ice has changed dramatically in recent years leading to the suggestion that a "tipping point" may have been reached in which strong positive feedbacks in the Arctic system accelerate ice retreat and lead to a new era of thinner and less extensive ice cover. However, the patchy observational record and considerable natural variability in the Arctic make it difficult to assess whether a tipping point has actually been reached.
Here, I present research on the future trajectory of Arctic summer sea ice from future climate model projections. In this work, we find that abrupt reductions in the ice cover are quite common. We analyze the factors that cause these abrupt transitions and find that they result from a preconditioning (thinning) of the ice pack, a triggering event, and positive feedbacks that amplify the ice retreat. These mechanisms are analyzed and the consequences of these abrupt transitions for other aspects of the climate system are discussed. Finally, some cautions are given regarding the use of climate models for addressing issues of future Arctic change.
References:
Observed Arctic change:
Stroeve, J.C., M.C. Serreze, F. Fetterer, T. Arbetter, W. Meier, J. Maslanik, and K. Knowles (2005), Tracking the Arctic's shrinking ice cover: Another extreme September minimum in 2004, Geophys. Res. Lett., 32, L04501, doi:10.1029/2004GL021810.
Rothrock, D.A., Y. Yu, and G.A. Maykut (1999), Thinning of the Arctic sea-ice cover, Geophys. Res. Lett., 26, 3469-3472. Polyakov, I.V. et al. (2005), One more step toward a warmer Arctic, Geophys. Res. Lett., 32, L17605, doi:10.1029/2005GL023740.
Role of sea ice in the climate system:
Hall, A. (2004), The role of surface albedo feedback in climate. J. Climate, 17, 1550-1568. Bitz, C.M., P.R. Gent, R.A. Woodgate, M.M. Holland, and R. Lindsay (2006), The influence of sea ice on ocean heat uptake in response to increasing CO2. J. Clim., 19, 2437-2450.
Projected Arctic change:
Holland, M.M. and C.M. Bitz, 2003: Polar amplification of climate change in coupled models, Clim. Dyn, 21,221-232, doi:00382-003-0332-6.
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Abstract:
The meridional overturning circulation (MOC) of the Atlantic Ocean is an important component of the climate system due to its role in meridional heat transport, and in the oceanic uptake of carbon dioxide and other gases. Interest in the MOC is heightened by evidence in the paleo-climate record that it has undergone abrupt changes in the past and by projections from climate models that suggest that its strength may decline in response to anthropogenic climate forcing. In this talk we will review the observational evidence for the present state of the Atlantic MOC, conceptual models for the relationship of the strength of the MOC to thermohaline and mechanical forcing, and results from climate model simulations of the response of the MOC to paleo-climate scenarios and future climate change.
Suggested reading:
Clark, P.U., N.G. Psias, T.F. Stocker, and A.J. Weaver (2002): The role of the thermohaline circulation in abrupt climate change. Nature, 415, 863-869.
Marotzke, J. (2000): Abrupt climate change and thermohaline circulation: mechanisms and predictability, Proc. Nat Acad. Sci, 97, 1347-1350, 2000
Talley, L.D., J.L. Reid, and P.E. Robbins (2003): Data-based meridional overturning streamfunctions for the global ocean. J. Climate, 16, 3213-3226.
Wood, R.A., M. Velinga, and R. Thorpe (2003): Global warming and thermohaline circulation stability. Phil. Trans. R. Soc. Lond A , 361, 1961-1975.
Wunsch, C. (2002): What is the thermohaline circulation? Science, 298, 1179-1180.
Abstract:
The radiative forcing from increased concentrations of long-lived greenhouse gases (LLGHGs) could induce significant changes in the climate system over the 21st century. The most comprehensive tools for simulating this climate change are fully coupled atmosphere-ocean general circulation models (AOGCMs). The Intergovernmental Panel on Climate Change (IPCC) has commissioned simulations from an ensemble of AOGCMs for analysis. One of the characteristic properties of an AOGCM is its sensitivity to higher levels of LLGHGs. The ensemble of AOGCMs assembled for the IPCC exhibits a large range in climate sensitivities. The IPCC uses simulations from this ensemble to estimate the range of climate response to projected increases in LLGHGs. The accuracy of the simulated radiative forcing has been evaluated as part of the current IPCC assessment. Several lines of evidence from this evaluation indicate that there are substantial discrepancies among the AOGCMs and between the AOGCMs and reference calculations. These differences have important implications for interpreting variations in forcing and response across the multi-model ensemble of AOGCM simulations assembled for the IPCC fourth assessment report. New mathematical methods could significantly improve accuracy of the radiative parameterizations in global models. These and other uncertainties in long-range climate projection could be avoided by focusing instead on committed climate change through the year 2050. Developing predictions of this committed change could also strengthen the links between climate science and public policy.
Suggested reading:
Collins, W.D., V. Ramaswamy, M.D. Schwarzkopf, Y. Sun, R.W. Portmann, Q. Fu, S.E.B. Casanova, J.-L. Dufresne, D.W. Fillmore, P.M.D. Forster, V.Y. Galin, L.K. Gohar, W.J. Ingram, D.P. Kratz, M.-P. Lefebvre, J. Li, P. Marquet, V. Oinas, Y. Tsushima, T. Uchiyama and W.Y. Zhong, 2006: Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the IPCC AR4. J. Geophys. Res., vol. 111, D14317, doi:10.1029/2005JD006713.
Abstract:
The equatorial Pacific is a region with strong negative feedbacks. Yet coupled general circulation models (GCMs) have exhibited a propensity to develop a significant SST bias in that region, indicating an unrealistic sensitivity in the coupled models to small energy flux errors that inevitably occur in the individual model components. Could this "hypersensitivity" exhibited in the coupled models be due to an underestimate of the strength of the negative feedbacks in this region? I will use this lecture to address this question and highlight the continuing uncertainties in our model's ability to accurately calculate the major feedbacks in the tropical region.
Suggested reading:
Abstract:
The largest known extinction in Earth's history took place approximately 250 million years ago at the Permian-Triassic boundary, where approximately 95% of marine and 75% of terrestrial species were lost. Associated with this event was an extended period of magma activity and global ocean anoxia. What caused such a catastrophic change in life? A number of hypotheses have been proposed to explain various aspects of the extinction and the climate of this time period. I will describe results from the first realistic fully coupled climate model simulation of this time period. I will show how global warming leads to significant changes to ocean circulation that leads to inhospitable conditions for marine life. I will also present results from a coupled climate atmospheric chemistry model for this time period. These atmospheric chemistry results provide an explanation for the demise of terrestrial life at this time. I will conclude my presentation with a discussion on the relevance of these results for future climate change.
Recommended Readings:
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Abstract:
This lecture will approach the variations in the climate system from the standpoint of energy flows. In the atmosphere the energy comes in as radiant energy and is converted into internal heat energy, latent energy, potential energy, kinetic energy, and perhaps even chemical energy. It is moved around, mainly by the atmosphere and oceans, stored and sequestered in oceans and ice, and ultimately radiated back to space. Chnages in atmospheric composition are significantly altering these natural flows of energy and changing the climate. A brief outline will be given of the greenhouse effect, the energy flows through the system, and the role of the climate system components. Then the discussion will turn to changes in these flows and observations of climate change. This includes radiative forcing, and heat imbalance of the planet, and the critical role of water.
Recommended Readings (pdf files available through: http://www.cgd.ucar.edu/cas/trenberth-publish.html) :
(1) Trenberth, K. E., and D. P. Stepaniak, 2004: The flow of energy through the Earth's climate system. Symons Lecture 2004. Quart. J. Roy. Meteor. Soc., 130, 2677-2701.
(2) Trenberth, K. E., 2004: Earth's Energy Balance. Encyclopedia of Energy, Cutler J. Cleveland (Ed. in Chief), Vol. 1, Elsevier Inc. 859-870.
(3) Trenberth, K. E., J. Fasullo, and L. Smith, 2005: Trends and variability in column-integrated water vapor. Clim. Dyn., 24, 741-758.
(4) Dai, A., K. E. Trenberth and T. Qian, 2004: A global data set of Palmer Drought Severity Index for 1870-2002: Relationship with soil moisture and effects of surface warming. J. Hydrometeor., 5, 1117-1130.
(5) Karl, T. R., and K. E. Trenberth, 2003: Modern global climate change. Science, 302, 1719.
(6) Trenberth, K. E., and D. P. Stepaniak, 2003: Co-variability of components of poleward atmospheric energy transports on seasonal and interannual timescales. J. Climate, 16, 3691-3705.
(7) Trenberth, K. E., and D. P. Stepaniak, 2003: Seamless poleward atmospheric energy transports and implications for the Hadley circulation. J. Climate, 16, 3706--3722.
(8) Trenberth, K. E., A. Dai, R. M. Rasmussen and D. B. Parsons, 2003: The changing character of precipitation. Bull. Amer. Meteor. Soc., 84, 1205?1217.
Trenberth, K. E., and J. M. Caron, 2001: Estimates of meridional atmosphere and ocean heat transports. J. Climate, 14, 3433Ð3443.
Abstract:
The potential impact of global climate changes on tropical cyclones has been brought into sharp focus by the recent catastrophic years of 1994 and 1995 in the North Atlantic and by record systems being recorded in Australia and the eastern Asian region. This lecture will address some of the recent research in this area and the implications for the next 30-50 years, with a focus on the North Atlantic. The lecture will contain three parts:
The attached paper, which is in the revision stage, provides an overall assessment of the state of the science in the North Atlantic, together with an assessment of the trends and variability in North Atlantic tropical cyclones. I also recommend the articles by Knutson and Tuleya (2004), Mann and Emanuel (2006), Oochi et al (2006) and Santer et al (2006) as referenced in the attached.
Suggested reading: