Understanding the dynamics of the tropical atmospheric circulation and ENSO
Stephen Zebiak, Columbia University
Interest in the tropical atmospheric circulation, particularly the near-surface winds, dates back several centuries. Here I will highlight research contributing to current understanding of the key physical processes controlling near-surface winds over the tropical oceans, and their relation to sea surface temperature in particular.
Contributions to the understanding of coupled ENSO dynamics will be reviewed in a specific context – that is, the development of the so-called Cane-Zebiak ENSO model. I will review some of the initial assumptions and motivations, key prior research, original results with this model, both anticipated and unexpected, and conclusions that could be drawn from the initial work with CZ to elucidate the essential physics of ENSO.
Observing El Niño
David Halpern, Jet Propulsion Laboratory
For centuries, the El Niño observing network was confined to the coastal region off Chile, Peru and Ecuador. A half century ago, when the theoretical foundation for the El Niño radically changed, the geographical domain for observing El Niño expanded to include the entire equatorial Pacific. The latitudinal width of the equatorial region was defined by ocean dynamics, which, fortunately, yielded a relatively small north-south region for observing El Niño. Observing El Niño requires knowledge of the mean and La Niña conditions, creating technical and resource challenges for data acquisition relevant for the multi-month El Niño phenomenon occurring at multi-year intervals in an ocean rich with large amplitude sub-monthly variability. The El Niño ocean environment represents a severe challenge to sustain in-situ surface wind and near-surface current measurements. The remoteness of the El Niño region from countries with adequate ocean observing resources is another challenge for observing El Niño. Observing El Niño contributed to a remarkable advancement in the predictability of El Niño through development of a global ocean general circulation model (OGCM) capable of replicating in-situ current observations, especially when the OGCM is constrained with sparse ocean observations. Just as the satellite global positioning system radically improved at-sea navigation compared to the sextant, innovative satellite technology enabled new observing opportunities to improve understanding of the El Niño. Sea surface temperature and ocean surface topography are routinely monitored from satellite to assess the El Niño life cycle. Probably the most critical variable to determine the El Niño life cycle is the surface wind vector. Satellite surface zonal wind vectors in the Pacific east of 150°W showed negligible difference between El Niño and La Niña events during 2000-2011, which is a result not prescribed by conventional wisdom. In four decades the community developed an El Niño observing network capable of providing information to enhance prediction skill of El Niño and La Niña events for 6-9 months in advance, but success over the past 15 years has not reached expectations. Sustaining an improved El Niño observing network with appropriate infusion of new scientific ideas and innovative technology for worldwide societal benefit requires the dedication of new explorers, who are in attendance.
Building the Road to ENSO Prediction
E.S. Sarachik, University of Washington
We recount the early days of modern equatorial oceanography when Mark Cane did his thesis on a model of the time dependent equatorial undercurrent. The key development was the ocean surface layer which responds directly to the imposed wind stress and its curl with horizontal currents and vertical velocities at the base of the surface layer. This layer was generalized by Cane and Zebiak to respond to anomalous heat fluxes which in the equatorial regions depends negatively on the anomalous SST. The only missing piece is the temperature of upwelled water which depends on the depth of the equatorial thermocline. All the processes which change anomalous SST in terms of the imposed wind stress were now in place. The response of the anomalous surface stresses to the anomalous surface temperature was then given by a consistent version of the surface Gill model. The next key development was specifying the climatology, a decision that was genius in its simplicity: it allowed the calculation of anomalies without having to calculate the climatology (which to this day cannot be calculated in the most complex coupled climate models because of undiagnosed but ever present biases). The resulting Zebiak-Cane model was the first to reasonably simulate the ENSO phenomenon. The leap to using the model as a prediction model (initialized solely by a history of the Florida State winds) was daring and led to the first successful first ENSO forecast by dynamical means. The ongoing history of ENSO prediction, especially the T-POP program will be briefly described.
How much does the climate vary on multi-decadal timescales and why?
Amy Clement, University of Miami
Towards Understanding the Dynamics of El Niño Complexity
Fei-fei Jin, University of Hawaii
ENSO exhibits rich variations in its intensity, its timescales and spatial patterns in sea surface temperature (SST) anomalies and other associated fields. The notion of different ENSO features of the so-called central Pacific and eastern Pacific ENSO types, and the recent recognition of the existence of combination-mode which comes from ENSO and annual-cycle interaction but has its own distinct time-space features, both further reveal the complex nature of ENSO. This talk will briefly review some recent progresses in understanding ENSO dynamics for its three basic aspects. We will discuss the roles of so-called state-dependent noise in generating extreme El Nino events, the roles of the annual cycle and ENSO interaction in generating rich ENSO timescales, and roles of the potential of multiple ENSO modes in generating different ENSO periodicity and patterns.
Does El Niño variability affect warming trends in the tropical Pacific?
Christina Karamperidou, University of Hawaii
The possibility of stronger El Niño events in a warming climate, and subsequently stronger teleconnections with global temperature and precipitation has received much attention in recent decades. At the same time, climate models project non-uniform warming of the tropical Pacific in response to climate change, namely more pronounced warming of the cold tongue compared to the warm pool. Then, the issue of ENSO and global warming is traditionally thought of as the issue of the response of ENSO feedbacks to pronounced cold tongue surface warming in addition to a shoaling of the thermocline. Here, we reverse the question, and explore the relationship between the characteristics of El Niño variability and the dominant ENSO feedbacks that climate models simulate and their projected warming pattern in the tropical Pacific. We find that models with stronger El Niño variability and a mean climate with the capacity to support stronger and more diverse El Niño events project less cold tongue warming in future climate, compared to models with decreased nonlinearities in the ENSO they simulate. Then the question becomes: do models exaggerate or mute the tropical warming signal based on what kind of El Niño they simulate? What are the mechanisms involved, and what is the impact on projected global climate?
Climate response to increased CO2 - effects of explicit representation of atmospheric convection and connections with past warm and hot climates
Eli Tziperman, Harvard University
(w/ Nathan P. Arnold, Mark Branson, Melissa A. Burt, Dorian S. Abbot, Zhiming Kuang, and David A. Randall)
The effect of clouds on climate remains the largest uncertainty in climate change predictions, due to the inability of global climate models to resolve essential small-scale cloud and convection processes. We compare pre-industrial and quadrupled CO2 simulations between a conventional GCM in which convection is parameterized and a "super-parameterized" model in which convection is explicitly simulated with a cloud permitting model in each grid cell. We find that the global responses of the two models to increased CO2 are broadly similar: both simulate ice-free Arctic summers, winter-time Arctic convection, and enhanced MJO activity. Super-parameterization produces significant differences at both CO2 levels, including greater Arctic cloud cover, further reduced sea ice area at high CO2, and a stronger increase with CO2 of the Madden-Julian oscillation. The tropical and Arctic responses are related to mechanisms offered to explain the permanent El Nino of the Pliocene, 2-5 Myr ago, and the warmth of the Arctic during the Eocene, 50 Myr. The representation of clouds and convection has an enormous impact on simulation of the climate system. This study addresses concerns that conventional parameterizations may bias the response of climate models to increased greenhouse gases. The broadly similar response of two models with parameterized and non-parameterized convection and clouds suggests that state-of-the-art predictions, based on parameterized climate models, may not necessarily be strongly biased in either direction (too strong or too weak warming). At the same time, large differences in simulated tropical variability and Arctic sea ice area suggest that improvement in convection and cloud representations remain essential.
Practical limits of seasonal-to-interannual predictability
Lisa Goddard, International Research Institute for Climate and Society, Columbia University
Regional Earth System Predictions: Extending the Mark Cane Legacy to Decision-Making
Ragu Murtugudde, University of Maryland
Seminal ENSO predictions by Mark Cane and Steve Zebiak set off an era of seasonal-to-interannual prediction efforts that have borne fruits in forecast applications in numerous sectors and have only raised expectations for usable and actionable climate information. Designer forecasts with regional specificity and linked products for water, food, energy, and health are now routinely being attempted with dynamical downscaling. Skepticism persists due to the garbage-in-garbage-out syndrome in downscaling global climate predictions and projections. Significant progress is made in representing the natural-human system interactions at global scales. Regional Earth System Models (RESM) have come of age and are demonstrably adding value to global predictions and projections and enhancing skills with multi-physics ensembles and detailed representations of land-atmosphere-ocean-hydrological-ecosystem interactions. This is a way forward to adaptive management of resources with interactive decision-support from RESM predictions and projections. These tools combined with social science, policy- and decision-making are a way forward to fulfilling the dream envisioned by Mark Cane and his colleagues while establishing the International Research Institute for Climate Prediction the early 1990. This talk will present an example of an RESM prediction system implemented over the Chesapeake Bay watershed to issues weather and climate predictions from days to seasons with linked products for Health, Harmful Algal Blooms, Fisheries, pathogens, and so on. There will be some discussion on aposiopesis.
Forecasting Infectious Disease Outbreaks
Jeffrey Shaman, Columbia University
Dynamic models of infectious disease systems abound and are used to study the epidemiological characteristics of disease outbreaks, the ecological mechanisms affecting transmission, and the suitability of various control and intervention strategies. The dynamics of disease transmission are non-linear and consequently difficult to forecast. Here, we describe combined model-inference frameworks developed for the prediction of infectious diseases. We show that accurate and reliable predictions of seasonal influenza outbreaks can be made using a mathematical model representing population-level influenza transmission dynamics that has been recursively optimized using ensemble data assimilation techniques and real-time estimates of influenza incidence. Operational real-time forecasts of influenza and Ebola are currently being generated.
Monsoon variability and prediction
Sulochana Gadgil, Indian Institute of Science
The focus of this talk is the interannual variation of the Indian summer monsoon rainfall (ISMR), which seems to govern the pulse of life of over a billion people even today. ENSO rules this tropical phenomenon as well, with most of the droughts being associated with the warm phase. At each meeting of the South Asian Climate Outlook Forum in April, the major point of discussion is the prediction of the Nino 3 .4 SST anomalies for the forthcoming summer, for which beautiful plumes are available from IRI. If a positive SST anomaly of reasonable magnitude is predicted (as in this year), almost all the models also predict deficit rainfall over most of the Indian region. Reports then appear in the media about the expected impact of the impending El Nino , such as one this May predicting a decrease of about 1.75% in the GDP i.e. about 30 billion US$. This is a reflection of the effective communication of the El Nino-success story.
Although ENSO is the most important factor in determining the interannual variation of the monsoon, in some years, the impact of ENSO can be neutralized , as became evident in 1997 when the monsoon rainfall was close to the normal despite the strong El Nino. The other major factor determining the interannual variation has been identified as the Equatorial Indian Ocean Oscillation (EQUINOO) involving a see-saw between deep convection over the eastern and western parts of the equatorial Indian Ocean. The association of ISMR with simultaneous values of the indices of the two modes explains all the extremes of ISMR and over 50% of the total variation. The question of whether it would be possible to predict the Indian rainfall during the summer monsoon season as well as parts of the season, on the basis of the links to these two modes is addressed.
The coupled models are not able to predict the evolution of EQUINOO as yet and almost no model can simulate its links with the Indian monsoon. From our experience of this year as well as in 2012, it appears that there are some problems in predictions of ENSO during the summer monsoon season as well. These issues and the interesting questions arising from the experience of 2014 such as the role of the coupling over the Pacific and the attribution of monthly anomalies of Indian rainfall to El Nino are briefly discussed.
Recent Hiatus in Global Warming and the Tropical Pacific Ocean Dynamical Thermostat
Tony Rosati, GFDL/NOAA
Although atmospheric greenhouse gases continue to increase, the global surface warming since around 2001 has remained steady. Some recent studies by Kosaka and Xie, 2013 and England et al, 2013 have identified the eastern tropical Pacific as contributor to the current global hiatus. It was noted that the increased intensity of the Pacific trade winds was sufficient to give rise to a cooling of the eastern Pacific SST and hence a pause in the surface warming.
In order to understand the mechanisms of the hiatus, numerical experiments with a coupled model were run. All experiments are 10 member ensembles using the GFDL CM2.1 model. One experiment was a standard historical run from 1861-2040 using observed and projected radiative forcings. A second experiment is identical to the first except the wind stress anomalies over the tropical Pacific are replaced with anomalies from observations for the period 1979-2013.
In order to separate the processes a second set of perturbation experiments off the control simulation are run. An additional stress is added to the model wind stress only over the tropical Pacific. These experiments show the “ocean dynamical thermostat” as noted by Cane and Clement. A novel part of this experiment is noting the time evolution of the eastern Pacific cold tongue transitioning from cold to warm and the mechanisms that lead to the change.
Communicating Climate Science
Heidi Cullen, Climate Central
Founded in 2008 by leading scientists and communications experts at Princeton, Yale and Stanford, Climate Central works to tackle the misperception that climate change is a distant thing – affecting other people and other places - by demonstrating the local and personal impacts of global warming. As part of its mission, Climate Central supports local television weathercasters in their efforts to educate viewers about climate change through its Climate Matters program. Launched in 2010 with support from the National Science Foundation, the program has grown into a network that includes more than 150 weathercasters from across the country. Climate Matters delivers customized climate information at the regional and local level to weathercasters, providing ready-to-use, broadcast quality graphics and analyses that put climate change into a local context. This talk will provide an overview of how TV meteorologists use climate information to educate local audiences about climate change. It will also discuss the importance of providing the larger climate context during extreme weather events, when audiences are particularly interested in the climate connection.
Rain Prayers, Climate Reconstruction, and the Disastrous Weather of the First American Colonies
Sam White, Ohio State University
The first century (1530s-1630s CE) of European efforts to colonize the present US and Canada left a grim record of death and disaster. Colonists froze, starved, and perished of mysterious illnesses. Relations with Native Americans broke down in violent struggles over food and resources. Missions and settlements were abandoned or survived by only the thinnest of margins. Histories of the period have often overlooked an important factor: When it came to both larger climatic trends and particular weather events, the first Europeans in North America had the worst possible timing. Their settlements coincided with some of the coldest winters and deepest droughts in the past several centuries, which contributed heavily to their failure. This paper will examine this history by focusing on a peculiar pattern of events recounted in numerous early encounter narratives: competition between Europeans and Native Americans to control the weather through prayer and magic. When reconsidered in light of recent archaeology, proxy-based climate reconstructions, and a closer reading of the historical sources, these events reveal much about the ecological and psychological pressures of climate fluctuations in preindustrial societies.
Sustainability policies in the HANDY model
Eugenia Kalnay, Univeristy of Maryland
(w/ Jorge Rivas and Safa Motesharrei)
In the paper “Human and Nature Dynamics (HANDY): Modeling inequality and use of resources in the collapse and sustainability of societies”, Ecological Economics, 101, 90-102, we developed a 4-variable “thought-experiment” model for interaction of humans and nature. We showed that it is possible to reach a sustainable steady state, but that overexploitation of either Labor (by having a very large inequality between Elites and Commoners) or Nature (by over-depletion) lead to societal collapse.
Here we develop rules designed to alert the government and guide policies to prevent a forthcoming collapse. They are based on three Sustainability Ratios, which have to be less or equal than 1 in order to have long-term sustainability:
The HANDY model has several parameters that can be reduced in order to maintain the sustainability ratios below 1. They are depletion of nature per capita, subsistence salary paid to the workers (Commoners), coefficient of inequality (ratio between the salary of Elites and Commoners), and the ratio between birth and death rates.
Computing the current value of three sustainability ratios allows testing their sensitivity to the parameters of the model that need to be reduced to maintain the sustainability ratios below 1. This can guide policies by alerting the government of the danger of current unsustainable paths that will lead to collapse, and pointing to the optimal way to change the parameters in order to reach the best path to sustainability. Even after a collapse has been avoided, it is possible to find the best path to a more plentiful, optimal sustainable future.
Science in the Impartial Service of Society: How Lessons Mark Taught Me Without Even Trying Shaped My Science Forever
Gidon Eshel, Bard College
Two outstanding gaps in the textbooks on the Dynamics of Climate
David Battisti, University of Washington
There are several outstanding gaps in our knowledge of the climate system. One of them is the lack of a definitive understanding of the processes that are fundamental for the ocean overturning circulation to be centered in the Atlantic Ocean (and not the Pacific). Count Rumford argued in 1798 that cooling at high latitudes caused water to sink and spread into the deep ocean, causing warm water from the tropics to be advected northward and giving rise to an overturning circulation. Stommel (1961) appears to be the first to examine the joint effects of freshwater and heat fluxes on the overturning circulation, giving rise to the possibility of “push vs. pull”. Warren (1983) examined the freshwater balance in the North Atlantic and argued that the sinking is located in the North Atlantic (and not in the North Pacific) because the wind-driven currents move salty water from the tropics to the high latitudes more efficiently in the Atlantic, and suggested the asymmetry in freshwater flux is ultimately due to the Rockies and Tibet. Alternatively Stocker and Wright (1991) argue the asymmetry in the freshwater flux into the North Atlantic and Pacific is due to freshwater export from the Atlantic to the Pacific, while Emil-Geay et al (2003) argue it is due to the large water vapor export from the Asian Monsoon into the N. Pacific. Finally, recent studies suggest that deep water formation should form in the Atlantic can arise simply because of the differences in the widths of the Atlantic and Pacific basins (Ferreira et al 2010). I will discuss recent studies that inform on each of these possibilities, and discuss a possible way forward for further progress.
Eastern tropical Pacific SSTs and the salinity contrast across the Isthmus of Panamá: If the Isthmus has been there for a long time, what climate change led to the salinity contrast?
Peter Molnar, University of Colorado, Boulder
(w/ Alberto M. Mestas-Nuñez)
Much paleoceanographic evidence suggests that before ~4 Ma, the salinity in the eastern Pacific differed little from that in the western Caribbean. At present, however, the contrast across Central America is 1-2, with the Caribbean the more saline. The standard explanation is that the Isthmus of Panamá emerged at ~3 Ma, but recent geologic work in Panamá suggests that the isthmus has been in place since 10 Ma, if not since 20 Ma. Moisture transport across Central America declines during El Niño events, if with a large seasonal difference. Given the wealth of paleoceanographic evidence for a warmer eastern Tropical Pacific at 4 Ma than today, as well as resemblances of pre-Ice Age climates to teleconnections associated with El Niño events, we argue by analogy with present conditions that the warm eastern Tropical Pacific at 4 Ma suppressed moisture transport from the Caribbean to the Pacific. Regressions of moisture transport against El Niño indices permit, but do not require, blockage of the Central American Seaway as the explanation for the change in salinity contrast across Central America.
Highlights of the WAIS Divide Core
Jeff Severinghaus, Scripps Institution of Oceanography, University of California, San Diego
Role of Seasonal Transitions and Westerly Jets in East Asian Paleoclimate
John Chiang, University of California, Berkeley
(w/ Inez Y. Fung, Chi-Hua Wu, Yanjun Cai, Jacob P. Edman, Yuwei Liu, Jesse A. Day, Tripti Bhattacharya, Yugarshi Mondal, and Clothilde A. Labrousse)
The summer rainfall climate of East Asia underwent large and abrupt changes during past climates, in response to precessional forcing, glacial-interglacial cycles as well as abrupt changes to the North Atlantic during the last glacial. However, current interpretations of said changes are typically formulated in terms of modulation of summer monsoon intensity, and do not account for the known complexity in the seasonal evolution of East Asian rainfall, which exhibits sharp transition from the Spring regime to the Meiyu, and then again from the Meiyu to the Summer regime. We explore the interpretation that East Asian rainfall climate undergoes a modulation of its seasonality during said paleoclimate changes. Following previous suggestions we focus on role of the westerly jet over Asia, namely that its latitude relative to Tibet is critical in determining the stepwise transitions in East Asian rainfall seasons. In support of this linkage, we show from observational data that the interannual co-variation of June (July-August) rainfall and upper tropospheric zonal winds show properties consistent with an altered timing of the transition to the Meiyu (Summer), and with more northward-shifted westerlies for earlier transitions. We similarly suggest that East Asian paleoclimate changes resulted from an altered timing in the northward evolution of the jet and hence the seasonal transitions, in particular the transition of the jet from south of the Plateau to the north that determines the seasonal transition from Spring rains to the Meiyu. In an extreme scenario – which we speculate the climate system tended towards during stadial (cold) phases of D/O stadials and periods of low Northern Hemisphere summer insolation – the jet does not jump north of the Plateau, essentially keeping East Asia in prolonged Spring conditions. We argue that this hypothesis provides a viable explanation for a key paleoproxy signature of D/O stadials over East Asia, namely the heavier mean δ18O of precipitation as recorded in speleothem records. The southward jet position prevents the low-level monsoonal flow – which is isotopically light – from penetrating into the interior of East Asia; as such, precipitation there will be heavier, consistent with speleothem records. This hypothesis can also explain other key evidences of East Asian paleoclimate changes, in particular the occurrence of dusty conditions during North Atlantic stadials, and the southward migration of the Holocene optimal rainfall.
Response of the equatorial Pacific to glacial-interglacial climate variations
David W. Lea, University of California, Santa Barbara
SST records and proxies developed over the last two decades have established the climate response of the tropical Pacific to glacial-interglacial variations. During glacials, the Indo-Pacific warm pool cooled by 2-3 °C relative to pre-Industrial, in agreement with coupled model runs. The primary forcing for this cooling was the reduction in radiative forcing – primarily due to lower atmospheric greenhouse gases (GHGs), but also increased ice albedo -- that occurred during glacial intervals. The magnitude and patterns of SST change cannot be explained by radiative forcing alone, however, and require additional feedbacks. First, SST changes in the eastern equatorial Pacific during the last three interglacials (MIS 5, 7 and 9) and during some glacials were larger than can be explained by climate forcing alone; enhanced equatorial warming (cooling) during warm interglacials (cold glacials) provides one potential explanation. Second, the warm pool was ~1 °C warmer during MIS 5e, the last interglacial 125 kyr BP; the influence of flooding of the Sunda Shelf during the high sea level stand is one possible hypothesis. This talk will examine the evidence for these inferences and consider how paleoclimate data and theory can inform projections of warm pool futures.
Tropical Pacific behavior during the Pleistocene ice ages
Athanasios Koutavas, City University of New York, Staten Island
In the context of Pleistocene glaciations this talk will address two questions: (1) Does the tropical Pacific undergo dynamic changes in response to orbital forcing? (2) Are tropical sea surface temperatures controlled mainly by CO2? Evidence from sediment cores will be used to argue that the answer to the first question is yes, while the answer to the second is no.
Holocene constraints on tropical Pacific dynamics
Julien Emile-Geay, University of Southern California
The Tropical Pacific exerts a large influence on climate and weather worldwide, and uncertainties in its response to external forcing are key to the spread in twenty-first climate projections. In particular, the relationship between El Nino-Southern Oscillation (ENSO), the seasonal cycle, and natural forcings remains an open question. Here we synthesize a pan- Pacific network of annually-resolved marine biocarbonates, which we compare to a comprehensive set of global climate model (GCM) simulations. Observations from discrete Holocene snapshots suggest important reductions in ENSO variability throughout the Holocene, most consistently during 3-5 ka BP. Such reductions are outside of the range predicted by GCMs under steady boundary conditions, and have no clear relationship to orbital forcing. Observations support a significant – though not exceptional – mid-Holocene increase in seasonality, as do models. No clear relationship is found between radiative forcing, seasonal amplitude, and ENSO variability, either in models or observations. The synthesis suggests that tropical Pacific climate is highly variable, that this variability is largely endogenous, and that the current generation of GCMs tends to underestimate the amplitude of such internal variations.
The role of the tropical Pacific in interannual and decadal timescale climate variability
Gerald A. Meehl, NCAR
The dynamic and mechanistic framework of the Tropospheric Biennial Oscillation (TBO), first formulated in 1987 and based on earlier work of Mark Cane and others, encompasses interannual variability of ENSO, the Asian-Australian monsoon, and what later became known as the Indian Ocean Dipole (IOD). Decadal variability of the TBO, related to why the TBO is not perfectly biennial, is connected to the Interdecadal Pacific Oscillation (IPO) that modulates interannual variability associated with the TBO. Processes in the tropical Pacific associated with the IPO, in turn, have contributed to the current hiatus of global warming, and link ocean mixing processes not only in the tropical and subtropical Pacific, but around Antarctica and in the North Atlantic as well. Decadal climate prediction with initialized climate models is shown to have some skill in predicting the IPO and the current global warming hiatus.
The Tropics in Future Climate Change
Richard Seager, Lamont-Doherty Earth Observatory, Columbia University
Back in 1997 Mark Cane published a paper that suggested that the response of the tropical Pacific Ocean to a positive radiative forcing was such that the eastern equatorial Pacific failed to warm or even cooled, an application of the "ocean dynamical thermostat" idea. Despite observational evidence of precisely this pattern of change in SST observations going back over a century, the idea was treated with by many with derision and contempt. 17 years later I revisit the idea. As of now, Nature has still failed to warm the eastern equatorial Pacific Ocean. In the same time coupled GCMs have steadfastly predicted preferential warming for the central to eastern equatorial Pacific Ocean and project that the east-west gradient will further weaken over the coming century. It ewill be argued that this disagreement reflects model tropical Pacific Oceans with SSTs that respond too strongly to direct radiative forcing and too weakly or wrongly to ocean dynamics and with too weak or wrong dynamical atmosphere-ocean coupling. For the historical period the coupled models already have tropical zonal SST asymmetries that are too weak and these asymmetries further weaken as GHGs increase the radiative forcing. These model biases have consequences. As I will show, in the case of East Africa, where wrong ocean dynamics cause chronic model error in simulating the regional climate, this might be the difference between model projections of a wetting climate and actual transition to a drier climate. The appeal is made to return to the tropical oceans as an active area of climate research. But please let's not motivate this by "model bias" (yawn) since it is the physics of tropical ocean dynamical control on regional climates across the globe that intrigues and motivates.
Twenty-First Century Tropical Pacific Climate Change: The Big Unknown
Mojib Latif, GEOMAR Helmholtz Centre for Ocean Research Kiel and University of Kiel
The Tropical Pacific Sector climate depicted some peculiar behavior during the early 21st century. A decadal cooling, unprecedented in observations/reanalysis data, was observed in the Equatorial Pacific, which contributed to the global warming hiatus, the Walker Circulation intensified, and the El Niño/Southern Oscillation was weak in amplitude and high-frequency in nature. Climate models basically failed to predict these changes. Further, 21st-century projections of Tropical Pacific climate largely differ from model to model, concerning both the mean state and interannual variability. Some models project a weaker zonal asymmetry in the Equatorial Pacific, others project the opposite. Some models project weaker interannual variability, others project stronger interannual variability. What do we learn from all this? What does this mean for model development? And finally, what does this mean to society?
Precipitation change projections: Dangerous ranges and fast-process diagnostics
David Neelin, University of California, Los Angeles
Projections of tropical precipitation change under global warming are known to have substantial uncertainties at the regional scale. Parameter sensitivity of the precipitation response tends to be smooth but substantially nonlinear in models for which this has been evaluated. An example of parameter sensitivity in the Community Earth System Model raises the question of whether particularly sensitive, nonlinear ranges — “dangerous ranges” —can contribute disproportionately to uncertainty. In this example, a set of fast-process diagnostics of relationships among variables at the fast time scales relevant to parameterized deep convection provide evidence for eliminating the dangerous range, hence a contribution to reduction in uncertainty. This raises the question of how extensively such a procedure could be replicated.
Narrowing the uncertainty of regional climate projection: A new challenge for coupled ocean-atmosphere dynamics
Shang-Ping Xie, Scripps Institution of Oceanography, University of California, San Diego
Global mean temperature has risen for the past century and is projected to rise even more in response to the increasing atmospheric CO2 concentrations. Precipitation change is of vital importance to societies but precipitation projections are intrinsically challenging as they change sign from one region to another. Recent studies show that in the tropics, radiatively forced changes in precipitation and atmospheric circulation are tightly coupled to spatial patterns of ocean surface warming. Mechanisms identified for SST pattern formation include those important in natural variability such as Bjerknes and WES feedbacks but there are also mechanisms unique to climate change such as the dynamic thermostat due to the equatorial upwelling. A better understanding of ocean-atmosphere coupling holds the key to building confidence in regional climate projections, much as it is to developing seasonal prediction in the case of ENSO. The talk examines the interactions between radiatively forced changes in the ocean and atmosphere, and discusses the implications for regional climate change.
Regimes and Climate Change
Tim Palmer, University of Oxford
Mark and I were brought together on many occasions by the workings of the World Climate Research Programme. At these WCRP meetings we could share our love of dynamics to better understand SI predictions. However, dynamical thinking played a much smaller role in WCRP's work on climate change - to the frustration of a number of us. My own work looking at how the Lorenz 63 model responds to external forcing was born out of a frustration with the perceived wisdom in climate change circles that dynamics was merely responsible for internal-variability noise that could be averaged out with a sufficiently long time filter. This led to a study of weather regime structures in the atmosphere, and the ability of models to simulate such regimes, some of which continues to the present day.
The recent hiatus in global warming has led some to reevaluate the role of dynamics in determining global warming. Here we consider the following questions. Could the hiatus be considered a climate regime? And, if so, is it possible that the hiatus regime could signal a negative dynamical feedback on climate? We attempt to answer these questions using the iconic Zebiak-Cane model.
On the Importance of Asking the Right Question
George Philander, Princeton University
What causes the Ice Ages? Milankovitch’s seminal answer, on the basis of scant observations, regards glaciers as isolated phenomena that wax and wane in response to local variations in sunlight. The data now available provide a different perspective -- glaciers are members of a suite of different phenomena in different parts of the globe, all interacting -- and raise the question: how does the climate respond to global Milankovitch forcing? That forcing has three main components that induce three separate “Ice Age” cycles, each with distinctive properties. Their superposition changes continually and affords tests for theories, tests for resolving disputes about the factors that determine ITCZ movements for example, and for improving climate models.
Perspectives on the role of the ocean in transient climate change.
John Marshall, Massachusetts Institute of Technology
The role of the ocean in setting the patterns and timescale of the transient response of the climate to anthropogenic greenhouse gas forcing is discussed. A simple framework for thinking about the problem is proposed in which the anthropogenic ocean temperature signal is forced from the surface by anomalous downwelling heat fluxes and damped at a rate controlled by a ‘climate feedback’ parameter. As the century proceeds, the framework suggests that the ocean will act to delay warming signals in the Southern Ocean and in the northern North Atlantic, but accelerate warming in the Arctic.
Tropical Cyclones and Global Climate Models: Entering a Golden Age of Research
Isaac Held, GFDL/NOAA
In the 1970s-1990s improving simulations of the macroturbulence of the midlatitude troposphere in global models gradually provided a framework, through a combination of comprehensive and more idealized modeling, for studying the dynamics underlying storm statistics. A similar transition towards simulation with global climate models playing a central role in research is occurring today regarding tropical cyclones. As in the extratropical case, some parts of the problem will be more recalcitrant than others, and research will continue for decades. But I will provide a few examples to argue that the time is ripe for rapid advances in our understanding of tropical cyclone statistics through global climate modeling, once again combining comprehensive and idealized simulations.