The problems that atmospheric and oceanic scientists are currently addressing concern a variety of phenomena. They range from hurricanes and other weather phenomena that develop over the course of a few days, to El Niño conditions that appear in the tropical Pacific Ocean over weeks and months, to the fate, over the next several decades and even centuries, of the carbon dioxide we emit into the atmosphere because of our industrial and agricultural activities. The research to address these problems can be highly interdisciplinary, requiring familiarity with mathematics, physics, chemistry, biology. Students in AOS are strongly encouraged to acquire such a broad background. However, the focus is on the basic tools that everyone uses, namely mathematical models that describe atmospheric and oceanic conditions.
      In principle, the behavior of the atmosphere and ocean is described by a set of nonlinear differential equations governing composition, temperature and motion of the air and seawater. The full governing equations are vastly too complicated to solve in any direct way. Thus a key step in the formation of a research project is determining the degree of simplification that may be appropriate. The approach adopted for a particular problem might be a treatment of an extremely idealized situation in which solutions to the equations can be obtained analytically or by some mixture of analytic techniques.
      At the other extreme, some problems are best attacked with numerical simulation models. These are attempts at fairly comprehensive treatment of realistic situations. For example, models of the global atmospheric circulation may involve taking the differential numerical approximations. A typical model might be discretized on a grid with 2 km spacing in the vertical and 300 km spacing in the horizontal. The equations are then numerically integrated forward in the with a time step of a few minutes. The result can be archived to represent a kind of synthetic "history" of the atmospheric flow in the model. This history can then be analyzed at various levels of sophistication, such as comparing the long-term mean "climate"" simulated by the model with comparable observations. Another possible analysis may elucidate detailed mechanisms involved in the model, such as an investigation into the role of latent heat release in producing atmospheric motions of various scales.
      Scientists in AOS are adopting this approach - using a suite of models of varying degrees of complexity and realism -- to study, not only atmospheric, but also oceanic phenomena such as currents, waves, and the dispersion of various chemicals. Some phenomena are neither strictly atmospheric, nor strictly oceanic, but involve, interactions between the two media. El Niño and La Niña are prime examples; they are being studied by means of coupled ocean-atmosphere models of varying degrees of complexity. Investigation of the Earth's carbon cycle, of exchanges of that gas between the atmosphere, the oceans and the biosphere, require coupled models of those various components of our planet.
      Unlike other branches of physical science, the phenomena of interest to meteorologists and oceanographers do not generally lend themselves to controlled laboratory experiments. However, the use of comprehensive numerical simulation models does allow a kind of "controlled experiment" to be performed. It is relatively straightforward to make some fundamental change in the simulation model, e.g., by removing all the clouds. The model can then be rerun and the new results compared with those from the standard run.
      Such experiments may be useful for gaining an understanding of the detailed physics of the atmospheric and oceanic circulation. This approach is also adopted when making predictions of the response of the climate to changes in atmospheric composition, for example, running the model with substantially increased carbon dioxide in the atmosphere.
      While much student research involves theoretical models of various sorts, another important aspect of research in the Program is focused directly on observations. It is fortunate that daily observations of wind, temperature, and humidity are taken on a routine basis throughout the world for operational weather forecasting purposes. These observations are archived for later use by meteorological researchers. Many research projects in the AOS Program have made use of these "historical" observations. In addition, researchers participate in special field programs organized by meteorologists to obtain observations at dense networks of stations for relatively short periods, and also participate in oceanographic expeditions to obtain measurements at sea. Collaborations with paleo-climatologists can involve trips to Antarctica to obtain data.

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Areas of Study
Climate Dynamics
The goals of climate dynamics research are to identify the physical and dynamical mechanisms which maintain climate and/or cause its variations, to discover and understand the predictability of the climate system on seasonal and longer time scales, and to evaluate the impact of human activity on the earth's climate. Important tools for climate dynamics research include a hierarchy of models for investigating the circulations of the atmosphere and ocean, and their interactions with the land. A wide range of theoretical and diagnostic methods also provide powerful insights into the climate system.

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Atmospheric Physics
Research in atmospheric physics is focused on understanding the complex nature of the interactions between various processes in the atmosphere (e.g., radiation transfer, convection and hydrologic cycle), and investigating the manner in which these determine climate on a range of space and time scales. Natural and human-induced perturbations e.g., greenhouse gases and aerosols, result in perturbations to the atmosphere leading to a forcing of climate change. The research, guided by fundamental physical principles, is conducted using a hierarchy of numerical models of varying sophistication, while the physical interpretations are aided by the use of observations.

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Chemical Oceanography
GFDL scientists are making use of certain chemical species, such as chlorofluorocarbons and radiocarbon, as tracers of the ocean circulation. They can assess the ocean's potential to take up heat from the atmosphere, an important part of the greenhouse warming problem, by studying tracer distributions in the ocean.

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Ocean Circulation
The Ocean is the 'slow' component of the climate system, and may play a controlling role in climate variability on timescales from weeks to millennia. Research in physical oceanography thus concerns both the fundamental dynamics of the ocean circulation, and its role in the climate system as a whole. Particular topics of interest include the role of eddies and turbulence in ocean circulation, interactions between the tropics and extratropics, ocean-atmosphere interactions, the dynamics El Niño, and the role of the ocean circulation in carbon cycling. Scientists and students utilize state-of-the art computer models and sophisticated mathematical models in their research.

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Water Cycle
Water couples the Earth's physical, chemical and biological systems and plays a key role in determining the flow of energy within the climate system. Evaporation and precipitation provide important sources of latent heat exchange between the atmosphere and ocean, while the presence of water vapor, clouds, and snow/ice regulates the transfer of radiative energy. Through a combination of observational and modeling analyses, our research strives to develop a greater understanding of the feedbacks within the water cycle that influence climate change and to assess the potential impacts of human activities on the water cycle.

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Storm Dynamics
Storm fronts, cyclones, and hurricanes are the building blocks of the daily weather. Recent progress in limited area modeling has been instrumental in improving short-range prediction skills. The circulation of fronts, the non-linear evolution of cyclones and the formation of hurricanes have been topics of intense investigation. Further research is underway to understand why some environments can spawn very destructive storms while others allow only weak systems to develop. Another important focus of current research is the process by which storms interact with atmospheric flow at smaller scales (e.g., individual convective cells) and larger scales (e.g., planetary waves).

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Atmospheric Circulation
The atmospheric circulation fluctuates over a broad spectrum of space and time scales, and involves a great variety of phenomena that range from those associated with daily changes in weather, to the seasonal cycle, the interannual variability of El Niño and La Niña, and longer-term fluctuations such as the North Atlantic Oscillation. To investigate the different facets of this variability, scientists at GFDL and in AOS are using a two-pronged approach: analyses of observational data sets; and experiments using numerical models that simulate the atmospheric circulation. Comprehensive daily records of atmospheric conditions, including the winds, in three-dimensions, constitute a firm basis for investigating various types of atmospheric behavior, for delineating the relationships between the various phenomena, and for evaluating their respective contributions to the time-averaged climate. Calculations with numerical models can serve to determine the effects of changes in the land and ocean surfaces beneath the atmosphere, and can also shed light on processes such as the heating and cooling associated with convection and radiative transfer.

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Atmospheric Chemistry
In research on the chemistry of both the natural and polluted atmosphere, powerful computer models of the atmospheric circulation are being used as the transport component of models of the global chemistry. These models are applied to the study of global biogeochemical cycles of nitrogen, carbon, and sulfur. In addition transport models are used in investigations of environmental issues ranging from acid deposition to changes in atmospheric levels of ozone and other greenhouse gases.

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Carbon Cycle
Only about half of the carbon dioxide we emit into the atmosphere remains there; the rest is either absorbed by the oceans or terrestrial ecosystem. Efforts to quantify uptake by each component, and in particular their regional variations, have been underway for several years using available atmospheric and oceanic carbon measurements. Changes in ocean biology induced by climate change are being modeled. The long-term fate of carbon dioxide in the atmosphere-ocean system is also being investigated. While it is uncertain whether future emissions of greenhouse gases can be sufficiently reduced to prevent climatic change, the efficacy of various technological approaches for the removal of some carbon dioxide from the atmosphere are being explored.

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Paleo-climate
Some sixty million years ago the poles were so warm that palm trees and crocodiles flourished there. Subsequently the Earth experienced gradual global cooling, presumably a consequence of the gradual drifting of the continents, that led to the appearance of continental glaciers in high northern latitudes about 3 million years ago. Since that time, the Earth's response to modest, periodic changes in the distribution of sunlight - because of variations in orbital parameters known as Milankovich cycles - amplified significantly. For the past million years, our planet has experienced dramatic recurrent Ice Ages. Paleo-climatologists in the department of geosciences establish detailed records of these climate fluctuations by analyzing ice-core records from Antarctica. Joint projects with AOS faculty members have as their goal explanations for the fascinatingly different climates of the past.

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Areas of Focus
Predicting El Niño
      El Niño was weak and persistent in 1992; very intense and brief in 1997. Forecasts on both occasions were poor. Scientists and students in the program are developing a hierarchy of coupled ocean-atmosphere models to explain, simulate and predict El Niño. Their results indicate that the Southern Oscillation between El Niño and its complement, La Niña, is analogous to a damped pendulum that is sustained by modest, random blows. The effect of a blow - for example, a burst of westerly winds that persists for a few weeks along the equator near the dateline - depends very much on the phase of the oscillation. At the right time it can amplify the swing significantly - that happened in 1997.
      Given that developments in the tropical Pacific depend on "noise" such as westerly wind bursts, how can El Niño be predicted? A recent graduate of the program, Andrew Wittenberg, tackled this problem by exploring probabilistic forecasts of the type shown in the figure. He used a model that simulates the Southern Oscillation to determine how it responds to different realizations of the observed atmospheric "noise". The results indicate how the risk of an El Niño changes with time.

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Hurricanes
      Hurricanes are powerful and destructive tropical storms that can devastate coastal regions and cause billions of dollars in damage. With this sort of destruction at stake, it is essential to be able to accurately predict the movement and intensity of individual storms so that proper preparation measures can be taken.
      A challenging goal in hurricane forecasting is accurate forecasts of intensity. Although there has been a steady decline in operational forecast track errors in the past 20 years, there has been little improvement in intensity prediction. Part of the problem in both statistically and dynamically based forecast methods, is the failure to take into account interactions between a hurricane and the ocean beneath it.
      At AOS, scientists are developing an improved version of the GFDL Hurricane Prediction System in which the model is coupled with a high-resolution version of the Princeton Ocean Model (POM). Run in parallel with the operational GFDL model during past hurricane seasons, this new model has shown great improvements in the intensity prediction of hurricanes.

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Clouds
      Clouds and their impact on climate are key uncertainties in estimating the effect of increased greenhouse gases on earth's climate. To tackle this challenging problem, scientists at AOS and GFDL are studying the complex interplay of clouds, radiation, and climate in relation to global climate change.
      Weather and climate modeling involves processes covering an extremely wide range of space and time scales, which makes it a particularly challenging research problem. Recently, a new class of models have been developed that, compared to older models, can cover a smaller area of the earth with a much higher concentration of gridpoints.
      These new models will be useful for examining the impact of smaller-scale features, such as clouds, on climate sensitivity.

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Links
Following are links to current projects that maintain websites:

GLobal Ocean Data Analysis Project (GLODAP)

Princeton Regional Ocean Forecast System (PROFS)

The Princeton Ocean Model (POM)