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Updated 12 October, 2003
OBSERVING , MONITORING , AND DOCUMENTING CLIMATE VARIABILITY AND CHANGE
Climatic Conditions over the United States:
Twelve initial sites of the U.S. Climate Reference Network (CRN) have been installed. As nationwide deployment is completed, this network of surface-based climate stations will provide long-term, benchmark high-quality reference observations of numerous variables, including temperature and precipitation, from 250 geographic regions.
Subsurface Ocean Observations:
In support of the international Argo collaboration to establish an eventual global network of drifting floats equipped with sensors for measuring the salinity and temperature of ocean water, the United States has (as of August 2002) contributed 185 of the 535 floats that are now operational. A total of 3,000 floats are planned, with 1,000 to be deployed by the United States.
A new sensor orbiting the Earth aboard the EOS-Terra satellite is now collecting the most detailed measurements ever made of sea-surface temperature and more than 40 other meteorological, biological, and hydrological parameters. Whereas ship measurements are made at only a limited number of points each day, the Moderate-Resolution Imaging Spectroradiometer (MODIS) is making measurements every day all over the globe. Comparison with surface measurements collected from ships and buoys indicates that the MODIS sensor measures sea-surface temperature to within about 0.25°C -- better than twice the accuracy of previous satellite observations, thus permitting earlier and more detailed detection of climate signals, for example precursors used in predicting El Niño events. Measurements of ocean temperatures and chlorophyll concentrations are revealing more detail and variability than previously available, while the MODIS chlorophyll fluorescence product adds a whole new dimension to studies of the marine and coastal ecosystems. With this new capability, scientists can observe ocean biology and ocean circulation together and obtain unprecedented views of the relationship between the physical and biological state of the ocean.
3.1. 1997-98 El Niño event, water temperature and
sea-surface topography in the equatorial Pacific Ocean
Monitoring Global Ocean Circulation:
The joint U.S./French oceanography satellite, Jason, was launched into orbit in December 2001. Jason joins the TOPEX/Poseidon satellite, expanding the set of observations of the global interactions occurring between the oceans and the atmosphere. Instruments on the satellite are mapping variations in the height of the ocean surface, which provides a means of monitoring the global ocean currents and ocean heat storage. These data are used routinely for initializing ocean circulation models for climate prediction. The decadal trend calculated from ocean height observations can be used to characterize global change in sea level.
Atmospheric Radiation Measurement:
Through improvements in measurement techniques and related climate model radiation codes, the Atmospheric Radiation Measurement (ARM) program has improved the agreement between measured and modeled instantaneous clear sky infrared fluxes from 20 Watts/m2 to 5 Watts/m2. The inclusion of the advanced radiation transfer code into climate models has resulted in extending the forecast period by 7 percent and reducing the computation time required to produce the forecasts.
PREDICTING AND SIMULATING SEASONAL TO INTERANNUAL CLIMATE VARIABILITY
Prediction of El Niño and La Niña Events:
Research on understanding the physical mechanisms and predictability of the climate has led to a new hypothesis and demonstration highlighting the role of variability on timescales of less than a season in determining the variations of the El Niño/Southern Oscillation (ENSO). Coupled model predictions that consider such episodic intra-seasonal variations have demonstrated significant improvements in making 3-6 month predictions of the seasonal variability of ENSO conditions. The results suggest that advancement in coupled model predictions of ENSO may be linked to consideration of intra-seasonal variations.
Skillful Simulation of Climate Fluctuations:
Analysis of an ensemble of Atmospheric Model Intercomparison Project (AMIP) simulations was carried out to determine the theoretical limit of predictability of the very disruptive 1988 drought and 1993 floods in the central U.S. The results indicated that model predictions of summertime conditions could become more skillful if antecedent soil moisture anomalies (i.e., extreme dry or wet conditions) that were associated with the strong ENSO event during the previous winter and spring were incorporated into the models.
Consequently, the ability to observe and successfully consider these anomalies in climate models is identified as another area for attention.
Improving Precipitation Forecasts:
An improved method for predicting seasonal precipitation was developed. By making maximum use of the sea-surface temperature information gathered from the world oceans, the factors that can contribute to improving predictability were identified. Preliminary tests suggest that overall predictive skill could increase by 10-20 percent, with most of the gain in the spring and summer, a time when predictability is traditionally at its lowest.
El Niño-Induced Disease Outbreak:
In a study in Peru, causal links between El Niño and bartonellosis (a deadly tropical disease caused by bites of sand flies) were identified, enabling demonstration of the possibility of predicting disease outbreaks based on predicted and observed changes in sea-surface temperature in the tropical Pacific Ocean. If this linkage is verified, predictions of bartonellosis outbreaks could be developed to enable the public health sector to take preventive measures. Development of health early warning systems may be facilitated through outreach activities and the use of routine forecasts by the International Research Institute for Climate Prediction (IRI).
Figure 3.2 Changes in cold-season extreme precipitation and mean annual snowpack based on ensemble regional climate simulations of current and mid-21st century climate conditions
UNDERSTANDINGAND MODELING CLIMATE PROCESSES AND PROJECTING CLIMATE CHANGE
Model Representation of Weather Systems:
Improvements in a very high resolution general circulation model that assimilates numerous satellite measurements have enabled much more accurate simulation of critical weather systems such as cyclones, fronts, and jet streams. This achievement will enable the model to take full advantage of high-resolution satellite data sets from the fleet of Earth Observing System (EOS) satellites. More accurate, high-resolution observations of surface winds (from the QuikSCAT satellite), and sea-surface temperature and precipitation (from the Tropical Rainfall Measuring Mission [TRMM] satellite) have led to increased lead times in the prediction of Atlantic hurricane intensity, track, and landfall on the southeastern and eastern U.S. coasts.
Projected Changes in Climate Extremes:
Simulations with a new hurricane model suggest that tropical cyclone intensities may increase under conditions of warming of tropical sea surface temperatures. The model projects that this warming, representative of the average projected change during the 21st century as a result of human-induced changes in atmospheric composition, results in an increase of approximately 5-10 percent in peak hurricane winds. However, considerable controversy still exists with respect to the correctness of such simulations in light of our inability to assess the veracity of these models due to the lack of consideration of the full complement of climate system changes under a warming scenario and inadequate observational data on hurricanes -- areas where continuing research is warranted.
Projected Changes in Climate Variations:
Analysis of climate model simulations indicates that a progressive warming of the tropical oceans induces major wintertime climate change over the Northern Hemisphere. The model results suggest that the warming of tropical waters, particularly in the western Pacific and Indian Oceans, is leading preferentially to an increase in one sign, or phase, of the North Atlantic Oscillation (NAO), a major pattern of climate variability.The trend toward this single phase of the NAO has resulted in the observed warming over much of the Eurasian continent, causing wetter winters in northern Europe and Scandinavia and drier winters in southern Europe and the Middle East. Research provides evidence that this trend is linked to a concurrent warming trend of the Indian and tropical Pacific Oceans; such oceanic warming is projected in coupled climate models to be a result of anthropogenic forcing. Researchers cannot say with certainty whether this trend toward a single phase of the NAO will continue, or whether it will revert to the opposite phase as part of a low-frequency oscillation.
Indications of Abrupt Climate Change in the Past:
Comparisons of sediment derived records of drift ice in the North Atlantic Ocean with proxies of changes in solar irradiance indicate that changes in solar radiation at certain times over the last 10,000 years may have affected the rate of formation of North Atlantic deep water, and thereby the strength of the Gulf Stream. Past changes of this type have had dramatic effects on the climates of countries bordering the North Atlantic Ocean. Also, results from a highly-idealized model of the tropical ocean-atmosphere system suggest that particular alignments of the Earth's orbital parameters can induce quite rapid (even abrupt) changes in the occurrence of El Niño events, sometimes causing a locking-in of an altered climate pattern for times as long as several centuries. Thus, climate variations on shorter timescales may be inexorably linked to longer-term climate changes.
Improving Key Climatic Features in Models:
To remedy systematic shortcomings in model simulations of key climatic features in the eastern Pacific Ocean (specifically the equatorial cold tongue of sea-surface temperatures, the inter-tropical convergence zone, and the extensive stratocumulus cloud decks off the west coast of South America), the Eastern Pacific Investigations of Climate (EPIC-2001) field campaign was carried out in the fall of 2001. During the intensive, eight-week observation, measurements of key processes were made from research aircraft, ships, and buoys. The data will be used to improve model representations of relevant processes in ways that are expected to improve the accuracy of predictions of anomalously wet and dry conditions over the Americas, as well as El Niño events.
Global Climate Model Development:
A major upgrade of the Community Climate System Model (CCSM) was completed. The upgrade incorporates new state-of-the-art ice, ocean, and land model components, numerous improvements in treatments of important atmospheric phenomena, as well as many algorithmic changes. To date, a 1,000-year simulation with the fully coupled CCSM-2 model has been completed. This simulation demonstrated critical new abilities to represent small-scale land, ocean, sea ice, and river runoff processes. A comprehensive suite of diagnostics describing how the new version addressed key uncertainties is forthcoming.