The Global Water Cycle
USGCRP Fiscal Years 2004-2005 Accomplishments

USGCRP
Program Elements

Atmospheric Composition

Ecosystems

Global Carbon Cycle

Decision-Support Resources Development and Related Research on Human Contributions and Responses

Climate Variability and Change

Global
Water Cycle

Variations of U.S. Drought Occurrence Related to Fluctuations in Sea Surface Temperatures in the North Atlantic [11]

Researchers analyzed precipitation data to identify spatial and temporal variation of drought occurrence in the conterminous United States during the 20th century. They found that these variations were largely (74%) explained by multi-decadal fluctuations in sea surface temperatures in the North Atlantic and North Pacific Oceans and by the long-term trend in Northern Hemisphere temperatures. The results suggest that persistence of the present warm conditions in the North Atlantic into the next decade may lead to continuation of the present western drought pattern, which is similar to that of the 1950s, or to development of a drought pattern similar to that of the 1930s.

North American Climate: Water Cycle and the Pacific Decadal Oscillation [2]

Two main characteristics distinguish the Pacific Decadal Oscillation (PDO) from the El Niño Southern Oscillation (ENSO). First, 20th century PDO "events" persisted for 20 to 30 years, while typical ENSO events persisted for 6 to 18 months. Second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics – but the opposite is true for ENSO. Major changes in northeast Pacific marine ecosystems have been correlated with phase changes in the PDO. Even in the absence of a full theoretical understanding of the PDO, information about its phase can improve season-to-season and year-to-year climate forecasts for North America because of its strong tendency for multi-season and multi-year persistence. For the past several years, North Pacific sea surface temperature variations have not consistently correlated with either the warm or cool phases of the PDO pattern (see top panel of Figure 9). The PDO index has been highly variable. The 1900 to 2004 time series of the PDO index is shown in the bottom panel of Figure 9. Monthly updates of the PDO index are available online.

Observed and Projected Changes in U.S. Snowpack and Runoff [3, 4, 6, 7, 9, 14, 15]

Observations from the past 5 decades indicate that the spring snowpack in the Pacific Northwest has declined significantly since the mid-20th century. At most of the mountain locations studied, declines in snow water equivalent (the depth of liquid water from the melted snow) coincide with significant increases in temperature, and have occurred despite increases in precipitation. The largest decreases have occurred at the lowest elevations, suggesting that moderate warming throughout the region may have raised the elevation of freezing level. These snowpack declines have been accompanied by an earlier annual peak in river runoff. Researchers have found that, across several hundred stream gages in mountainous western North America from New Mexico to Alaska, as well as New England, the snowmelt runoff season has come earlier in recent decades, with average timing shifts (by several measures) of 1 to 2 weeks earlier. These long-term timing shifts have been most strongly related to changes in seasonal temperatures during the snowmelt runoff season, and have not been significantly related to winter and spring precipitation. These results have significant implications for water resource managers since snowmelt provides much of the water used during summer for irrigation, energy production, municipal and industrial water supply, fish and river ecosystem protection, recreation, and other uses.

Additional studies have addressed possible implications of potential future greenhouse gas-induced climate change on the mountain snowpack and runoff in the western United States. These studies used the Intergovernmental Panel on Climate Change (IPCC) business-as-usual greenhouse gas emission scenario to drive multiple simulations of the Parallel Climate Model (PCM). In one study, annual snowpack was projected to be 20 to 70% smaller by the middle of the 21 st century when the PCM output was downscaled (i.e., translated from coarse to fine resolution) with the MM5 regional climate model. Somewhat smaller snowpack reductions were found when the downscaling was performed using the Regional Spectral Model. Using the same PCM simulations, other studies suggest that reservoir levels may decline by approximately one-third in the Colorado River Basin by the end of the 21 st century and that streamflow timing in most western North American streams might be roughly a full month earlier. However, there is significant uncertainty associated with downscaling climate projections to fine resolutions. Moreover, different results may be obtained if other global climate models or forcing scenarios are used. For example, the PCM projects less warming from greenhouse gas increases than most other climate models.

Projections of Global-Scale Runoff and Soil Moisture Changes [10]

Scientists have also examined the potential for significant global-scale changes in river discharges and soil wetness over the rest of the 21 st century due to greenhouse warming. Their analysis of model projections, based on a typical greenhouse gas emission scenario, indicates that the discharges from Arctic rivers such as the Mackenzie may increase by up to 20% (of the pre-Industrial Period level) by the middle of the 21 st century and by up to 40% or more in a few centuries. In the tropics, the discharges from the Amazon and Ganga-Brahmaputra rivers are also projected to increase substantially. However, the projected changes in runoff from other tropical and many mid-latitude rivers are smaller (on a percentage basis), with both positive and negative signs. For soil moisture, the results of this study indicate reductions during much of the year in many semi-arid regions of the world, such as southwestern North America, northeastern China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. The projected reduction is particularly large during the dry season (on a percentage basis). From middle to high latitudes of the Northern Hemisphere, this study projects soil moisture to generally decrease during the summer growing season but increase in winter.

North American Monsoon Experiment Field Campaign

Most summer precipitation around the world is driven by strong solar heating of the land surface. The consequent large summer land-ocean temperature contrasts lead to summer monsoon circulations typically characterized by a systematic reversal of wind patterns and very strong rainfall events. The North American summer monsoon is one major component of the world's monsoon systems, the others being the Asian-Australian monsoon and the African monsoon systems. These systems interact with the El Niño/ La Niña cycles, among others. The North American Monsoon Experiment (NAME) field campaign was conducted in, over, and around the southwestern United States and northern Mexico during the summer of 2004 and involved internationally coordinated research. NAME aims at determining the sources and limits of predictability of summer precipitation from the monsoon over North America. The field campaign was motivated by previous and ongoing diagnostic and modeling studies, which identified processes contributing to the variability of monsoonal circulation, convection, and precipitation. The field experiment significantly enhanced the spatial and temporal resolution of observations of those processes available for the monsoon region. The results of NAME, currently being analyzed, will be used to attempt to improve warm-season weather predictions in 2006 and beyond.

Madden-Julian Oscillation and Floods [1]

Recent studies have revealed relationships between the Madden-Julian Oscillation (MJO) and precipitation that may provide an important key to prediction of flooding over southwest Asia. The MJO (also referred to as the 30-60 day or 40-50 day oscillation) is thought to be one of the main intra-seasonal fluctuations that explain weather variations in the tropics. The MJO is associated with variations in surface and upper-level wind fields, sea surface temperature, and cloudiness/rainfall. It affects the entire tropical troposphere, but is most evident in the Indian and western Pacific oceans with an active (wet) phase and an inactive (dry) phase. The MJO is strongest in the eastern Indian Ocean, when its wind anomalies extend over southwest Asia. A 22-year record of precipitation observations over southwest Asia shows that there is a 55% increase in daily precipitation when the MJO is in its active phase. The effect of the MJO is quite consistent from year to year, with more rain attributed to the MJO as it circumnavigates the globe in each of the 22 years in the record. These findings indicate that the evolutions of storms over southwest Asia and resultant precipitation could conceivably be forecast with some skill for 3-week periods.

Glacier Mass-Balance Records Show a Retreating Trend in Alaska Glaciers [5]

CCSP-supported scientists continue to monitor long-term glacier mass balance at three benchmark glaciers in Washington and Alaska, each in a different climate regime. Winter accumulation, summer ablation, and net mass balance are measured using accepted glaciological techniques. The data collected are posted on the Internet. The mass balance records extend for 50 years at South Cascade Glacier, Washington, and 38 years at Gulkana and Wolverine Glaciers, both in Alaska. These records are among the longest in North America. The data are used to understand glacier-related hydrologic processes and to improve the quantitative prediction of water resources, glacier-related hazards, and the consequences of climate change. Consistent with the observed warming, for the past quarter century these glaciers have experienced almost continuous negative net balances, indicative of the glacier retreat observed in the glaciated regions of the Pacific Northwest and Alaska (see Figure 10).

New North American Assimilation System and Reanalysis [13]

Knowledge of the land surface states (e.g., soil moisture, snowpack, evaporation) and the ability to model these states have been major focus areas of CCSP-supported research. The North American Land Data Assimilation System project has created a 1/8-degree modeling system over the continental United States that provides this important land surface information for the purpose of initializing weather and climate models. (see an article from the Journal of Geophysical Research)

North American Regional Reanalysis [12]

A 25-year North American Regional Reanalysis (NARR) of historical climate data, covering the period October 1978 through December 2003, was completed in 2004. The regional reanalysis provides a wide range of high-resolution, daily water cycle analysis products such as precipitation, relative humidity, soil moisture, and snow data fields for the 25-year period at 32-km grid scale over North America. The NARR represents advances in regional models and data assimilation that include assimilation of precipitation, direct assimilation of radiances, additional data, and recent developments in modeling, particularly land-surface components. The NARR is a major improvement in both resolution and accuracy over previous reanalysis products.

Global Energy and Water Cycle Experiment – Coordinated Enhanced Observing Period

With support from CCSP agencies, the international Global Energy and Water Cycle Experiment (GEWEX) has coordinated modeling activities and production of new data sets to aid water cycle research. A major GEWEX initiative, the Coordinated Enhanced Observing Period (CEOP), has brought together global observations and model outputs in a consistent framework and provided them to researchers through the Internet. These integrated data sets have enabled new efforts to determine whether the water cycle is accelerating as a result of global change. CEOP is an example of the type of international coordination needed to support the Global Earth Observing System of Systems.

Coupling between Soil Moisture and Seasonal Precipitation [8]

A project supported by CCSP and performed by the GEWEX Global Land/Atmosphere System Study compared climate models to determine the influence of soil moisture on the ability of the model to predict seasonal precipitation and temperature. Although the models differed in the strength of the simulated coupling between soil moisture and precipitation, they were consistent in identifying areas around the globe where knowledge of soil moisture conditions led to enhanced seasonal climate prediction capability (see Figure 11).

International Cooperation on Integrated Global Water Cycle Observations

A CCSP-supported report, The Integrated Global Water Cycle Observations Theme was developed under the framework of the Integrated Global Observing Strategy Partnership and issued in 2004. The report provides a framework for guiding decisions on priorities and strategies for water cycle observations. It also promotes strategies to facilitate the acquisition, processing, and distribution of data products needed for effective management of the world's water resources. The report was developed following extensive international review and workshops held in the United States, Europe, and Japan, and was co-funded by CCSP and the Japan Aerospace Exploration Agency.