Assessing the Oceanic Water Cycle with an Integrative Approach

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ABSTRACTOCE-0647949Though many programs purport to study the global water cycle, none have properly addressed its largest component, the oceans. The oceans are by far the largest reservoir of water on Earth, the source of most evaporation and the sink of most precipitation. Trends in ocean salinities over the past 40 years provide indications of a changing global water cycle. However, quantitative assessment of the water cycle over the vast oceanic areas remains challenging because of the scarcity of data. In this study, a support is requested to utilize data from new ocean flux climatologies, river flow data, growing oceanic salinity monitoring networks and an assimilative global ocean circulation model to examine the mean state, seasonal cycle, trends and variability in the water cycle over the global ocean.Intellectual impact: It is generally assumed that global warming will enhance the water cycle, due to the greater vapor carrying capacity of warmer air. The vapor pressure of water is about 15mb at the mean global temperature of 14oC and increases about 1mb/oC at this temperature. Thus, a 1oC temperature rise may enhance the water cycle by ~ 7%, assuming transport by the wind is not changed dramatically. This is about 10cm/year for an evaporation minus precipitation (E-P) difference of 1.5m/yr. This is the sort of evaporation increase inferred from salinity trends in the subtropical Atlantic over the past 40 years. The suggested integrative approach combines new flux data and a data assimilating ocean circulation model to address the following four general issues regarding the oceanic water cycle:(1) What are the mean and seasonal patterns of net water exchange between atmosphere, ocean and land? What meridional fluxes and inter-basin transports of freshwater are implied for the oceans? (2) What are the consequences of river water discharges vs. rainfall for upper ocean thermohaline structure? What changes in upper-ocean mixing result from evaporative or precipitative forcing and how does upper ocean mixing affect the oceans ability to absorb, store, transport and release heat and freshwater? What pathways and timescales are found for salinity anomalies generated by surface flux changes? How do these feedback on climate?(3) What are the seasonal and interannual variations in the land and ocean storage of freshwater and how do these affect the upper-ocean salinity distribution? Are changes in evaporation and rainfall due to variations in winds patterns, SST, or atmospheric humidity?(4) Which regions of the ocean show the most sensitive and interesting response to freshwater forcing? Which regions would be best for an ocean/atmosphere process study during the upcoming Aquarius salinity satellite mission (2009 launch)?Broader impacts: Advancing our understanding of the water cycle will be of great benefit to society, as anticipation of trends in water supplies is fundamental for planning allocation of water resources. The ocean is key to this understanding; the addition of only one percent of Atlantic rainfall would double the discharge of the Mississippi river. This project will develop ties between oceanographers, surface flux experts and modelers to study the ocean-atmosphere-land exchanges of freshwater for the global ocean. Any knowledge gained will improve our understanding of the water cycle, and its effects on the ability of the ocean to absorb, store and transport heat. Thus, an improved understanding of this key component of the climate system will be achieved, thereby leading to greater chances of predicting future changes. Finally, the training of one graduate student in this important area will be supported, and a web site on the oceanic water cycle and help identify likely sites for salinity constrained ocean-atmosphere exchange process studies will be developed.This Project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) Program.