Exploring Recent Changes of Ocean Salinity Distributions in the Context of Climate Change

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Because the hydrologic cycle remains one of the least well understood aspects of the climate system, and freshwater forcing the most probable cause for differences among climate models, the proposed research will provide an important observational foundation for evaluating ongoing and future changes in Earth's climate. Over the past fifty years, ocean salinity distributions have been changing and the accumulating observational record is beginning to suggest some trends of near global extent. Various studies have individually documented salinity increases in tropical and subtropical surface waters of the Indo-Pacific, Atlantic and Mediterranean in recent decades; while over the same time period, the intermediate and deep water masses, ventilated at higher latitudes in both hemispheres, have become increasingly fresh. The proposed research will investigate whether these apparently large-scale and hemispherically symmetric shifts in ocean salinity distribution can provide evidence of a change in the planetary hydrologic cycle. It will engage in comparing the observed rates of freshwater gains and losses in the ocean to results from climate model simulations. Reflecting model predictions that the hydrologic cycle may have climatically important effects on the thermohaline circulation (THC), the study will evaluate the impacts of the observed changes in salinity and freshwater distributions on ocean density. A systematic census of ocean property content changes will be conducted for the Pacific, Atlantic, Arctic, and Nordic Seas from 30 degrees S to 90 degrees N utilizing new climatology products and tools. Volumetric integrals of salt will be used to identify first order changes in ocean freshwater transports over the instrumental record and to estimate E-P anomalies for the ventilated ocean layers. These anomalies will be compared to the latest evaporation and precipitation climatology products derived from other sources. Combining the Bergen Coupled Model with observations, the time-dependent formation and subduction rates for Salinity Maximum Waters will be investigated in conjunction with researchers at Laboratoire d' Oceanographie Dynamique et de Climatologie (LODYC) in Paris, France. The observational analyses will also be compared to results from NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) coupled ocean-atmosphere models. These comparisons will assess the degree to which the model projected changes in salinity resemble the observed changes and provide perspective on whether they represent a signal of any significance against the background variability of the climate system. Finally, the amplitude of the dramatic freshening of the water column that has occurred in the northern North Atlantic will be assessed in the context of threshold values for the THC, as determined from model simulations. Broader Impacts: The broader impacts of this study are likely to be threefold. First, it will establish collaboration among researchers from several groups, i.e., modelers and observationalists from WHOI, GFDL, and LODYC. Second, the model-data comparisons will constitute a powerful tool for calibrating climate simulations and may be a significant stimulus for further model experiments to diagnose the mechanisms of hydrologic changes. Third, the larger research community will benefit from the expansion of the HydroBase quality-controlled database and gridded climatology products which will result from this work.