Abstract
A novel assessment of recent changes in air-sea freshwater fluxes has been conducted using a surface temperature-salinity framework applied to four atmospheric reanalyses. Viewed in the T-S space of the ocean surface, the complex pattern of the longitude-latitude space mean global Precipitation minus Evaporation (PME) reduces to three distinct regions. The analysis is conducted for the period 1979-2007 for which there is most evidence for a broadening of the (atmospheric) tropical belt. All four of the reanalyses display an increase in strength of the water cycle. The range of increase is between 2%-30% over the period analysed, with an average of 14%. Considering the average across the reanalyses, the water cycle changes are dominated by changes in tropical as opposed to mid-high latitude precipitation. The increases in the water cycle strength, are consistent in sign, but larger than in a 1% greenhouse gas run of the HadGEM3 climate model. In the model a shift of the precipitation/evaporation cells to higher temperatures is more evident, due to the much stronger global warming signal. The observed changes in freshwater fluxes appear to be reflected in changes in the T-S distribution of the Global Ocean. Specifically, across the diverse range of atmospheric reanalyses considered here, there was an acceleration of the hydrological cycle during 1979-2007 which led to a broadening of the ocean's salinity distribution. Finally, although the reanalyses indicate that the warm temperature tropical precipitation dominated water cycle change, ocean observations suggest that ocean processes redistributed the freshening to lower ocean temperatures.
Highlights
Variations in the air-sea fluxes of heat and freshwater are an important influence on water mass properties and ocean circulation on multiple time and space scales
Decreasing salinity trends exceeding 20.2 pss/50 years are found in precipitation-dominated regions such as the Western Pacific Warm Pool and the North Pacific subpolar region [e.g., Durack and Wijffels, 2010; Skliris et al, 2014]. These patterns are consistent with expected surface freshwater flux changes associated with intensification of the global hydrological cycle. Such changes are predicted by climate models [Held and Soden, 2006; Durack et al, 2012] but have proved hard to confirm from observations due to the difficulty in obtaining sufficiently accurate precipitation and evaporation data sets over much of the global ocean
Changes in Precipitation minus Evaporation (PME) Mean PME fields for the four reanalyses are shown in Figures 1a, 1c, 1e, and 1g and in T-S space in panels b, d, f and h
Summary
Variations in the air-sea fluxes of heat and freshwater are an important influence on water mass properties and ocean circulation on multiple time and space scales. Decreasing salinity trends exceeding 20.2 pss/50 years are found in precipitation-dominated regions such as the Western Pacific Warm Pool and the North Pacific subpolar region [e.g., Durack and Wijffels, 2010; Skliris et al, 2014]. These patterns are consistent with expected surface freshwater flux changes associated with intensification of the global hydrological cycle. In this study we take an alternative approach of examining air-sea flux variability in temperature-salinity (T-S) space The advantage of such an approach is that it provides a more physically instructive link to water mass transformation processes and changes in the global thermohaline circulation.
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