The importance to continue and enhance spaceborne salinity observing capability to study ocean-water cycle-climate linkages
Tong LeeSimon YuehDetlef StammerGary LagerloefAïda Alvera AzcarateJacqueline BoutinNicolás ReulRoberto SabiaAntonio Turiel
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In-situ observing system has provided the capability to monitor multi-decadal changes of salinity in the open ocean and on large scales. However, in-situ platforms are inadequate to monitor salinity changes in marginal seas and coastal oceans as well as salinity variations on mesoscales. Monitoring longer-term changes of salinity in these regions and scales are important to the studies of terrestrial-ocean water cycle linkage, cross-shelf exchanges, coastal-open ocean connection, energy transfer, and biogeochemistry. Satellite measurements of sea surface salinity (SSS) have demonstrated their values to enhance salinity observing capability in these regions and scales. This presentation highlights the accomplishments of satellite SSS, especially in studying salinity variations for regions and scales not well resolved by in-situ platforms. Examples will be provided to emphasize the synergy of satellite and in-situ salinity observing systems to investigate the linkage of open-ocean and marginal sea salinity in relation to longer-term changes in the climate and water cycle. Recognizing this need, the Global Climate Observing System (GCOS) Implementation Needs (Belward et al.2016) suggested Action 032: Ensure the continuity of space-based SSS measurements. Sustaining satellite SSS observing capability, enhancing spatial resolution, and improving accuracy (especially in high-latitude oceans) are important to studying the linkages of the ocean with the water cycle and climate variability.Keywords:
Water cycle
Ocean observations
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The SMOS and AquariusISAC-D satellite missions will begin a new era to map the global sea surface salinity (SSS) field and its variability from space within the next twothree years. They will provide critical data needed to study the interactions between the ocean circulation, global water cycle and climate. Key scientific issues to address are (1) mapping large expanses of the ocean where conventional SSS data do not yet exist, (2) understanding the seasonal and interannual SSS variations and the link to precipitation, evaporation and sea-ice patterns, (3) links between SSS and variations in the oceanic overturning circulation, (4) air-sea coupling processes in the tropics that influence El Nino, and (4) closing the marine freshwater budget. There is a growing body of oceanographic evidence in the form of salinity trends that portend significant changes in the hydrologic cycle. Over the past several decades, highlatitude oceans have become fresher while the subtropical oceans have become saltier. This change is slowly spreading into the subsurface ocean layers and may be affecting the strength of the ocean's therrnohaline overturning circulation. Salinity is directly linked to the ocean dynamics through the density distribution, and provides an important signature of the global water cycle. The distribution and variation of oceanic salinity is therefore attracting increasing scientific attention due to the relationship to the global water cycle and its influence on circulation, mixing, and climate processes. The oceans dominate the water cycle by providing 86% of global surface evaporation (E) and receiving 78% of global precipitation (P). Regional differences in E-P, land runoff, and the melting or freezing of ice affect the salinity of surface water. Direct observations of E-P over the ocean have large uncertainty, with discrepancies between the various state-of-the-art precipitation analyses of a factor of two or more in many regions. Quantifying the climatic influence of the oceanic water cycle requires more accurately resolving the net air-sea water flux. Measuring global SSS trends on seasonal to interannual timescales by satellite is fundamental to this problem because the SSS trends represent detectable time-integrated signals of the variable marine hydrological cycle. Satellite measurements, coupled with an array of in situ observations, will provide global synoptic SSS fields for the first time history. These data will provide a strong constraint on climate models and data assimilation efforts, which must properly represent the freshwater budget in terms of E-P, ocean advection and surface layer mixing in order to accurately simulate the true ocean state. The SSS fields will allow us to quantify the covariability between the SSS and the strong seasonal E-P cycle in the tropics and high latitudes. Field measurement campaigns to exploit satellite and in situ measurements to close the seasonal E-P cycle over an ocean region are being considered. Lastly the satellite systems will monitor and trace the large long-lived SSS anomalies from year to year that have the potential to influence El Nino and the large scale ocean circulation.
Water cycle
Ocean observations
Global Change
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