As articulated in the report of the US CLIVAR Salinity Working Group (2008), no part of the climate system is as important to society as the global hydrological cycle; yet we lack key understanding of its major element, the ocean. The ocean is the source of nearly all terrestrial precipitation, and the ocean itself is the recipient of most precipitation. Significant shifts in rainfall patterns associated with climate change will likely cause disruptions in agriculture and freshwater supplies for civilization. Ocean general circulation models used to examine such changes often require artificial salinity restoring terms to maintain stable solutions. Thus, it is of great importance to improve our abilities to monitor, understand, and model the water cycle over the oceans (See March, 2008 issue of Oceanography magazine; the table of contents is appended to the reference list). As upper ocean salinity (UOS) is an important variable that indicates the intensity of water exchange between ocean and atmosphere and has direct impact on the ocean’s mass distribution, mixing rates, and associated interior circulation, improved observation systems for salinity and better understanding of the processes that control it are needed for progress in understanding the oceanic water cycle. In this document we propose a suite of field activities designed to improve our understanding of the physical mechanisms controlling upper ocean salinity. The planned work coincides with the US-Argentine Aquarius/SAC-D and European SMOS surface salinity satellite missions (planned for launch in 2010 and 2009 respectively) as well as the continuing, global array of ARGO profiling floats, all of which provide excellent new tools to monitor ocean salinity.
SPURS-1: A Subtropical Field and Modeling Program
In order to leverage other observational resources associated with the Atlantic Meridional Overturning Circulation program (AMOC Planning team, 2007), we propose that the initial focus of an observational and modeling effort be directed towards the strong sea surface salinity maximum of the subtropical North Atlantic that is subducted into the thermocline and forms the lower limb of the subtropical/tropical shallow overturning circulation (ShallMOC). This region approximately includes the latitudes of 15°- 30°N and 30°-50°W. We call this program Salinity Processes in the Upper-Ocean Regional Study (SPURS), with the overarching goal of assessing the relative importance of all six terms in the eq. 1.
Mesoscale: The highest resolution is within two 200 km boxes, one centered on the salinity maximum in the N. Atlantic (the outcropping region) and one centered approximately 10° of latitude to the south of the maximum (the edge of the SSS > 37 region that marks the high salinity bowl), where the surface salinity gradients are large and E−P is a maximum. Within the mesoscale box observations will resolve horizontal scales from <10 km to the full 200 km scope of the box. The observational program may include:
• Ship-based underway thermosalinograph and undulating profiler measurements; ADCP, XBT/XCTD transects; AUV work; ASIP; PAL; and microstructure profilers.
• Ship deployment of gliders, surface drifters, EM-APEX. A possible approach is a yearlong glider survey that connects the 2 mesoscale boxes, carried out by ~8 gliders. The surface drifters can be enhanced with ADCP and wind sensors, to determine the surface structure.
• The use of profiling floats (30 km resolution in each box) and gliders, some equipped with microstructure and/or EM sensors, to determine the near surface and subsurface structure.
• Flux buoys (one located south of the S-max, where the biggest differences in surface flux estimates is found, and acoustic wind observations from floats and gliders.
• Estimates of eddy fluxes from a central flux mooring in each box. In the northern box, this requires a new flux mooring; in the southern box, the use of a Pirata mooring already in place can be used (enhancements to its sensor suite might be required)
Small-scale: (<10 km), < 1 day. Here the effort would be dedicated to estimation of the microscale dissipation parameters ε and χ in as many places as possible using enhanced gliders, profiling floats, and shipboard measurements. The focus would be on sub-mesoscale and internal/inertial-wave induced mixing and processes such as salt fingers. The surface skin can be studied with upward-going profilers. The data will be used to make estimates of turbulent mixing at scales from several hundred kilometers down to the dissipation scale of millimeters.More