The tropical eastern pacific fresh pool (epfp) is characterized by some of the highest precipitation rates and the lowest sea surface salinities found in the open ocean; IT also has one of the…

This federal contract award is a $289,443.64 grant to the Woods Hole Oceanographic Institution, a private, non-profit research organization, from the National Aeronautics and Space Administration (NASA). The grant is for research to test and improve hypotheses about the drivers of sea surface salinity variability in the Tropical Eastern Pacific Fresh Pool, leveraging new satellite salinity data. The research will involve field observations collected during the Salinity Processes in the Upper ocean Regional Study (SPURS-2) program. The grant will support a postdoctoral investigator to analyze the regional salinity dynamics and shepherd the data into a permanent NASA archive. This award is not associated with a set-aside designation.

The tropical eastern pacific fresh pool (epfp) is characterized by some of the highest precipitation rates and the lowest sea surface salinities found in the open ocean; IT also has one of the strongest annual cycles of sea surface salinity. The wealth of new salinity data from the aquarius, smos and smap satellite instruments has provided new insights into the regional dynamics affecting sea surface salinity in the epfp. Recent studies enabled by satellite salinity measurements have yielded hypotheses about how precipitation, evaporation, and ocean dynamics work together to drive variability in SSS at different times and locations in and around the epfp. This proposal seeks to test and improve upon these previous satellite-driven hypotheses with field observations collected during spurs-2. One of the important hypotheses posed prior to spurs-2 is that northward ekman transport associated with the westward trade winds is persistently pushing the freshwater deposited under the (northern hemisphere) itcz northward. Thus, when the itcz is moving northward in spring, the low SSS region and the itcz precipitation move northward together, but when the itcz precipitation is moving southward in the fall, the low SSS moves northward, despite the fact that the itcz and its persistent high precipitation rates are moving southward (YU, 2015). This hypothesis is based partly on a zonally-averaged analysis, a perspective that neglects zonal gradients and zonal advection. A complicating factor in the northeast tropical pacific is that there is an appreciable zonal gradient associated with the epfp, and there are intense zonal currents (the north equatorial counter current and north equatorial current). The seasonal cycles of these currents are complicated; guimbard ET AL. (2017) hypothesize that they are a controlling factor in the seasonal cycle of the epfp. The local surface-layer salinity budget at the spurs-2 mooring site can provide important insights into the regional dynamics of the epfp at a site near the mean location of its westward edge. Preliminary analysis of satellite and in situ data at the spurs-2 central mooring site (10 n, 125 w) suggests that both of the above hypothesized mechanisms are important, but that, in addition to local freshwater fluxes, vertical mixing across the very strong and shallow main pycnocline and halocline may be an equally important factor. We will draw on all of the available spurs-2 data and modelling activities to address the question, what are the physical processes responsible for the location, magnitude, and maintenance of the low surface salinity region in the eastern tropical pacific ocean? The analysis will also involve comparison of the regional SSS dynamics in the spurs-1 and spurs-2 experiments. Most of the work will be done by a postdoctoral investigator, who will be jointly mentored by the CO-is. Another important objective is to shepherd final, quality-controlled data into a permanent NASA archive (PO.daac).