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In this special issue of Oceanography we explore the results of SPURS-2, the second Salinity Processes in the Upper-ocean Regional Study (SPURS), conducted in the eastern equatorial Pacific. SPURS is an ambitious multiyear field program to study surface salinity in evaporation-​dominated (SPURS-1) and precipitation-dominated (SPURS-2) regions of the global ocean. The primary goal was to further our understanding of the global oceanic freshwater cycle through investigation of the physical processes controlling the upper-ocean salinity balance: air-sea interactions, transport, and mixing. With the advent of satellites capable of measuring sea surface salinity, such as NASA’s Aquarius instrument and the Soil Moisture Active Passive (SMAP) satellite, as well as the European Space Agency’s Soil Moisture and Ocean Salinity (SMOS) platform, a near-synoptic view of such processes has become possible (Figure 1). To take full advantage of such observations, we need to understand the link between upper-ocean dynamics and the oceanic freshwater cycle.

One part of the Salinity Processes in the Upper-ocean Regional Study (SPURS) field campaign focused on understanding the physical processes affecting the evolution of upper-ocean salinity in the region of climatological maximum sea surface salinity in the subtropical North Atlantic (SPURS-1). An upper-ocean salinity budget provides a useful framework for increasing this understanding. The SPURS-1 program included a central heavily instrumented mooring for making accurate measurements of air-sea surface fluxes, as well as other moorings, Argo floats, and gliders that together formed a dense observational array. Data from this array are used to estimate terms in the upper-ocean salinity and heat budgets during the SPURS-1 campaign, with a focus on the first several months (October 2012 to February 2013) when the surface mixed layer was becoming deeper, fresher, and cooler. Specifically, we examine the salinity and temperature balances for an upper-ocean mixed layer, defined as the layer where the density is within 0.4 kg m–3 of its surface value. The gross features of the evolution of upper-ocean salinity and temperature during this fall/winter season are explained by a combination of evaporation and precipitation at the sea surface, horizontal transport of heat and salt by mixed-layer currents, and vertical entrainment of fresher, cooler fluid into the layer as it deepened. While all of these processes were important in the observed seasonal (fall) freshening at this location in the salinity-maximum region, the variability of salinity on monthly-to-intraseasonal time scales resulted primarily from horizontal advection.

Author Posting. © Oceanography Society, 2008. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 21, 4 (2008): 34-45.