Oceanography the Official Magazine of the Oceanography Society in the Spurs Region on Seasonal to Interannual Time Scales

Surface salinity variations and processes affecting surface salinity in the high-salinity region of the subtropical North Atlantic (the SPURS-1 area) are investigated by combining data from in situ observations and satellite remote-sensing measurements. On temporal average, the surface freshwater flux term (evaporation minus precipitation) in the SPURS-1 region increases mixed-layer salinity. Oceanic advection plays the largest role in compensating this salinity increase. On seasonal time scales, mixed-layer salinity increases from April to August and decreases from September to March. This seasonal evolution of the mixed-layer salinity is largely controlled by the freshwater flux term, with vertical entrainment playing a secondary role. The domainaveraged oceanic advection and diffusion terms do not show significant seasonal cycles. The sum of all estimated salinity budget terms largely captures salinity variations on interannual time scales. Unlike the seasonal cycle, variations in freshwater flux, oceanic advection, and vertical entrainment all contribute to interannual variations in surface salinity. Oceanic advection plays a larger role in salinity changes during 2008–2012, whereas the surface freshwater flux term dominates surface salinity evolution during 2004–2007 and in 2013. Although evaporation in the SPURS-1 region dominates the mean freshwater flux, precipitation plays a larger role in interannual variations of the freshwater flux. Separating the advection term into geostrophic and Ekman components indicates that the Ekman component dominates the total advection term. The effect of Ekman advection on salinity changes in the SPURS-1 region is closely linked to the spatial distribution of salinity anomalies. Therefore, it is important to understand largescale forcing changes. INTRODUCTION Temperature and salinity are the two fundamental ocean state variables. In contrast to extensive studies of ocean temperature, ocean salinity has received much less attention, mainly due to lack of data, but also because ocean salinity is generally perceived to have no direct influence on ocean-atmosphere interaction. However, through modification of oceanic density fields, salinity can impact ocean circulation and mixing (e.g., Fedorov et al., 2004; Huang et al., 2005), which in turn affects ocean temperature. Thus, salinity can play a substantial role in ocean-atmosphere interaction and the global climate system, particularly at lower latitudes where the existence of a barrier layer, defined as the layer between the bottom of a shallow halocline and the top of the thermocline, has been observed (Lukas and Lindstrom, 1991; Sprintall and Tomczak, 1992; Godfrey et al., 1999) and at high latitudes where salinity dominates thermohaline circulation (de Boyer Montegut et al., 2007). Formation and erosion of the barrier layer greatly influences surface mixed-layer dynamics. It impacts entrainment of cooler thermocline waters into the surface mixed layer, and thus regulates heat and momentum exchanges between the ocean and the atmosphere. The ocean also acts as a salt reservoir, whose salt content is conserved over time. Thus, the distribution of ocean salinity can be used to estimate freshwater flux (e.g., evaporation and precipitation) and transport as well as ocean mixing processes (Lukas and Lindstrom, 1991). Ocean salinity can also be used as an indicator of the strength of the water cycle. Durack and Wijffels (2010) assembled all available salinity data for the period 1950–2000 and found a sea surface salinity increase in evaporationdominated regions and a decrease in precipitation-dominated regions. The resemblance of the spatial pattern of salinity change to the mean salinity field suggests an amplification of the global water cycle (i.e., strengthening of the processes responsible for mean surface salinity distribution; Schmitt, 2008; Durack et al., 2012). This resemblance also indicates that changes in ocean salinity are more robust indicators of global water cycle changes than estimated changes in evaporation minus precipitation (E–P). During recent decades, large-scale changes of salinity have been observed in certain regions. A number of studies have described freshening at high latitudes (e.g., Wong et al., 1999; Bindoff and McDougall, 2000; Curry et al., 2003; Curry and Mauritzen, 2005; Josey and Marsh, 2005), while Curry et al. (2003) found a systematic increase in salinity at low latitudes of the North Atlantic Ocean. However, these changes have not been explained. Understanding the physical processes governing these Salinity Processes in the Upper-ocean Regional Study Mixed-Layer Salinity Budget