Formation of the South Pacific Shallow Salinity Minimum: A Southern Ocean Pathway to the Tropical Pacific

Abstract

In the eastern South Pacific Ocean, at a depth of about 200 m, a salinity minimum is found. This minimum is associated with a particular water mass, the ''Shallow Salinity Minimum Water'' (SSMW). SSMW outcrops in a fresh tongue (S min ) centered at about 45ЊS. The S min appears to emanate from the eastern boundary, against the mean flow. The watermass transformation that creates SSMW and S min is investigated here. The S min and SSMW are transformed from saltier and warmer waters originating from the western South Pacific. The freshening and cooling occur when the water is advected eastward at the poleward side of the subtropical gyre. Sources of freshening and cooling are air-sea exchange and advection of water from south of the subtropical gyre. A freshwater and heat budget for the mixed layer reveals that both sources equally contribute to the watermass transformation in the mixed layer. The freshened and cooled mixed layer water is subducted into the gyre interior along the southern rim of the subtropical gyre. Subduction into the zonal flow restricts the transformation of interior properties to diffusion only. A simple advection/diffusion balance reveals diffusion coefficients of order 2000 m 2 s Ϫ1 . The tongue shape of the S min is explained from a dynamical viewpoint because no relation to a positive precipitation-evaporation balance was found. Freshest S min values are found to coincide with slowest eastward mixed layer flow that accumulates the largest amounts of freshwater in the mixed layer and creates the fresh tongue at the sea surface. Although the SSMW is the densest and freshest mode of water subducted along the South American coast, the freshening and cooling in the South Pacific affect a whole range of densities (25.0-26.8 kg m Ϫ3 ). The transformed water turns northward with the gyre circulation and contributes to the hydrographic structure of the gyre farther north. Because the South Pacific provides most of the source waters that upwell along the equatorial Pacific, variability in South Pacific hydrography may influence equatorial Pacific hydrography. Because one-half of the transformation is found to be controlled through Ekman transport, variability in wind forcing at the southern rim of the subtropical gyre may be a source for variability of the equatorial Pacific.

Contribution of Ekman pumping to the changes in properties and volume of the Eastern South Pacific Intermediate Water

“…Additionally, the trend of change from 2000 onward matches the beginning of a decade of deceleration in sea surface warming on the global scale, most likely due to an increase of the SP trade winds (England et al 2014). An acceleration of downward EP velocity is favorable for amplifi ed ESPIW formation (Karstensen 2004;Schneider et al 2003;Tsuchiya and Talley 1998), and the EP trend from 2000 onward leads us to the explanation of the increase in volume and area of ESPIW, which in turn contributed to the ventilation of areas of the SP of intermediate depth. Similarly, Pérez et al (2000) associated variable wind stress curl with variability in the ventilation of North East Atlantic Central Water, while Waugh (2014) associated it with the descent in age of Subantarctic and Subtropical Mode Waters.…”

“…Analysis of the P-E balance using ERA-Interim data for the region 42.5°-48.0°S, 76°-85°W (SASW before being subducted) during 2000-2012 revealed only a small nonsignifi cant positive statistical trend. Analysis based on differences between early Argo and 1990s hydrographic data and changes in sea surface height suggested the circulation of the SP subtropical gyre increased during 1993-2004(Roemmich et al 2007). This intensifi cation of the gyre circulation reduced salinity in the upper 200 m of the water column in the eastern SP, along 32°30'S by 0.1, via amplifi ed transport of fresher SASW from higher to middle latitudes (Schneider et al 2007).…”

“…In the eastern SP, SASW is much fresher than in the west because of the heavy precipitation and freshwater runoff from Chilean Patagonia. Thus, this creates a low-salinity tongue that stretches from the coast of South America toward the central SP at around 45°S (Karstensen 2004).…”

“…In the Eastern SP, north of the region of STC, there is an upper minimum-salinity water mass referred to as Shallow Salinity Minimum Water (Reid 1973;Tsuchiya & Talley 1998;Karstensen 2004) or as ESPIW (Emery & Meincke 1986). A conceptual diagram, shown in Figure 1, illustrates both the subduction process at the region of STC and the formation of central water masses.…”

Are there inescapable issues prohibiting the use of terrain-following coordinates in climate models?

“…ROMS and CARS show very consistent features, although there are significant differences in the coastal areas of Western Pacific and off the west coast of South America (Fig.10). The deeper Z20 in ROMS off the South American West Coast may result from the under-resolved upwelling or, more likely, from an inadequate representation of subsurface ocean processes like the subduction of temperature anomalies (Karstensen, 2004;Yeager and Large, 2004). The deep bias south of 20…”