2. Oceanographic background - Formation of intermediate and deep waters in the Southern Ocean and advection into the Pacific and Atlantic Oceans
The Southern Ocean is the formation site for much of the global deep and intermediate waters, including Antarctic Intermediate Water (AAIW) and Upper and Lower Circumpolar Deep Water (U/L-CDW). The extent to which these intermediate and deep waters penetrate into lower latitudes in the Pacific and Atlantic Oceans varies depending on formation sites, bathymetry, and the prevalence of other water masses.
AAIW is reflected in salinity minima in depth profiles, although this erodes with northward transport, and it is found at neutral densities ~27 < γη < ~27.4 (Orsi et al., 1995; Sloyan & Rintoul, 2001; Talley et al., 2011; Bostock et al., 2013). AAIW flows northward to the tropical South Pacific (~10-15° S) where it mixes with Equatorial Pacific Intermediate Water, itself derived from the mixing of AAIW and Pacific Deep Water (PDW) (Bostock et al., 2013). Atlantic AAIW is diluted by mixing during northward transport, as evidenced by increased salinity, and reaches a maximum extent slightly beyond the equator (Suga & Taley, 1995). Tropical Atlantic AAIW is found at a depth of around 700-800 m dicernible by low salinity (~34.20-34.40) and high O2­ (≥ 190 µmol kg-1) south of 20° S. Basin-scale circulation patterns result in more O2-depleted AAIW north of 20° S impacted by organic matter respiration and vertical mixing (Suga & Talley, 1995).
The deep waters of the Pacific Ocean comprise UCDW, LCDW and PDW, the latter of which is largely found at similar densities as UCDW (γη < 28.0; T > 1.5° C; S < 34.65 PSU) and is formed as the colder, more saline and more oxygen rich LCDW (γη > 28.0) is upwelled and mixed throughout Pacific (e.g. Orsi et al. 1995; Sloyan & Rintoul, 2001; Kawabe & Fujio, 2010; Talley et al. 2011). Densities occupied by UCDW transition to predominantly PDW in the tropical South to subtropical North Pacific, with UCDW reaching further north in the western Pacific. LCDW extends into the North Pacific along the western side of the basin, and the northernmost reaches are characterized by elevated [Si(OH)4] (≥170 µmol kg-1) (Kawabe & Fujio, 2010). CDW enters the South Atlantic below AAIW and can be dominant as far north as ~25° S, but the relative contribution of CDW weakens as the water mass continues northward with CDW accounting for ≤ 50% of deep waters in the tropical Atlantic (Larqué et al., 1997).