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).