Subantarctic Mode Water

Subantarctic mode water (SAMW) is an important water mass in the Earth's oceans. It is formed near the Subantarctic Front on the northern flank of the Antarctic Circumpolar Current. The surface density of Subantarctic Mode Water ranges between about 1026.0 and 1027.0 kg/m3 and the core of this water mass is often identified as a region of particularly low stratification.

Another important facet of SAMW is that silicate (an important nutrient for diatoms) is relatively depleted relative to nitrate. This depletion can be tracked over much of the globe, suggesting that SAMW helps set the blend of nutrients delivered to low-latitude ocean ecosystems, and thus determines the balance of species within these ecosystems.

SAMW is a very homogenous layer that forms north of the Subantarctic Front and is also referred to as a pycnostad. Its uniformity can be attributed to convective overturning that also serves to ventilate it resulting in the high dissolved oxygen value of >6ml/l.

It has slightly less dissolved oxygen than the surface water layer above it, but greater dissolved oxygen to the water masses surrounding it. It has some variability in temperature, salinity and density from a west to east gradient in the Pacific Ocean. From west to east, the density increases from 26.9 mg/cm3 to 27.1 mg/cm3, the temperature decreases from 8.5 C to 5.5 C, and the salinity decreases from 34.62 ppt to 34.25 ppt (psu) In the region where the Peru-Chile Undercurrent flows above the SAMW, the SAMW can be distinguished as having locally-characteristically low phosphorus, silicate and other nutrient concentrations in comparison. It moves by the transference of heat energy via the Subtropical anticyclonic gyre and retains its individuality as differentiated with the less-salty Antarctic Intermediate Water below it and the more highly oxygenated surface water above it. The oxygen maximum portion of SAMW sinks at 28˚S to 700m and lifts back to 500m around 15˚S after oxygen levels decreased.

SAMW acts as an oxygenator for mid oceanic depths in the Southern oceans. Near the surface it picks up atmospheric oxygen and carbon dioxide and then sinks, or subducts near the Indian Ocean, contributing to the Indian subtropical gyre and cooling and contributing to the Antarctic Circumpolar Current (ACC).