Sea Ice-Driven Variability in the Pacific Subantarctic Mode Water Formation Regions

Abstract

Well-mixed mode waters that form in the north of the Southern Ocean are particularly important to the ocean absorption of heat and CO2 from the atmosphere. Two types of mode water form in the Pacific sector of the Southern Ocean: a Central pool and a Southeast pool. Both have shown significant year-to-year variability in recent decades. Variability in the regions where these waters form is shown to be due to changes in air-sea fluxes, horizontal advection, and the upward transport of deeper water. Transport of freshwater into the mode water formation regions is shown to be correlated with year-to-year changes in sea ice area in the Ross Sea and in the Amundsen/Bellingshausen seas. The results suggest that it takes around 6 months for sea ice melt from the Amundsen/Bellingshausen seas to reach the southeast mode water region, and up to 2 years for freshwater from the Ross Sea to reach both mode water formation sites. In 2015, Amundsen/Bellingshausen sea ice was particularly high, leading to more freshwater being transported to the southeast mode water site the following spring/summer. A huge decrease in winter sea ice in 2016 then caused the opposite, and salinity at the formation site was unusually high.