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

Subantarctic Mode Water (SAMW) forms north of the Subantarctic Front, in regions of deep winter mixed layers, and is important to the absorption and storage of anthropogenic CO2 and heat. Two SAMW pools exist in the Pacific, a lighter Central mode (CPSAMW), and a denser Southeast mode (SEPSAMW). Both have experienced significant interannual variability in thickness and properties in recent years. We compute mixed layer temperature and salinity budgets for the two SAMW formation regions, to determine the relative contribution of processes driving variability in the properties of mixed layers that subduct to form SAMW. The dominant drivers of temperature and salinity variability are shown to be surface fluxes, horizontal advection, and entrainment of deeper water. Salt advection into each SAMW formation region is found to be strongly correlated with changes in sea ice area in the northern Ross Sea, with lags of up to 2 years. Further correlation is found between meridional salt advection in the southeast Pacific formation regions, and sea ice area in the northern Amundsen/Bellingshausen seas, suggesting that freshwater derived from sea ice melt reaches the SEPSAMW formation region within 6 months. In 2016, strong advective freshening of the SEPSAMW formation region, linked to increased winter sea ice in the Amundsen/Bellingshausen seas, led to anomalously fresh mixed layers. However, a regime change in Antarctic sea ice in 2016 resulted in a subsequent lack of the usual advective freshening in the SEPSAMW formation region, driving increased salinity of the mixed layer the following year. 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. Temperature and salinity variability at Pacific Subantarctic Mode Water (SAMW) formation sites is driven by surface fluxes, advection and entrainmentFreshwater anomalies advected from the Ross Sea take up to 2 years to reach the Pacific SAMW formation regionsAnomalies in Amundsen/Bellingshausen sea ice in 2016-2017 drove salinity anomalies at the nearby southeast Pacific SAMW formation site