Australia: The Land Where Time Began

A biography of the Australian continent 

Southern Ocean Overturn Upper Branch – Water-Mass transformation by Sea Ice

A continuous thermodynamic transformation of the buoyancy of seawater is required by ocean overturning circulation. In the Southern Ocean a pathway for the upwelling of Circumpolar Deep Water from mid depth without strong Diapycnal mixing is provided by the steeply sloping isopycnals (Toggweiler & Samuels, 1995; Wolfe & Cessi, 2011; Nikurashin & Vallis, 2012), where it is transformed directly by surface fluxes of heat and freshwater and splits into an upper and lower branch (Speer, Rintoul & Sloyan, 2000; Marshall & Speer, 2012; Talley, 2013). The role of sea ice in the upper branch is less well understood, in part due to a paucity of observations of the thickness and transport of sea ice (Ren, Speer & Chassingnet, 2011; Tamura et al., 2011), while it is believed that brine rejection from sea ice contributes to the lower branch (Jacobs, 2004). In this paper Abernathy et al., present the results of their study in which they quantified the sea ice freshwater flux by use the Southern Ocean State Estimate, which is a state-of-the-art assimilation of data incorporating millions of ocean and ice observations. They then compared the relative roles of atmosphere, sea ice, and glacial freshwater fluxes, heat fluxes, and mixing of the upper ocean in transforming buoyancy within the upper branch, by use of the water-mass transformation framework (Walin, 1982). They found that sea ice is a dominant term, with differential brine rejection and the melting of ice transforming upwelled Circumpolar Deep Water at a rate of ~22 x 10⁶ m³ s^-1. It is implied by these results that there is a prominent role for Antarctic sea ice in the upper branch and suggest that there is a tight coupling between residual overturning and sea ice transport that is wind-driven.

Sources & Further reading

1. Abernathey, R. P., I. Cerovecki, P. R. Holland, E. Newsom, M. Mazloff and L. D. Talley (2016). "Water-mass transformation by sea ice in the upper branch of the Southern Ocean overturning." Nature Geosci 9(8): 596-601.