Ice Ocean Feedbacks

Wintertime ice-ocean feedbacks in the Southern Ocean

Collaborators: Ethan Campbell, Stephen Riser, and Annie Wong

A schematic illustrating the coupling between wintertime sea ice growth and deep ocean ventilation in the Southern Ocean. As sea ice grows, it releases a flux of high salinity water into the ocean, Fs, which deepens the mixed layer and entrains heat from the thermocline, Fent. This entrained heat provides a negative feedback to ice growth.

A fundamental feature of the sea-ice-covered Southern Ocean is the vertical arrangement of cold, fresh surface water over warm, salty deep water. In the wintertime, this configuration facilitates a negative feedback to sea ice growth. When sea ice grows, it extracts freshwater from the surface ocean, leaving behind dense brine that sinks and churns up sub-surface heat. There are two major consequences to this convective process:

  1. The upward flux of heat limits the growth of sea ice. This is the primary reason sea ice in the Southern Ocean is generally thinner than in the Arctic. In extreme cases, the heat that is mixed up from the deep ocean is sufficiently large that melts the overlying sea ice cover leading to the formation of open-ocean polynyas.
  2. The flip side to the above process is deep ocean cooling. The cooling of the deep ocean is an essential step towards the formation of Antarctic Bottom Water, the dense water mass that occupies most of the global abyssal ocean. Equally important, this process helps to insulate the ice-fringed coastline of Antarctica.

The bulk of my thesis work was focused on better understanding the coupling between winter sea ice growth and deep ocean ventilation across the Southern Ocean. To this end, I used in situ under-ice ocean data, mainly from Argo floats and instrumented seals, to characterize the vertical stratification across the Antarctic and to diagnose the efficiency with which sea ice may extract heat from the deep ocean.

Figure showing the ratio of vertical temperature and salinity difference across the winter pycnocline in the Antarctic sea ice zone. Red colors highlight areas where the halocline is relatively weak and the thermocline is relatively strong. Temperature and salinity measurements represent bin-averaged profile data from under-ice Argo floats, instrumented seals, and shipboard measurements.

One key revelation from this work is that the thermodynamic coupling between winter sea ice growth and ocean ventilation varies significantly across the sea ice zone. On one extreme is the southeastern Weddell Sea, near Maud Rise, which features a weak halocline but a relatively strong thermocline. At the other extreme are strongly stratified regions near the sea ice edge, where the entrainment of heat is so weak that sea ice growth is essentially decoupled from the deep ocean. Since prior studies on this subject have largely focused on the Weddell Sea, this observational analysis provides a more nuanced understanding of under-ice processes in the Southern Ocean. For further details, please refer to our publication below.

Publication: Wilson, E. A., S. C. Riser, A. Wong and E. C. Campbell (2019), Winter upper ocean stability and ice-ocean feedbacks in the sea-ice-covered Southern Ocean, Journal of Physical Oceanography, 49, 1099–1117, doi:10.1175/JPO-D-18-0184.1

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Earle Wilson
Postdoctoral Scholar