Ice Ocean Feedbacks

Wintertime feedbacks between sea ice and the upper ocean in the Southern Ocean

Collaborators: Ethan Campbell, Stephen Riser, and Annie Wong

A schematic illustrating the coupling between wintertime ice growth and deep ocean ventilation in the Southern Ocean. As sea ice grows, it releases flux of salt to the ocean, Fs, which deepens the mixed layer and entrains heat from the thermocline, Fent. This entrained heat is a negative feedback to ice growth. In extreme cases, this ventilted heat may melt the existing sea ice cover and create open-ocean polynyas.

The bulk of my thesis work was focused on the coupling between winter sea ice growth and deep ocean ventilation across the Southern Ocean sea ice zone. This coupling describes the negative feedback that occurs when sea ice growth triggers the entrainment of heat from the winter pycnocline. To characterize this process, I utilized recently collected under-ice upper ocean data, mainly from Argo floats and instrumented seals. This was one of the first studies to explore this under-ice dataset in its entirety. This work was also the first circumpolar evaluation of wintertime sea ice-ocean feedbacks in the Southern 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. Here, the coupling between winter sea ice growth and thermocline ventilation is strong enough to neutralize the surface cooling by the atmosphere. At the other extreme are strongly stratified regions near the sea ice edge, where heat entrainment is so weak that sea ice growth is essentially decoupled from the deep ocean. Since prior studies on this subject have largely been focused on the Weddell Sea, this observational analysis provides a more nuanced understanding of under-ice processes in the Southern Ocean.

Another aspect of this works explores the circumstances under which heat fluxes from the deep ocean may eliminate the winter ice cover and lead to the formation of a winter polynya. This was running idealized simulation using 1-D sea ice-ocean model. From these simulations, we deduce that offshore winter polynyas are only feasible in the eastern Weddell Sea. Even then, the initiation of a winter polynya in this region requires a culmination of extreme circumstances, specifically the abrupt application of storm-force winds over an abnormally weak halocline.

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