My Research Focus: Polar Regions in the Anthropocene
Polar climate systems are changing.
The seasonal see-saw of sea ice from Arctic to Antarctic is Earth's single largest mode of annual variation. In just a single year 7% of Earth's surface is covered and then un-covered by frozen sea water. The areas periodically capped by sea ice, the polar oceans, therefore have their own uniquely variable sunlight, surface cover, and life. Together, they have an area larger than all rainforests, all urban areas, and all warm deserts - combined.
Polar oceans are changing faster than anywhere else on the planet. Since 1970, the Arctic surface has warmed by 2 degrees, and annual Arctic sea ice volumes have declined by more than 60%. The coupled Arctic climate and ecological system is entering a state not observed in human history. While Southern Ocean waters warm rapidly, the Antarctic sea ice cover has been stable, kept in place by the presence of the Antarctic Ice Sheet and the surrounding Antarctic circumpolar current.
Both remains devilishly difficult to observe, model, and understand. The state of both the "new" Arctic system, and the "old" Antarctic, in past, present, and future are highly uncertain. My research focuses on the many open questions about these coupled system, seen in terms of their sea ice, ocean, ecology, and human systems.
The seasonal see-saw of sea ice from Arctic to Antarctic is Earth's single largest mode of annual variation. In just a single year 7% of Earth's surface is covered and then un-covered by frozen sea water. The areas periodically capped by sea ice, the polar oceans, therefore have their own uniquely variable sunlight, surface cover, and life. Together, they have an area larger than all rainforests, all urban areas, and all warm deserts - combined.
Polar oceans are changing faster than anywhere else on the planet. Since 1970, the Arctic surface has warmed by 2 degrees, and annual Arctic sea ice volumes have declined by more than 60%. The coupled Arctic climate and ecological system is entering a state not observed in human history. While Southern Ocean waters warm rapidly, the Antarctic sea ice cover has been stable, kept in place by the presence of the Antarctic Ice Sheet and the surrounding Antarctic circumpolar current.
Both remains devilishly difficult to observe, model, and understand. The state of both the "new" Arctic system, and the "old" Antarctic, in past, present, and future are highly uncertain. My research focuses on the many open questions about these coupled system, seen in terms of their sea ice, ocean, ecology, and human systems.
What you see below is a work in progress as of December 2020. For updated research information, please see my CV or Google Scholar profile.
Sea ice is made of floes.
|
Sea ice is a mosaic, formed from a myriad of individual pieces, called floes. These floes range in horizontal extent from meters to tens of kilometer across. Still, all IPCC-class sea ice models treat sea ice as a continuum. Until recently, no models contained evolving information about the distribution of floe sizes.
During my PhD, I developed a model of sea ice that includes floe behaviors (Horvat and Tziperman, 2015), like floe-size-dependent melting and fracture by waves. We used this new model to examine previous hypotheses about the scaling behavior of sea ice floe size (Horvat and Tziperman, 2017). |
Sea ice floes in the Labrador Sea (C.Bitz)
|
With Cecilia Bitz, Baylor Fox-Kemper, Lettie Roach, and others, we are now implementing the floe-size model in sea ice and climate models (e.g., Roach et al, 2018). The primary use of this model will be to examine how ocean surface waves can affect the sea ice cover now and in a thinner, more fractured Arctic future.
Arctic sea ice is thinning
Significant scientific and public attention is paid to the precipitous decline in September sea ice minimum extent. That sea ice is also fundamentally changing as it retreats. In the past, the Arctic was covered in thick, multi-year ice, but is now primarily thin, seasonal ice that is covered in melt ponds in summer.
|
The most important consequence of this change in sea ice? A dramatic reduction in the albedo of the sea ice that remains.
The left panel shows projected changes to summer sea ice albedo in the CESM Large ensemble. As a result of surface melt ponds and thinning, the albedo of sea ice is expected to decline by more than 70%. In the last 30 years the annual sea ice cover has declined by less than 15%, but the sea ice thickness has decreased by more than 50%. With Cecilia Bitz and Chris Polashenski, we concluded that the Arctic ice-albedo feedback may be driven by thickness. |
Recent change has revealed a new ecological regime
Areas of the July Arctic Ocean that permit under-ice blooms.
|
Ice-covered regions are often considered biological deserts, as sea ice is a strong reflector of solar radiation. In July 2011, however, scientists observed a "massive" phytoplankton bloom underneath sea ice. There, ocean chlorophyll concentrations ranked among the highest ever found on Earth. (Others have recorded similar biological activity).
Heading a collaboration with scientists at Cambridge University and the University of Reading, I showed that thinning Arctic sea ice provides the necessary solar radiation for these blooms to occur in the modern Arctic (Horvat et al, 2017). The signature of these biological events has been uncovered in measurements of atmospheric iodine (Cuevas [et al, incl. C Horvat], 2018). |
We are now considering how local factors control these blooms. With David Rees Jones, Daniela Flocco, and Ken Golden, we explain that the timing and initiation of under-ice blooms may be tied to the fractal dimension of melt ponds on the ice surface (paper in review).
A ponded sea ice surface in the Chukchi Sea (from Don Perovich)
Turbulence Energized at the Edge of Sea Ice Floes
The ice-covered polar oceans are places unlike any other. For most areas of the world's oceans, energy enters at large physical scales (like the atmospheric mesoscale) and cascades to smaller scales before being lost to mixing and turbulence.
In sea-ice-covered regions, the difference between ice and ocean can lead to severe differences in the properties of the upper ocean (i.e. available potential energy), yet on near-infinitesimal scales. How the energy injected at these fine horizontal scales produces the Arctic Ocean we observe today is the basis for my NOAA Climate and Global Change Fellowship.
We've attacked this problem from two directions:
In general we find the the scale of the cracks and floes determines the response of the upper ocean. In particular, in the melt season there is strong dependence of sea ice melt on the size of individual floes. We've started to parameterize these effects for use in floe-size-sensitive sea ice models (Horvat and Tziperman, 2018)
In sea-ice-covered regions, the difference between ice and ocean can lead to severe differences in the properties of the upper ocean (i.e. available potential energy), yet on near-infinitesimal scales. How the energy injected at these fine horizontal scales produces the Arctic Ocean we observe today is the basis for my NOAA Climate and Global Change Fellowship.
We've attacked this problem from two directions:
- What happens in the melt season, when the upper-ocean solar forcing across a floe edge can vary by two orders of magnitude? (Horvat, Tziperman, and Campin 2016)
- What happens in the freeze-up season, when salt is rejected along cracks and fissures in the largely-frozen sea ice (Horvat and Fox-Kemper, in prep).
In general we find the the scale of the cracks and floes determines the response of the upper ocean. In particular, in the melt season there is strong dependence of sea ice melt on the size of individual floes. We've started to parameterize these effects for use in floe-size-sensitive sea ice models (Horvat and Tziperman, 2018)
List of publications
On the theory of the floe size distribution:
On floe size effects on upper-ocean turbulence:
On under-ice ecology:
On floe size observations:
- A. Roberts, E. Hunke, S. Kamal, W. Lipscomb, C. Horvat, and W. Maslowski. A Variational Model for Sea Ice Ridging in Earth System Models, Part I: Theory. J. Adv. Model Earth Sys. 2018.
- L. Roach, C. Horvat, S. Dean, and C. Bitz. An emergent sea ice floe size distribution in a global coupled ocean-sea ice model. J. Geophys. Res. Oceans. 2018.
- C. Horvat and E. Tziperman. The evolution of scaling laws in the sea ice floe size and thickness distribution. J. Geophys. Res. Oceans. 2017.
- C. Horvat and E. Tziperman. A prognostic model of the sea-ice floe size and thickness distribution, The Cryosphere. 2015.
On floe size effects on upper-ocean turbulence:
- C. Horvat and E. Tziperman. Understanding melting due to ocean eddy heat fluxes at the edge of sea-ice floes. Geophys. Res. Lett. 2018. doi:10.1029/2018GL079363.
- C. Horvat, E. Tziperman, and J.M. Campin. Effects of the floe size distribution on ocean eddies and sea ice melting. Geophys. Res. Lett. 2016,
On under-ice ecology:
- C. Cuevas, N. Maffezzoli, J. Corella, A. Spolaro, P. Vallelonga, [et al., incl. C. Horvat]. Rapid increase in atmospheric iodine levels in the North Atlantic since the mid-20th century. Nature Communications, 2018.
- C. Horvat, D. Rees Jones, S. Iams, D. Schroeder, D. Flocco, D. Feltham. Prediction and timing of sub-ice phytoplankton blooms in the Arctic Ocean. Science Advances, 2017.
- C. Horvat, D. Flocco, D. Rees Jones, L. Roach. The distribution of solar energy under ponded first-year sea ice. in review.
On floe size observations:
- B. Hwang, J. Wilkinson, E. Maksym, H.C. Graber, A. Schweiger, C. Horvat, et al.. Winter-to-summer transition of Arctic sea ice breakup and floe size distribution in the Beaufort Sea. Elem Sci Anth, 2017.
- C. Horvat, R. Tilling, L. Roach, C. Bitz, B. Fox-Kemper, et al. The Sea Ice Floe Size Distribution Reconstructed From Satellite Altimetry: Theory, Climatology, and Model Comparison. in prep.
