High-Resolution Thermal Imaging in the Antarctic Marginal Ice Zone: Skin Temperature Heterogeneity and Effects on Heat Fluxes

This article was published on 14 September 2023.

šŸ‘¤ Ippolita Tersigni, Alberto Alberello, Gabriele Messori, Marcello Vichi, Miguel Onorato, Alessandro Toffoli


Insufficient in situ observations from the Antarctic marginal ice zone (MIZ) limit our understanding and description of relevant mechanical and thermodynamic processes that regulate the seasonal sea ice cycle. Here we present high-resolution thermal images of the ocean surface and complementary measurements of atmospheric variables that were acquired underway during one austral winter and one austral spring expedition in the Atlantic and Indian sectors of the Southern Ocean. Skin temperature data and ice cover images were used to estimate the partitioning of the heterogeneous surface and calculate the heat fluxes to compare with ERA5 reanalyses. The winter MIZ was composed of different but relatively regularly distributed sea ice types with sharp thermal gradients. The surface-weighted skin temperature compared well with the reanalyses due to a compensation of errors between the sea ice fraction and the ice floe temperature. These uncertainties determine the dominant source of inaccuracy for heat fluxes as computed from observed variables. In spring, the sea ice type distribution was more irregular, with alternation of sea ice cover and large open water fractions even 400 km from the ice edge. The skin temperature distribution was more homogeneous and did not produce substantial uncertainties in heat fluxes. The discrepancies relative to reanalysis data are however larger than in winter and are attributed to biases in the atmospheric variables, with the downward solar radiation being the most critical.Ā 

Key Points

  • Thermal images of the ocean surface were used to compute heat fluxes over the Antarctic marginal ice zone (MIZ) in winter and spring.
  • The MIZ was a compound of several ice types with strong thermal gradients in winter and more homogeneous temperature in spring.
  • The comparison of heat fluxes against reanalyses points toward biases due to the skin temperature in winter and solar radiation in spring.

Plain Language Summary

The Southern Ocean stores and release more heat than any other latitude band on the planet, making it a major element of the global climate. In the Antarctic, air-sea heat exchange is mediated by the seasonal sea ice cycle, which forms an unsteady and composite interface. In situ measurements are serendipitous in the region and models are poorly constrained. Here, we present a set of high-resolution thermal images of the uppermost ocean layer (skin temperature) and atmospheric variables acquired underway from the icebreaker S.A. Agulhas II in winter and spring. Observations, and heat fluxes derived from them, are compared with reanalysis, which are model predictions adjusted with assimilated observations different from the ones we collected. In winter, the sea ice shows a neat separation between several ice types with sharp gradients of surface temperature. The reanalysis captures the mean skin temperature, but this is due to error compensation, which leads to inaccuracies in heat fluxes. In spring, sea ice is a disordered mixture of ice types and open water with a homogeneous thermal distribution. Uncertainties in skin temperature have smaller effects on the heat fluxes modeled by the reanalysis. Differences between reanalysis and observations are dominated by biases in solar radiation.

Full article can be retrievedĀ here.Ā