Abstract
Abstract. Recent low summer sea ice extent in the Weddell Sea raises questions about the contributions of dynamic and thermodynamic atmospheric and oceanic energy fluxes. The roles of snow, superimposed ice, and snow ice are particularly intriguing, as they are sensitive indicators of changes in atmospheric forcing and as they could trigger snow–albedo feedbacks that could accelerate ice melt. Here we present snow depth data and ice core observations of superimposed ice and snow ice collected in the northwestern Weddell Sea in late austral summer 2019, supplemented by airborne ice thickness measurements. Texture, salinity, and oxygen isotope analyses showed mean thicknesses of superimposed and snow ice of 0.11±0.11 and 0.22±0.22 m, respectively, or 3 % to 54 % of total ice thickness. Mean snow depths ranged between 0.46±0.29 m in the south to 0.05±0.06 m in the north, with mean and modal total ice thicknesses of 4.12±1.87 to 1.62±1.05 m and 3.9 to 0.9 m, respectively. These snow and ice properties are similar to results from previous studies, suggesting that the ice's summer surface energy balance and related seasonal transition of snow properties have changed little in past decades. This is supported by our additional analyses of the summer energy balance using atmospheric reanalysis data and by melt onset observations from satellite scatterometry showing few recent changes.
Highlights
After more than 3 decades of highly variable, but slowly increasing, Antarctic summer sea ice coverage, ice extent has strongly declined between austral summer 2016/17 and 2018/19, most notably in the Weddell Sea (Parkinson, 2019; Turner et al, 2020)
3.1 Sea ice and snow conditions In February and March 2019, the northwestern Weddell Sea was characterized by the presence of at least three different ice regimes known from previous studies (Haas et al, 2008) and visible in satellite synthetic-aperture radar (SAR) imagery (Fig. 1): (i) heavily deformed ice near the coast of the Antarctic Peninsula and along the Larsen Ice Shelf; (ii) east of that, a band of younger, thinner, and less deformed ice originating from the Ronne Ice Shelf; and (iii) in the very east, older, strongly deformed, thick ice originating from the southeastern Weddell Sea (Filchner Ice Shelf)
Our results showed similar sea ice properties to those found in the few previous studies that have been carried out since the 1980s and 1990s, and in particular we did not find unusual amounts of meteoric ice, i.e., of superimposed ice or snow ice which are sensitive indicators of the sea ice surface energy balance and the relation between snow depth and ice thickness, respectively
Summary
After more than 3 decades of highly variable, but slowly increasing, Antarctic summer sea ice coverage, ice extent has strongly declined between austral summer 2016/17 and 2018/19, most notably in the Weddell Sea (Parkinson, 2019; Turner et al, 2020). Earlier seasonal sea ice retreat, anomalous surface winds, and strong atmosphere– ocean coupling caused stronger heat absorption by the upper ocean, more southward Ekman transport of warmer surface waters, and the warming of the ocean mixed layer, amplifying the sea ice loss (Meehl et al, 2019; Turner et al, 2020) These studies suggest a significant change in the coupled atmosphere–ice–ocean system in the Weddell Sea, the quantitative contribution of the individual components is not yet clear, nor is it clear how they might affect seasonal dynamic and thermodynamic sea ice properties
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