10 Oct 2022
10 Oct 2022
Status: this preprint is open for discussion.

On the drivers of regime shifts in the Antarctic marginal seas

Verena Haid, Ralph Timmermann, Özgür Gürses, and Hartmut H. Hellmer Verena Haid et al.
  • Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany

Abstract. Recent studies found evidence for a potential future tipping point when the density of Antarctic continental shelf waters, specifically in the southern Weddell Sea, allows the onshore flow of warm waters of open ocean origin. A cold-to-warm shift in the adjacent ice shelf cavities entails a multiplication of ice shelf basal melt rates and can possibly trigger instabilities in the ice sheet. From a suite of numerical experiments, aimed to force such a regime shift on the continental shelf, we identified the density balance between the shelf waters formed by sea ice production and the warmer water at the shelf break as the deciding element for a tipping into a warm state. In our experiments, this process is reversible but with evidence for hysteresis behaviour. Using HadCM3 20th-century output as atmospheric forcing, the resulting state of the Filchner-Ronne cavity depends on the initial state. In contrast, ERA Interim forcing pushes even a warm initialisation into a cold state, i.e., the system back to the cold side of the reversal threshold. However, it turns out that for forcing data perturbations of a realistic magnitude, a unique and universal recipe for triggering a regime shift in Antarctic marginal seas does not exist. Whether or not any given forcing or perturbation yields a density imbalance and thus allows for the inflow of warm water depends on the interplay between bottom topography, mean ocean state, sea ice processes, and atmospheric conditions.

Verena Haid et al.

Status: open (until 07 Dec 2022)

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  • RC1: 'Comment on egusphere-2022-1044', Anonymous Referee #1, 06 Dec 2022 reply

Verena Haid et al.

Verena Haid et al.


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Short summary
Recently, it was found that cold-to-warm changes in the Antarctic shelf sea are possible and lead to higher melt rates of ice shelves. In modeling experiments, we found that if the highest density in front of the ice shelf becomes lower than the density of the warmer water off-shelf at the deepest access to the shelf, the off-shelf water will flow onto the shelf. Our results also indicate that this change will offer some, although not much resistance to reversal and constitutes a tipping point.