So why are Antarctic sea ice losses running so slow this year?
Well the temperature anomalies for Antarctica have been 1.5 to 3 Celsius colder than average for months now. That could be one reason. Maybe the La Nina has some effect as well?
Also gerontocrat has his theory that increased melting of the Antarctic ice sheet causes increased sea ice formation but there's probably not much melting going on there at this time of year, perhaps thats more of a factor in the southern hemisphere summer.
First, icebergs and the basal side of Antarctic ice shelves melt year round, and the first linked reference about HadGEM3-GC3.1 projections related to climate responses from the modeled increasing Antarctic iceberg and ice shelf melt. Key identified issues include that:
- Increased meltwater is increasing (temporarily) sea ice extent while reducing AABW production; both of which contribute to a slowing of the MOC.
- Increase snowfall at lower latitudes of Antarctica is applying more driving force on key marine glaciers; which will cause both ice velocities and iceberg calving to increase; and increased snow that falls directly into the ocean also contributes to both a slowdown of the MOC and increased advection of warm CDW towards key marine glacier grounding lines.
Shona Mackie; Inga J. Smith; Jeff K. Ridley; David P. Stevens and Patricia J. Langhorne (2020), "Climate response to increasing Antarctic iceberg and ice shelf melt, J. Climate 1–70;
https://doi.org/10.1175/JCLI-D-19-0881.1https://journals.ametsoc.org/jcli/article/doi/10.1175/JCLI-D-19-0881.1/353964/Climate-response-to-increasing-Antarctic-icebergAbstract: "Mass loss from the Antarctic continent is increasing, however climate models either assume a constant mass loss rate, or return snowfall over land to the ocean to maintain equilibrium. Numerous studies have investigated sea ice and ocean sensitivity to this assumption and reached different conclusions, possibly due to different representations of melt fluxes. The coupled atmosphere-land-ocean-sea ice model, HadGEM3-GC3.1, includes a realistic spatial distribution of coastal melt fluxes, a new ice shelf cavity parametrization and explicit representation of icebergs. This makes it appropriate to revisit how increasing melt fluxes influence ocean and sea ice, and to assess whether responses to melt from ice shelves and icebergs are distinguishable. We present results from simulated scenarios of increasing meltwater fluxes and show that these drive sea ice increases and, for increasing ice shelf melt, a decline in Antarctic Bottom Water formation. In our experiments, the mixed layer around the Antarctic coast deepens in response to rising ice shelf meltwater, and shallows in response to stratification driven by iceberg melt. We find similar surface temperature and salinity responses to increasing meltwater fluxes from ice shelves and icebergs, but mid-layer waters warm to greater depths and further north when ice shelf melt is present. We show that as meltwater fluxes increase, snowfall becomes more likely at lower latitudes, and Antarctic Circumpolar Current transport declines. These insights are helpful for interpretation of climate simulations that assume constant mass loss rates, and demonstrate the importance of representing increasing melt rates for both ice shelves and icebergs."
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Second, the second linked reference indicates that some of the recent high Antarctic sea ice extents have been due to both natural variability and to recent changes in wind-driven sea ice transport that move sea ice northward into the Southern Ocean.
Zhang, L., Delworth, T. L., Cooke, W., & Yang, X. (2018). Natural variability of Southern Ocean convection as a driver of observed climate trends. Nature Climate Change. doi:10.1038/s41558-018-0350-3,
https://doi.org/10.1038/s41558-018-0350-3https://www.nature.com/articles/s41558-018-0350-3Abstract: "Observed Southern Ocean surface cooling and sea-ice expansion over the past several decades are inconsistent with many historical simulations from climate models. Here we show that natural multidecadal variability involving Southern Ocean convection may have contributed strongly to the observed temperature and sea-ice trends. These observed trends are consistent with a particular phase of natural variability of the Southern Ocean as derived from climate model simulations. Ensembles of simulations are conducted starting from differing phases of this variability. The observed spatial pattern of trends is reproduced in simulations that start from an active phase of Southern Ocean convection. Simulations starting from a neutral phase do not reproduce the observed changes, similarly to the multimodel mean results of CMIP5 models. The long timescales associated with this natural variability show potential for skillful decadal prediction."
Extract: "However, we cannot conclude that internally generated SO deep convection is the only driver, even in recent observations. The SO deep-convection change could work together with various other mechanisms identified in earlier studies, such as wind-driven ice transport and cold/warm-temperature advection, and anthropogenic surface freshening due to an amplified hydrological cycle and ice-sheet melting. As mentioned above, the surface wind trend favours warm SST and decreasing sea ice over the Antarctic Peninsula through warm advection and over the Amundsen– Bellingshausen seas through enhanced vertical mixing caused by anomalous negative wind stress curl. Our model also shows that the long-lasting westerly winds over the SO induce upwelling and a spin-up of the AABW cell, which in turn generates the warm SST. The surface freshwater changes due to shifted storm tracks and melting ice sheet in future may slow down the SO MOC, which also cannot be excluded. It is also possible that melting of land-based ice sheets, a process usually not included in climate models, could cause surface freshening and the subsequent suppressed convection and SST cooling."
