2022-12-15 テキサス大学オースチン校(UT Austin)
このモデルは、氷が鋭角に海に接する垂直氷河前線に特化して設計された初めてのもので、学術誌『Geophysical Research Letters』に発表されました。このモデルは、アラスカの氷河前線が従来の100倍もの速さで融解しているという最近の観測結果を反映しています。研究者によれば、このモデルは、あらゆる地球気候モデルの重要な要素である海洋モデルと氷床モデルの両方を改善するために使用することができる。
4年前、ラトガース大学環境生物科学部のレベッカ・ジャクソンは、海洋センサーを積んだロボットカヤックを、人が踏み込むことのできないアラスカのルコント氷河の400メートル以内に送り込んだ。その結果、予想外の結果が得られた。ルコント氷河の前線は、既存の氷河融解モデルが予測する100倍の速さで融解していたのだ。このデータをもとに、シュルツはグエン、ピラー両氏と協力して、より優れたモデルを開発した。グリーンランドの氷河の前線が海にぶつかる際の急傾斜を考慮し、融解速度を記述するための新しい方程式を検討した。
＜関連情報＞
- https://news.utexas.edu/2022/12/15/greenlands-glaciers-might-be-melting-100-times-as-fast-as-previously-thought/
- https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL100654
海成氷河の垂直氷前線に対する改良された観測制約付き融解率パラメタリゼーション An Improved and Observationally-Constrained Melt Rate Parameterization for Vertical Ice Fronts of Marine Terminating Glaciers
K. Schulz, A. T. Nguyen, H. R. Pillar
Geophysical Research Letters Published: 20 September 2022
DOI:https://doi.org/10.1029/2022GL100654
Black lines show the vertical distribution of observed fjord (a) temperature (°C) and (b) salinity near LeConte glacier. Black lines in (c–f) show average plume properties of the 200 member ensemble with γT = 10−3 m s−1
Abstract
Submarine melting at Greenland’s marine terminating glaciers is a crucial, yet poorly constrained process in the coupled ice-ocean system. Application of Antarctic melt rate representations, derived for floating glacier tongues, to non-floating marine terminating glaciers commonly found in Greenland, results in a dramatic underestimation of submarine melting. Here, we revisit the physical theory underlying melt rate parameterizations and leverage recently published observational data to derive a novel melt rate parameterization. This is the first parameterization that (a) consistently comprises both convective- and shear-dominated melt regimes, (b) includes coefficients quantitatively constrained using observational data, and (c) is applicable to any vertical glacier front. We show that, compared to the current state-of-the-art approach, the scheme provides an improved fit to observed melt rates on the scale of the terminating front, offering an opportunity to incorporate this critical missing forcing into ocean circulation models.
Key Points
- Convective instabilities may govern vertical ice front melting at low ambient currents but are ignored in existing parameterizations
- We propose a novel melt parameterization for vertical fronts continuous across convective and shear regimes constrained by observations
- The contribution of background melting as opposed to melting within subglacial discharge plumes might be larger than previously thought
Plain Language Summary
Where Greenland’s glaciers terminate in the ocean, the relatively warm waters in the fjords melt the ice. This is a very important process, as the rate of melt determines how fast the glaciers are losing mass and inject freshwater into the ocean, which contributes to sea level rise and can change ocean currents. Unfortunately, it is still difficult to calculate how much glacial ice is melted by the warm ocean around Greenland, as it is unfeasible to measure the small melting processes so close to the calving glacier front. Up to now, melt rate calculations rely on estimates for floating glacier tongues in Antarctica, which are more accessible, but it has become increasingly apparent that important differences exist for these two cases. In this study, we try to find a better way to calculate melt rates for marine terminating glaciers with vertical fronts, by reconsidering the underlying physics of submarine melt, and by using observations of submarine melt waters near a vertical glacier front in Alaska.
