A new state-dependent parameterization for the free drift of sea ice
A new state-dependent parameterization for the free drift of sea ice
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DOI：
10.5194/tc-16-533-2022
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发表时间：
2022-02
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通讯作者：
Charles Brunette;L. Tremblay;R. Newton
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作者：
Charles Brunette;L. Tremblay;R. Newton
抽象的。对海冰运动的自由漂移估计是产生结合浮标和卫星得出的海冰运动矢量的无缝观测记录所必需的。我们发展了一种新的海冰自由漂移的参数化方法，该方法基于风强迫、风转角、海冰状态变量(厚度和浓度)以及对洋流的估计。考虑到风-冰-海洋传输系数的空间分布与海冰厚度的空间分布具有相似的结构，我们采用标准的自由漂移方程，引入了一个随冰厚度线性变化的风-冰-海洋传输系数。结果表明，以浮标日漂移数据为真，平均偏差为−0.5 cm S−1(低速偏差)，均方根误差为5.1 cm S−1。这意味着与极地探路者数据集中使用的自由漂移估计值相比，漂移速度误差减少了35%(Tschudi等人，2019b)。与厚度相关的传递系数提供了海冰漂移速度的季节性和长期趋势的改善，与7月(1月)相比，5月份(10月份)的漂移速度最小(最大)，而恒定的传递系数参数化仅跟随平均地面风应力的峰值。在1979年至2019年期间，这个新模式的海冰漂移趋势为每十年+0.45厘米S−1，而浮标观测的海冰漂移趋势为每十年+0.39厘米S−1，而在具有恒定传递系数的自由漂移参数化(−0.09厘米S−1每十年)或极地探路者自由漂移输入数据(−0.01厘米S−1每十年)方面基本上没有趋势。通过最小二乘拟合得到的最佳转角为25∘，其漂移方向的平均误差和均方根误差分别为+3和42∘。从极小化过程得到的海流估计解决了主要的大尺度特征，如Beaufort Gyre和跨极漂移流，并与根据ECCO、Glorys和PIOMAS冰洋再分析得到的海况估计以及来自海洋动力学地形的地转流的估计很好地一致，均方根差分别为2.4、2.9、2.6和3.8 cm S−1。最后，重复对时间序列的两个小节(2000年前和2000年后)的分析清楚地表明，波弗特环流(特别是沿阿拉斯加海岸线)在2000年后加速，环流在2000年后扩大，同时海冰覆盖变薄，观测到的冰漂移速度和洋流加速。这一新的数据集公开用于补充合并的基于观测的海冰漂移数据集，其中包括卫星和浮标漂移记录。
Abstract. Free-drift estimates of sea ice motion are necessary to produce a seamless observational record combining buoy and satellite-derived sea ice motion vectors. We develop a new parameterization for the free drift of sea ice based on wind forcing, wind turning angle, sea ice state variables (thickness and concentration), and estimates of the ocean currents. Given the fact that the spatial distribution of the wind–ice–ocean transfer coefficient has a similar structure to that of the spatial distribution of sea ice thickness, we take the standard free-drift equation and introduce a wind–ice–ocean transfer coefficient that scales linearly with ice thickness. Results show a mean bias error of −0.5 cm s−1 (low-speed bias) and a root-mean-square error of 5.1 cm s−1, considering daily buoy drift data as truth. This represents a 35 % reduction of the error on drift speed compared to the free-drift estimates used in the Polar Pathfinder dataset (Tschudi et al., 2019b). The thickness-dependent transfer coefficient provides an improved seasonality and long-term trend of the sea ice drift speed, with a minimum (maximum) drift speed in May (October), compared to July (January) for the constant transfer coefficient parameterizations which simply follow the peak in mean surface wind stresses. Over the 1979–2019 period, the trend in sea ice drift in this new model is +0.45 cm s−1 per decade compared with +0.39 cm s−1 per decade from the buoy observations, whereas there is essentially no trend in a free-drift parameterization with a constant transfer coefficient (−0.09 cm s−1 per decade) or the Polar Pathfinder free-drift input data (−0.01 cm s−1 per decade). The optimal wind turning angle obtained from a least-squares fitting is 25∘, resulting in a mean error and a root-mean-square error of +3 and 42∘ on the direction of the drift, respectively. The ocean current estimates obtained from the minimization procedure resolve key large-scale features such as the Beaufort Gyre and Transpolar Drift Stream and are in good agreement with ocean state estimates from the ECCO, GLORYS, and PIOMAS ice–ocean reanalyses, as well as geostrophic currents from dynamical ocean topography, with a root-mean-square difference of 2.4, 2.9, 2.6, and 3.8 cm s−1, respectively. Finally, a repeat of the analysis on two sub-sections of the time series (pre- and post-2000) clearly shows the acceleration of the Beaufort Gyre (particularly along the Alaskan coastline) and an expansion of the gyre in the post-2000s, concurrent with a thinning of the sea ice cover and the observed acceleration of the ice drift speed and ocean currents. This new dataset is publicly available for complementing merged observation-based sea ice drift datasets that include satellite and buoy drift records.