Feasibility of estimating sea surface height anomalies from surface ocean currents and winds

L. O'Neill, D. Chelton, Ernesto Rodríguez, R. Samelson, A. Wineteer
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Abstract

We propose a method to reconstruct sea surface height anomalies (SSHA) from vector surface currents and winds. This analysis is motivated by the proposed satellite ODYSEA, which is a Doppler scatterometer that measures coincident surface vector winds and currents. If it is feasible to estimate SSHA from these measurements, thenODYSEA could provide collocated fields of SSHA, currents, and winds over a projected wide swath of ∼1700 km. The reconstruction also yields estimates of the low-frequency surface geostrophic, Ekman, irrotational and non-divergent current components and a framework for separation of balanced and unbalanced motions. The reconstruction is based on a steady-state surface momentum budget including the Ekman drift, Coriolis acceleration, and horizontal advection. The horizontal SSHA gradient is obtained as a residual of these terms, and the unknown SSHA is solved for using a Helmholtz-Hodge Decomposition given an imposed SSHA boundary condition. We develop the reconstruction using surface currents, winds, and SSHA off the U.S. west coast from a 43-day coupled ROMS/WRF simulation. We also consider how simulated ODYSEA measurement and sampling errors and boundary condition uncertainties impact reconstruction accuracy. We find that temporal smoothing of the currents for periods of 150 hours is necessary to mitigate large reconstruction errors associated with unbalanced near-inertial motions. For the most realistic case of projected ODYSEA measurement noise and temporal sampling, the reconstructed SSHA fields have an RMS error of 2.1 cm and a model skill (squared correlation) of 0.958 with 150-hour resolution. We conclude that an accurate SSHA reconstruction is feasible using information measured by ODYSEA and external SSHA boundary conditions.
从表层洋流和风估算海面高度异常的可行性
我们提出了一种从矢量海面流和风重建海面高度异常(SSHA)的方法。该卫星是一个多普勒散射计,可测量重合的海面矢量风和海流。如果可以根据这些测量结果估算出 SSHA,那么 ODYSEA 就可以在预计的 1700 公里宽的扫描范围内提供 SSHA、海流和风的同位场。重建还可以估算出低频表层地转、埃克曼、非旋转和非发散海流成分,以及平衡和非平衡运动的分离框架。重建基于稳态地表动量预算,包括埃克曼漂移、科里奥利加速度和水平平流。水平 SSHA 梯度是作为这些项的残差而获得的,在施加 SSHA 边界条件的情况下,使用亥姆霍兹-霍奇分解法求解未知 SSHA。我们利用 ROMS/WRF 43 天耦合模拟的美国西海岸海面洋流、风和 SSHA 进行重建。我们还考虑了模拟 ODYSEA 测量和采样误差以及边界条件不确定性对重建精度的影响。我们发现,有必要对 150 小时内的海流进行时间平滑处理,以减轻与不平衡近惯性运动相关的巨大重构误差。在预计的 ODYSEA 测量噪声和时间采样最现实的情况下,重建的 SSHA 场的均方根误差为 2.1 厘米,150 小时分辨率的模型技能(相关平方)为 0.958。我们的结论是,利用 ODYSEA 测量信息和外部 SSHA 边界条件重建精确的 SSHA 是可行的。
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