An instantaneous M₂ internal tide model from SWOT

The new wide-swath Surface Water and Ocean Topography (SWOT) altimetry mission measures two-dimensional sea surface height (SSH) along a 120-km swath with a spatial resolution of 2 km. We explore SWOT's capability of observing internal tides using the recently released 75 days of SWOT data from 7 September to 21 November 2023. We demonstrate that we can extract mode-1 M₂ internal tides from such a short data record by our new mapping technique. SWOT-75d represents a 75-day instantaneous model. Nadir-30y is constructed using 30 years of nadir altimetry data from 1993 to 2022 and represents a climate normal. The nadir altimetry data in 2023 are used for model evaluation. Despite its large errors, SWOT-75d reveals the basic features of the global mode-1 M₂ internal tide field, and causes positive variance reduction in regions of strong internal tides. Nadir-30y performs better overall, but SWOT-75d performs better in the tropical South Atlantic Ocean, the central North Pacific Ocean, and the Melanesian region. Evaluation using seasonally subsetted altimetry data reveals that M₂ internal tides have significant temporal variations. SWOT-75d performs the best in fall, because the model is constructed using data largely in fall. SWOT-75d has large phase anomalies, which are spatially smoothed and used to adjust the phases in Nadir-30y. The phase-adjusted model can better make internal tide correction for SWOT and its performance is improved by 20%. Our results demonstrate that (a) mode-1 M₂ internal tides can be extracted from 75 days of SWOT data by our mapping technique, and (b) the instantaneous internal tide model can be used to improve internal tide correction for SWOT.新型宽幅天基地表水与海洋地形（Surface Water and Ocean Topography, SWOT）测高任务，可沿120千米幅宽以2千米的空间分辨率获取二维海面高度（sea surface height, SSH）观测数据。本研究利用2023年9月7日至11月21日期间公开的75天SWOT实测数据，探究了该任务观测内潮的能力。通过自主研发的新型制图技术，我们证实仅依靠如此短时长的观测序列，即可提取出一阶M₂内潮信号。SWOT-75d为75天瞬时内潮模型；天顶-30年（Nadir-30y）则基于1993至2022年共计30年的天底测高数据构建，代表气候标准态。2023年的天底测高数据用于模型验证。尽管SWOT-75d存在较大误差，但其仍揭示了全球一阶M₂内潮汐场的基本特征，并在强内潮活动区域实现了正向方差缩减。整体而言，天顶-30年模型表现更优，但SWOT-75d在热带南大西洋、北太平洋中部以及美拉尼西亚海域的表现更为出色。利用按季节分箱的测高数据开展的验证结果显示，M₂内潮存在显著的时间变化特征。由于SWOT-75d的构建数据主要采集于秋季，该模型在秋季的表现最佳。SWOT-75d存在较大的相位异常，我们对其进行空间平滑处理后，用于修正天顶-30年模型的相位。经相位修正后的模型可更好地为SWOT任务开展内潮校正，校正性能提升了20%。本研究结果证实：（a）借助本研究提出的制图技术，可从75天的SWOT观测数据中提取出一阶M₂内潮信号；（b）该瞬时内潮模型可用于优化SWOT任务的内潮校正流程。