Wave influence on altimetry sea level at the coast

Abstract

Nowadays, in most altimetry sea level products, data are only retrieved up to 5–10 km from the coasts. The need to extend the satellite sea level record closest to land has led to dedicated studies in order to define and develop altimetry processing algorithms adapted to coastal ocean conditions. Among the different issues that strongly affect the performance of satellite altimetry as we approach the coast, the robustness of the correction related to the different states of sea surface waves (called sea state bias) is suspected to be one of the most important. Moreover, waves undergo a series of transformations when propagating from deep-water to shallow water, that add a dynamical contribution to the sea level, known as wave set-up and set-down. This signal impacts altimetry-measured coastal sea level in a way that remains poorly quantified and understood. Here, through a case study, we explore the potential to use the combination of shore-based camera video and tide gauges as a validation and analysis tool for coastal altimetry. The results show that measurements of sea level fluctuations on annual scales is similar between all three datasets. The analysis of the sea level data from both the tide gauge and the video camera which are co-located reveals that the physical contribution due to wave set-up and wave set-down accounts for 2% of the total sea level variations at monthly scale. We observe the loss of quality of the altimetry-derived significant wave height at ∼10 km from the coast, confirming the associated loss of accuracy in the sea state bias correction. Finally, this study highlights the critical need to co-locate the various measuring tools and devices at the coast with the satellite ground tracks if we want to optimally exploit coastal altimetry up to the coastline.