Tianning Tang (University of Manchester, UK); Stephen Roberts (University of Oxford, UK); Thomas Adcock (University of Oxford, UK)

On September 16th, 2023, an anomalous 10.88 mHz seismic signal was observed globally, persisting for 9 days. One month later an identical signal appeared, lasting for another week. Several studies have theorized that these signals were produced by seiches which formed after two landslide-generated mega-tsunamis in an East Greenland fjord. This theory is supported by seismic inversions, and analytical and numerical modeling, but no direct observations have been made. Here, we present primary observations of this phenomenon using data from the Surface Water Ocean Topography mission. Other oceanographic processes such as tides are ruled out through direct tidal estimates made from the SWOT data which are compared to FES2022 and in-situ gauge observations. These gauge observations are also used to rule out Ekman transport. We validate the seiche theory of previous authors and independently estimate its initial amplitude at 7.9 m using Bayesian machine learning and seismic data. This study demonstrates the value of SWOT for studying fast oceanic processes and extreme events, while also highlighting the need for specialized methods to address the altimetric data’s limitations, namely temporal sparsity. We highlight some of the challenges of using SWOT in narrow fjords and share our procedure for post-processing these data. The SWOT data and approaches will help in understanding future unseen extremes driven by climate change.