Reconstructing regional 20th century sea level changes from tide gauges using the Analog Method

Anthropogenic climate change induces sea level changes (SLC) that must be accurately estimated to improve understanding of both past and future changes, facilitate timely adaptation and mitigate coastal risk. The rate and acceleration of global mean sea level and the associated uncertainty has been thoroughly assessed for the period since 1900. For the period since 1993, regional assessments have been produced, leveraging tide gauge records and satellite altimetry, allowing nations to understand and adapt more appropriately to local sea-level changes. However, improved regional timeseries are needed to robustly detect potential accelerations in local SLC.

This study proposes a novel data-driven approach for reconstructing regional SLC from tide gauges. We use a Reduced-Space Ensemble Kalman Smoother associated with the statistical Analog Prediction. This method, named RedAnDA, has been previously applied to reconstruct past temperature and salinity fields in the tropical Pacific, with good results. In this work, RedAnDA derives empirical orthogonal functions from satellite altimetry to extrapolate spatial features of the variability, as well as Analogs to predict monthly SLC associated with interannual-to-decadal variability. The uncertainty is quantified from the spread within the ensemble and takes various components into consideration, such as non-linearity in the dynamics or sampling issues. Tide gauge and altimetry input datasets are pre-processed (for instance for vertical land motion) using state-of-the-art methods. 

The RedAnDA performance is assessed by comparing the reconstruction to altimetry and existing tide gauge reconstructions, to evaluate our results over the recent period. In comparison to other reconstruction methods, RedAnDA can assess monthly changes associated with interannual variability over the 20th century, relying only on observational-based information. We further test the method by doing reconstructions which only assimilate 50-75% of the tide gauges, using the remaining ones for validation. These different tests show that RedAnDA can provide important additional regional information on SLC in the 20th century, including new estimates of the acceleration in regional SLC.