Improved capabilities of global ocean reanalyses for analysing sea level variability near the Atlantic and Gulf of Mexico Coastal U.S.

Abstract

Realistic representation of monthly sea level anomalies in coastal regions has been a challenge for global ocean reanalyses. This is especially the case in coastal regions where sea levels are influenced by western boundary currents such as near the U.S. Atlantic Coast and the Gulf of Mexico. For these regions, most ocean reanalyses compare poorly to observations. Problems in reanalyses include errors in data assimilation and horizontal resolutions that are too coarse to simulate energetic currents like the Gulf Stream and Loop Current System. However, model capabilities are advancing with improved data assimilation and higher resolution. Here, we show that some current-generation ocean reanalyses produce monthly sea level anomalies with improved skill when compared to satellite altimetry observations of sea surface heights. Using tide gauge observations for coastal verification, we find the highest skill associated with the GLORYS12 and HYCOM ocean reanalyses. Both systems assimilate altimetry observations and have eddy-resolving horizontal resolutions (1/12°). We found less skill in three other ocean reanalyses (ACCESS-S2, ORAS5, and ORAP6) with coarser, though still eddy-permitting, resolutions (1/4°). The operational reanalysis from ECMWF (ORAS5) and their pilot reanalysis (ORAP6) provide an interesting comparison because the latter assimilates altimetry globally and with more weight, as well as assimilating ocean observations over continental shelves. We find these attributes associated with improved skill near many tide gauges. We also assessed an older reanalysis (CFSR), which has the lowest skill likely due to its lower resolution (1/2°) and lack of altimetry assimilation. ACCESS-S2 likewise does not assimilate altimetry, although its skill is much better than CFSR and only somewhat lower than ORAS5. Since coastal flooding is influenced by sea level anomalies, the recent development of skilful ocean reanalyses on monthly timescales may be useful for better understanding the physical processes associated with flood risks.

Assessment of water levels from 43 years of NOAA’s Coastal Ocean Reanalysis (CORA) for the Gulf of Mexico and East Coasts

“…Along the Mid-Atlantic Coast, the GIA trends unresolved by GLORYS12 can be as large as 2 mm yr -1 (Harvey et al, 2021). Besides unresolved processes in GLORYS12, its resolution (1/12°) could explain some disparities in the trends for estuaries like the Chesapeake Bay area where water level gauges are far removed from the nearest-ocean model grid point (Feng et al, 2024).…”

“…Previously, global ocean reanalyzes using baroclinic models showed poor skill in simulating interannual sea level variability along the East Coast of North America (Piecuch et al, 2016;Long et al, 2021;Widlansky et al, 2023). GLORYS12 possesses a much higher spatial resolution (1/12°) than the reanalyzes previously assessed by those studies, and it seems to well simulate oceanic processes around North America (Amaya et al, 2023;Feng et al, 2024). However, none of the models currently used for global ocean reanalyzes assimilate land-based gauge observations of water levels (sometimes referred to as tide gauge data).…”

Skill assessment of seasonal forecasts of ocean variables

“…The dotted areas indicate where the ACC skill is different at the 90% significant level. Feng et al (2024). The different skill gain patterns between SST and OHC/SSH can be attributed to different factors: i) the SST forecast in dynamical models benefit from the memory of the OHC in the initial conditions, which will be included in the persistence of OHC itself but not in the persistence of SST; ii) the impact of the atmospheric component, which is stronger in SST than in the subsurface.…”