Ocean mass, sterodynamic effects, and vertical land motion largely explain US coast relative sea level rise

Global mean sea-level (GMSL) rise is a key indicator of climate warming, but regional relative sea-level (RSL) rise is more crucial for coastal planning and decision-making. Regional sea-level changes often deviate significantly from GMSL rise due to various processes. During the satellite altimeter era (1993-present), regional geocentric sea-level (GSL) rates have exceeded GMSL rise in some areas, while being near zero in others. Local vertical land motion (VLM) further complicates these regional patterns. Accurate projections of future regional RSL changes require a deep understanding of these contributing processes and their temporal and spatial variations. The U.S. coastlines showcase this regional variability. The East Coast experiences faster sea-level rise than the West Coast, with additional local variations. Previous research linked high rates on the East Coast to VLM, glacial isostatic adjustment (GIA), and ocean sterodynamic processes, with variations north and south of Cape Hatteras. The highest rates are in the western Gulf of Mexico, largely due to subsidence from subsurface fluid withdrawal. The Pacific Coast shows rates lower than the global average, partly due to decadal climate variability influenced by the Pacific Decadal Oscillation (PDO). Despite improvements in our understanding due to expanded sea-level observing networks, gaps and limitations remain that hinder reliable future RSL assessments. This study aims to explain RSL trends along U.S. coastlines during the satellite altimeter era (1993-2018) by combining contributions from VLM, sterodynamic effects, and ocean mass changes at 55 tide-gauge locations. The goal is to determine if these processes fully account for observed RSL trends and to identify areas where our understanding is lacking.

Numerous studies have investigated sea-level trends along U.S. coastlines. Research has linked the high rates along the U.S. East Coast to VLM and GIA, as well as ocean sterodynamic processes, with varying influences north and south of Cape Hatteras where the Gulf Stream separates from the coast. Studies also highlight the role of high subsidence rates in the western Gulf of Mexico due to subsurface fluid withdrawal. Conversely, the Pacific Coast's slower rise is partially explained by decadal climate variability related to the PDO. However, despite these advances, gaps in understanding persist, impacting the ability to reliably project future RSL changes.

The study employs a sea-level budget equation to assess the contributions of different processes to RSL trends at 55 U.S. tide-gauge locations from 1993 to 2018. The equation incorporates: 1. **Contemporary Mass Redistribution (CMR):** GSL trend from changes in land ice mass and terrestrial water storage, using GRD patterns from Frederikse et al. (2020). 2. **Sterodynamic Effects (SD):** Trend in steric sea level and regional sea-level changes from ocean dynamics, estimated using the Estimating the Circulation & Climate of the Ocean (ECCO) framework. 3. **Glacial Isostatic Adjustment (GIA):** RSL trend from GIA, accounting for effects on both GSL and VLM, using an ensemble of GIA models from Caron et al. (2018). 4. **Vertical Land Motion (VLM):** VLM trend observed by GPS, with the GIA component subtracted to avoid double-counting. A GPS imaging technique was used to estimate VLM at tide-gauge locations, averaging GPS observations weighted by record length and distance. The equation is: *RSL_TG*(r) = CMR(r) + SD(r) + GIA_RSL(r) − (VLM(r) − GIA_VLM(r)) + RES(r) Where GIA is separated into GIA effects on sea level (GIA_RSL) and GIA effects on VLM (GIA_VLM) to prevent double counting. A similar equation is used for altimetry-derived GSL trends (GSL_ALT), excluding VLM. The residual (RES) represents unexplained variation. The U.S. coast is divided into three regions: Northeast, Southeast (including Gulf Coast), and West Coast. Linear trends and uncertainties are computed for each component, accounting for serial correlation. Process-based uncertainties are incorporated into the overall uncertainty estimates for the sum of contributors and residuals. Comparisons are made between tide gauge and altimeter data to validate the findings. The distance to the nearest GPS station is also analyzed for its influence on residual magnitude.

The study successfully explains RSL trends within uncertainty estimates at 47 of 55 tide gauges. Key findings include: * **Regional Patterns:** Clear spatial patterns in sea-level rise are observed and explained. The West Coast rise is primarily driven by CMR with minimal sterodynamic influence. High subsidence rates on the Gulf Coast are linked to subsurface fluid extraction. On the Atlantic Coast, sterodynamic and CMR rates are similar, with a larger GIA role northward. * **Residual Analysis:** While many locations have residuals not statistically different from zero, regional biases are observed, particularly in the Northeast, indicating potential systematic issues. These biases suggest that one or more of the process-based estimates in the Northeast might need reassessment. * **VLM and Sterodynamic Effects:** Analysis suggests that VLM and sterodynamic estimates are likely contributors to larger residuals or regional biases. The discrepancies between tide-gauge and altimeter trends suggest a role for local VLM processes. However, there is no clear correlation between the distance from tide gauges to the nearest GPS station and residual magnitude. * **Altimeter Comparison:** Comparisons with altimeter data (GSL) show general agreement, particularly on the West Coast. The Northeast region, however, exhibits consistent overestimation of the reconstructed trend compared to altimeter measurements, likely due to overestimation of sterodynamic effects. * **Limitations:** While many locations' RSL trends are explained by the combined contributions of CMR, sterodynamic effects, and VLM, significant uncertainties persist, especially in VLM estimates due to the lack of collocated GPS stations and short, inconsistent GPS records. The study also notes limitations in estimating sterodynamic effects due to observational constraints.

The study's success in explaining RSL trends at most locations highlights the importance of accounting for CMR, sterodynamic effects, and VLM. The regional biases identified, particularly in the Northeast, underscore the need for continued improvements in the accuracy of process-based estimates, especially for sterodynamic effects. The observed discrepancies between tide-gauge and altimeter trends, primarily driven by VLM uncertainties, highlight the need for denser and more consistently measured GPS data. The large number of locations showing agreement within uncertainty margins provides confidence in the employed methodologies, while the remaining unexplained variance points toward the need for refined modeling and improved observations.

This study provides a comprehensive assessment of the processes contributing to RSL rise along the U.S. coast (1993-2018). The methodology successfully explains observed trends at most locations, highlighting the critical role of CMR, sterodynamic effects, and VLM. However, regional biases and unexplained residuals reveal the need for improved data collection and modeling, especially regarding VLM and sterodynamic effects. Future research should focus on refining the process-based estimates to reduce uncertainties and improve projections for future sea-level rise.

The study acknowledges several limitations: the lack of collocated GPS stations at many tide-gauge locations introduces uncertainties in VLM estimations. The short record lengths and inconsistencies of GPS data further compound these uncertainties. Estimating coastal sterodynamic effects also presents challenges due to limitations in observational data. The study also notes potential biases or inaccuracies in the process-based estimates, particularly in the Northeast region, which require further investigation.