Seasonal Cycle in Sea Level Across the Coastal Zone

Abstract

Data from tide gauges and satellite altimeters are used to provide an up-to-date assessment of the mean seasonal cycle in sea level ($\zeta$) over most of the global coastal ocean. The tide gauge records, where available, depict a $\zeta$ seasonal cycle with complex spatial structure along and across continental boundaries, and an annual oscillation dominating over semiannual variability, except in a few regions (e.g., the northwestern Gulf of Mexico). Comparisons between tide gauge and altimeter data reveal substantial root-mean-square differences and only slight improvements in agreement when using along-track data optimized for coastal applications. Quantification of the uncertainty in the altimeter products, inferred from comparing gridded and along-track estimates, indicate that differences to tide gauges partly reflect short-scale features of the seasonal cycle in proximity to the coasts. We additionally probe the $\zeta$ seasonal budget using satellite gravimetry-based manometric estimates and steric terms calculated from the World Ocean Atlas 2023. Focusing on global median values, the sum of the estimated steric and manometric harmonics can explain $\sim$ 65% (respectively 40%) of the annual (semiannual) variance in the coastal $\zeta$ observations. We identify several regions, for example, the Australian seaboard, where the seasonal $\zeta$ budget is not closed and illustrate that such analysis is mainly limited by the coarse spatial resolution of present satellite-derived mass change products. For most regions with a sufficiently tight budget closure, we find that although the importance of the manometric term generally increases with decreasing water depth, steric contributions are non-negligible near coastlines, especially at the annual frequency.