Using a recently developed 1/12th degree regional ocean model, we establish a link between U.S. East Coast sea level variability and offshore upper-ocean heat content change. This link manifests as a cross-shore mass redistribution driven by an offshore thermosteric sea level response to subsurface warming or cooling. Approximately 50\% of simulated monthly to inter-annual coastal sea level variance south of Cape Hatteras can be statistically accounted for by this mechanism, realized as a function of regional ocean hypsometry, gyre scale warming, and the depth-dependence of density change. This response to offshore warming explains the non-stationarity of U.S. East coast sea level covariance, specifically observed and modeled behavior after $\sim$ 2010. Since approximately 2010, elevated rates of sea level rise south of Cape Hatteras can be partly explained as the result of shore-ward mass redistribution due to offshore sub-surface warming within the North Atlantic subtropical gyre. These results reveal a mechanism that connects local coastal sea level to a broader region and identifies the influence of regional heat content changes on coastal sea level. This analysis presents a framework for identifying new regions that may be susceptible to enhanced sea level rise due to ocean warming and helps bridge the gap between quantifying large scale change and anticipating local coastal impacts like flooding and storm surge.

Sea-level change threatens the U.S. East Coast. Thus, it is important to understand the underlying causes, including ocean dynamics. Most past studies emphasized links between coastal sea level and local atmospheric variability or large-scale circulation and climate, but possible relationships with local ocean currents over the shelf and slope remain largely unexplored. Here we use 7 years of in-situ velocity and sea-level data to quantify the relationship between northeastern U.S. coastal sea level and variable Shelfbreak Jet transport south of Nantucket Island. At timescales of 1-15 days, southern New England coastal sea level and transport vary in anti-phase, with magnitude-squared coherences of ~0.5 and admittance amplitudes of ~0.3 m Sv-1. These results are consistent with a dominant geostrophic balance between along-shelf transport and coastal sea level, corroborating a hypothesis made decades ago that was not tested due to the lack of transport data.

The daily rise and fall of the tides are intimately familiar to those living on the coast. However, due to sea-level rise, what communities in the United States now experience as the highest, lowest, and average water levels on a typical day no longer corresponds to the official definitions of high tide, low tide, and mean sea level, respectively. Water levels now regularly exceed official high tide along parts of the Chesapeake Bay, Gulf Coast, and Puerto Rico. In other words, yesterday's high tide is becoming today's new normal. This demonstrates how sea-level rise is radically redefining the American shore.

Changes in the seasonal sea level cycle (SSLC) can modulate the flooding risk along coastlines. Here, we use harmonic analysis to quantify changes in the amplitude and phase of the annual component of the sea level cycle at 663 tide gauge locations along the global coastline where long records are available. We identify coastal hotspots by applying clustering methods revealing coherent regions with similar patterns of variability in the annual sea level cycle (ASLC). Results show that for most tide gauges the annual amplitude reached its maximum after 1970 and its peak typically occurs during the fall season of the respective hemisphere. Many tide gauges exhibit non-stationarity in the annual cycle in terms of amplitude and/or phase. For example, at 125 tide gauges we find significant trends in the amplitude (either increasing or decreasing) while several sites (36 in total), mostly in the Mediterranean and around Pacific islands, experienced phase changes leading to shifts in the timing of the peak of the annual cycle by more than a month. Our results highlight the importance of accounting for potential non-stationarity in seasonal mean sea level (MSL) cycles along coastlines.

The Gulf Stream is a vital limb of the North Atlantic circulation that influences regional climate, sea level, and hurricane activity. Given the Gulf Stream’s relevance to weather and climate, many studies have attempted to estimate trends in its volumetric transport from various datasets, but results have been inconclusive, and no consensus has emerged whether the current is weakening with climate change. Here we use Bayesian analysis to jointly assimilate multiple datasets from the Florida Straits to quantify uncertainty and change in Gulf Stream volume transport since 1982. We find with virtual certainty (probability $P>99\%$) that Gulf Stream volume transport through the Florida Straits declined by $1.2 \pm 1.0$ Sv in the past 40 years (95\% credible interval). This represents the first unequivocal evidence for a recent multidecadal decline in this climate-relevant component of ocean circulation.

Rates of sea-level rise are increasing across the global ocean. Since $\sim 2008$, sea-level acceleration is particularly pronounced along the US Gulf of Mexico coastline. Here we use model solutions and observational data to identify the physical mechanisms responsible for enhanced rates of recent coastal sea-level rise in this region. Specifically, we quantify the effect of offshore subsurface ocean warming on coastal sea-level rise and its relationship to regional hypsometry. Using the Estimating the Circulation and Climate of the Ocean (ECCO) Version 5 ocean state estimate, we establish that coastal sea-level changes are largely the result of changes in regional ocean mass, reflected in ocean bottom pressure, on interannual to decadal timescales. These coastal ocean bottom pressure changes reflect both net mass flux into and out of the Gulf, as well as internal mass redistribution within the Gulf, which can be understood as an isostatic ocean response to subsurface offshore warming. We test the relationships among coastal sea-level, ocean bottom pressure, and subsurface ocean warming predicted by the model using data from satellite gravimetry, satellite altimetery, tide gauges, and Argo floats. Our estimates of mass redistribution explain a significant fraction of coastal sea-level trends observed by tide gauges. For instance, at St. Petersburg, Florida, this mass redistribution accounts for $>$ 50\% of the coastal sea-level trend observed over the 2008-2017 decade. This study elucidates a physical mechanism whereby coastal sea-level responds to open-ocean subsurface warming and motivates future studies of this linkage in other regions.

Satellite observations are used to establish the dominant magnitudes, scales, and mechanisms of intraseasonal variability in ocean dynamic sea level (ζ) in the Persian Gulf over 2002-2015. Empirical orthogonal function (EOF) analysis applied to altimetry data reveals a basin-wide, single-signed intraseasonal fluctuation that contributes importantly to ζ variance in the Persian Gulf at monthly to decadal timescales. An EOF analysis of Gravity Recovery and Climate Experiment (GRACE) observations over the same period returns a similar large-scale mode of intraseasonal variability, suggesting that the basin-wide intraseasonal ζ variation has a predominantly barotropic nature. A linear barotropic theory is developed to interpret the data. The theory represents Persian-Gulf-average ζ () in terms of local freshwater flux, barometric pressure, and wind stress forcing, as well as ζ at the boundary in the Gulf of Oman. The theory is tested using a multiple linear regression with these freshwater flux, barometric pressure, wind stress, and boundary ζ quantities as input, and as output. The regression explains 70%+/-9% (95% confidence interval) of the intraseasonal variance. Numerical values of regression coefficients computed empirically from the data are consistent with theoretical expectations from first principles. Results point to a substantial non-isostatic response to surface loading. The Gulf of Oman ζ boundary condition shows lagged correlation with ζ upstream along the Indian Subcontinent, Maritime Continent, and equatorial Indian Ocean, suggesting a large-scale Indian-Ocean influence on intraseasonal variation mediated by coastal and equatorial waves, and hinting at potential predictability. This study highlights the value of GRACE for understanding sea level in an understudied marginal sea.

Atmospheric rivers (ARs) effect inland hydrological impacts related to extreme precipitation. However, little is known about the possible coastal hazards associated with these events. Here we elucidate high-tide floods (HTFs) and storm surges during ARs on the US West Coast during 1980—2016. HTFs and landfalling ARs co-occur more often than expected from random chance. Between 10%—63% of HTFs coincide with landfalling ARs, depending on location. However, only 2%—15% of ARs coincide with HTFs, suggesting that ARs typically must co-occur with anomalously high tides or mean sea levels to cause HTFs. Storm surges during ARs are interpretable in terms of local wind, pressure, and precipitation forcing: meridional wind and barometric pressure are the primary drivers of storm surge, but precipitation can also make secondary contributions. This study highlights the relevance of ARs to coastal impacts, clarifies the drivers of storm surge during ARs, and identifies future research directions.

The Florida Current marks the beginning of the Gulf Stream at Florida Straits, and plays an important role in climate. Nearly continuous measurements of Florida Current transport are available at 27N since 1982. These data are too short for assessing possible multidecadal or centennial trends. Here I reconstruct Florida Current transport during 1909-2018 using probabilistic methods and principles of ocean physics applied to the available transport data and longer coastal sea-level records. Florida Current transport likely declined steadily during the past century. Transport since 1982 has likely been weaker on average than during 1909-1981. The weakest decadal-mean transport in the last 110 y likely took place in the past two decades. Results corroborate hypotheses that the deep branch of the overturning circulation declined over the recent past, and support relationships observed in climate models between the overturning and surface western boundary current transports at multidecadal and longer timescales.

Reconstructions of Common-Era sea level are informative of relationships between sea level and natural climate variation, and the uniqueness of modern sea-level rise. Kench et al. recently reconstructed Common-Era sea level in the Maldives, Indian Ocean, using coral microatolls. They reported periods of 150-500 yr when sea level fell and rose at average rates of 2.7-4.3 mm/yr. These periods coincided with intervals of cooling and warming inferred from proxy reconstructions of sea-surface temperature (SST) and radiative forcing (ref. 2, Fig. 2). Kench et al. reasoned that these 0.6-1.4-m centennial-scale sea-level fluctuations were driven by climate, specifically thermal contraction and expansion of seawater. In contrast to previous studies, Kench et al. argued that modern rates and magnitudes of sea-level rise caused by climate change have precedent during the Common Era. We use principles of sea-level physics to argue that pre-industrial radiative forcing and SST changes were insufficient to cause thermosteric sea-level (TSL) trends as large as reported for the Maldives.

Identifying the causes for historical sea-level changes in coastal tide-gauge records is important for constraining oceanographic, geologic, and climatic processes. The Río de la Plata estuary in South America features the longest tide-gauge records in the South Atlantic. Despite the relevance of these data for large-scale circulation and climate studies, the mechanisms underlying relative sea-level changes in this region during the past century have not been firmly established. I study annual data from tide gauges in the Río de la Plata and stream gauges along the Río Paraná and Río Uruguay to establish relationships between river streamflow and sea level over 1931-2014. Regression analysis suggests that streamflow explains 59%+/-17% of the total sea-level variance at Buenos Aires, Argentina, and 28%+/-21% at Montevideo, Uruguay (95% confidence intervals). A longterm streamflow increase effected sea-level trends of 0.71+/-0.35 mm/yr at Buenos Aires and 0.48+/-0.38 mm/yr at Montevideo. More generally, sea level at Buenos Aires and Montevideo respectively rises by (7.3+/-1.8)x10^-6 m and (4.7+/-2.6)x10^-6 m per 1 m^3 s^-1 streamflow increase. These observational results are consistent with simple theories for the coastal sea-level response to streamflow forcing, suggesting a causal relationship between streamflow and sea level mediated by ocean dynamics. Findings advance understanding of local, regional, and global sea-level changes, clarify sea-level physics, inform future projections of coastal sea level and the interpretation of satellite data and proxy reconstructions, and highlight future research directions.