Climate warming does not force sea-level rise (SLR) at the same rate everywhere. Rather, there are spatial variations of SLR superimposed on a global average rise. These variations are forced by dynamic processes 1-4 , arising from circulation and variations in temperature and/or salinity, and by static equilibrium processes 5 , arising from mass redistributions changing gravity and the Earth's rotation and shape. These sealevel variations form unique spatial patterns, yet there are very few observations verifying predicted patterns or fingerprints 6 . Here, we present evidence of recently accelerated SLR in a unique 1,000-km-long hotspot on the highly populated North American Atlantic coast north of Cape Hatteras and show that it is consistent with a modelled fingerprint of dynamic SLR. Between 1950Between -1979Between and 1980Between -2009, SLR rate increases in this northeast hotspot were ∼3-4 times higher than the global average. Modelled dynamic plus steric SLR by 2100 at New York City ranges with Intergovernmental Panel on Climate Change scenario from 36 to 51 cm (ref. 3); lower emission scenarios project 24-36 cm (ref. 7). Extrapolations from data herein range from 20 to 29 cm. SLR superimposed on storm surge, wave run-up and set-up will increase the vulnerability of coastal cities to flooding, and beaches and wetlands to deterioration.We test the hypothesis that a statistically significant observed northeast hotspot (NEH) of accelerated SLR exists by determining its position and dimensions and comparing them with model projections 1-4 . We explore correlations between rate changes of observed NEH SLR and of climate indices potentially relevant to NEH formation.In the late twentieth century, sea levels were relatively low along the North American east coast, particularly north of Cape Hatteras 8,9 . Sea-surface gradients sloped down towards the coast away from the Gulf Stream and its continuation to the northeast, the North Atlantic Current 10 . The sharp pressure gradients balance the Coriolis force to sustain these narrow and strong geostrophic currents, leading to low coastal sea levels.These low levels could rise with warming and/or freshening of surface water in the subpolar north Atlantic, where less dense water inhibits deep convection associated with the Atlantic Meridional Overturning Current (AMOC). The AMOC weakens and pressure gradients along the North American east coast decrease, raising sea levels. The models considered here simulate this dynamic SLR using Intergovernmental Panel for Climate Change (IPCC) Special Report on Emissions Scenarios warming scenarios 2-4 and/or assumed freshening scenarios 1,4 . Gyre system weakening by changes in the North Atlantic Oscillation 11,12 (NAO) could also reduce sea-level gradients and raise sea levels.To establish the observed NEH, we analyse tide-gauge records along the North American Atlantic coast for increasing rates of SLR (see Methods and Supplementary Information). With leastsquares linear regression, rates of SLR were found for the first an...
Onshore volume transport (Stokes drift) due to surface gravity waves propagating toward the beach can result in a compensating Eulerian offshore flow in the surf zone referred to as undertow. Observed offshore flows indicate that wave-driven undertow extends well offshore of the surf zone, over the inner shelves of Martha's Vineyard, Massachusetts, and North Carolina. Theoretical estimates of the wave-driven offshore transport from linear wave theory and observed wave characteristics account for 50% or more of the observed offshore transport variance in water depths between 5 and 12 m, and reproduce the observed dependence on wave height and water depth.During weak winds, wave-driven cross-shelf velocity profiles over the inner shelf have maximum offshore flow (1-6 cm s Ϫ1) and vertical shear near the surface and weak flow and shear in the lower half of the water column. The observed offshore flow profiles do not resemble the parabolic profiles with maximum flow at middepth observed within the surf zone. Instead, the vertical structure is similar to the Stokes drift velocity profile but with the opposite direction. This vertical structure is consistent with a dynamical balance between the Coriolis force associated with the offshore flow and an along-shelf "Hasselmann wave stress" due to the influence of the earth's rotation on surface gravity waves. The close agreement between the observed and modeled profiles provides compelling evidence for the importance of the Hasselmann wave stress in forcing oceanic flows. Summer profiles are more vertically sheared than either winter profiles or model profiles, for reasons that remain unclear.
A new type of alongshore progressive wave with periods and alongshore wavelengths of the order of 102 seconds and meters, respectively, has been observed in the surf zone. These periods fall into the lower end of the much studied infragravity frequency band previously shown to contain surface gravity edge and leaky waves. However, their short wavelengths (more than an order of magnitude smaller than the mode 0 edge wave) distinguish these new waves from surface gravity waves. In addition, they are only observed in the presence of mean longshore current and they change celerity (O(1 m/s)) and direction with the mean current. Alongshore wavenumber-frequency spectra clearly identify these waves, distinct from edge and leaky waves, by their approximately linear dispersion line at wavenumbers greater than the mode 0 edge wave dispersion curve. Their rms horizontal velocities can exceed 30 cm/s. These waves are shown to be consistent with a model [Bowen and Holman, this issue] of vorticity waves generated by the shear instability of the mean longshore current. 18,03118,032
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.