From shelfbreak to shoreline: What is the relationship between coastal sea level and local ocean dynamics?

The US East Coast is a heavily populated coastline, and models project that it will experience faster-than-global rates of sea-level rise in the coming century. Sea level changes due to several factors, such as ocean thermal expansion, melting of land ice, and ocean dynamics. The latter is one of the least well-known sea-level components. While several studies have shown a relationship between Gulf Stream transport and coastal sea level South of Cape Hatteras, such causal linkage to large-scale open-ocean circulation has not been shown for coastal sea level North of Cape Hatteras, possibly because coastal sea level here is more strongly related to local circulation over the shelf and the slope. Here we use 7 years of hourly velocity data from the Ocean Observatory Initiative (OOI) Coastal Pioneer Array to test this hypothesis. The Array is located at the New England shelfbreak ~75 nautical miles south of Martha’s Vineyard, and is composed of seven site moorings, spread from the shelf to offshore of the shelf break. Each mooring contains, among other instruments, an upward-looking ADCP, which measures three-dimensional velocities throughout the water column. This unique dataset, together with the dense network of coastal tide gauges, allows for a great case study of coastal sea level and local ocean dynamics.

With a spectral coherence analysis we investigate the spatiotemporal relationship between coastal sea level and current velocities. We see that, after removing the effects of tides and the inverted barometer, low-frequency coherence between coastal sea level and the shallower moorings becomes apparent. The (spatial) structure of this relationship is revealed by quantifying correlation between sea level and current velocity as a function of distance between the Array and tide gauges. These results point to a geostrophic relationship between along-shelf transport and coastal sea level, corroborating hypotheses made decades ago that have never been tested, due to the lack of dedicated observations of shelf circulation. Additionally, our results suggest that focusing only on large-scale circulation, such as the Gulf Stream or the overturning, may not be satisfactory for understanding the most basic dynamics essential for making meaningful projections of the future.