The connection between coastal sea level and local ocean dynamics, and its relation to high-tide flooding along southern New England (U.S.)

According to NOAA’s Annual High Tide Flooding Outlook [1], the number of high-tide flooding days along the US East coast has increased  rapidly in recent years. High-tide flooding, also known as nuisance flooding, identifies floods that can occur in the absence of storms, for example, simply due to above-normal water levels.. Due to sea-level rise, it is predicted that, by 2050, coastal communities across the U.S. will experience high-tide flooding on average 45 to 85 days per year. Predicting the frequency of future coastal flooding is vital for the development and maintenance of coastal cities. Here we discuss the role of  local ocean dynamics to coastal flooding.

Along the Northeast US coast, an important driver of coastal sea-level variability is ocean dynamics, which includes large-scale circulation, such as the Gulf Stream, but also to smaller local ocean currents. An important circulation feature in this region is the Shelf break jet (SBJ). The SBJ flows equatorward from the Labrador Sea towards the Gulf Stream at Cape Hatteras following the shelf break along the Northeast US coast. We use velocity data from the Ocean Observatory Initiative (OOI) Coastal Pioneer Array and tide-gauge data during 2014-2022 to establish the connection between coastal sea level and local ocean circulation over the shelf and the slope. Located at the New England shelf break,  about 75 nautical miles south of Martha’s Vineyard, the Array 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. A spectral coherence and admittance analysis reveal that, after removing the effects of tides and the inverted barometer, about 30% of the coastal sea-level variance in the 1—15-day band in this region is related to the SBJ transport. This relationship has a clear spatial pattern: we find significant coherence between SBJ transport and coastal sea level from the South of New England to as far south as the Delaware coast, depending on frequency.

Since this frequency band coincides with the frequency variability of storm surges, we pose the question: “Are any of the flood events registered in this region related to SBJ variability”? To answer this question, we focus on 6 tide gauges stations along southern New England, which feature the highest coherence with SBJ transport in the 1—15-day band. When the jet-related variability is regressed off the tide-gauge sea level data over these frequencies, the number of minor flood days reduces. Thus, a fraction of coastal floods in these locations might be related to SBJ variability. This simple exercise highlights the importance of considering local ocean dynamics when projecting future coastal flooding.

 

Reference:

[1] https://tidesandcurrents.noaa.gov/high-tide-flooding/annual-outlook.html