On the modulation and alongcoast coherence of United States East Coast decadal sea level variability

Abstract:

United States East Coast tide gauges show: 1) multidecadal periods of enhanced decadal sea level variability (up to 8 cm amplitude) and 2) limited coherence in sea level across Cape Hatteras at decadal frequencies. No convincing explanation for these observations has been proposed to date.

Here, we apply spectral methods to detrended sea level measurements at 12 United States East Coast tide gauges with long, nearly continuous, records. Tide gauges south of Portland, Maine and north of Key West, Florida exhibit significant peaks in power at 10-15 year periods that are modulated in time and space. Although frequency resolution is limited by tide gauge record lengths, we identify two distinct, significant, spectral peaks at 11.7 and 13.8 year periods at many locations. By fitting the superposition of two sine curves at these periods to each sea level time series, we are able to explain up to 30% of the total variance and 50-94% of the variance in the 10-15 year band for tide gauges between Portland and Key West. This simple model successfully captures the modulation of decadal sea level variability at ~80 year periods and the along-coast variation in the phase of the modulated signal. The model also predicts that the amplitude of decadal sea level variability will increase over the next three decades in many locations, most prominently south of Cape Hatteras.

We propose that observed United States East Coast decadal sea level variability, and its apparent limited coherence across Cape Hatteras, reflects the local expression of two coherent, quasi-decadal modes. This improved understanding of spatial and temporal structure of sea level variability may inform interpretations of geographic shifts in sea level “hotspots” over the past few decades.