Decadal Shift of NAO-Linked Interannual Sea Level Variability along the U.S. Northeast Coast

Abstract

Recent studies have linked interannual sea level variability and extreme events along the U.S. northeast coast (NEC) to the North Atlantic Oscillation (NAO), a natural internal climate mode that prevails in the North Atlantic Ocean. The correlation between the NAO index and coastal sea level north of Cape Hatteras was weak from the 1960s to the mid-1980s, but it has markedly increased since around 1987. The causes for the decadal shift remain unknown. Yet understanding the abrupt change is vital for decadal sea level prediction and is essential for risk management. Here we use a robust method, the Bayesian dynamic linear model (DLM), to explore the nonstationary NAO impact on NEC sea level. The results show that a spatial pattern change of NAO-related winds near the NEC is a major cause of the NAO–sea level relationship shift. A new index using regional sea level pressure is developed that is a significantly better predictor of NEC sea level than is the NAO and is strongly linked to the intensity of westerly winds near the NEC. These results point to the vital importance of monitoring regional changes of wind and sea level pressure patterns, rather than the NAO index alone, to achieve more accurate predictions of sea level change along the NEC.

How is New England Coastal Sea Level Related to the Atlantic Meridional Overturning Circulation at 26° N?

“…To assess the contribution of local forcing to the relationship between T moc and (Figure 1), we consider T ek and loc time series in Figure 2a. Note that loc explains 41% of the variance and T ek explains 39% of the T moc variance on nonseasonal monthly timescales during 2004-2017, consistent with past studies arguing that local forcing contributes importantly to and T moc variability on these timescales (e.g., Andres et al, 2013;Heimbach et al, 2011;Kenigson et al, 2018;Li et al, 2014;Piecuch & Ponte, 2015;Piecuch et al, 2016;Pillar et al, 2016;Woodworth et al, 2014;Zhao & Johns, 2014). For example, Woodworth et al (2014) argue that local winds are a dominant driver of interannual sea level variability along the northeast American Atlantic coast.…”

“…Our interpretation is similar to the recent paradigm due to Volkov et al (), whereby anticorrelation between the Atlantic overturning at 26° N and sea level across the Mediterranean Sea results from an analogous atmospheric teleconnection and local wind forcing. More broadly, by elucidating the anticorrelation between coastal sea level and the overturning circulation and the role of local atmospheric forcing, this study complements past papers establishing the importance of local air pressure and alongshore wind stress associated with large‐scale climate modes as drivers of coastal sea level variability in this region on these timescales (Andres et al, ; Kenigson et al, ; Li et al, ; Noble, ; Piecuch & Ponte, ; Piecuch et al, ; Sandstrom, ; Thompson, ; Woodworth et al, ). Our findings clarify that anticorrelation observed between coastal sea level and the overturning circulation does not reflect a causal relationship in the sense of geostrophy, but rather arises from ageostrophic processes forced by temporally coherent, spatially separated local atmospheric forcing mechanisms.…”

“…However, tide gauge data and reanalysis wind and pressure estimates are available for longer time periods (Compo et al, ; Laloyaux et al, ; Poli et al, ), and future studies should analyze relationships between New England sea level and Ekman transports across the subtropical Atlantic to see if similar results are found over longer periods going back in time. Such an analysis is timely, because the relationship between U.S. northeast coast sea level and the NAO is nonstationary—the two quantities are significantly correlated on interannual timescales during 1987–2012, but uncorrelated over 1970–1986 (Andres et al, ; Kenigson et al, ). Moreover, given the frequency dependence of the oceanic response to atmospheric forcing (e.g., Frankignoul et al, ), it is unclear to what extent our results for shorter periods apply more generally to longer timescales.…”

“…The p a and ⃗ distributions associated with periods of anomalous T ek and loc values (Figure 4), especially the p a centers of action near the Azores High, are reminiscent of the NAO (Hurrell, 1995), West Atlantic Index (Kenigson et al, 2018), and Arctic Oscillation (Thompson & Wallace, 1998) teleconnection patterns (cf. Figure S15).…”

“…The reverse occurs during negative NAO states, with AMOC weakening at 26° N and sea level increasing over the Mediterranean (Volkov et al, ). Since previous studies highlight the influence of local air pressure and alongshore winds (partly tied to the NAO) in driving sea level changes on the New England coast (Andres et al, ; Kenigson et al, ; Li et al, ; Noble, ; Piecuch & Ponte, ; Piecuch et al, ; Sandstrom, ; Thompson, ; Woodworth et al, ), it stands to reason that instances of anticorrelation between New England coastal sea level and AMOC monitored at 26° N could result from a similar teleconnection process.…”