Global sea level reconstruction for 1900–2015 reveals regional variability in ocean dynamics and an unprecedented long weakening in the Gulf Stream flow since the 1990s

Abstract

Abstract. A new monthly global sea level reconstruction for 1900–2015 was analyzed and compared with various observations to examine regional variability and trends in the ocean dynamics of the western North Atlantic Ocean and the US East Coast. Proxies of the Gulf Stream (GS) strength in the Mid-Atlantic Bight (GS-MAB) and in the South Atlantic Bight (GS-SAB) were derived from sea level differences across the GS. While decadal oscillations dominate the 116-year record, the analysis showed an unprecedented long period of weakening in the GS flow since the late 1990s. The only other period of long weakening in the record was during the 1960s–1970s, and red noise experiments showed that is very unlikely that those just occurred by chance. Ensemble empirical mode decomposition (EEMD) was used to separate oscillations at different timescales, showing that the low-frequency variability of the GS is connected to the Atlantic Multi-decadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC). The recent weakening of the reconstructed GS-MAB was mostly influenced by weakening of the upper mid-ocean transport component of AMOC as observed by the RAPID measurements for 2005–2015. Comparison between the reconstructed sea level near the coast and tide gauge data for 1927–2015 showed that the reconstruction underestimated observed coastal sea level variability for timescales less than ∼5 years, but lower-frequency variability of coastal sea level was captured very well in both amplitude and phase by the reconstruction. Comparison between the GS-SAB proxy and the observed Florida Current transport for 1982–2015 also showed significant correlations for oscillations with periods longer than ∼5 years. The study demonstrated that despite the coarse horizontal resolution of the global reconstruction (1∘ × 1∘), long-term variations in regional dynamics can be captured quite well, thus making the data useful for studies of long-term variability in other regions as well.

Western boundary circulation and coastal sea-level variability in Northern Hemisphere oceans

“…In the Pacific, tide gauges in the region west of Kii, where the sea level is less affected during large meander periods, should be preferred. That tide gauges can be used to infer past open-ocean dynamics is a result in line with some recent studies which have reconstructed openocean sea level (and hence ocean dynamics) using the global tide gauge network and advanced statistical methods (Ezer and Dangendorf, 2020;Dangendorf et al, 2019Dangendorf et al, , 2021.…”

Has the Gulf Stream Slowed or Shifted in the Altimetry Era?

“…Studies which average in stream‐following coordinates (Dong et al., 2019; Rossby et al., 2014; W. Z. Zhang et al., 2020 and this study) do not find significant changes of the GS transport at the Oleander transect. Thus, the “slowdown” of the GS reported by some previous studies (Caesar et al., 2018; Ezer & Dangendorf, 2020; Smeed et al., 2018; Yang et al., 2016) may not be due to a weakened GS but the destabilization of the GS path.…”

North American East Coast Sea Level Exhibits High Power and Spatiotemporal Complexity on Decadal Timescales

“…For example, in Figure 1b, at the Battery tide gauge (New York City) shows a shift to more pronounced quasi‐decadal variability around 1960–1970. This shift is particularly evident in tide gauges north of Cape Hatteras, and is apparent in earlier studies utilizing long east coast tide gauge records (e.g., Ezer & Dangendorf, 2020; Piecuch et al., 2017; Woodworth et al., 2017). However, epochs of amplified and damped quasi‐decadal variability are evident in most tide gauge records along the east coast (Figures 1c and ).…”

“…Over shorter (interannual) periods, the SST tripole has been found to be strongly correlated with North Atlantic sea surface height (an “SSH tripole,” with out‐of‐phase coastal sea level variability across Cape Hatteras) (Volkov et al., 2019). Investigations of the relationship between the SST and SSH tripoles and ocean dynamics at decadal timescales, including those governing Gulf Stream variability (e.g., Ezer, 2019; Ezer & Dangendorf, 2020; Hameed et al., 2018; Nigam et al., 2018), are likely to be informative.…”