Detecting changes in the transport of the Gulf Stream and the Atlantic overturning circulation from coastal sea level data: The extreme decline in 2009-2010 and estimated variations for 1935-2012

Recent studies reported weakening in the Atlantic Meridional Overturning Circulation (AMOC) and in the Gulf Stream (GS), using records of about a decade (RAPID project) or two (altimeter data). Coastal sea level records are much longer, so the possibility of detecting climatic changes in ocean circulation from sea level data is intriguing and thus been examined here. First, it is shown that variations in the AMOC transport from the RAPID project since 2004 are consistent with the flow between Bermuda and the U. S. coast derived from the Oleander measurements and from sea level difference (SLDIF). Despite apparent disagreement between recent studies on the ability of data to detect weakening in the GS flow, estimated transport changes from 3 different independent data sources agree quite well with each other on the extreme decline in transport in 2009-2010. Due to eddies and meandering, the flow representing the GS part of the Oleander line is not correlated with AMOC or with the Florida Current, only the flow across the entire Oleander line from the U.S. coast to Bermuda is correlated with climatic transport changes. Second, Empirical Mode Decomposition (EMD) analysis shows that SLDIF can detect (with lag) the portion of the variations in the AMOC transport that are associated with the Florida Current and the wind-driven Ekman transport (SLDIF-transport correlations of ~ 0.7-0.9). The SLDIF has thus been used to estimate variations in transport since 1935 and compared with AMOC obtained from reanalysis data. The significant weakening in AMOC after ~ 2000 (~ 4.5 Sv per decade) is comparable to weakening seen in the 1960s to early 1970s. Both periods of weakening AMOC, in the 1960s and 2000s, are characterized by faster than normal sea level rise along the northeastern U.S. coast, so monitoring changes in AMOC has practical implications for coastal protection.