Reconstruction of the Gulf Stream from 1940 to the Present and Correlation with the North Atlantic Oscillation

Abstract

In this study, the Gulf Stream (GS)’s response to the North Atlantic Oscillation (NAO) is investigated by generating an observation-based reconstruction of the GS path between 70° and 50°W since 1940. Using in situ data from the World Ocean Database (WOD), SeaDataNet, International Council for the Exploration of the Sea (ICES), Hydrobase3, and Argo floats, a harmonized database of more than 40 million entries is created. A variational inverse method implemented in the software Data Interpolating Variational Analysis (DIVA) allows the production of time series of monthly analyses of temperature and salinity over the North Atlantic (NA). These time series are used to derive two GS indices: the GS north wall (GSNW) index for position and the GS delta (GSD) index as a proxy of its transport. This study finds a significant correlation (0.37) between the GSNW and the NAO at a lag of 1 year (NAO preceding GS) since 1940 and significant correlations (0.50 and 0.43) between the GSD and the NAO at lags of 0 and 2 years between 1960 and 2014. The authors suggest this 2-yr lag is due to Rossby waves, generated by NAO variability, that propagate westward from the center of the NA. This is the first reconstruction of GS indices over a 75-yr period based on an objective method using the largest in situ dataset so far.

Properties of baroclinic Rossby waves in the North Atlantic from eddy-resolving simulations of ocean circulation

“…Following the state of the art, the time lag between a specific NAO phase (+ or -) and its consequence on the GS path and transport still remains a question on which there is no perfect consensus at this stage. As a matter of fact, the bulk of estimates ranges between 0 and 2 years considering both GS path and transport (Watelet et al, 2017).…”

“…Still, the major part of GS indices are computed on a yearly basis, for both position and transport, which leaves room for deeper investigation. We thus used the monthly GS Delta index (GSD, proxy for its transport) from Watelet et al (2017), already averaged on running 12 months, and used their monthly GS North Wall index (GSNW, representative of its position) that we averaged the same way. This allows the computation of correlations with NAO using monthly time lags, as shown in Figure 7.…”

“…The chosen period allows for a 13-year spin-up of the simulation. We decided to focus on the latitude 39°N which was considered representative of the average GS path from our SST fields in Watelet et al (2017). Here, indeed, we only examine the zonal component of the Rossby waves travelling across the NA.…”

“…Regarding longitudes, we chose to analyse propagating signals from 30 to 70°W, i.e. between the approximate center of the NA at 39°N where we can expect the NAO signal to be carried to the ocean through wind forcing and the western limit of the GS region as defined in Watelet et al (2017).…”

The Distinction Between the Gulf Stream and Its North Wall

“…The estimates of the NW position obtained by Watelet et al's method are very different from the NW positions determined by subjective analysis, as seen in Figure 6 of Watelet et al (). Additionally, the NW index of Watelet et al () has different relationship to the North Atlantic Oscillation than the index derived from the subjective analysis (Taylor & Stephens, ), with significant correlations only at lag 0 instead of for lags of 0, 1, and 2 years (Hameed et al, ). The likely reason for this difference is that Watelet et al's method conflates the positions of the NW and SSF.…”