Secular change and inter-annual variability of the Gulf Stream position, 1993–2013, 70°−55°W

Abstract

The Gulf Stream (GS) is the northeastward-flowing surface limb of the Atlantic Ocean's meridional overturning circulation (AMOC) “conveyer belt” that flows towards Europe and the Nordic Seas. Changes in the GS position after its separation from the coast at Cape Hatteras, i.e., from 75°W to 50°W, may be key to understanding the AMOC, sea level variability and ecosystem behavior along the east coast of North America. In this study we compare secular change and inter-annual variability (IAV) of the Gulf Stream North Wall (GSNW) position with equator-ward Labrador Current (LC) transport along the southwestern Grand Banks near 52°W using 21 years (1993–2013) of satellite altimeter data. Results at 55°, 60°, and 65°W show a significant southward (negative) secular trend for the GSNW, decreasing to a small but insignificant southward trend at 70°W. IAV of de-trended GSNW position residuals also decreases to the west. The long-term secular trend of annual mean upper layer (200m) LC transport near 52°W is positive. Furthermore, IAV of LC transport residuals near 52°W along the southwestern Grand Banks are significantly correlated with GSNW position residuals at 55°W at a lag of +1-year, with positive (negative) LC transport residuals corresponding to southward (northward) GSNW positions one year later. The Taylor-Stephens index (TSI) computed from the first principal component of the GSNW position from 79° to 65°W shows a similar relationship with a more distal LC index computed along altimeter ground track 250 located north of the Grand Banks across Hamilton Bank in the western Labrador Sea. Increased (decreased) sea height differences along ground track 250 are significantly correlated with a more southward (northward) TSI two years later (lag of +2-years). Spectral analysis of IAV reveals corresponding spectral peaks at 5–7 years and 2–3 years for the North Atlantic Oscillation (NAO), GSNW (70°−55°W) and LC transport near 52°W for the 1993–2013 period suggesting a connection between these phenomena. An upper-layer (200m) slope water volume calculation using the LC IAV rms residual of +1.04Sv near 52°W results in an estimated GSNW IAV residual of 79km, or 63% of the observed 125.6km (1.13°) rms value at 55°W. A similar upper-layer slope water volume calculation using the positive long-term, upper-layer LC transport trend accounts for 68% of the mean observed secular southward shift of the GSNW between 55° and 70°W over the 1993–2013 period. Our work provides additional observational evidence of important interactions between the upper layers of the sub-polar and sub-tropical gyres within the North Atlantic over both secular and inter-annual time scales as suggested by previous studies.