Multidecadal variability of potential temperature, salinity, and transport in the eastern subpolar North Atlantic

Abstract

The Extended Ellett Line (EEL) hydrographic section extends from Scotland to Iceland crossing the Rockall Trough, Hatton‐Rockall Basin, and Iceland Basin. With 61 full‐depth stations at a horizontal resolution of 10–50 km, the EEL samples the upper limb of the Atlantic Meridional Overturning Circulation flowing across the Iceland‐Scotland Ridge into the Nordic Seas. The Rockall Trough has been sampled nearly four times per year from 1975 to 1996, and the full section annually since 1996. The EEL is an exceptionally long‐time series of deep ocean temperatures and salinities. This study extends prior work in the Rockall Trough, and examines for the first time 18 year records in the Iceland and Hatton‐Rockall Basins. We quantify errors in the time series from two sources: observational errors and aliasing. The data quality and annual sampling are suitable for observing interannual to decadal variability because the variability exceeds our error estimates. The upper waters of all three basins are cooler/fresher from 1997 to 2001, warmer/more saline 2001–2006, and cooler/fresher from 2006 to 2014. A reference level for geostrophic shear is developed heuristically and by comparison with sea‐surface altimetry. The mean northward transport in the upper waters is 6.7 ± 3.7 Sv and there is a 6.1 ± 2.5 Sv southward flow below the thermocline. Although the magnitude of the Iceland Basin overturning circulation (4.3 ± 1.9 Sv) is greater than in the Rockall Trough (3.0 ± 3.7 Sv), the variability is greater in the Rockall Trough. We discuss the results in the context of our understanding of drivers of variability.

The GEOVIDE cruise in May–June 2014 reveals an intense Meridional Overturning Circulation over a cold and fresh subpolar North Atlantic

“…Based on EN4 ocean reanalysis, they detected negative temperature anomalies in the surface waters, which they related to anomalous air-sea heat fluxes. Conversely, Holliday et al (2015), who found evidence of similar cooling and freshening in the Iceland basins from 2010-2011 to 2014, hypothesized a remote source of those anomalies, writing that "the eastern SPNA is once again being influenced by cold, fresh western subpolar water". We will discuss both hypotheses in this study.…”

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

“…The DWBC has traditionally been considered the sole conduit for the lower limb of the AMOC. However, this assumption has been challenged by observational and modeling studies that reveal the importance of interior, and boundary, pathways (e.g., Bower et al 2009;Holliday et al 2009;Stramma et al 2004;Xu et al 2010;Lozier et al 2013).…”

Transports and pathways of overflow water in the Rockall Trough

“…Previous geostrophic calculations within the Rockall Trough have used a mid-depth level of no motion (Ellett and Martin, 1973;Holliday et al, 2000;Holliday et al, 2015). A comparison between geostrophic-derived and satellite-derived velocities, as well as the net transport balance within the basin, suggests that the assumption of zero flow at 1200 m is reasonable (Holliday et al, 2000;Holliday et al, 2015). Additionally, analysis of historical moorings at stations F and M on the EEL (Figure 3 for location) show a flow reversal at around this depth (Holliday et al, 2000).…”

“…To calculate absolute geostrophic velocities and transports, an assumption about a level of no/known motion must be made. Previous geostrophic calculations within the Rockall Trough have used a mid-depth level of no motion (Ellett and Martin, 1973;Holliday et al, 2000;Holliday et al, 2015). A comparison between geostrophic-derived and satellite-derived velocities, as well as the net transport balance within the basin, suggests that the assumption of zero flow at 1200 m is reasonable (Holliday et al, 2000;Holliday et al, 2015).…”