The Extended Ellett Line: Discoveries from 65 years of marine observations west of the UK

. Shallow and deep branches of the Meridional Overturning Circulation pass through the Rockall Trough and Iceland Basin where measurements of salinity and temperature have been made for 65 years. There is a very small number of decadal-scale time series in the world ocean, so this long-term data collection represents an unusually rich resource for climate science. The early data sets of surface temperature and salinity collected by ocean weather ships provided a previously unseen picture of the annual cycle of these properties as well as multiyear variability. In 1975, regular, repeated sampling of the full-depth deep ocean began to reveal the variability of water masses and details of their circulation. Here, we describe the history of sampling in the region and the main scientific discoveries about ocean circulation and variability made using these data. Continuing sustained observing of temperature, salinity, nutrients, and carbon from ships will contribute to the international focus in the subpolar gyre over the next decade.

represents an unusually rich resource for climate science. The early data sets of surface temperature and salinity collected by ocean weather ships provided a previously unseen picture of the annual cycle of these properties as well as multiyear variability.
In 1975, regular, repeated sampling of the full-depth deep ocean began to reveal the variability of water masses and details of their circulation. Here, we describe the history of sampling in the region and the main scientific discoveries about ocean circulation and variability made using these data. Continuing sustained observing of temperature, salinity, nutrients, and carbon from ships will contribute to the international focus in the subpolar gyre over the next decade.

INtRODuCtION
Across the world ocean, there are a relatively small number of places where we have long-term (multidecadal) records of full-depth, deep ocean temperature and salinity. Many of the long deep-ocean records that do exist are around the fringes of the North Atlantic, and they often began as environmental surveys for fisheries management (Dye et al., 2012). Remote sensing, Argo floats, and ocean gliders have greatly increased the amount of marine data available globally in the upper 2 km, but there is still much that these long deep-ocean time series can tell us that other observations and models cannot. The Extended Ellett Line    (Ellett and Jones, 1994). Lumby had devised a surface sampler that towed a thermometer and   with the instruments, leading to very noisy data; however, David Ellett was able to process and calibrate most, but not all the data sets collected (Holliday, 2002). In recognition of the contribution that this quiet, modest, and kind man had made to the field of oceanography, the section became known as the  some 20 and 30 years later. Ellett (1980Ellett ( , 1982 (Read and Gould, 1992;Cunningham and Haine, 1995;Sy et al., 1997). Ellett (1980Ellett ( , 1982 Ellett and Jones, 1994). gray shading represents ± 1 standard deviation.  However, it was a further 20 years before the hypothesis of shifts in the polar front was more fully explored.
Although Ellett and Martin (1973) had detected a high salinity current on the edge of the Scottish continental shelf, it was first fully described in Booth and Ellett (1983)  3.0 ± 2.1 Sv was in the slope current (Holliday et al., 2000). The interannual variability of the slope current transport was linked to periods of enhanced flow of unusually warm water into the North Sea with subsequent effects on the ecosystem there (Reid et al., 2001). Data from the early WOCE cruises revealed that the pathways of the two main fronts of the North Atlantic Current both passed through the Iceland Basin, but there was a distinct zonal shift in the location of the fronts compared to a similar survey in 1991 (Pollard et al., 2004).
The data also showed that some of the warm, saline Rockall Trough water recir-  (Holliday, 2003). Instead, the key factor influencing the properties was the position of the subpolar front to the south and west of the Rockall Trough. When the subpolar front was situated further east, the trough contained a higher proportion of relatively cooler and fresher subpolar gyre water; conversely, when the front was further west, the region was influenced more strongly by warm, saline subtropical water. The zonal shift in the subpolar front was later shown to arise from a North Atlantic-wide change in the circulation and strength of the subpolar gyre that profoundly influences regional ecology (Hátún et al., 2005(Hátún et al., , 2009  higher salinity and replenishing nutrients (Huthnance, 1995;Hydes et al., 2003).
The shelf waters sampled by the EEL exhibit an interannual variability of up to 1.0-1.5°C (Inall et al., 2009)