Interannual variability of the northwestern Iberia deep ocean: Response to large‐scale North Atlantic forcing

Abstract The oceanic hydrography of the north‐easternmost region of the North Atlantic subtropical gyre has been monitored since 2003 by three sections extending between 100 and 200 nautical miles from the Spanish NW and N coast into the Atlantic and the Bay of Biscay. The sections were occupied twice a year from 2003 to 2010, annually after that, and measure the whole water column (>5000 m). Correlation of series in the vertical and among sections, autocorrelation and estimates of the effect of the noise induced by the mesoscale field, all indicate that observed signatures are robust changes of water masses at the regional scale. The hydrographic time series are not characterized by smooth trends but instead by shifts that persist through consecutive cruises. The most notable features include a shift to more saline central waters around 2005 after which they remained stable, and a decrease in thermohaline properties of the Labrador Sea Water from autumn 2008 to 2010. Years with a strong winter North Atlantic Oscillation (NAO) index are characterized by shifts in thermohaline properties across most of the intermediate levels, with the most notable event being the warming and increasing salinity that followed the large NAO index drop of 2010. The observations are consistent with current understanding of the large‐scale functioning of the North Atlantic, which predicts a northeastward expansion of subtropical temperate waters in the eastern boundary as a response to NAO forcing. The observed variability is discussed in relationship to large‐scale circulation.