Deep ocean hydrographical variability and circulation in the north-west Iberia

Abstract

Oceanic hydrography of the north-easternmost region of the North Atlantic subtropical gyre has been monitored since 2003 for the whole water column (>5000 m) by three sections extending zonally 200 nautical miles (nm) westwards of Cape Finisterre (43°N, western Iberia margin), and meridionally 100 nm off Cape Ortegal (8°W, southwestern Bay of Biscay) and 100 nm off Santander (3°47'W, southeastern Bay of Biscay), allowing for the analysis of the deep ocean variability from seasonal to interannual scales. Semiannual time series of the Finisterre oceanographic section from 2003 to 2010 have shown that all water masses down to the permanent thermocline (2000 dbar) exhibit a consistent seasonal signature in their thermohaline properties with notable asymmetry between the slope region and the outer ocean. In summertime, Mediterranean Water (MW) gets tightly attached against the slope and is uplifted, reinforcing its thermohaline signature and diminishing its presence at the outer ocean. In wintertime the situation reverses, MW seems to detach from the slope and spreads out to the open ocean, even being observed a secondary branch around the Galicia Bank. Thermohaline seasonality at depth shows values up to 0.4°C and 0.08 in salinity at the lower MW, of the order of 20% of the overall interannual variability observed during the whole period. Decomposition of thermohaline changes at isobaric levels to changes along isoneutral surfaces and changes due to vertical displacements helped analyze the physical processes behind the observed seasonality in terms of (1) the large-scale seasonality of the subtropical gyre in response to the seasonal migration of the subtropical high pressure system, (2) the continental slope dynamics, characterized by summer upwelling, winter development of the Iberian Poleward Current and Mediterranean water spreading, and (3) the possible influence of seasonal changes of water mass properties at their formation sources. Once signals were de-seasonalized, time series of property interannual changes of the different water masses were analyzed. 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 timeseries 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. These observations are consistent with current understanding of the large-scale functioning of the North Atlantic, which predicts a northeastwards expansion of subtropical temperate waters in the eastern boundary as a response to negative NAO forcing. February and September cruises in 2008, exhibiting the typical winter/summer thermohaline distinct structures and also having a quite complete set of direct velocity measurements ---Lowered Acoustic Current Profiler (LADCP), ship hull ADCP and an active mooring line--- were explored to estimate transports across the section and compare different methods available. Three regions were evidenced, the continental shelf break-slope characterized by northwards transport, southwards circulation across most of the passage to the Galician Bank summit and then northwards flow again west of the seamount. Currents across the passage from Finisterre to the Galician Bank were markedly barotropic so mesoscale structures evidenced by geostrophy were being advected across the passage.