Search

Six summer surveys conducted from 2001 to 2005 and in 2012 by the Spanish Institute of Oceanography (IEO) reveal that the hydrographic early summer scenarios around the Balearic Islands are related to the winter atmospheric forcing in the northwestern Mediterranean Sea. The Balearic Islands (western Mediterranean Sea) lie at the transition between the southern, fresher, newly arrived Atlantic Waters (AW) and the northern, saltier, resident AW. The meridional position of the salinity driven oceanic density front separating the new from the resident AW is determined by the presence/absence of Western Intermediate Water (WIW) in the Mallorca and Ibiza channels. When WIW is present in the channels, the oceanic density front is found either at the south of the islands, or along the Emil Boudot escarpment. In contrast, when WIW is absent, new AW progresses northwards crossing the Ibiza channel and/or the Mallorca channel. In this later scenario, the oceanic density front is closer to the Balearic Islands. A good correspondence exists between standardized winter air temperature anomaly in the Gulf ofLions and the presence of WIW in the channels. We discuss the use of a regional climatic index based on these parameters to forecast in a first-order approach the position of the oceanic front, as it is expected to have high impact on the regional marine ecosystem., Sí

A key challenge in oceanography is to capture and quantify processes that happen on short time scales, seasonal changes and inter-annual variations. To address this problem the P&O European Ferries Ltd. Ship MV Pride of Bilbao was fitted with a FerryBox from 2002 to 2010 and data returned to NOC in real time providing near continuous measurements between UK (Portsmouth) and Spain (Bilbao) of temperature, salinity, chlorophyll-fluorescence and oxygen. Additional monthly samples were collected on manned crossings. Over 6000 samples were analysed for nitrate (nitrate and nitrite) concentrations. The timing of nitrate concentration increases (with winter mixing) and decreases (with the spring bloom) are different on and off shelf and in autumn nitrate concentrations remain high on the shelf. Off shelf in the Bay of Biscay, the mixed layer depth assessed using Argo floats, was found to vary from 212 m in relatively mild winters (such as 2007/2008) to 476 m in cold winters (2009/2010). Years with deeper mixing were associated with an increase in nitrate concentrations in the surface waters (~3 µmol l-) and the increased vertical nutrient supply resulted in higher productivity the following spring. Bloom progression could be seen through the increase in oxygen anomaly and decrease in nitrate concentrations off shelf prior to changes further north on the shelf and phytoplankton growth was initiated as shoaling begins. The full dataset demonstrates that ships of opportunity, particularly ferries with consistently repeated routes, can deliver high quality in situ measurements over large time and space scales that currently cannot be delivered in any other way.

Seasonality of hydrographical properties at depth in the western Iberian margin (Eastern North Atlantic) is analysed from a 2003–2010 timeseries of a semi-annual oceanographic section extending ~ 200 nm off Cape Finisterre (43° N). All waters masses down to the whole extent of the permanent thermocline (2000 dbar) show a consistent seasonal signature in their termohaline properties and there is a notable asymmetry between the slope region and the outer ocean (in the surroundings of the Galicia Bank). There is overall cooling and freshening of East North Atlantic Central Waters in summertime, which is larger and deeper-reaching on the slope. In summertime, Mediterranean Water 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 developing 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 helps to analyse 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 and subsequent anomalies in Ekman transport and wind stress curl, (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.

We investigate the effect of strong meteorological perturbations in early spring on the success of mackerel (Scomber scombrus) recruitment in the N/NW Iberian area (southern Bay of Biscay) for the period 1999–2008. In 2000, the year of the most pronounced recruitment failure on record, two consecutive multidisciplinary surveys sampled hydrographic conditions and mackerel eggs, larvae and post-larvae over the main mackerel spawning grounds of the north and northwest coast of the Iberian Peninsula. Analysis of egg and larval abundance and birthdates based on the otoliths of mackerel juveniles caught between July and October 2000 showed that there were no survivors from the early spring spawns, indicating a massive loss of early spawning effort. Moreover, the abundance of 1-year-old mackerel estimated from an acoustic survey carried out in 2001 was the lowest observed within the 1999–2008 time series. This low or null survival from the early spawns in 2000 could be due to the meteorological and oceanographic conditions of that spring, in particular two storm events in April after a relatively calm March. The first storm event from the north caused strong local wind in the southern Bay of Biscay but a weak oceanographic response. The second storm event from the southwest was mainly felt west of Galicia and caused a notable increase in shelf currents and a shift of the hydrographical structure along the shelf. Detailed analysis of strong wind pulses in early spring within the historical recruitment record suggests that strong local turbulence generated by high wind speeds and advection of larvae caused by the enhancement of shelf currents can contribute to reduced recruitment. Our observations indicate that, in 2000, both mechanisms were present.

Mixed layer depth (MLD) variability from seasonal to decadal time scales in the Bay of Biscay is studied in this work. A hydrographic time series running since 1991 in the study area, a climatology of the upper layer vertical structure based on the topology of this temperature profile time series and a one-dimensional water column model have been used for this purpose. The prevailing factors driving MLD variability have been determined with detail, and agreement with observations is achieved. Tests carried out to investigate climatological profile skill to reproduce the upper layer temporal evolution have demonstrated its ability to simulate variability at seasonal time scales and reproduce the most conspicuous events observed. This has enabled us to carry out a reconstruction of the MLD variability for the last 60 years in the study area. Favourable sequence of intense mixing events explains interannual differences and cases of extraordinary deepening of winter mixed layer. The negative phase of the Eastern Atlantic pattern seems to determine important interannual variability through intense episodes of cooling and mixing as in winter 2005 in the Bay of Biscay. Low-frequency variability is also observed. A very striking and unexpected shallower winter MLD during the 1970s and 1980s than those observed from 1995 has been found. Simulation results support this counter-intuitive outcome of shallower winter mixed layers concurrent with generalized upper water warming trends reported on several occasions for the area. The long-term trends in MLD seem related with decadal variability in the North Atlantic Oscillation, being in phase and opposition with other deepening-shallowing cycles found from subtropical-to-subpolar areas in the North Atlantic., Sí

A method for characterizing the upper ocean structure is developed. Each temperature (density) profile is fitted by an ideal function based on the assumption that the permanent and seasonal thermoclines can be approximated respectively by steady state and transients of turbulent-diffusive processes and that the mixed layer can advance sharply under external forcing. The ideal profile is composed of two pieces joined at the mixed layer depth (MLD). The upper part is a constant; the part below the MLD is a product of an exponential decay and a Gaussian, representing the seasonal thermocline and decaying asymptotically to a straight line that describes the permanent thermocline. The composition of an exponential decay and a Gaussian accurately fits a wide family of solutions of the diffusion equation and includes the case of a shift of the boundary. The ideal fit for each profile relies on six adjustable parameters including the MLD. As the function is non-linear and non-differentiable, a Differential Evolution optimization algorithm is proposed to make the fitting. The solution gives a good estimate of the MLD based on the topology of the profile. It also provides a measure of the gradient and the shape of each profile, which are intuitive parameters for characterizing the upper ocean structure with direct applicability in ecosystem models. The algorithm is applied to a time series of monthly conductivity–temperature–depth (CTD) profiles from a hydrographical station in the southern Bay of Biscay. The construction of a local climatology of the vertical structure evolution (mixed layer development) is presented as a practical application. Other potential uses of the method are also discussed.