The MORENA Project

Shelf-ocean exchanges and transport processes along the continental margin in the European coastal upwelling region

A.F.G. Fiuza* (Inst. de Oceanografia, Univ. de Lisboa-GODFUL, Portugal)
F. Perez (Inst. Investigaciones Marinas-IIM, Vigo, Spain)
J.A. Johnson (Univ. East Anglia-UEA, UK)
J. Cabanas (Inst. Espanol de Oceanografia-IEO, Vigo, Spain)
L.P. Roed (Nansen Environm. Rem. Sensing Center-NERSC, Oslo, Norway)
T. Sherwin (Univ. Coll. North Wales-UCNW UK)
I. Robinson (Univ. Southampton-SUDO, UK)
A.C. Duarte (Univ. Aveiro-UDA, Portugal)
P. Davies (Univ. Dundee-DUCE, UK)
G. Savidge (Queen's Univ. Belfast-QUB, UK)

Introduction

The MORENA Project ("Multidisciplinary Oceanographic Research in the Eastern Boundary of the North Atlantic") is sponsored by the CEC MAST-2 Programme and has a general objective to measure, understand and model shelf-ocean exchange in a typical coastal upwelling region of the eastern boundary layer of the subtropical ocean. This is being attained through a multidisciplinary approach aimed at the quantitative understanding of the physical, chemical and biological processes involved in the transfer of matter (including salt, particulates, nutrients, organic compounds, biomass), momentum and energy across and along the shelf, the shelf break and the slope, in the Iberian region of the European Atlantic.

MORENA has the following components:
Observations, Modelling and Combined Analysis.

Observations

Two oceanographic cruises have already taken place in the MORENA study region off the west coast of the Iberian Peninsula (40 degrees - 43 degrees N, coast - 11 degrees W), with the objective of measuring physical, chemical and biological parameters, describing transfer processes across and along the shelf, the shelf break and the slope, before and after the upwelling season. The first, "MORENA-1", was carried out in May-June 1993 onboard the R/V "Cornide de Saavedra", provided by IEO. During this cruise, the whole region was surveyed with a grid of CTD/rosette stations, continuous underway measurements were carried out, three surface drifters were launched and four long-term rigs with current meters were deployed (two in the deep sea off western Galicia and two on the slope off Portugal). The second cruise, "MORENA 2" was conducted in November 1993 onboard the R/V "Haakon Mosby", provided by the Geophysical Institute, University of Bergen, with the cooperation of NERSC. CTD/rosette stations covering the southern two-thirds of the MORENA region were made, two long-term current meter moorings were deployed in deep waters off Portugal (at 41 degrees N), fourteen surface drifters were launched and a detailed SeaSoar/ADCP survey was conducted, together with continuous underway measurements of several parameters.

In May 1994, the two offshore long-term moorings deployed in May 1993 off western Galicia (near 42 degrees 20'N), were recovered by IEO with a Spanish vessel; IEO then deployed another two similar moorings on the continental slope at 42 degrees 13'N and made a CTD section along that line. Also in May 1994, using the R/V "Capricornio", provided by IPIMAR (Portuguese Institute for Maritime Research), GODFUL recovered the slope moorings at 41 degrees N and made a CTD section along this MORENA southern current meter line.

The structures of the hydrological fields in both cruises indicate general conditions of coastal convergence and northward flow in the upper few hundred meters. An example is provided by the sea surface temperature map represented in Fig. 1, which was prepared from data received and processed at the GODFUL Space Oceanography Centre. A relatively warm (and saline) tongue extended northward over the upper continental slope. This northward flow of subtropical waters was confirmed by the trajectories of the drifters deployed during "MORENA 2" and is a manifestation of the poleward autumn-winter coastal current identified by Ambar et al. (1986) and Frouin et al. (1990) off western and northern Iberia. The subsurface temperature fields during "MORENA 1" (e.g. at 50m) show a similar northward warm and saline intrusion, indicating a general resemblance of the oceanographic conditions at the Central Water levels in the study region before and after the summer upwelling season.

Figure 1: Satellite-derived sea surface temperature (degrees C) distribution obtained during the "MORENA 2" cruise. NOAA 12, 15 November 1993, 08:55 UT.

The circulation at greater depths during these cruises, revealed by the temperature and salinity distributions and by baroclinic geostrophic calculations at the levels of the upper and lower cores of Mediterranean Water (Ambar and Howe, 1979), was constituted by a well defined northward undercurrent over the slope.

The "MORENA 3" experiment is to take place in July-August 1994, at the peak of the upwelling season, and will be mainly dedicated to the detailed mapping of the space-time structure of a cross-shelf filament and to an investigation of its role in shelf-ocean exchange, using a combination of CTD, XBT, SeaSoar/ADCP, and other underway observations, as well as physical, chemical and biological parameters measured with moored instrumentation. A study of internal solitary waves will also be conducted. The summer situation is illustrated in the sea surface temperature map of Fig. 2, which shows a typical configuration with cold upwelling water over the shelf and upper slope, and filaments extending offshore for more than 100 km. Such filaments transport cool, low salinity, phytoplankton-rich waters across the continental margin (Fiuza 1983, 1984; Sousa and Bricaud, 1992).

Figure 2: Sea surface temperature (degrees C) in the MORENA region at 18:40 UT on 9th July 1994.

Satellite remote sensing is a major component of MORENA. Extensive use is made of sea surface temperature based upon NOAA/AVHRR and ERS-1/ATSR data, NIMBUS-7/CZCS-derived phytopigment concentrations, microwave imaging of internal wave trains using ERS-1/SAR, and altimetry from ERS-1 and TOPEX-Poseidon data.

Models

MORENA includes the development of advanced numerical and laboratory models, to provide a framework for understanding the circulation on the continental margin and the nature of the shelf-deep sea exchange processes in the study region, and for interpreting the field data.

Work is being developed at UEA to produce a primitive equation numerical model of the MORENA region extended from the Gulf of Cadiz to the Celtic Sea and from the Iberian and French Atlantic coasts out beyond the shelf break. The model is forced by seasonal wind stress and surface heat and Salt fluxes, and will include run-off from rivers and outflow from the Mediterranean.

Layered numerical models are under development at NERSC. A one active layer, thermohaline, nonlinear, reduced gravity model, including the effect of bottom topography on the first baroclinic mode, has already been implemented. A long run in which the model is forced with a constant northerly wind has been completed; large filaments with an offshore extent of 200 km were evident after a few months of simulation time. An important finding is that filament structures with realistic length scales and surface configurations (Fig. 2) may be produced even with a straight coast and flat bottom. However, the filaments in the model are clearly modified by the introduction of bottom topography. Further, the model response suggests that the filaments are mainly due to the frontal instability mechanism described by McCreary et al. (1991), i.e., in the wake of the barotropic instability due to coastal upwelling. The filament spacing is about 120 km, which agrees well with satellite imagery (e.g., Fiuza, 1983).

A laboratory model has been constructed and used at DUCE to collect and provide experimental data with which other modelling results can be compared and validated, and to carry out further laboratory tests, as appropriate to investigate flow phenomena revealed by the field measurements. Some changes will be made in order to simulate the actual situation off the Iberian Peninsula more. Experiments will be conducted with the addition of a bank placed on the shelf (near the shelf break) for direct comparison with the NERSC models. Furthermore, to simulate tidal effects, the rotation rate can be made time dependent. A bigger rotating platform will be built, together with a rectangular channel with a source-sink arrangement.

Combined Analysis

A combination of the information obtained and analysed separately under the observational and modelling components is being carried out in the frame of MORENA, consisting of (a) validation of model results against analysed in situ and satellite observations, (b) validation of satellite data, (c) comparison of results derived from the idealized experiments with the different models, (d) an inter-model comparison of the different model results from the simulation experiment, and (e) utilization of data assimilation techniques.

References

Ambar I., A.F.C. Fiuza, T. Boyd and R. Frouin, 1986: Observations of a Warm Oceanic Current Flowing Northward Along the Coasts of Portugal and Spain During Nov-Dec 1983. EOS Trans., Am. Geophys. Union, 67 (44) 1054.

Fiuza A.F.G., 1983: Upwelling Patterns Off Portugal, p. 85-98 in Coastal Upwelling: its Sediment Record. E. Suess and J. Thiede (eds.), Plenum Pub. Co.

Fiuza A.F.G., 1984: Dinamica e Hidrologia das Aguas Costeiras Portuguesas. Doctoral Dissertation, Universidade de Lisboa, 294 pp.

Frouin R., A.F.G. Fiuza, I. Ambar and T.J. Boyd, 1990: Observations of a Poleward Surface Current Off the Coasts of Portugal and Spain During Winter. J. Geophys. Res., 95 (C1), 679-691.

McCreary J.P., Y. Fukamachi and P.K. Kundu, 1991: A Numerical investigation of Jets and Eddies near Eastern Ocean Boundaries. J. Geophys. Res., 96 (C2), 2515-2534.

Sousa F. and A. Bricaud, 1992: Satellite-Derived Phytoplankton Structures in the Portuguese Upwelling Area. J. Geophys. Res., 97 (C7), 11343-11356.

J.Johnson@uea.ac.uk