Eric D. Barton

The Transition Zone of the Canary Current Upwelling Region.

Abstract Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.

The influence of island-generated eddies on chlorophyll distribution: a study of mesoscale variation around Gran Canaria

This study reports hydrographic and biological observations from three cruises where cyclonic and anticyclonic eddies were observed downstream of Gran Canaria island. Based on field data and remote sensing images (AVHRR and CZCS), two mechanisms associated with island- generated eddies, largely responsible for the formation and distribution of chlorophyll around the Canary Islands, are proposed. First, nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies might represent important sources of primary production in the oligotrophic waters of the Canary region. Second, eddies are responsible for the horizontal transport and distribution of chlorophyll originating near the islands or off the African coast. Water with high chlorophyll content, resulting from island stirring or local upwelling at the flanks of the islands, is incorporated into cyclonic eddies in their development and subsequently transported downstream. On the other hand, anticyclonic eddies can also entrain water rich in chlorophyll when interacting with the offshore boundary of the African coastal upwelling. This chlorophyll will be advected southward as the eddy drifts. The recurrence of cyclonic and anticyclonic eddies, together with the presence of upwelling filaments throughout the year, must have important biological consequences in the formation and transport of organic matter in the Canary region. 0 1997 Elsevier Science Ltd. All rights reserved

Development, persistence, and variability of upwelling filaments off the Atlantic coast of the Iberian Peninsula

The development, persistence, and variability of upwelling filaments off the Atlantic coast of the Iberian Peninsula are examined by means of advanced very high resolution radiometer infrared imagery observed between 1982 and 1990. These observations indicate that the regime is broadly similar to that observed in the California Current system and is closely related to the large scale wind climatology of the subtropical gyre. Upwelling generally starts in late May or early June and persists until late September or early October. In May or June, a narrow band of cold water of quite uniform width is observed along much of the west coast of the Iberian Peninsula. This band has a “fringed” appearance; that is it consists of many narrow “fingers” of cool water extending 20–30 km offshore. The major filament structures generally do not begin to form until late July or August. The filaments appear first as bulges in the upwelling front. These bulges grow offshore to form filaments that reach their maximum length (200–250 km) in September. The lengths of the filaments gradually decrease until the filaments become relatively rare in late October. Typically, five or six fully developed filaments are observed off the Iberian Peninsula late in the upwelling season. Most of these are associated with major topographic features of the region, in particular the large capes which are common to the north and south of the peninsula. It is therefore postulated that the dominant dynamical processes related to filament formation off Iberia is topographic forcing. The exceptions are two major filaments commonly observed along the more regular coastline of northern Portugal. It is hypothesized that these filaments are formed by flow instabilities resulting in meandering of the southward flowing jet. These instabilities may possibly be initiated by the large capes of northern Spain.

Offshore wind forcing in the Gulf of Tehuantepec, Mexico: The asymmetric circulation

Since the early surveys carried out by the Eastern Tropical Pacific (EASTROPIC) and Scripps Tuna Oceanographic Research (STOR) projects in the tropical Pacific off Mexico, the northerly winds which blow over the Gulf of Tehuantepec were described as an important factor controlling the dynamics of this coastal ocean. In January-February 1989 an international team carried out the experiment Tehuano, designed to study the response of the gulf to these wind pulses. The complete evolution of the coastal ocean after an event of moderate intensity was observed. The forcing is characterized by a mostly symmetric, fan- shaped, offshore wind jet, which in turn produces a remarkably asymmetric upper ocean response. While analytical results based on Ekman theory forced by a symmetric offshore wind predict the formation of a symmetric dipolar circulation, the observed flow consists mainly of a large (200 km in diameter) anticyclonic warm-core eddy in the western gulf, with only a weak cyclonic counterpart in the eastern gulf. Intense surface cooling under the wind jet is caused by entrainment of subsurface water into the upper layer. The thermocline in the west deepens with the development of the eddy, which is formed initially by the advection of warm surface waters from west of the gulf. East of the axis of the wind, the mixed layer deepens due to wind-induced entrainment, while, at the same time, shoaling and compression of the deeper isotherms by curl-induced upwelling (Ekman pumping) strengthen the thermocline.

Spatial patterns of wind and sea surface temperature in the Galician upwelling region

[1] Cape Finisterre is the most northwest point in the Galician region and separates the meridional west and zonal north coasts of Galicia. The wind field there has high spatial and temporal variability throughout the year. No clear seasonal signal is evident; upwelling and nonupwelling patterns alternate in all seasons. Two outstanding features of the Galician region are persistent upwelling near Cape Finisterre even when not present farther south and alternation of strong upwelling on north and west coasts. Up to now, explanations have relied upon particular dynamics of oceanic flow past Finisterre. We find that major features of upwelling around Finisterre are related to strong spatial structure in the wind field. Analysis of QuikScat wind data for July 1999 to May 2001 shows strong repeatable patterns in the synoptic wind field. These wind patterns emerge as the combination of the two dominant modes in a complex empirical orthogonal function (CEOF) analysis representing over 85% of the variance. Summer wind patterns give rise to characteristic distributions of upwelling along the coast and favor development of filaments in particular locations. The wind measured at Finisterre itself, often used as a general indicator of upwelling conditions around the Galician coast, is not always representative of the overall wind field. The relevant wind fields allowed a qualitative explanation of temperature structure seen in sea surface temperature images, and of differences in both the upwelling and downwelling regimes between the two years. INDEX TERMS: 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; 4279 Oceanography: General: Upwelling and convergences; 4516 Oceanography: Physical: Eastern boundary currents; 4504 Oceanography: Physical: Air/sea interactions (0312); KEYWORDS: SeaWinds, coastal upwelling, sea surface temperature, air-sea coupling, Iberia, wind patterns Citation: Torres, R., E. D. Barton, P. Miller, and E. Fanjul, Spatial patterns of wind and sea surface temperature in the Galician upwelling region, J. Geophys. Res., 108(C4), 3130, doi:10.1029/2002JC001361, 2003.

Eddy development and motion in the Caribbean Sea

Eddy motion in the Caribbean Sea is described on the basis of sea level anomalies deduced from ERS-1 altimetry data corrected with TOPEX/Poseidon data during the 15 months of the Exact Repeat Mission (October 1992 to December 1993). Both cyclones and anticyclones were observed in the satellite data as anomalies originating in the Venezuelan Basin or entering the Caribbean through the Antillean passages, mainly the St. Lucia Channel, Anegada Passage, and north of Trinidad. The diameter of the eddies ranged from a few tens of kilometers to 700 km. Advection speeds were typically 20–30 cm s−1 and the eddies were energetic (kinetic energy > 0.6 m2 s−2). Their lifetime of 3–4 months was determined, in general, by their interaction with topography. Most eddy activity was eroded and disappeared at the Central American Rise area, although a few eddies crossed into the Cayman Sea through the Chibcha Channel. Some eddies also entered the Cayman Sea from outside the Caribbean through the Windward Passage. The Panama-Colombia Gyre was evident only during the tropical rainy season. A large cyclonic eddy was formed there during the period of maximum precipitation, when strong meridional salinity and wind speed gradients occurred. Eddy production in the central Caribbean appears to be associated with the interaction of the meandering Caribbean Current and the strong wind curl.

Vertical structure, turbulent mixing and fluxes during Lagrangian observations of an upwelling filament system off Northwest Iberia

Abstract In August 1998, a recurrent filament located near 42°N off Galicia was sampled as part of the OMEX-II project. Lagrangian and other observations were made on the shelf where the filament arose and offshore in the filament itself under upwelling favourable but fluctuating winds. The shelf drift experiment monitored a change from southward to weak northward net flow as the winds decreased to zero. Shipborne ADCP measurements showed that the shelf was supplying decreasing volumes of water to the filament as the wind speeds decreased. At the shelf edge the internal tide was larger than can be explained by local forcing and there were many unusually large high frequency internal waves with a quasi-sinusoidal form. Turbulence observations revealed enhanced dissipation rates and vertical eddy diffusion coefficients within the shelf thermocline (of order 1 cm 2 s −1 ), which appeared to be caused by the breaking of internal wave. A second Lagrangian experiment was executed in the filament some 120 km offshore, which again coincided with a period of wind relaxation. Cross-sections revealed a double cold core and that the offshore flow was limited to a thin surface layer. Substantial onshore flow occurred below 50 m in the centre of the filament, while the strongest and deepest offshore flow coincided with its northern boundary. Turbulent kinetic energy dissipation rate measurements showed very weak mixing below 15 m in the filament core, but enhanced mixing at its boundaries. Four mixed layer drifters released in the filament initially indicated convergence at its southern boundary, marked by strong temperature and salinity contrasts. After the wind became more favourable for upwelling, the drifters accelerated. One drifter traced the full extent of the filament, while the other three escaped from it and began to circulate cyclonically over 28 days in a 100 km diameter loop back towards their release point. Although strong mesoscale activity linked the shelf and ocean regimes, offshore transport in the filament was weak at the time of the experiment and vertical and horizontal re-circulations on a variety of time scales were important. There was sufficient vertical mixing in the thermocline to cause it to thicken and draw some heat into the lower layers during the summer months on the shelf. The amount of heat involved was too little to have a significant impact on the development of a filament over a typical lifetime of a week.

Eddy and deep chlorophyl maximum response to wind-shear in the lee of Gran Canaria

The physical and biological properties of the warm wake of Gran Canaria were examined during a survey carried out in June 1998. The sampling region was dominated by the presence of a warm triangular region downwind the island and an anticyclonic eddy spun off the island. Convergent and divergent frontal regions were generated by the wind shear zones extending along either side of the sheltered region of the warm wake. With increasing distance from shore, evidence of convergent/divergent frontal regions weakened, but the influence of the eddy increased. Both structures, frontal regions and the eddy, clearly altered the vertical phytoplankton biomass distribution as indicated by chlorophyll-fluorescence. Downwelling on the convergent boundary moved the 26.2 kg m � 3 isopycnal and its associated deep chlorophyll maximum (DCM) below the 1% light zone. Upwelling at the divergent boundary not only elevated the DCM with its associated isopycnal but also, because of the increased light levels, allowed a shift in the DCM to higher (deeper) density surfaces (26.4 kg m � 3 ). However, the highest integrated chlorophyll occurred in the

Upwelling filaments off Cap Blanc: Interaction of the NW African upwelling current and the Cape Verde frontal zone eddy field?

[1] The hydrographical and dynamical properties of the upwelling filaments forming off Cap Blanc (Mauritania) are investigated using remotely sensed and in situ data collected in April/May 2009 during the strongest upwelling season. The area is situated at the southern edge of the NW African upwelling system, where the Cape Verde Frontal Zone (CVFZ) separates warmer, saltier North Atlantic Central Water (NACW) and cooler, fresher South Atlantic Central Water (SACW). Sea surface temperature images indicated the presence of an upwelling filament extending >280 km offshore, rooted over the Cap Blanc promontory and entrained around a warm-core anticyclonic eddy. After this filament started to decay, a new cold filament developed at the approximate same location. High resolution Moving Vessel Profiler (MVP) and Acoustic Doppler Current Profiler (ADCP) surveys of these mesoscale structures revealed that both filaments were carrying South Atlantic Central Water (SACW) offshore through an intense jet-like flow. Similarity of the relative vorticity structure across the filament with that of the tangent eddy suggested that the latter was responsible for the offshore current. Tracking of this eddy in altimetric data demonstrated that it originated from the CVFZ, as implied by its hydrographic structure. Altimetric data also revealed that another anticyclonic structure centered over the Cap Blanc promontory was responsible for the northwestward advection of SACW into the base of the filament. The results support the idea that some upwelling filaments can be produced by the interaction of an external eddy field, including topographic eddies, with the upwelled water.

Spatial variability of planktonic invertebrate larvae in the Canary Islands area

In October 1991, invertebrate larvae abundances were analysed to study the influence of the disturbance of the Canary Current flow by the Canary Islands archipelago on the variability of larval distribution. Two transects and two time-series stations located to the north (non-perturbed zone) and the south (perturbed zone) of the Canary Islands were sampled. Oceanographical data showed a highly stratified water column and zonally uniform salinity and temperature seaward of the African upwelling in the non-perturbed zone, while the perturbed zone presented strong turbulence in the form of mesoscale eddies. Invertebrate larval abundances were lower for most taxa studied in the non-perturbed zone and northern time-series station. Significant differences (P < 0.001) of invertebrate larval abundance between the two zones sampled were found. Decapod larvae were the most abundant larval group in both zones. Stations located in eddy structures presented the highest values of larval densities. Specifically, the larvae collected at Station 18, located in the core of an anticyclonic eddy, represented 60 ± 18% of total larvae collected in the south transect. Finally, our results suggest that eddies, mainly anticyclonic eddies, act as a strong larval retention zone south of the islands, and that there is a local northward transport from the Canary Islands.