Walter Zenk

Directly measured mid-depth circulation in the northeastern North Atlantic Ocean

The circulation of water masses in the northeastern North Atlantic Ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes. But the ocean circulation at depths below the reach of satellite observations is difficult to measure, and only recently have comprehensive, direct observations of whole ocean basins been possible. Here we present quantitative maps of the absolute velocities at two levels in the northeastern North Atlantic as obtained from acoustically tracked floats. We find that most of the mean flow transported northward by the Gulf Stream system at the thermocline level (about 600 m depth) remains within the subpolar region, and only relatively little enters the Rockall trough or the Nordic seas. Contrary to previous work, our data indicate that warm, saline water from the Mediterranean Sea reaches the high latitudes through a combination of narrow slope currents and mixing processes. At both depths under investigation, currents cross the Mid-Atlantic Ridge preferentially over deep gaps in the ridge, demonstrating that sea-floor topography can constrain even upper-ocean circulation patterns.

A census of Meddies tracked by floats

Recent subsurface float measurements in 27 Mediterranean Water eddies (Meddies) in the Atlantic are grouped together to reveal new information about the pathways of these energetic eddies and how they are often modified and possibly destroyed by collisions with seamounts. Twenty Meddies were tracked in the Iberian Basin west of Portugal, seven in the Canary Basin. During February 1994 14 Meddies were simultaneously observed, 11 of them in the Iberian Basin. Most (69%) of the newly formed Meddies in the Iberian Basin translated southwestward into the vicinity of the Horseshoe Seamounts and probably collided with them. Some Meddies (31%) passed around the northern side of the seamounts and translated southwestward at a typical velocity of 2.0 cm/s into the Canary Basin. Some Meddies observed there were estimated to be up to ∼5 yr old. Four Meddies in the Canary Basin collided with the Great Meteor Seamounts and three Meddies were inferred to have been destroyed by the collision. Overall an estimated 90% of Meddies collided with major seamounts. The mean time from Meddy formation to a collision with a major seamount was estimated to be around 1.7 yr. Combined with the estimated Meddy formation rate of 17 Meddies/yr from previous work, this suggests that around 29 Meddies co-exist in the North Atlantic. Therefore during February 1994 we observed about half of the population of Meddies.

Large Lenses of Highly Saline Mediterranean Water

Isolated compact anticyclonic eddies or salt lenses were found in the Canary Basin. Hydrographic surveys of three such lenses show large anomalies of salinity and temperature (∼0.8, 2.5°C). They are centered at ∼1100 m, have a vertical extent of up to 900 m and radii of ∼50 km. Current meter records indicate anticyclonic velocities up to 29 cm s−1. Fine structure with vertical scales of ∼20 m and less, possibly due to intrusive decay, appears at the outer edges of the lenses whereas the centers are free of such structure. The probability of finding a salt lens at any station in the Canary Basin is fairly high (∼0.08).

The Cape Cauldron: A regime of turbulent inter-ocean exchange

Combining in-situ Lagrangian intermediate depth velocity measurements from the KAPEX (Cape of Good Hope Experiments) float program with sea-surface height data, this study reviews the inter-ocean exchange mechanisms around southern Africa. In the southeastern Cape Basin, a highly energetic field of coexisting anticyclonic and cyclonic eddies is documented. Agulhas Rings of typically 200 km diameter are observed to merge, split, deform, and to reconnect to the Agulhas Retroflection. Concomitant, slightly smaller cyclones are observed to drift across the northwestward migration path of the Agulhas Rings. These cyclones, with typical diameters of 120 km, are formed within the Cape Basin along the African shelf, inshore of the Agulhas Current, and in the subantarctic region south of Africa. The data suggest the annual formation of 3–6 long-lived Agulhas Rings that eventually cross 5°E longitude, while approximately twice the number of rings occur in the southeastern Cape Basin. Within this region, cyclones outnumber anticyclones by a factor of 3:2. Both cyclones and anticyclones extend through the upper thermocline into the intermediate depth layer. Mean drifts of anticyclones are 3.8±1.2 cm s−1 to the northwest, while cyclones follow a west–southwestward route at 3.6±0.8 cm s−1. Transport estimates suggest that the intermediate depth layer in the southeastern Cape Basin is primarily supplied from the east (approximately 9 Sv), with minor direct inflow from the Atlantic to the west and south. Cyclone/anticyclone interaction is surmised to result in vigorous stirring and mixing processes in the southeastern Cape Basin, which necessitates a review of the traditional concept of Indo-Atlantic inter-ocean exchange. We propose to limit the concept of “isolated Agulhas Rings embedded in a sluggish Benguela Drift” to the northwestern Cape Basin and beyond, while linking this regime to the Agulhas Retroflection proper through a zone of turbulent stirring and mixing in the southeastern Cape Basin, named for the first time the “Cape Cauldron” hereinafter.

The Flow of Antarctic Bottom Water into the Brazil Basin

The total transport of Antarctic Bottom Water across the Rio Grande Rise, including the western boundary, the Vema Channel, and the Hunter Channel is estimated from hydrographic measurements across these pathways. The contribution of the Vema Channel is greatest at 3.9 × 106 m3 s−1, which is very close to earlier estimates. The western boundary current contribution is 2.0 × 106 m3 s−1 and that of the Hunter Channel 0.7 × 106 m3 s−1. The lower values outside the Vema Channel are offset by the important source of mass they form to the lower density classes of bottom water. About 40% of the flow is concentrated in the highest density class representing the source of Weddell Sea Deep Water to the Brazil Basin. The flow structure is characterized by horizontal and vertical recirculation.

The Madeira Mode Water

Quasi-homogeneous layers in vertical profiles of temperature and salinity in the eastern North Atlantic near Madeira indicate the existence of a subtropical Mode Water in the Eastern Basin. Temperature sections show a maximum horizontal extent of at least 500 km. The frequency distribution analysis of homogeneous layers in a historical XBT dataset shows a Mode Water formation region near and to the north of Madeira. This Mode Water is found at increasing depths and displaced to the west and southwest during the course of the year after its formation by wintertime convection. It disappears almost completely, due to mixing, before the next winter. Volume estimates suggest that this Madeira Mode Water in the eastern Atlantic accounts for 15–20% of the total Central Water formation in the corresponding density range as obtained from tracer studies in the North Atlantic gyre.

The intermediate depth circulation of the western South Atlantic

The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally-buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 10(6)m(3)s(-1)), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.

Reconstructed Mediterranean Salt Lens Trajectories

The existence of energetic anticyclonic mid-depth vortices of Mediterranean Water (meddies) questions the validity of a conventional advective–diffusive balance in the eastern Atlantic subtropical gyre. A mesoscale experiment in the Azores–Madeira region reveals a link of these meddies to large-scale subsurface meanders. For the first time it is shown that meddies may have strong surface vorticity, indicative of a generation process involving the Azores Current—a deep reaching near-surface jet.

Changes in the CFC inventories and formation rates of upper labrador sea water, 1997-2001

Abstract Chlorofluorocarbon (component CFC-11) and hydrographic data from 1997, 1999, and 2001 are presented to track the large-scale spreading of the Upper Labrador Sea Water (ULSW) in the subpolar gyre of the North Atlantic Ocean. ULSW is CFC rich and comparatively low in salinity. It is located on top of the denser “classical” Labrador Sea Water (LSW), defined in the density range σΘ = 27.68–27.74 kg m−3. It follows spreading pathways similar to LSW and has entered the eastern North Atlantic. Despite data gaps, the CFC-11 inventories of ULSW in the subpolar North Atlantic (40°–65°N) could be estimated within 11%. The inventory increased from 6.0 ± 0.6 million moles in 1997 to 8.1 ± 0.6 million moles in 1999 and to 9.5 ± 0.6 million moles in 2001. CFC-11 inventory estimates were used to determine ULSW formation rates for different periods. For 1970–97, the mean formation rate resulted in 3.2–3.3 Sv (Sv ≡ 106 m3 s−1). To obtain this estimate, 5.0 million moles of CFC-11 located in 1997 in the ULSW in the...

Currents, fronts and eddy fluxes in the Canary Basin

Hydrographic data from two cruises in the Canary Basin (Meteor 57, July 1981; Poseidon 86, April 1982) are analysed with respect to current distribution and lateral heat flux in the Azores-Madeira region. The first part of the data base consists of long transects of XBT and G.E.K. measurements between Cape Finisterre (North West Spain) and the northern Canary Basin, where several year-long current meter records exist. Further information is obtained by thermosalinograph surface data and by expendable current profilers (XCP). Geostrophic currents are derived from XBT profiles, using the tight temperature-salinity relationship in the depth range of the Warmwassersphare. The results compare well with the G.E.K. and XCP current observations. The second part consists of CTD data from an eddy resolving, box-shaped CTD survey (500 × 500 km2) centered at the mooring location (33°N, 22°W), The observations are supplemented by satellite-buoy trajectories. Horizontal parameter distribution is shown in terms of objectively contoured maps. Bands of spatially enhanced energetic structures, seen in the long transects are further resolved by the box survey as a deep jet-like current system cross the Canary Basin in a west-east direction. Associated with this Azores Current is a frontal zone with near-surface temperature and salinity steps of order 2 K and 0.3 practical salinity units. The dynamic topography field can be decomposed into a linear background field, a Rossby wave and a mesoscale eddy field. We find that major contributions to the meridional eddy heat flux are confined to the vicinity of the Azores current frontal zone. It is shown that the principal balance in the temperature equation is between heating by the mean horizontal advection terms and cooling by the eddy flux divergence.