On the origin and the spreading of the shallow Mediterranean water core in the Iberian basin

The origin and the spreading of the shallow Mediterranean water core (Ms) in the Iberian basin is discussed with a quasi-synoptic hydrographic data set enhanced by chlorofluoromethane (CFM) measurements. Its characteristic density level is found to be σt = 27.4. Characterized by high temperature and CFM values, Ms enters the Iberian basin in the region of Cape St Vincent between depths of 500–750 dbar. A heat anomaly of >11.8 × 109 J m−2 is chosen as the boundary between the presence of Ms and the background field. The core is found in a tongue-like shape as well as in separate isolated eddies of both cyclonic and anticyclonic circulation. Using the optimum multiparameter analysis (Tomczak and Large, 1989, Journal of Geophysical Research, 94, 16141–16149), the North Atlantic Central Water (NACW), which mixes with the Mediterranean outflow to form Ms, turned out to be in the mean 1°C warmer and 0.11 saltier than in regions with minor Mediterranean influence. This points to the Gulf of Cadiz as the origin of Ms, where the Mediterranean oufflows is in contact with NACW of the appropriate characteristics

Variability in the Deep Western Boundary Current in the equatorial Atlantic at 44°W

The variability of the North Atlantic Deep Water (NADW) was studied by ten hydrographic repeat sections taken along 44 degreesW off Brazil between September 1989 and March 1994. This data set allowed for the first time to describe the seasonal signal in the Deep Western Boundary Current at the equator from hydrographic data. Annual and semiannual layer thickness modulations were observed similar to such signals in transport time series, however with a time lag of 2 months. A comparison of the interannual variability of the Labrador Sea Water component of the NADW at 44 degreesW at the equator with the formation region indicated a time lag of 13 to 17 years. The effective spreading velocities in the Labrador Sea Water are in the range 2 to 5 cms(-1) for the tropical Atlantic Ocean

Spreading of overflow water from the Greenland to the Labrador Sea

In 1996, about 320 kg of SF6 were introduced in the center of the Greenland Sea gyre. We use this signal together with the CFC distribution to follow the spreading of Greenland gyre water from the Denmark Strait through the Irminger Basin and the Labrador Sea to the Grand Banks. In the summer of 2003 Denmark Strait Overflow Water tagged with deliberately released SF6 could be traced throughout the Irminger Basin to the central Labrador Sea, confirming that water with potential density of 28.045 contributes to the Denmark Strait Overflow. The upper limit of the transfer time from the central Greenland Sea to the Labrador Sea was found to be 7 years. This study suggests that roughly 4 kg of excess SF6 has been transported over the Denmark Strait and confirm earlier reported transport through the Faroe Bank Channel. These results should be considered when using SF6 as a transient tracer

Tracer signals of the intermediate layer of the Arabian Sea

In 1995, hydrographic and chlorofluorocarbon (CFCs, components F11, F12) measurements were carried out in the Gulf of Aden, in the Gulf of Oman, and in the Arabian Sea. In the Gulf of Oman, the F12 concentrations in the Persian Gulf outflow (PGW) at about 300m depth were significantly higher than in ambient surface water with saturations reaching 270%. These high values could not be caused by air-sea gas exchange. The outflow was probably contaminated with oil, and the lipophilic character of the CFCs could then lead to the observed supersaturations. The intermediate F12 maximum decreased rapidly further east and south. At the Strait of Bab el Mandeb in the Gulf of Aden, the Red Sea outflow (RSW) was saturated with F12 to about 65% at 400m depth, and decreased to 50% while descending to 800m depth. The low saturation is not surprising, because the outflow contains deep and intermediate water masses from the Red Sea which were isolated from the surface for some time. The tracer contributions to the Arabian Sea for Indian Central Water (ICW) and PGW are about equal, while below 500m depth the RSW contribution greatly exceeds ICW. Modeling the CFC budget of the Arabian Sea, the inflow of ICW north of 12 degrees N is estimated to be 1-6 Sv, depending mainly on the strength of the flow of Red Sea Water into the Arabian Sea

The Mediterranean water tongue and its chlorofluoromethane signal in the Iberian Basin in early summer 1989

Hydrographic observations from the Iberian Basin demonstrate the variability of water masses in upper and intermediate layers. The surveyed area embraces the internal front between water masses from higher latitudes and the Mediterranean outflow, exhibits several isolated Mediterranean eddy (meddy) structures at middepth, and displays the virtual source region for the Mediterranean Water (MW) tongue off the Portuguese continental slope. The description is enhanced by additional chlorofluoromethane measurements, which show anomalously high concentrations at middepth, due to mixing of MW with the overlying Atlantic waters in the Gulf of Cadiz. The geostrophic stream function shows several meddylike features that not only are remarkably extended in the depth range of the MW, but are also correlated with surface height anomalies

Coastal upwelling velocities inferred from helium isotope disequilibrium

In the framework of SOPRAN, two of the main gloabal Eastern Boundary Current Upwelling Systems (EBUS) have been investigated, off the coasts of Mauritania in the northern Atlantic and of Peru in the southern Pacific. The upwelling in the EBUS is driven by alongshore winds causing an offshore transport of surface waters. The upwelled water typically exhibits high concentrations of climate relevant gases such as CO2, N2O and halogenated compounds. The oceanic upwelling velocities, however, are too small (in the order of 10-5 m/s) to be measured directly. Here we use oceanic measurements of the helium-3/helium-4 isotopic ratio as an indirect means to infer these velocities. The water that upwells into the oceanic mixed layer from below is typically enriched in the lighter isotope helium-3. This excess of helium-3 originates from venting of primordial helium through hydrothermal activity. Helium data have been collected on four cruises within the coastal upwelling regions off Mauritania and Peru. Near the coast, the helium derived upwelling velocities are in good agreement with the wind driven flow calculated from Ekman theory. At some locations in the open ocean, however, the helium method results in much higher vertical velocities compared to the wind derived Ekman divergence. This enhanced upwelling might be attributed to eddy activity. Both advective and turbulent (derived from microstructure measurements) fluxes of nutrients into the mixed layer are determined. In coastal upwelling regions, these fluxes play a key role in fostering ocean primary productivity