Description: The distributions and transports of deepwater masses at the western boundary in the tropical Atlantic off Brazil have been studied on three surveys along 35 degrees W and 5 degrees S and one at 10 degrees S. Transports are obtained from direct measurements of the velocity fields (Pegasus profiling system and lowered acoustic Doppler current profiler) and from geostrophic computations. Using chlorofluoromethane (CFM) and hydrographic distributions, four water masses could be identified forming the North Atlantic Deep Water (NADW) system. Two of these have a high CFM content, the ''shallow upper NADW'' (SUNADW) and the ''overflow lower NADW'' (OLNADW). These exhibit the highest velocity signals at 35 degrees W, where distinct flow cores seem to exist; most of the southeastward flow of the SUNADW (centered around 1600 m) occurs 320 km offshore between 3 degrees 09'S and 1 degrees 50'S (9.7 +/- 3.3 Sv); farther north in that section, a highly variable reversing flow is found in a second velocity maximum. The transport of OLNADW (centered around 3800 m) of 4.6 +/- 2.6 Sv is guided by the Parnaiba Ridge at 1 degrees 45'S, 35 degrees W. The water masses located between the two CFM maxima, the Labrador Sea Water (LSW) and the LNADW old water mass (LNADW-old), did not show any persistent flow features, however, a rather constant transport of 11.1 +/- 2.6 Sv was observed for these two layers. The total southeastward flow of the NADW at 35 degrees W showed a transport of 26.8 +/- 7.0 Sv, if one neglects the reversing SUNADW north of 1 degrees 50'S. At 5 degrees S the flow of all deepwater masses shows vertically aligned cores; the main southward transport occurred near the coast (19.5 +/- 5.3 Sv). The boundary current is limited offshore by a flow reversal, present in all three surveys, but located at different longitudes. At 10 degrees S a southward transport of 4.7 Sv was observed in November 1992. However, the section extended only to 32 degrees 30'W, so that probably a significant part of the flow has been missed. An important result is the large transport variability between single cruises as well as variability of the spatial distribution of the flow at 35 degrees W, which could lead to large uncertainties in the interpretation of single cruise observations. Despite these uncertainties we suggest a circulation pattern of the various deepwater masses near the equator by combining our mean transport estimates with other observations.

Description: 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.

Description: Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean, The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Provencal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Provencal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600-1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10(6) m(3) s(-1)), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero-Provencal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Provencal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior

Description: During May - August, 1997, the distributions of dissolved methane and CCl3F (CFC11) were measured in the Atlantic between 50° and 60°N. In surface waters throughout the region, methane was observed to be close to equilibrium with the atmospheric mixing ratio, implying that surface ocean methane is tracking its atmospheric history in regions of North Atlantic Deep Water formation. Despite the different atmospheric history and ocean chemistry of CH4 and CFC11, their spatial distribution patterns in the water column are remarkably similar. One-dimensional distributions have been simulated with an advection-diffusion model forced by the atmospheric histories. The results suggest that the similar patterns result from the increasing input of CH4 and CFC11 to newly formed deep waters over time, combined with the effect of horizontal mixing and the oxidation of methane on a 50 year time scale.

Description: Four World Ocean Circulation Experiment (WOCE) repeat cruises (October 1990 to March 1994) in the tropical Atlantic off Brazil are used to study the spatial and temporal evolution of the chlorofluorocarbon (CFC) (components CFC-11 and CFC-12) and tritium signal in the upper North Atlantic Deep Water (NADW). Its shallowest part, located in the tropical Atlantic around 1600-m depth, is the shallow upper North Atlantic Deep Water (SUNADW). It is characterized by a distinct tracer maximum, which is presumably received through winter time convection in the subpolar North Atlantic. Here we discuss the tracer fields and the temporal evolution of the tracer signal of the SUNADW in the tropical Atlantic along two meridional sections at 44 degrees and 35 degrees W and two zonal sections at 5 degrees and 10 degrees S off Brazil. The spatial and temporal development of the tracer field in the tropical Atlantic as well as the correlation with hydrographic parameters show that the temporal tracer change being due to the arrival of "younger" water is disturbed by other processes. In particular, the impact of variable mixing and spreading pathways on the observed tracer variability in the SUNADW is evident in the observations.

Description: We compare estimates of the anthropogenic CO2 content of seawater samples from the subpolar North Atlantic Ocean calculated on the basis of a back-calculation technique with measurements of the chlorofluorocarbon CFC-11. Estimated anthropogenic CO2 concentrations are in the range 10–80 µmol kg-1, while CFC-11 concentrations cover the full range from below detection limit to 〉 5 pmol kg-1 in waters at atmospheric equilibrium. The majority of the data points show a linear correlation between anthropogenic CO2 concentrations and CFC-11 saturation, which can only be explained by the strongly advective nature of the North Atlantic Ocean. Only deep eastern basin samples deviate from this general observation in that they show still significant concentrations of anthropogenic CO2 where CFC-11 is no longer detectable. In order to remove the influence of the Revelle factor reflected in the anthropogenic CO2 concentrations we have calculated 'excess' pCO2, showing an even tighter linear correlation with atmospheric equilibrium concentrations of CFC-11.