An additional deep-water mass in Drake Passage as revealed by 3He data

We present 3He data froma repeat section across Drake Passage, fromthree sections off the South American continent in the Pacific, at 28?S, 35?S, and 43?S, and fromthree sections in the Atlantic, eastward of the Malvinas, close to 35?W, and near the Greenwich Meridian. In Drake Passage, a distinct high-3He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88?W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high-3He feature can be ruled out. We explain the feature by addition of high-3He waters present at the 43?S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep- Sea Research I 49 (2002) 681), that is strongly related to the high-3He feature. Employing multiparameter water mass analysis (including 3He as a parameter), we find that deep waters as met at the 43?S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35?W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.071.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3He signature in Drake Passage is essentially confined north of the Polar Front. Further downstreamthe signature crosses this front, to the extent that at 35?W the contributions south and north of it are of similar magnitude. At the same time, the 3He levels north of the front are reduced due to a substantial admixture of low-3He North Atlantic Deep Water, such that 3He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow fromthe Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass. r 2003 Elsevier Ltd. All rights reserved

Temporal Evolution of CFC 11 and CFC 12 Concentration in the Ocean Interior

We study the temporal evolution of concentrations of the chlorofluorocarbons CFC 11 and CFC 12 in the ocean, under the assumption of circulation and mixing being invariant in time. This allows us to define a time‐invariant age distribution for a given point in the ocean, where the age is defined as time since the last contact with the atmosphere occurred. This concept is evaluated for a number of fundamental situations. We deduce a tendency for low CFC 11 and CFC 12 concentrations in advective regimes to increase exponentially in time and for concentrations near to a solubility equilibrium with atmospheric concentrations to increase rather more linearly. The apparent saturations, i.e., the ratios of interior to mixed‐layer CFC concentrations, increase monotonically in time, typical rates being 5–10% per decade. The theoretical results are compatible with time trends found in repeated CFC observations in the ocean. Diagrams on the temporal evolution for different age distributions are presented for the period 1970–2000, which can serve as a general orientation. The diagrams furthermore can provide time corrections for quasi‐synoptic evaluation of CFC observations taken over an extended period of time and assist in constructing time‐dependent CFC boundary conditions for numerical models of ocean circulation

A meridional 14C and 39Ar section in northeast Atlantic deep water

14C, 39Ar, and complementary hydrographic and nutrient data are presented for deep water below 2500 m depth, from stations along a meridional section (8°S to 45°N) through the Romanche Trench and along the deep northeast Atlantic basins (F/S Meteor, cruise 56, leg 5). The large-scale 14C distribution along the section is resolved at the 14C data precision of ±2‰. Bottom water Δ14C decreases by 6‰ from the equator to 45°N, and farther up there is a weak Δ14C minimum (−123‰) over much of the section. The 14C data are interpreted as giving a turnover time of about 30 years for the waters below the depth of the 14C minimum (∼4250 m). It is found that water of 1.50±0.05°C potential temperature enters the East Atlantic from the west through the Romanche Trench (sill depth about 4000 m), and a preliminary value for the inflow rate of 3.6×106 m3/s is deduced. This rate greatly exceeds estimated deep inflow rates through the Vema fracture zone or across the northern boundary of the East Atlantic. 39Ar data that cover an entire deep-ocean circulation system are presented for the first time. The observed 14C and 39Ar distributions are mutually consistent. Transit times from the source regions to the equator for water from northern and southern deepwater sources are estimated to be about 170 and 105 years, respectively, and the 39Ar concentration of young Antarctic Bottom Water is deduced as 60±7% modern. The 39Ar-14C correlation in the ocean appears to be affected by mixing of waters of different age and by more efficient raising of 39Ar in the deepwater formation processes

Baroclinic Flow and Transient-Tracer Fields in the Canary–Cape Verde Basin

Simulated transient-tracer distributions (tritium, 3H3, freons) on the isopycnal horizons σ0=26.5 and 26.8 kg m−3 are presented for the East Atlantic, 10° −40°N. Tracer transport is modeled by employing a baroclinic flow field based on empirical data in a kinematic isopycnal advection-diffusion numerical model, in which winter convection is taken as the mechanism of communication with the ocean surface layer, and the isopycnal diffusivity is a free parameter. Diapucnic transport is ignored. The simulations employ time-dependent tracer boundary conditions, which are constructed on the basis of available observations. Simulations are compared to data obtained on a meridional section in 1981 (F/S Meteor, cruise 56/5). Best simulations were obtained by means of a subjective optimization procedure. On both levels, the observed distributions and the best simulated distributions agree well. The fact that the surface boundary conditions and interior distributions of the tracers are distinctly different leads us to the conclusion that our model provides a consistent description of upper main-thermocline ventilation and interior transport Surface-water densities in February are found to represent adequately the winter outcrop boundaries with an uncertainty of about ±300 km across. The required isopycnal diffusivity south of 29°N is 1700 m2 s−1, and 2900 m2 s−1 further north (+70/−40%). Interior transport is found to be predominantly advective. Advective ventilation across 30.5°N east of 33°W amounts to only 12% and 40% for the 26.5 and 26.8 horizons of the total ventilation rates reported by Sarmiento. The North Atlantic/South Atlantic Central Water boundary near 15°N is found to be predominantly determined by advection

A transatlantic tritium section near 40°N, 1971

We report, numerically and in graphical form, measured tritium concentrations from five hydrographic stations in the North Atlantic. Fairly homogeneous concentrations are observed in a surface layer typically 400 m deep. In the thermocline, concentrations decrease steadily down to the a σθ = 27.3 density horizon, and are more variable further down. The tritium in the lower part of the thermocline originates from the Subarctic Intermediate Water and the Mediterranean Water. There is a relative tritium maximum associated with the Mediterranean Water on the easternmost station of the section. In the deep water (σθ > 27.8), concentrations east of the Midatlantic Ridge are close to the limit of detection down to 2500 m, and undetectable further down, while west of the ridge tritium is found throughout the water column. The deep water tritium is associated with the deep-water advective cores of Arctic origin. The present tritium data can serve as northern boundary values in attempts to use tritium in studies of the North Atlantic main thermocline dynamics. The present data together with data from the literature point to a general division of the North Atlantic main thermocline into two layers separated by an isopycnal surface near σθ = 27.3