Chantal Andrié

The deep currents in the Eastern Equatorial Atlantic Ocean

[1]xa0In the Atlantic Ocean, equatorial deep jets have been observed in the central and western equatorial basin. Here, we present three meridional velocity sections carried out in boreal summer 2000 in the eastern equatorial Atlantic. At that time, the deep equatorial jets were observed to weaken or to vanish in the Gulf of Guinea toward the African coast. At 10°W, the comparison with currents measured one year before, in boreal summer 1999, along with tracer distributions confirm the existence of an eastward route along the equator of part of the North Atlantic Deep Water.

Tritium in the western Mediterranean Sea during 1981 Phycemed cruise

Abstract We report on simultaneous hydrological and tritium data taken in the western Mediterranean Sea during April 1981 and which implement our knowledge of the spatial and temporal variability of the convection process occurring in the Northern Basin (Gulf of Lion, Ligurian Sea). The renewal time of the deep waters in the Medoc area is calculated to be 11 ± 2 years using a box-model assymption. An important local phenomenon of “cascading” off the Ebro River near the Spanish coast is, noticeable by the use of tritium data. In the Sardinia Straits area tritium data indicate very active mixing between 100 and 500 m depth. The tritium subsurface maxima in Sardinia Straits suggests the influence of not only the Levantine Intermediate Water (LIW) but also an important shallower component. In waters deeper than 500m, an active mixing occurs between the deep water and the LIW via an intermediate water mass from the Tyrrhenian Sea by “salt-fingering”. Assuming a two end-member mixing. We determine the deep tritium content in the Sardinia Channel to be 1.8 TU. For comparison, the deep tritium content of the Northern Basin is equal to 1.3 TU. Tritium data relative to the Alboran Sea show that a layer of high tritium content persists all along its path from Sardifia to Gibraltar on a density surface shallower than the intermediate water. The homogeneity of the deep tritium concentrations between 1200 m depth and the bottom corroborate the upward “pumping” and westward circulation of deep waters along the continental slope of the North African Shelf. From the data measured in the Sardinia Straits and in the Alboran Sea, and upper limit of the deep advection rate of the order of 0.5 cm s−1 is estimated.

CFC time series in the deep water masses of the western tropical Atlantic, 1990-1999

Abstract A unique CFC time series was collected along the 35°W meridian in the tropical Atlantic during eight cruises from 1990 to 1999, in order to investigate the large-scale variability of the circulation of the North Atlantic Deep Water (NADW) through transient tracer distributions. Within the upper NADW (UNADW), it appears that CFC distributions, are highly variable; cores of maximal CFC concentrations are closely associated with salinity and are principally caused by the local dynamics of equatorial and extra-equatorial deep jets within the three particular regimes of the 3–1°S, 1°S–1°N and 1–3°N bands. CFC distribution within the lower NADW (LNADW) is less heterogeneous and linked to oxygen. A double core has been observed for 1999, possibly due to the recirculation of the deep flow constrained by the circulation of the underlying Antarctic Bottom Water (AABW). The variability of the maximal concentrations presumably results from the seasonal or semi-annual variability of the regional deep circulation. For both layers, the temporal evolution of the mean concentrations has been traced throughout the 1990s. At the UNADW level, the temporal increase of the CFC concentration within the 3°S–3°N band is mainly due to the transient signal from the atmosphere. This increase is clearly dominant compared with the local dynamics. Similar behavior has been observed for the LNADW in the 3°S–1°N band, corresponding to the equatorial channel. An attempt has been made to access the transit time for the NADW pathway from the Labrador and Nordic Seas to the tropics. The mean transit time around 25xa0yr for the UNADW has been compared to other evaluations from hydrographic measurements.

On the crossing of the equator by intermediate water masses in the western Atlantic ocean: Identification and pathways of Antarctic Intermediate Water and Upper Circumpolar Water

The flow of intermediate water masses across the equator in the Atlantic Ocean is of fundamental interest in the context of the global meridional circulation cell associated with the formation of the North Atlantic Deep Water. This paper describes the flow and pathways of the Antarctic Intermediate Water (AAIW) and the Upper Circumpolar Water (UCPW) at between 500- and 1200-m depths in the western equatorial Atlantic (5s - 730). These have been deduced from hydrological and geochemical tracer (nutrients and chlorofluorocarbons) data sets from CITHER 1 (W LAtalante, January-March 1993), ETAMBOT 1 (W Le Noroit, September-October 1995), and ETAMBOT 2 (Wv Edwin Link, April-May 1996) cruises. Both the AAIW and UCPW enter, on the isopycnals Oe = 27.25 (676 -I 36 dbar) and Oe = 27.40 (919 f 35 dbar), respectively, the equatorial belt as narrow, northwestward flows around the northeast tip of Brazil near 5s. During transit within this zone the core properties of UCPW erode more than those of AAIW. Flow patterns of both the water masses show westward spreading and eastward recirculations on either side of the equator. Temporal variations in spreading and recirculation occur at both levels, but they are more pronounced at the AAIW level, in agreement with earlier observations in the upper layers. At the northern boundary of the equatorial belt (730) the AATW flows along the western boundary while the UCPW, instead, recirculates into the interior of the ocean.

Variability of AABW properties in the equatorial channel at 35°W

[1]xa0Hydrological data and tracer (silicate, oxygen and CFCs) analyses have been performed in order to describe the time and space variability of the characteristics and transport of the Antarctic Bottom Water (AABW) entering the equatorial channel at 35°W. The 35°W sections were sampled in the tropical Atlantic during several cruises from 1993 to 1999 as part of the international WOCE and CLIVAR efforts. Data for the 1990s reveal a slight warming trend superimposed on significant variability due to the seasonal changes of AABW transports. Data from previous cruises made in the 1970s and 1980s complete the time-series and show that, over the last three decades, the maximal AABW input occurred in the early 1990s.

The arrival of a new Labrador Sea Water signal in the tropical Atlantic in 1996

[1]xa0We describe results inferred from the tracking, in the tropical Atlantic, of CFC signals linked to the intensification of convection processes in the Labrador Sea since 1988. CFC and hydrology series (1993–1997) off French Guiana (7°N) have been investigated in order to analyse LSW characteristics and their variability. A dramatic increase of the CFC concentrations below 2000 m at the level of the classical LSW has been observed in April 1996. Using CFC and salinity data, the comparison of the (1993–1997) series at 7°N with a (1989–1999) series at 35°W indicates that the sporadic 1996 event has been followed in the equatorial Atlantic in the late 90s by the arrival of a more continuous newly ventilated LSW flow. This study evaluates a transit time from the Labrador basin to the tropics of less than one decade, in agreement with direct current measurements.