Spring and winter water mass composition in the Brazil-Malvinas Confluence

Hydrographic data of the Confluence 1 cruise collected during austral spring (November 1988) have been analyzed to estimate relative mixing proportions of the various water masses of the Brazil-Malvinas Confluence region using a multiparameter analysis. Seven source water types (SWT) are identified in this region, and all are retained for the analysis: Thermocline Water (TW), Subantarctic Surface Water (SASW), Antarctic Intermediate Water (AAIW), Upper Circumpolar Deep Water (UCDW), North Atlantic Deep Water (NADW), Lower Circumpolar Deep Water (LCDW) and Weddell Sea Deep Water (WSDW). Tracers selected are temperature, salinity, dissolved oxygen and nutrients. Mixing proportions are quantified and plotted along five zonal sections at 35.4, 36.5, 37.9, 41 and 41.6S. The solution obtained during the springtime cruise is consistent with the wintertime (September 1989) data set (Maamaatuaiahutapu et al., 1992): both show the large local recirculation of AAIW and the separation of NADW from the coast south of the thermocline front. However, noticeable changes in water mass mixing proportions can be detected between the winter of 1989 and the preceding spring. The seasonal change for the upper layers of TW and SASW is related to temporal and spatial fluctuations of the thermohaline front. The marked differences in SWT proportions between the two seasons occur at the same location for TW, SASW and AAIW; suggesting that the upper waters have a large impact on the AAIW movement. The deep waters undergo great spatial changes between the two cruises. The variation of the deep convergence position (revealed by the variation of spatial occupancy of the CDW and NADW) seems influenced by the movement of the thermocline front

Age determination of mixed water masses using CFC and oxygen data

We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section I5 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48 degrees-55 degrees E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55 degrees E after 15-20 years. It takes ICW some 25-30 years to reach 110 degrees E

Deep Lenses of Circumpolar Water in the Argentine Basin

Three deep anticyclonic eddies of a species only reported once before [ Gordon and Greengrove, 1986 ] were intersected by hydrographic lines of the World Ocean Circulation Experiment (WOCE) and South Atlantic Ventilation Experiment (SAVE) programs in the Argentine Basin. The vortices are centered near 3500 m depth at the interface between North Atlantic Deep Water and Bottom Water. They have ∼1500-m-thick cores containing Lower Circumpolar Deep Water and a dynamic influence that may span up to two thirds of the water column. As one eddy was observed just downstream of the western termination of the Falkland Escarpment, a destabilization of the deep boundary current by the sudden slope relaxation is suggested as a potential cause of eddy formation. Besides isopycnal interleaving at the eddy perimeters, strongly eroded core properties in the upper parts of the lenses, associated with low density ratios, hint at double diffusion at the top of the structures as another major decay mechanism. The presence of an eddy in the northern Argentine Basin shows the possibility for a northward drift of the vortices, in this basin at least. Deep events in recent current measurements from the Vema Channel are presented that raise the question of further equatorward motion to the Brazil Basin

Characterizing swells in the southern Pacific from seismic and infrasonic noise analyses

International audienceA temporary network of 10 broad-band seismic stations has been installed in French Polynesia for the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). All the seismic stations were installed either on volcanic islands or on atolls of the various archipelagos of French Polynesia in a manner which complements the geographic coverage provided by the regional permanent stations. The primary aim of PLUME is to image the upper mantle structures related to plate motion and hotspot activity. However, because of its proximity to all sites, the ocean is responsible for a high level of noise in the seismic data and we show that these data can also be used to analyse ocean wave activity. The power spectral density (PSD) analyses of the seismic data recorded in French Polynesia show clear peaks in the 0.05– 0.10 Hz band (periods between 10 and 20 s), which corresponds to swell frequencies. Clear peaks in this frequency band are also observed in infrasonic data recorded on Tahiti. Ground motion analysis shows that the swell-related seismic noise (SRSN) is linearly polarized in the horizontal plane and its amplitude decreases rapidly with the distance from the shore. The microseismic and the infrasonic 'noise' amplitudes show very similar variations from station to station and both are strongly correlated with the swell amplitudes predicted by the National Oceanic and Atmospheric Administration (NOAA), wind-forced, 'WaveWatch' models. The swell direction can be estimated from SRSN polarization analysis but this has to be done with care since, for some cases, the ground motions are strongly controlled by the islands' anisometric shapes and by swell refraction processes. We find cases, however, such as Tahiti or roughly circular Tuamotu atolls, where the azimuth of the swell is in good agreement with the seismic estimates. We, therefore, demonstrate that the SRSN and the infrasonic signal observed in French Polynesia can be used in such cases as a proxy for swell amplitude and azimuth. From the continuous analysis of the data recorded in 2003 at the permanent seismic station PPTL in Tahiti, transfer functions have been obtained. This could provide a way to quantify the swell activity during the last two decades and, therefore, assist in the investigation of climate changes

Contexte océanographique de l'archipel des Australes

International audienc

Floating marine debris surface drift: Convergence and accumulation toward the South Pacific subtropical gyre

International audienc

Transports of the Brazil and Malvinas currents at their confluence

Geostrophic transports of the western boundary currents at the Brazil/Malvinas Confluence in the South Atlantic Ocean are estimated from the data set of the Confluence 3 cruise (February 1990) with a nonlinear inverse model which takes into consideration density, current meter and wind data, and dynamical (planetary vorticity, Ekman, mass conservation) constraints. Inversions are carried out with two initial different levels of no motion at 1500 m (Case A) and at 3000 m (Case B). Consistencies of the water volume transports provided by both inversions are analyzed and compared to previous estimates. Current meter constraints are applied in the Malvinas Current region where a total transport of 45 +/- 7 Sv (1 Sv = 10(6) m(3) s(-1)) is given by both inversions. Within the Brazil Current region, discrepancies between both inversions appear. Case A provides a total transport of 30 +/- 7 Sv while case B gives a total transport of 56 +/- 8 Sv. In the first two layers (0-1000 m; 1000-2000 m), case B (53 Sv) gives larger transport than case A (32 Sv). North of the Confluence and at the North Atlantic level, water is found to flow northward in case A at a rate of 3.4 +/- 2 Sv and southward in case B at a rate of 3 +/- 3 Sv. Case B results are more in agreement with our present knowledge of water mass circulation in the Confluence region than case A results. Eastward transports at the Brazil-Malvinas Confluence are estimated to be 20 +/- 7 Sv and 30 +/- 7 Sv for cases A and B, respectively. Compared to the total transports of the Brazil and Malvinas Currents, these estimates suggest that most of the water supplied to the Confluence area recirculates within the Brazil Current and the Malvinas Current regions