General Hydrography of the Beagle Channel, a Subantarctic Interoceanic Passage at the Southern Tip of South America (original) (raw)
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Caceres et al Ocean Dynamics 2006
Measurements of velocity and density profiles were used to describe the tidal and mean flow structure across and along a sill in Refugio Channel, a fjord-like inlet in Southern Chile (43.9°S). These are the first oceanographic measurements of any kind effected in Refugio Channel. Current profiles were obtained with a 307.2-kHz acoustic Doppler current profiler during two semidiurnal cycles along a repeated triangular circuit. Two alongchannel transects formed the sides of the triangle that crossed the sill and were identified as the western and eastern transects. One cross-channel transect, the base of the triangle, was located on the seaward side of the sill. Density profiles were obtained at the corners of the triangle. The longitudinal mean flow in the western transect showed a two-layer exchange structure over the landward side of the sill. The structure of net seaward flow at the surface and landward flow at depth was disrupted by the sill in such a way that over the seaward side of the sill, only seaward flow was observed throughout the water column. This likely resulted from the blocking of landward net flow by the sill. In the eastern transect, two-layer exchange dominated over most of the transect and was consistent with the observed density profiles. Over the seaward side of the sill, a surface layer, ∼10 m deep, flowed landward as a third layer. This feature should have been caused by river input further seaward (to the north) and produced a surface convergence region over the sill. In terms of tidal flows, the greatest tidal current amplitudes were 40 cm s −1 over the sill as the flow accelerated through the reduced cross-sectional area of the channel. Near-surface flow convergences were identified over both along-channel transects.
Deep Sea Research Part I: Oceanographic Research Papers, 2003
Hydrographic data from a cruise at 24.5 N in the subtropical North Atlantic are used to calculate and examine the fluxes across the section. The components of the fluxes of heat, freshwater, oxygen and nutrients through the section are analysed. After the variables are separated into section average, baroclinic profile and anomalies, the fluxes are separated into an Ekman component and its barotropic compensation, baroclinic and horizontal components. The baroclinic contribution due to the meridional overturning circulation (MOC) is responsible for the largest amount of heat transport with warmer waters flowing poleward and cooler deep waters flowing equatorward. The Ekman component and its barotropic compensation is the second-most important component of heat transport. The MOC transports about 25% of the combined meridional heat transport by atmosphere and ocean at 24 N. The Ekman and baroclinic components are responsible for the northward salt transport. Salt conservation for the Atlantic north of 24.5 N suggests that there is a net precipitation between the Bering Strait and the 24.5 N section of about 0.5 Sv. Horizontal transport is the major contribution to the southward oxygen flux, as oxygen-poor water flows northward in the Florida Straits and oxygen-rich water flows southward in mid-ocean. Baroclinic transport is the main factor in the southward transport of all nutrients: low concentration upper water flows northward and higher concentration deep water flows southward.
Earth and Planetary Science Letters, 2013
The lithium/magnesium (Li/Mg) molar ratios, radiocarbon measurements (D 14 C) and Nd-isotopic composition (eNd) of the aragonite skeleton of a branching cold-water coral (CWC) species Madrepora oculata collected alive in the Bay of Biscay at $ 691 m water depth were investigated to reconstruct a robust record of the mid-depth water mass dynamics between 1950 and 1990 AD. Temperature estimates based on the skeletons Li/Mg molar ratios reveal small decadal changes of about 1 1C at thermocline depth synchronous to and of similar amplitude as surface temperature anomalies. D 14 C measurements shows quasi-decadal oscillations of 15% around pre-bomb D 14 C average value of À 59 7 6% and post-bomb D 14 C of À 12 7 6%, which most likely reflect decadal changes of water mass exchange across the thermocline. The coral eNd values remain in narrow ranges of À 11.9 to À 10.2, similar to the isotopic composition of East North Atlantic Central Water, but show highest values in the late 1950s, and early 1960s, 1970s, and 1980s. The punctuated changes of the coral Nd-isotopic composition may thus reflect periods of particular enhanced advection of temperate intermediate water (mid-depth Subpolar Gyre/Mediterranean Sea Water). Altogether, our robust multi-proxy record provides new evidence that Northern Hemisphere atmospheric variability (such as, North Atlantic Oscillation and East Atlantic pattern) drives changes not only in the thermocline but also in the middepth water-mass advection patterns in the Northeastern Atlantic margin. However, the interannual variability of our record remains to be tested.
The Pacific-Atlantic connection: Biogeochemical signals in the southern end of the Argentine shelf
Journal of Marine Systems, 2016
The Cape Horn Current transports low-salinity waters from the SE Pacific Ocean into the Atlantic, which are transported further north by the Malvinas current. Biogeochemical signals of this connection were studied by characterization of dissolved organic matter (DOM) by determination of dissolved organic carbon (DOC), fluorescent dissolved organic matter (FDOM c), and DOM humification index (HIX). Further, inorganic nutrients, salinity, temperature, stable isotopic composition of particulate organic nitrogen (δ 15 N) and chlorophyll a (Chla) were measured in the southern end of the Argentine shelf in March 2012. Three water types were characterized: waters of the Beagle Channel (BCW), coastal waters (CW) and oceanic waters (OW). Highest values of ammonium, DOC, FDOM c and HIX were found in BCW, the lowest in OW, suggesting that terrigenous input is a main source of ammonium and refractory carbon, which is supported by a highly significant inverse correlation of these parameters with salinity. In turn, lowest concentrations of nitrate, silicate and phosphate were found in BCW and CW, and highest in OW, with highly significant correlations of these nutrients with salinity, indicating the contribution of the saltier, nutrient-rich Antarctic Circumpolar Current (ACC) to the Pacific-Atlantic connection system. In general there was an inverse distribution pattern between Chla and those nutrients contributed by the ACC, which is consistent with the transition from coastal waters to the low-silicate, high-nitrate, low-chlorophyll, iron-limited setting of the Subantarctic oceanic waters north of the Polar Front. In contrast, in the low-salinity, internal BCW, high values of ammonium, DOC, HIX and FDOM indicate continental inputs, likely including iron complexes, which could have led to the observed high Chla values. δ 15 N values were positive in the study region, and same as ammonium, reached a maximum in the inner part of the BCW, declining towards OW. This does not support a previous assumption that rainfall on the SE Pacific could be the source of ammonium and hence explain negative δ 15 N values previously found in the northern Drake Passage. The highly significant inverse correlations of ammonium, FDOM c , HIX, and DOC with salinity suggest that continental runoff rather than wet deposition is an important source of ammonium and DOM in the Pacific-Atlantic connection.
Journal of Geophysical Research, 2001
A World Ocean Circulation experiment Hydrographic Program section along 66øW in the North Atlantic was made in 1997. In addition to the usual variables (hydrographic and tracer) measured in WOCE onetime sections, we made lowered acoustic doppler current profiler (LADCP) measurements at nearly all of the stations. The section closed off a portion of the western North Atlantic to the west of the line, making a closed volume for constraining the circulation. In addition, the deep portions of the Caribbean do not communicate with the rest of the basin. By combining mass, silica, and LADCP information as constraints an inverse calculation obtained reveals a strong eastward transport in the Gulf Stream bounded by westward flowing water on either side. Within these energetic flows we see evidence for recently ventilated Classical Labrador Sea Water, which has not reached the Deep Western Boundary Current north of Puerto Rico in any significant amounts. Within the Caribbean our major new finding is a deep cyclonic circulation below sill depth in excess of 100 times the inflow of deep Atlantic water through the major deep sill: the Anegada-Jungfern Passage. The signature of the deep Atlantic source water is most prominent in CFCs and both a bottom and mid-depth maximum are present in the Caribbean to the south of Puerto Rico. Off the coast of Venezuela, however, only the deeper CFC maximum is found. For the entire section the net overturning circulation, heat flux, and freshwater fluxes are all consistent with expectations based on water mass formation and air-sea exchanges to the west of our section, but the annual mean air-sea fluxes of heat and freshwater from Comprehensive Ocean-Atmosphere Data Sets appear somewhat too small in comparison with our singlesection result. 1. Introduction This cruise was part of the World Ocean Circulation Experiment (WOCE) Hydrographic Program's (WHP) global, onetime survey of the oceans. The effort to study the global ocean with state of the art instrumentation for highest-quality measurements has been completed in all of the major ocean basins ending with the North Atlantic. As part of this effort, two meridional legs were planned for summer of 1997 aboard the R/V Knorr at 52 ø (WHP line designator A20) and 66øW (A22) as part of the U.S. Atlantic Circulation and Climate Experiment. These lines were done back to back beginning off the coast of Newfoundland, Canada, working southward along 52W to the coast of Suriname, with a port stop between legs in Trinidad. Our leg (Figure 1) worked northward from the coast of Venezuela to Puerto Rico and thence northwards (along 66øW) to the continental shelf south of Cape Cod, Massachusetts. Basic instrumentation and principal investigators for both meridional legs were the same with small changes in at-sea personnel. This report is for the second of the two meridonal legs (A22): the only WHP line in the global survey that sampled in the Caribbean Sea. During the International Geophysical Year (IGY) in the mid-1950s and again in 1985, hydrographic stations were made to the ocean bottom in the region outside of the Caribbean along nominal longitudes of 66øW. Changes in water properties between these two occupations reflect longterm changes in the deep waters of the western North Atlantic [Joyce and Robbins, 1996]. Measurements made in 1997 along 66øW and 52øW were analyzed to document further the evolving system [Joyce et al. 1999], which is being strongly influenced by temporal changes in the Labrador Sea. Our efforts in this report will focus on the cruise data along 66øW in 1997; we present vertical sections of the basic hydrographic data including nutrients, clorofluorocarbons (CFCs), and lowered acoustic Doppler current profiler (LADCP) measurements in section 2. We discuss some of the water mass differences from the Caribbean to Atlantic proper and present a new multibeam bathymetric map of the 22,095 22,096
South Atlantic meridional fluxes
Deep Sea Research Part I: Oceanographic Research Papers, 2013
The properties of the meridional overturning circulation (MOC) and associated meridional heat transport (MHT) and salt fluxes are analyzed in the South Atlantic. The oceanographic data used for the study consist of Expendable bathythermograph (XBT) data collected along 27 sections at nominally 351S for the period of time 2002-2011, and Argo profile data collected in the region. Previous estimates obtained with a shorter record are improved and extended, using new oceanographic sections and wind fields. Different wind products are analyzed to determine the uncertainty in the Ekman component of the MHT derived from their use. Results of the analysis provide a 9-year time series of MHT, and volume transport in the upper layer of the MOC. Salt fluxes at 351S are estimated using a parameter introduced by numerical studies, the M ov that represents the salt flux and helps determine the basin scale salt feedback associated with the MOC. Volume and heat transport by the western and eastern boundary currents are estimated, and their covariablity is examined. Analysis of the data shows that the South Atlantic is responsible for a northward MHT with a mean value of 0.54 7 0.14 PW. The MHT exhibits no significant trend from 2002 to 2011. The MOC varies from 14.4 to 22.7 Sv with a mean value of 18.1 7 2.3 Sv and the maximum overturning transport is found at a mean depth of 1250 m. Statistical analysis suggests that an increase of 1 Sv in the MOC leads to an increase of the MHT of 0.04 7 0.02 PW. Estimates of the M ov from data collected from three different kinds of observations, contrary to those obtained from models, feature a positive salt advection feedback (M ov o 0) suggesting that freshwater perturbations will be amplified and that the MOC is bistable. In other words, the MOC might collapse with a large enough freshwater perturbation. Observations indicate that the mean value of the Brazil Current is À 8.6 7 4.1 Sv at 241S and À 19.4 7 4.3 Sv at 351S, increasing towards the south. East of 31E, the northward flowing Benguela Current and Agulhas rings have a net northward transport of 22.5 7 4.7 Sv. No significant correlation is observed between the MOC and the Brazil Current transport, and most of the compensation derives from the eastern boundary and interior transports. Products from the Ocean general circulation model For the Earth Simulator (OFES) are used to validate methodology used to extend the XBT record, and to aid in the interpretation of the observed findings.
Meridional Circulation in the Tropical North Atlantic
1993
A transatlantic CTD/ADCP section nominally located at 1 1 0 N was carried out in March 1989. In this paper relative geostrophic velocities are computed from these data via the thermal wind balance, with reference level choices based primarily on water mass distributions. Mass is conserved by requiring the geostrophic transport to balance the sum of the Ekman and shallow western boundary current transports. A brief overview of the meridional circulation of the upper waters resulting from these analysis techniques is presented, and indicates a North Brazil Current transport of nearly 12 Sv. Transports of the shallow waters are found to support the results of Schmitz and Richardson (1991) who found nearly half of the Florida Current waters to be derived from the South Atlantic. Schematic circulation patterns of the NADW and AABW are also presented. The deep waters of the western basin are dominated by a cyclonic recirculation gyre, consisting of a southward DWBC transport of 26.5 ± 1.8 Sv, with nearly half of this flow returning northward along the western flank of the MAR. A particularly notable result of the deep western basin analysis is the negligible net flow of middle NADW. Although the northward flows of upper and lower NADW along the western flank of the MAR are believed to be associated with the local recirculation gyre, the northward flow of middle NADW, which nearly balances the southward flow of this water mass along the western boundary, may be derived from the eastern basin of the South Atlantic. The deep waters of the eastern basin are also dominated by a large cyclonic recirculation gyre, consisting primarily of lower NADW and supplemented by middle NADW and AABW. Each of these water masses, as well as the upper NADW, have small net northward flows within the eastern basin. The AABW most likely enters the eastern basin by means of the Vema Fracture Zone, while the lower NADW enters primarily through the Kane Gap. Although the components of the horizontal circulation discussed above agree well with results from previous CTD, current meter, and float studies, the meridional overturning cell (5.2 ± 1.6 Sv) and the net heat flux (2.3 ± 1.6 x 1014 W) calculated in this study are considerably lower, and the net freshwater flux (-0.60 ± 1.5 Sv) is slightly higher than previous estimates. These discrepancies may be attributed to: (1) differences in methodologies, (2) the increased resolution of this section (as compared to earlier IGY sections), and (3) temporal (including decadal, synoptic, and most importantly, seasonal) variability. Annual average meridional overturning (12 Sv), heat flux (11 x 1014 W), and freshwater flux (-0.35 Sv), are computed based on annual average Ekman and NBC transports, temperatures, and salinities, and agree well with most previous annual estimates. The large difference between the March and the annual estimates is indicative of the importance of seasonal variability within the tropical North Atlantic.
Mixing of shelf, slope and Gulf Stream water over the continental slope of the Middle Atlantic Bight
Deep Sea Research Part I: Oceanographic Research Papers, 1993
Abstraet--A combination of water masses was observed over the continental slope between Cape Hatteras and Hudson Canyon following the passage of a Gulf Stream meander It was largely made up of fluid discharged from the Gulf Stream in the wake of the meander and also contained intrusions of Middle Atlantic Bight (MAB) shelf water and slope water Its most prominent dynamical feature was an anticyclonic eddy that had an initial diameter of about 100 km but contained relatively weak currents, not greater than 30 cm s -2 in magmtude. Discharged Gulf Stream water made up the eddy's core, and a band of entrained shelf water circulated at its margin This band was connected with another entrained shelf water band that flowed along the northern margin of the Gulf Stream Transport of the shelf water at the edge of the eddy was estimated to be 1 3 x 105 m 3 s-l, which is comparable with the estimated alongshore transport of water over the MAB shelf Vigorous mLxing of both shelf water bands with surrounding discharged Gulf Stream water ~s indicated by the large differences of the latter's T/S properties from those found within the Gulf Stream These differences were used to estimate salt fluxes into the shelf water band at the edge of the anttcyclonlc eddy, with results in the range of 1 9-6.6 × 10 -6 g salt cm -z s -1 . Density ratios and Richardson numbers determined from hydrographic data suggest that these salt fluxes were primanly the result of mixing by double dfffuswe processes, and that shear-reduced turbulence combined with double diffusion to effect vertical mixing at the margins of the shelf water band next to the Gulf Stream *Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S A. tNaval Underwater Systems Center, Newport, RI 02841, U.S.A ~tGraduate School of Oceanography, University of Rhode Island, Kingston, RI 02881, U S.A 1063 1064 J H CHURCHILL et al