Northern and southern water masses in the equatorial Atlantic (original) (raw)
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Nutrients in the Atlantic thermocline
Journal of Geophysical Research, 1985
A set of maps are presented of nutrient distribution on isopycnal surfaces in the North and tropical Atlantic Ocean main thermocline. The data used in producing these maps are from the Transient Tracers in the Oceans (TTO) North Atlantic Study and Tropical Atlantic Study, an associated German study (Meteor 56/5), two cross-Atlantic sections from cruise 109 of the Atlantis II, and the GEOSECS program. The nutrient distributions reflect primarily the sources at the northern and southern outcrops of the isopycnal surfaces, the in situ regeneration due to decomposition of sinking organic materials, and the interior physical processes as inferred from thermocline models and the distribution of conservative properties such as salinity. However, silica also exhibits behavior that cannot be explained by in situ regeneration. A simple phenomenological model suggests that cross-isopycnal advection and mixing in the equatorial region may play an important role in the nutrient dynamics. These data should prove of great value in constraining models of physical as well as biogeochemical processes. GEOSECS data. Broecker and Takahashi [1981] studied potential temperature, salinity, oxygen, silica, nitrate, barium, alkalinity, and "NO" as well as radioactive tracers •'•C and Copyright 1985 by the American Geophysical Union. Paper number 5C0431. 0148-0227/85/005C-0431 $05.00 226Ra, on the a•-32.05 surface in the Intermediate Water range. McDowell [1982] analyzed temperature, salinity, and oxygen on several potential density levels that lie in the North Atlantic thermocline and below and calculated the contributions from various source waters at each location in the North Atlantic. Sarmiento et al. [-1982a] described the distributions of salinity, potential vorticity, and tritium in 1972 on six potential density levels in the North and tropical Atlantic thermocline. The distribution of potential vorticity was also investigated by McDowell et al. [1982]. This study is an extension of the work by Sarmiento et al. [1982a] to nutrients. Here we have produced a set of water mass property maps and property-property plots on the same six potential density surfaces that were investigated in the earlier study. Our aim is to point out major water sources and major physical/biogeochemical processes acting on the Atlantic thermocline water. In this context, property-property plots can complement spatial distribution maps in important ways in that they tend to eliminate features caused by pure advection and to highlight the water masses involved in the circulation. Moreover, they point to the occurrence of crossisopycnal and nonconservative processes, which are not readily appreciated from spatial distribution maps. We shall look at five water properties: salinity, oxygen, apparent oxygen utilization (AOU), silica, and nitrate. Spatial distribution maps are produced for each quantity for each layer. The maps are produced by the objective mapping technique developed and described by Sarmiento et al. [-1982b]. This technique was designed to produce maps of gross distributions of properties on a quasi-horizontal surface, and its characteristic feature is an anisotropic autocorrelation function with a greater zonal extent. For the details of the mapping scheme, the reader is referred to the above paper. DATA The data used for this study were obtained during the GEOSECS Atlantic Study, Meteor cruise 56, leg 5 (R. Schlitzer et al., unpublished manuscript, 1985), the Transient Tracers in the Oceans North and Tropical Atlantic studies (TTO NAS and TAS respectively), and the two cross-Atlantic sections from cruise 109 of the Atlantis II [Roeromich and Wunsch, 1985•. The distribution of stations is shown in Figure 1. The TTO, Meteor, and Atlantis II cruises combined provide us with a coverage of the North and tropical Atlantic that is almost comparable in density with the International Geo
Variability of Water Structure in the Equatorial Atlantic
2000
Characteristics of water masses in the intermediate and deep layers of the ocean are considered. A compar- ative analysis of fine thermohaline structure and the main characteristics of different water masses has been carried out. A hypothesis has been put forward that variations in the temperature of near-bottom waters at the equator recorded in 2000 is a climatic response to
In the Gulf of Guinea (4OW), the equatorial upwelling, generated by the divergence of surface current meridional components, appears only in northern summer (June to c September) south of the equator and exhibits a respectable enrichment in nitrate and phosphate at the sea surface. But at the equator properly so-called, the surface layer is enriched also in nutrients by the vertical mixing by vertical shear between the Equatorial Undercurrent and the surface Equatorial Current. The zonal advection from the african coastal zone, put forward to explain the equatorial fertility, does I not agree with the observed surface distributions of nitrate.
On the intermediate and deep water flows in the South Atlantic Ocean
Journal of Geophysical Research, 1997
A multiparameter analysis is applied on zonal and meridional hydrographic sections obtained for the South Atlantic Ventilation Experiment (SAVE) to determine the spreading and mixing of water masses in the South Atlantic Ocean, focusing our interest on the large-scale flow of intermediate and deep waters. The method utilizes all information from the hydrographic data set including temperature, salinity, dissolved oxygen, and nutrient fields. Mixing proportions are quantified and plotted along the eight sections considered. Results show no evidence of a primats' route of Antarctic Intermediate Water along the western boundary of the South Atlantic. In the eastern basin the eastward extension of the Upper North Atlantic Deep Water (NADW) in the Guinea Basin following the cyclonic subequatorial gyre is confirmed. In the Angola Basin a weak but thick NADW core layer is observed in conjunction with very little presence of Lower Circumpolar Deep Water (LCDW). High LCDW concentrations in Cape Basin are indicative of the communication of this basin to cold water sources in the south. The method is sensitive enough to detect for instance the presence of the Congo River Plume in the Angola Basin or the influence of the Weddell Sea Deep Water in the vicinity of the Romanche and Chain Fracture Zones in the equatorial region. In conjunction with the multiparameter analyses along SAVE sections, an analysis of components of the residual vector R indicates a middepth minimum in the RN/P utilization ratio. Both a suitable explanation for the minimum and the potential consequences for the multiparameter analyses of South Atlantic water mass circulation are still to be found.
Deep-sea Research Part I-oceanographic Research Papers, 2020
This study presents a water mass analysis along the JC150 section in the subtropical North Atlantic, based on hydrographic and nutrient data, by combining an extended optimum multiparameter analysis (eOMPA) with a Lagrangian particle tracking experiment (LPTE). This combination, which was proposed for the first time, aided in better constraining the eOMPA end-member choice and providing information about their trajectories. It also enabled tracing the water mass origins in surface layers, which cannot be achieved with an eOMPA. The surface layers were occupied by a shallow type of Eastern South Atlantic Central Water (ESACW) with traces of the Amazon plume in the west. Western North Atlantic Central Water dominates from 100-500 m, while the 13 °C-ESACW contribution occurs marginally deeper (500-900 m). At approximately 700 m, Antarctic Intermediate Water (AAIW) dominates the west of the Mid-Atlantic Ridge (MAR), while Mediterranean Water dominates the east with a small but non-negligible contribution down to 3500 m. Below AAIW, Upper Circumpolar Deep Water is observed throughout the section (900-1250 m). Labrador Sea Water (LSW) is found centered at 1500 m, where the LPTE highlights an eastern LSW route from the eastern North Atlantic to the eastern subtropical Atlantic, which was not previously reported. North East Atlantic Deep Water (encompassing a contribution of Iceland-Scotland Overflow Water) is centered at ~2500 m, while North West Atlantic Bottom Water (NWABW, encompassing a contribution of Denmark Strait Overflow Water) is principally localized in the west of the MAR in the range of 3500-5000 m. NWABW is also present in significant proportions (> 25 %) in the east of the MAR, suggesting a crossing of the MAR possibly through the Kane fracture zone. This feature has not been investigated so far. Finally, Antarctic Bottom Water is present in deep waters throughout the section, mainly in the west of the MAR.
Journal of Geophysical Research, 1999
The flow of intermediate water masses across the equator in the Atlantic Ocean is of fundamental interest in the context of the giobai meridional circulation C{2II abbOldl•t•U Wl[11 [11• • o i l• vv Antarctic Intermediate Water (AAIW) and the Upper Circumpolar Water (UCPW) at between 500-and 1200-m depths in the western equatorial Atlantic (5øS -7ø30'N). These have been deduced from hydrological and geochemical tracer (nutrients and chlorofluorocarbons) data sets from CITHER 1 (• L'Atalante, January-March 1993), ETAMBOT 1 (• Le Noroit, September-October 1995), and ETAMBOT 2 (• Edwin Link, April-May 1996) cruises. Both the AAIW and UCPW enter, on the isopycnals 60-27.25 (676 ß 36 dbar) and 60 = 27.40 (919 ß 35 dbar), respectively, the equatorial belt as narrow, northwestward flows around the northeast tip of Brazil near 5øS. 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 v•iations 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 bound•y of the equatorial belt (7ø30'N) the AAIW flows along the western boundary while the UCPW, instead, recirculates into the interior of the ocean. complex exchanges between the intermediate waters and the overlying warm waters, on one hand, and the underlying cold waters, on the other hand, occur mainly in the western equatorial Atlantic. However, notwithstanding the importance of the intermediate water to the thermohaline circulation between the North and South Atlantic Oceans, this layer, except for a few recent studies [Suga and Talley, 1995; Bub and Brown, 1996], has received little attention. The northward spreading AAIW can be traced by its salinity minimum accompanied by a high-oxygen anomaly [Wast, 1935]. The oxygen maximum disappears as it meets the strong oxygen minimum characteristic of the equatorial zone [Reid, 1989]. Just beneath the salinity minimum in the lower part of the intermediate layer there is a potential temperature minimum that has been identified by Reid [1989] as the signature of the Upper Circumpolar Water (UCPW). Thus the AAIW and UCPW constitute the deepest layer of the relatively warm Atlantic waters. The equatorial circulation within this layer, lying approximately between 500 and 1200 m, differs from that of the overlying South Atlantic Central Water layer [Stramma and Schott, 1996]. The trajectories of the floats at 800 m depth in the study of Richardson and Schmitz [1993] also reveal the complex pathways of the water masses in the western tropical Atlantic. Nevertheless, in the equatorial Atlantic, the AAIW and UCPW are often treated as a single layer [Stramma and Schott, 1996], and even now the circulation of UCPW has not been differentiated from that of the AAIW. For example, Tsuchyia et al. [1994] consider the temperature minimum and the associated silicate maximum north of 21øS as the lower boundary of the AAIW rather than the northward extension of the UCPW silicate maximum. 20,911 20,912 OUDOT ET AL.' INTERMEDIATE WATER MASSES IN WESTERN ATLANTIC The large amount of data, in particular, on the geochemical (nutrients and chlorofluorocarbons) tracers, collected during the trans-Atlantic CITHER 1 cruise on World Ocean Circulation Experiment Hydrographic Programme (WHP) lines A6 and A7 and the ETAMBOT cruises in the western equatorial Atlantic, gave us an opportunity to study and differentiate the circulation patterns of AAIW and UCPW in the equatorial Atlantic, especially on its western boundary. The two trans-Atlantic Conductivity-temperature-depth-oxygen (CTDO2) tracer sections with closely spaced stations along 7ø30'N and 4ø30'S (WHP lines A6 and A7) and the two meridional sections along 3ø50'W and 35øW between these two latitudes were occupied in January-March 1993 (CITHER 1 cruise; Figure la)[Andrig et al., 1998; Arhan et al., 1998; Oudot et al., 1998]. The western equatorial Atlantic basin stations of the CITHER 1 cruise were reoccupied 0 ß ß ß ß 1000 ß ß ß ß 0
Intermediate layer water masses in the western tropical Atlantic Ocean
Journal of Geophysical Research, 1996
Intermediate layer water masses are defined according to temperaturesalinity relationships derived from conductivity-temperature-depth (CTD) observations measured during four 1990-1991 Western Tropical Atlantic Experiment (WESTRAX) hydrographic surveys. The intermediate layer, bounded by density surfaces of sigma theta 26.00 and 27.65 (approximately 150 and 1300 m deep, respectively), is conveniently divided into upper and lower layers by the relatively low salinity Antarctic Intermediate Water (AAIW) which is centered at sigma theta 27.25 (approximately 700 m deep). Approximately 604-5% of the region's waters are traced to a southern hemisphere origin, indicating the importance of AAIW in the western tropical Atlantic's water mass structure. The southern source water masses, South Atlantic Central Water and AAIW, enter the WESTRAX region (west of 44øW and between the equator and 9øN) as part of the subthermocline North Brazil Current. Depending on the season, all or part of these southern waters retrofiect anticyclonicMly through the region and flow eastward into the North Equatorial Undercurrent. The primary northern source water mass, North Atlantic Central Water, enters the northeastern corner of the WESTRAX region as part of a cyclonic branch of the North Equatorial Current (NEC) and converges with the southern water. This meeting produces mixture water masses which make up 454-4% of the region in volume and are predominantly of a southern nature. The patterns of the mixture water masses which fill the areas between the source water masses suggest the importance of lateral mixing in this ocean region. Further, some of the mixture water in the upper layer appears to be part of the NEC, suggesting southern water recirculation in the tropical Atlantic gyre. A time dependent water mass box model of advective and mixing transports is used to suggest that lateral mixing do•ninates vertical mixing by a ratio of approximately 10 to 1. Typical box model results for the fall-winter 1990-1991 period indicate that 13 Sv of mixture water masses are produced through mixing (a sum of 9 Sv and 4 Sv from southern and northern source water masses, respectively), while a net 17 Sv of mixture water masses are exported from the region. Atlantic Experiment (WESTRAX) was conducted during 1990 and 1991 to address these issues [Brown et al., 1992]. As part of this program, five major hydrographic and velocity surveys were carried out in the region west Copyright 1996 by the American Geophysical Union. Paper number 95JC03372. 0148-0227/96/95JC-03372509.00 of 44øW and bounded by the equator and 15øN. In this paper we describe some of these observations in terms of the seasonal evolution of the structure of intermediate layer water masses. In a companion paper we will describe and discuss the transports of these water masses through the WESTRAX region (F. L. Bub and W. S. Brown, manuscript in preparation, 1996)o Historical potential temperature-salinity (O-S) relationships for water types and water masses of the western tropical Atlantic Ocean have been proposed by Sverdrup e! al. [1942], Mamayew [1975], Emery and Dewar [1982], and Emery and Meincke [1986]. The water masses of the western tropical Atlantic are bracketed, both geographically and in O-S space (Figure 1), by waters which Emery and Dewar [1982] associate with the
Journal of Geophysical Research, 1999
This paper presents CFC and nontransient tracer observations in the western equatorial Atlantic Ocean on repeated sections along 7°30'N, the 35°W meridian, and a transect crossing the Ceara Rise. Three World Ocean Circulation Experiment cruises have been carried out in this area, in February-March 1993 (CITHER 1) and September-October 1995 and April-May 1996 (ETAMBOT 1 and 2). Together with the
On mean and seasonal currents and transports at the western boundary of the equatorial Atlantic
Journal of Geophysical Research, 1993
Current measurements from two consecutive yearlong deployments of three moored stations at the western end of the equator in the Atlantic, along 44øW, are used to determine the northwestward flow of warm water in the upper several 100 m and of the southeastward counterflow of North Atlantic Deep Water (NADW). Measurements from three acoustic Doppler current profilers (ADCPs) looking upward from 300 m toward the surface allowed calculation of a time series of upper layer transports over 1 year. Mean transport through the array for the upper 300 m is 23.8 Sv with an annual cycle of only +3 Sv that has its maximum in June-August and minimum in northern spring. Estimated additional mean northwestward transport in the range 300-600 m is 6.7 Sv, based on moored data and shipboard Pegasus and lowered ADCP profiling. In the de•th range 1400-3100 m a current core with maximum annual mean southeastward speed of 30 cm s--is found along the continental slope that carries an estimated upper NADW transport of 14.2-17.3 Sv, depending on the extrapolation used between the mooring in the core and the continental slope. This transport is higher than off-equatorial estimates and suggests near-equatorial recirculation at the upper NADW level, in agreement with northwestward mean flow found about 140 km offshore. Below 3100 m and above the 1.8øC isotherm, only a small core of lower NADW flow with speeds of 10-15 cm s -1 is found over the flat part of the basin near 1.5øN, clearly separated from the continental slope by a zone of near-zero mean speeds. Estimated transport of that small current core is about 4.5 Sv, which is significantly below other estimates of near-equatorial transport of lower NADW and suggests that a major fraction of lower NADW may cross the 44øW meridian north of the Ceara Rise. Intraseasonal variability is large, although smaller than observed at 8øN near the western boundary.