Source Water Masses Research Articles

The Deep Western Boundary Current: tracers and velocities

In the Deep Western Boundary Current (DWBC) mean velocities obtained by the F11/F12 dating method are far smaller (1–2 cm s −1) than direct velocity measurements (5–20 cm s −1). To resolve this discrepancy, a simple box model is presented that uses the ideas of Pickart et al. (1989, Physical Oceanography, 19, 940–951) to parametrize turbulent diffusion of the current with its surroundings. In contrast to previous models, however, the boundary conditions include all water masses forming the lower part of the DWBC (Denmark Strait Overflow Water, Iceland Scotland Overflow Water and Northeast Atlantic Water). The model-derived mean velocity of the DWBC leads to tracer concentrations that have to fit the observed F11 and F12 distributions, the F11/F12 ratios, and the tritium distributions. Moreover, the model area is extended from south of the Faroe bank along the continental margin of the American continent to 10°S. The model assumes uniform velocity and uniform turbulent mixing along the flow path of the DWBC, and enhanced turbulent mixing in the vicinity of the current compared to the ocean's interior allows the surrounding waters, which remain motionless, to accumulate tracers. The highest mean velocity of the DWBC, which results in model F12, F11, and 3H distributions as well as F11/F12 ratios, compatible to measurements of these tracers along the western boundary, are 4.8 cm s −1. Variations in the composition of the DWBC as well as changes in the time history of the source water masses do not increase the range of the model velocities.

Distribution of antarctic zooplankton around Elephant Island during the austral summers of 1988, 1989, and 1990

Year to year variation and vertical distributions of epipelagic Zooplankton around Elephant Island and King George Island were examined with samples collected with bongo nets and a 1 m2 MOCNESS during the austral summers (Jan–Feb.) of 1988, 1989 and 1990. Copepods were the major components of epipelagic Zooplankton (in numbers) with dominance of Metridia gerlachei (1988 and 1989) and small calanoids and cyclopoids (1990). Euphausiids and salps were the next most abundant groups. The percent composition of euphausiids decreased from 1988 to 1990 while that of salps increased. The abundance of salps exceeded euphausiids and major taxa of copepods in 1990. Local patches of polychaetes and amphipods were also found. Statistically significant annual variations with increased numbers in 1990 were found by analyses of variance in total abundance, abundances of copepods, salps, chaetognaths and amphipods, but abundances of euphausiids, polychaetes and fishes showed no significant annual variations. When the study area was divided geographically, horizontal variability in abundance within each year showed no significance in total abundance, abundances of copepods, euphausiids, amphipods and fishes, but significance in salps, polychaetes and chaetognaths. Results of site clustering based on covariances of abundances of eleven major taxa were well matched, though not perfect, with the distribution of surface water temperatures which could be used as a tracer of source water masses suggesting that spatial variation was related to hydrodynamic conditions. Factor analyses showed that annual and spatial variations in abundance were mainly caused by only two taxa, Metridia gerlachei and salps (mostly Salpa thompsoni). These two taxa were responsible for about 60% and 30% of total variance, respectively, and were useful indicators of the interannual variation. That is, 1988 and 1989 were the years of M. gerlachei, and 1990 was the year of salps. From vertically stratified MOCNESS samples, it was shown that the major taxa in this study area were active vertical migrators. While most samples obtained by relatively shallow tows (uppermost 100 m depth) were composed of exclusively one or two taxa, those from relatively deep tows (down to 200 m) showed various patterns of vertical stratification suggesting that the patterns of vertical migration were species specific. Species specific ontogenetic vertical migration associated with elevated habitat temperatures also seemed to be responsible for the annual variation in zooplankton distribution in the upper water column.

An oceanographic flip‐flop: deep water exchange in Fortune Bay, Newfoundland

Observations are presented of deep‐water exchange in a large coastal embayment on the south coast of Newfoundland. The bay has several outer sills not unlike those found in fjords. Outside the bay, two submerged channels on the continental shelf are reservoirs for two different deep‐water masses which may flow over separate sills to replace deep water within the bay. The deep water is renewed approximately twice a year. On average, exchange occurs both during the half‐year period centered on midwinter and during the half‐year period centered on late summer, with the water properties of the inflow in each period being dominated by one of the two source water masses in the shelf channels. The system therefore occupies one of two renewal states for much of the year. The transition between states appears to be induced largely by seasonal variations in the wind forcing over the shelf.

Hydrographic patterns in McMurdo Sound, Antarctica and their relationship to local benthic communities

Measurements of hydrographic parameters (temperature, salinity, nitrate, nitrite, phosphate, chlorophyll a, phaeophytin, and oxygen) in McMurdo Sound, Antarctica during spring, 1984, before the regional phytoplankton bloom, and summer, 1984, after the peak of the bloom, indicate the several processes contribute to changes in the vertical and horizontal structure of the water column. Regional variation in the source of water masses within the Sound, ice cover patterns, and meltwater from the Ross Ice Shelf and nearby continental glaciers result in east-west and north-south gradients in the thermohaline, nutrient, and productivity characteristics of the Sound. These patterns are also related to the extremely variable structure and productivity of shallow water benthic macrofaunal communities in McMurdo Sound. Hydrographic patterns during Spring (November) were indicative of conditions at the end of winter prior to the spring phytoplankton bloom. The water column was nearly isothermal with temperatures near or below the surface freezing point of seawater with only a slight salinity increase with depth. Salinity was lower in the west Sound than in the east, probably in response to glacial meltwater input from the Ross Ice Shelf and/or terrestrial sources. Nutrient levels were high and nearly homogenous throughout the Sound. Chlorophyll a was low (<1.0 μg/l) throughout most of the Sound, but was lowest in the western sound, as expected from the circulation pattern (Barry and Dayton 1988). Oxygen was uniformly low during spring. The summer hydrographic distributions, estimated from samples collected during the decline of the regional plankton bloom, were dramatically different than in during spring. Both the salinity and temperature were vertically stratified at all sites, particularly in the west Sound. Temperatures near the surface were well above the freezing point and occasionally near or above 0°C. Near surface salinity in the western Sound was nearly fresh (0.4 ppt) at some locations in the southwestern Sound. Chlorophyll a was high throughout the Sound relative to spring concentrations, and nutrient levels (NO3, PO4) were strongly depressed near the surface, due mainly to phytoplankton uptake rather than by dilution. Primary productivity estimates based on the summer nitrate and phosphate deficits over 90 days were 1.96–2.02 and 0.39–1.02 gCm-2d-1 for the east and west sound, respectively. Nutrient ratios indicated that glacial meltwater from the Ross Ice Shelf and/or nearby terrestrial sources may be an important component of the summer meltwater input to the western Sound. Enhanced water column stability due to this input may prolong the maintenance of high water column stability as this water mass flows northward and result in particularly high productivity in northern McMurdo Sound.

Neodymium isotopic variations in seawater

New data for the direct measurement of the isotopic composition of neodymium in Atlantic Ocean seawater are compared with previous measurements of Pacific Ocean seawater and ferromanganese sediments from major ocean basins. Data for Atlantic seawater are in excellent agreement with Nd isotopic measurements made on Atlantic ferromanganese sediments and are distinctly different from the observed compositions of Pacific samples. These results clearly demonstrate the existence of distinctive differences in the isotopic composition of Nd in the waters of the major ocean basins and are characteristic of the ocean basin sampled. The average ε Nd(0) values for the major oceans as determined by data from seawater and ferromanganese sediments are as follows: Atlantic Ocean, ε Nd(0) ≅ −12 ± 2 ; Indian Ocean, ε Nd(0) ≅ −8 ± 2 ; Pacific Ocean, ε Nd(0) ≅ −3 ± 2 . These values are considerably less than ε Nd(0) value sources with oceanic mantle affinities indicating that the REE in the oceans are dominated by continental sources. The difference in the absolute abundance of 143Nd between the Pacific and Atlantic Oceans corresponds to ∼10 6 atoms 143Nd per gram of seawater. The correspondence between the 143Nd/ 144Nd in seawater and in the associated sediments suggests the possible application of this approach to paleo-oceanography. Distinctive differences in ε Nd(0) values are observed in the Atlantic Ocean between deep-ocean water associated with North Atlantic Deep Water and near-surface water. This suggests that North Atlantic Deep Water may be relatively well mixed with respect to Nd isotopic composition whereas near-surface water may be quite heterogeneous, reflecting different sources for surface waters relative to deep water. This suggests that it may be possible to distinguish the sources of water masses within an ocean basin on the basis of Nd isotopic composition. The Nd isotopic variations in seawater are used to relate the residence time of Nd and mixing rates between the oceans.

The meridional distribution of silicate in the western atlantic ocean

Meridional sections of silicate, salinity, and potential temperature have been constructed that run through the western basins of the Atlantic Ocean. A comparison of these distributions shows 3hat extrema (maxima or minima) in vertical plots of silicate versus depth can be traced back to sources of water masses that are rich or poor in silicate. The dominant features are a silicate maximum at intermediate depths (≈ 1000 metres) that is associated withtthe northerly movement of Antarctic Intermediate Water; and an increase in silicate towards the bottom, in the deepest water, which is associated with the northerly movement of Antartiic Bottom Water. Both these waters are rich in silicate. In the northernmost basins (the Labrador and Newfoundland basins) a decrease in silicate towards the bottom is caused by the overflow of Norweigaan Sea Water, of low silicate content, through the Denmark Strait.

Chemical characteristics of water masses in the Amerasian Basin of the Arctic Ocean

Detailed vertical profiles of hydrographic data and routine chemical parameters (oxygen, nutrients) were taken in the Amerasian basin of the Arctic Ocean and in the Bering Strait and Chukchi Sea. Correlations of these chemical profiles with water mass sources and mixing processes are postulated. The chemical parameters show that the water of the sharp temperature maximum at about 75-meter depth is chemically a mixture of the arctic surface water and core water from about 160 meters. This core water corresponds to the temperature minimum and nutrient maximum and is of Bering Strait-Chukchi Sea origin. Chemical parameters clearly show that the temperature maximum and minimum simply reflect differing thermal regimes existing in the Chukchi. Extremely close vertical sampling has shown intriguing detail in the oxygen, temperature, salinity, and nutrient profiles in the area of the very sharp oxygen minimum occurring at about 205 meters. Temperature-salinity and chemical plots of the close-interval data identify a core of water at approximately 230 meters that is mixing above with Bering core water at 160 meters and below with the Atlantic core water at about 500 meters. Thus the oxygen minimum appears to be at least partly due to advection of shelf water into the basin rather than to in situ oxidation alone.

Source regions of oxygen maxima in intermediate depths of the Arabian Sea.

Oxygen maxima, in relation to σt salinity maxima and minima, and other hydrological structural features, have been examined along three meridional sections of the Indian Ocean. These relations have provided a background for the interpretation of the water mass sources of oxygen maxima of the whole Indian Ocean. After grouping these oxygen maxima according to density, their salinities have been used to identify mixing circuits in which the following waters are involved: from the south (1) South Indian Central, (2) Subtropical oxygen maximum, (3) Antarctic Intermediate; from the east (4) Equatorial Frontal water; and from the north (5) Persian Gulf, and (6) Red Sea. The principal routes whereby oxygen-rich mixtures of these waters enter the Arabian Sea, during the south-west monsoon, have been determined. The directions of flow along several of these routes agreed with measured directions of current flow. Where these currents disagreed the measured current was generally very weak.