R.T. Pollard

The deepening of the wind-Mixed layer

Abstract A simple model is given that describes the response of the upper ocean to an imposed wind stress. The stress drives both mean and turbulent flow near the surface, which is taken to mix thoroughly a layer of depth h, and to erode the stably stratified fluid below. A marginal stability criterion based on a Froude number is used to close the problem, and it is suggested that the mean momentum has a strong role in the mixing process. The initial deepening is predicted to obey where u. is the friction velocity of the imposed stress, N the ambient buoyancy frequency, and t the time. After one-half inertial period the deepening is arrested by rotadeon at a depth h = 22/4 u.{(Nf)+ where f is the Coriolis frequency. The flow is then a “mixed Ekman” layer, with strong inertial oscillations superimposed on it. Three quarters of the mean energy of the deepening layer is found to be kinetic, and only one-quarter potential. Heating and cooling are included in the model, but stress dominates for time-scales of ...

Southern Ocean deep-water carbon export enhanced by natural iron fertilization

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol-1 was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom6. The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

Vivaldi 1991 - A study of the formation, circulation and ventilation of Eastern North Atlantic Central Water

Abstract A synoptic, hydrographic data set comprising 32 full depth CTD casts and 2500 CTD/SeaSoar profiles to 500 m is used to describe the θ/S properties and circulation of Central Water east of the mid-Atlantic Ridge and between 39°N and 54°N. Eastward transport of 20 × 106 m3 s−1 in the North Atlantic Current turns entirely northwards to the west of 54°N, 20°W. This transport consists in the upper layers of Western North Atlantic Water freshened at temperatures below 10°C by mixing with SubArctic Intermediate Water. Northern and Southern branches of the North Atlantic Current are well defined and both turn northwards west of 20°W. A further 10 × 106 m3 s−1 of Eastern North Atlantic Water forms and recirculates anticyclonically to the west of Spain south of the North Atlantic Current and north of 40°N. Eastern North Atlantic Water is most weakly stratified east of 20°W and there is clear correlation between weakly stratified pycnostads and positive salinity anomalies relative to Western North Atlantic Water. Thus Eastern North Atlantic Water is a winter Mode Water in which strong winter cooling has increased the density and hence also the salinity anomaly at a given temperature. Near the southern entrance to the Rockall Trough there is evidence that salinities are also increased by Mediterranean Water influence. Circulation south of the North Atlantic Current is complex. There is no evidence for direct ventilation southwards across 40°N where water properties (θ/S, potential vorticity and CFC-113) and historical data all indicate westward ventilation east of 24°W, with weak southward ventilation occurring further west, in the vicinity of the Azores. The circulation pattern suggested is remarkably similar to that proposed by Helland-Hansen and Nansen in 1926 (The eastern North Atlantic, Geophysiske Publicajoner, 4, 1–76), with anticyclonic circulation of colder Eastern North Atlantic Water north of 40°N meeting warmer water from south of 40°N circulating cyclonically north of the Azores Current. The distribution of pycnostads and θ/S properties between 20°W and 35°W north of the Azores indicates alternate bands of Western and Eastern North Atlantic Water moving eastward and westward respectively, including evidence for westward motion immediately south of the Southern branch of the North Atlantic Current, possibly by westward propagation of anticyclonic eddies containing deep pycnostads.

Physical controls on biogeochemical zonation in the Southern Ocean

The primary control on the N–S zonation of the Southern Ocean is the wind-induced transport of the Antarctic Circumpolar Current (ACC). The ACC divides the Southern Ocean into three major zones: the Subantarctic Zone (SAZ) north of the ACC; the ACC transport zone; and the zone south of the ACC (SACCZ). The zone of ACC transport is most often subdivided into two zones, the Polar Frontal Zone (PFZ) and the Antarctic Zone (AAZ), but it may be appropriate to define more subzones or indeed only one at some longitudes. To maintain geostrophic balance, isopycnals must slope upwards to the south across the ACC, thus raising nutrient-rich deep water closer to the surface as one goes polewards. In addition, silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate-rich bottom water. Surface silicate concentrations therefore decrease northwards from high levels in the SACCZ to low levels in the SAZ. Within the SAZ and PFZ and even in the northern part of the AAZ, silicate levels may drop to limiting levels for siliceous phytoplankton production during summer. Nitrate concentrations also decrease northwards, but only become limiting in the Subtropical Zone north of the SAZ. The second circumpolar control is the changing balance of stratification, with temperature dominating near-surface stratification in the SAZ and salinity dominating further south because of fresh water input to the surface from melting ice. This results in circumpolar features such as the subsurface 2°C temperature minimum and the subduction of the salinity minimum of Antarctic Intermediate Water, which are often but not always associated with frontal jets and large transports. The transport of the ACC is dynamically constrained into narrow bands, the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude. Thus it is not the fronts that are circumpolar, but the total ACC transport and scalar properties of the salinity and temperature fields. Evidence of summer silicate and nitrate uptake in all zones (SAZ, PFZ and AAZ) shows that there is productivity despite their high-nutrient low-chlorophyll status. Blooms covering large areas (say 400 km across) in the PFZ and AAZ are found in the vicinity of submarine plateaux, which suggest benthic iron fertilizatio

A Method for Calibrating Shipmounted Acoustic Doppler Profilers and the Limitations of Gyro Compasses

Abstract Calibration of shipmounted acoustic Doppler profiles by a series of 90° turns during periods of GPS navigation provides estimates of misalignment angle φ and scaling factor A with standard deviations less than 0.2° and 0.3%. A varies by 1% with depth, and differs between bottom and water track modes, but φ is independent of these factors. Day to day variations in φ and A, however, are as large as 1° and 2%, the former because of long-period wander of the ships gyro compass, the latter possibly because of variations in Doppler spectra in different conditions. The gyro compass also shows short-period bias of 2° after a 90° turn, with 0.5° bias persisting for over 20 minutes. All these errors indicate that the limit of current accuracy both along and athwartships is about 0.05 m s−1 for a ship speed of 5 m s−1.

Water mass properties and fluxes in the Rockall Trough, 1975-1998

A time series of a standard hydrographic section in the northern Rockall Trough spanning 23 yr is examined for changes in water mass properties and transport levels. The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500 m) Wyville–Thompson ridge. It is one pathway by which warm North Atlantic upper water reaches the Norwegian Sea and is converted into cold dense overflow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (ENAW), which is warmer and saltier than the subpolar mode waters of the Iceland Basin, which also contribute to the Nordic Sea inflow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to the north, which prevents through flow but allows recirculation within the basin. The Rockall Trough experiences a strong seasonal signal in temperature and salinity with deep convective winter mixing to typically 600 m or more and the formation of a warm fresh summer surface layer. The time series reveals interannual changes in salinity of ±0.05 in the ENAW and ±0.04 in the LSW. The deep water freshening events are of a magnitude greater than that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reservior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of which 3.0 Sv is carried by the shelf edge current) but shows a high-level interannual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baroclinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the observations, and no evidence is found of a seasonal signal.

Decadal changes of the θ−S relationship of the Eastern North Atlantic Central Water

Abstract Observations of the eastern North Atlantic mode waters spanning a 20-year period were assembled including previously unpublished observations and historical data, to investigate spatial and temporal variations. Along 42°N, the freshening of salinity on the isopycnal σθ = 27.1 is shown to persist until 1990 and to be followed by an abrupt increase in 1991 to levels not seen since the late 1970s. Some suggestions are made of mechanisms 1970s. Some suggestions are made to mechanisms that may be involved in the variability.

Circulation pathways and transports of the Southern Ocean in the vicinity of the Southwest Indian Ridge

Circulation pathways and transports of the Antarctic Circumpolar Current (ACC) and Agulhas Return Current (ARC) around the complex bathymetry of the Southwest Indian Ridge and Del Cano Rise have been established from two hydrographic surveys and two-year long moorings. After crossing the Southwest Indian Ridge in a 110 Sv (1 Sv = 106 m3s−1) flow concentrated at 48°S, the ACC fragments before being reconcentrated into several branches by the bathymetry. In particular 30–40 Sv turns north around the eastern flank of the Del Cano Rise, turns west as part of anticyclonic flow round the Del Cano Rise finally returning eastward as part of the Crozet Front. Eddy variability from both satellite data and moorings is low over the bathymetry of the Del Cano Rise, so the strong currents found between the Del Cano Rise and the Crozet Plateau are believed to be permanent features. This largely barotropic, time-independent transport is the major pathway for the ACC to enter the Crozet Basin. The anticyclonic flow also transfers water from the Mozambique Basin across the Southwest Indian Ridge to the south side of the Del Cano Rise. Given the evidence for fragmentation of the ACC where it crosses the Southwest Indian Ridge and the concentration of several jets into the Crozet Front from the ACC, Subtropical Front and Agulhas Return Current, it is misleading to relate standard frontal definitions such as the Polar Front and Subantarctic Front to the major transport pathways.

On the physical structure of a front in the Bellingshausen Sea

Abstract The structure of a front observed at 67°S in Austral spring in the Bellingshausen Sea is described. The front extended to full depth, and has been identified by Read et al. (1995) as the southernmost major front of the Antarctic Circumpolar Current where Circumpolar Deep Water (CDW) outcrops. In this paper the structure of the front is examined on scales down to 10 km using a closely spaced CTD section and down to 3 km from underway surveys with SeaSoar and an Accustic Doppler Current Profiler (ADCP) which resolved the density and velocity structure, respectively, in the top 400 m. A seasonal halocline was present throughout the survey area on both sides of the front, at the base of a mixed layer 50–70 m deep. The signature of the previous winters mixed layer depth could be seen in the weak stratification between 70 and 150 m. The subsurface temperature minimum within the winter mixed layer was significantly colder south of the front (−1.7°C) than north of it (−1.1°C). The front, defined by eastward velocities greater than 10 cm s−1, was 70–80 km wide with flow at speeds of up to 50 cm s 1 in the surface layer. The zone within which water mass changes occurred was narrower, about 40 km, contained within the zone of eastward flow and extended down to 1000 m. The strong velocities in the frontal jet caused differential advection of surface properties from upstream (west) of the survey area, resulting in sharp cross-frontal gradients of salinity and chlorophyll a. Chlorophyll was largest (over 5 mg m−3) in a band barely 10 km wide that extended along-front for well over 100 km and was tightly constrained along the southern flank of the front where isopycnals outcropped from the winter mixed layer into the surface layer across the seasonal halocline. The large values of chlorophyll could only be maintained by advection into the survey area from an unknown source region upstream (Boyd et al., 1995). Patches of high chlorophyll were found only south of the front, and it is hypothesised that productivity south of the front is a consequence either of some property of the upwelling CDW or of water that had been under ice, identified by the −1.7°C subsurface temperature minimum. Temperature anomalies on the 27.4 kg m−3 isopycnal indicate that patches of water south of the front had broken off from the southern flank of the front, so it is probable that the bloom was spreading southwards from the front within the survey area rather than northwards from the ice edge.

Water masses and circulation pathways through the Iceland Basin during Vivaldi 1996

Quasi-synoptic data from late 1996 spanning the subpolar North Atlantic have been used to determine the major pathways of the North Atlantic Current (NAC) at that time. High spatial resolution allows fronts to be accurately positioned on each SeaSoar section. A clearly defined front of the NAC (the Southern Branch) turns north at around 25°W and continues through the middle of the Iceland Basin as far as 60°N, 20°W. A second branch (the Northern Branch or SubArctic Front) turns north around 30°W and retroflects westward north of 54°N to re-enter the Irminger Basin and become part of the Irminger Current up the western side of the Reykjanes Ridge. A third, eastward branch turns sharply northwest at the mouth of the Rockall Trough to skirt the southwestern margin of Hatton Bank. This branch carries a tongue of saline eastern North Atlantic water (ENAW) over Hatton Bank and in consequence ENAW covers the whole of the Hatton and Rockall Banks as well as the Rockall Trough, bounded in the west by the Southern Branch. The most saline water, found in Rockall Trough, spills out into the northern Iceland Basin between Rockall and Lousy Banks. This saline, weakly stratified tongue can be traced westward to the south of Iceland continuing southwestward along the eastern flank of the Reykjanes Ridge. Subarctic Intermediate Water is carried into the Iceland Basin, creating a fresh tongue bounded east and west by the more saline ENAW over Hatton Bank and the eastern flank of the Reykjanes Ridge respectively.