Harry Leach

Deep carbon export from a Southern Ocean iron-fertilized diatom bloom

Fertilization of the ocean by adding iron compounds has induced diatom-dominated phytoplankton blooms accompanied by considerable carbon dioxide drawdown in the ocean surface layer. However, because the fate of bloom biomass could not be adequately resolved in these experiments, the timescales of carbon sequestration from the atmosphere are uncertain. Here we report the results of a five-week experiment carried out in the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current, during which we tracked sinking particles from the surface to the deep-sea floor. A large diatom bloom peaked in the fourth week after fertilization. This was followed by mass mortality of several diatom species that formed rapidly sinking, mucilaginous aggregates of entangled cells and chains. Taken together, multiple lines of evidence—although each with important uncertainties—lead us to conclude that at least half the bloom biomass sank far below a depth of 1,000 metres and that a substantial portion is likely to have reached the sea floor. Thus, iron-fertilized diatom blooms may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments.

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.

Sustained monitoring of the southern ocean at Drake Passage: Past achievements and future priorities

Drake Passage is the narrowest constriction of the Antarctic Circumpolar Current (ACC) in the Southern Ocean, with implications for global ocean circulation and climate. We review the long-term sustained monitoring programs that have been conducted at Drake Passage, dating back to the early part of the twentieth century. Attention is drawn to numerous breakthroughs that have been made from these programs, including (1) the first determinations of the complex ACC structure and early quantifications of its transport; (2) realization that the ACC transport is remarkably steady over interannual and longer periods, and a growing understanding of the processes responsible for this; (3) recognition of the role of coupled climate modes in dictating the horizontal transport and the role of anthropogenic processes in this; and (4) understanding of mechanisms driving changes in both the upper and lower limbs of the Southern Ocean overturning circulation and their impacts. It is argued that monitoring of this passage remains a high priority for oceanographic and climate research but that strategic improvements could be made concerning how this is conducted. In particular, long-term programs should concentrate on delivering quantifications of key variables of direct relevance to large-scale environmental issues: In this context, the time-varying overturning circulation is, if anything, even more compelling a target than the ACC flow. Further, there is a need for better international resource sharing and improved spatiotemporal coordination of the measurements. If achieved, the improvements in understanding of important climatic issues deriving from Drake Passage monitoring can be sustained into the future.

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.

Mesoscale Subduction at the Antarctic Polar Front Driven by Baroclinic, Instability

Abstract A study of mesoscale subduction at the Antarctic Polar Front (PF) is conducted by use of hydrographic data from a high-resolution, quasi-synoptic survey of the front. The geostrophic velocity and isopycnal potential vorticity (PV) fields are computed, and the ageostrophic flow diagnosed from the semigeostrophic omega equation. It is found that the ageostrophic circulation induced by baroclinic instability counteracts the frontogenesis and frontolysis effected by the confluence and difluence, respectively, of the geostrophic velocity field. Though the sense of the ageostrophic circulation is reversed repeatedly along the front, the existence of PV gradients along isopycnals leads to a net cross-front “bolus” transport. In response to a reversal of this gradient with depth (a necessary condition for the onset of baroclinic instability), the bolus transport is northward at the protruding temperature minimum layer that characterizes the PF, and southward above. This net cross-front overturning circul...

On the seasonal development of mesoscale variability : the influence of the seasonal pycnocline formation

Abstract The hypothesis is raised and tested that the formation of the seasonal pycnocline in a baroclinic field leads to a temporal increase of mesoscale spatial variability during the heating season; the reasoning is that the process of seasonal pycnocline formation sets up a necessary condition for baroclinic instability, the vertical reversal of the isopycnic potential vorticity gradient (IPVG). This hypothesis is confirmed by analysis of hydrographic data collected repeatedly in high horizontal and vertical resolution along a section running from the Azores towards Greenland; the analysis reveals a temporal increase during the heating season of (1) mesoscale horizontal variability—most prominent in the region of the North Atlantic Current eddy field; and (2) the tendency of the IPVG to change sign vertically. Comparison with altimeter sea surface height data indicates that an increase of mesoscale variability during summer is an overall feature of the mid-latitude surface-intensified western boundary currents.

Mixing in cyclonic eddies in the Antarctic Circumpolar Current

Observations over a period of 39 days of the increasing minimum core temperature of Winter Water trapped within a mature cyclonic eddy in the Antarctic Circumpolar Current encouraged us to assess the mixing processes which might lead to this temperature rise. Using diffusive heat budgets for alternatively diapycnic (∼vertical) and then isopycnic (∼horizontal) mixing combined with the observed rate of warming allowed upper limits for diapycnic and isopycnic diffusivities to be inferred. This gave values of (3.3 ± 0.8) × 10−4m2s−1 and 87 ± 20m2s−1 for the diapycnic and isopycnic diffusivities respectively. These were then in turn applied to the isopycnic temperature distributions of a juvenile eddy observed earlier in the cruise and integrated forward in time. While both forms of diffusion undoubtedly play a role in modifying the temperature, it was the horizontal diffusivity which was better able to reproduce the tighter θS relationship and the horizontal spread of the temperature minimum observed in the mature eddy

Upper-Ocean Eddy Transports of Heat, Potential Vorticity, and Volume in the Northeastern North Atlantic—“Vivaldi 1991”

Abstract Mesoscale eddies in the northeast North Atlantic were investigated using the SeaSoar towed CTD and ADCP data from the 1991 Vivaldi cruise. These data cover an area of 1700 km × 1500 km between 39° and 54°N and between 35° and 10°W. To maximize statistical significance, but retain the possibility of determining north–south gradients, statistics of eddy quantities were calculated separately for the northern and southern halves of the cruise area. The mean flow in the south is essentially zero; in the north the flow is dominated by the North Atlantic Current (NAC) with a mean speed of 6.5 cm s−1. The eddy kinetic energy in the south, 205 cm2 s−2, is, however, only slightly less than in the north, 272 cm2 s−2. The eddy momentum transports, or Reynolds stresses, u′υ′, show a poleward decrease, corresponding to an acceleration of the mean eastward flow associated with the NAC of 0.03 cm s−1 day−1. The eddy heat transports, u′T′, are not significantly different from zero in the south but show a clear po...

Isopycnic modeling of the North Atlantic heat budget

A 2° resolution version of the Miami Isopycnic Coordinate Ocean Model is used within an idealized basin to examine the ocean heat budget. Simplified seasonal wind stress and thermohaline forcing, based on observational data of the North Atlantic, are used to drive the model. Following a 30-year spin-up, the model reaches a state of little annual heat content change, at which point a detailed study of the heat budget is performed. The heat budget is split into the components of surface forcing, diffusion, Ekman pumping, Ekman transport, and non-Ekman advection. Both annual and seasonal results are obtained. Analysis of the heat budget reveals different annual balances to exist in different ocean regions. In the subpolar gyre the principal balance is between cooling caused by surface fluxes and warming due to geostrophic advection. However, in the subtropical gyre, where net surface fluxes are small, Ekman pumping balances geostrophic advection. As such, the Ekman pumping is seen to be important for supplying the necessary heat for subduction to take place. An investigation of the Ekman compensatory flow is undertaken. It is shown that the exact temperature and, consequently, the depth of this flow are important for determining the Ekman heat content change. The results here tentatively suggest this depth to be in the upper thermocline. Subduction rates are calculated and shown to be reasonable. The seasonal heat budget is dominated everywhere by surface fluxes. All other terms have negligible seasonal cycles, except for the Ekman terms, which exhibit a limited seasonal variation. In doing so the Ekman terms are seen to control the seasonal cycle of the transport of heat. This is due to their dependency on not only the time variation in wind stress but also the degree of stratification of the upper ocean.

An empirical approach to improving tidal predictions using recent real-time tide gauge data

Harmonic tidal prediction methods are often problematic in estuaries owing to the distortion of tidal fluctuations in shallow water, causing disparity between predicted and observed sea levels. The UK National Tidal and Sea Level Facility attempted to reduce prediction errors for the short-term forecasting of High Water (HW) extremes using three alternative techniques to the Harmonic Method in the Bristol Channel, where prediction errors are relatively large. A simple procedure for correcting Harmonic Method HW predictions using recent observations (referred to as the Empirical Correction Method) proved most effective and was also successfully applied to sea-level records from 42 of the 44 UK Tide Gauge Network locations. It is to be incorporated into the operational systems of the UK Coastal Monitoring and Forecasting Partnership to improve UK short-term sea level predictions.