Richard G. Williams

Inferring the Subduction Rate and Period over the North Atlantic

Abstract The annual rate at which mixed-layer fluid is transferred into the permanent thermocline—that is, the annual subduction rate Sann and the effective subduction period 𝒯eff—is inferred from climatological data in the North Atlantic. From its kinematic definition, Sann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer. Geostrophic velocity fields, computed from the Levitus climatology assuming a level of no motion at 2.5 km, are used; the vertical velocity at the base of the mixed layer is deduced from observed surface Ekman pumping velocities and linear vorticity balance. A plausible pattern of Sann is obtained with subduction rates over the subtropical gyre approaching 100 m/yr—twice the maximum rate of Ekman pumping. The subduction period 𝒯eff is found by viewing subduction as a transformation process converting mixed-layer fluid into stratified thermocline fluid. The effective ...

The Ekman transfer of nutrients and maintenance of new production over the North Atlantic

The maintenance of new production requires a supply of nutrients to the euphotic zone to o⁄set the loss through biological export. The dynamical supply of nutrients is usually discussed in terms of the vertical transfer from nutrient-rich, deep waters. However, the horizontal transfer is important in regions of downwelling over subtropical gyres, where nutrients may be transported across the intergyre boundaries by the surface Ekman drift or geostrophic eddies from the neighbouring nutrient-rich, upwelling regions. The Ekman transfer of nitrate to the euphotic layer is diagnosed from climatology over the North Atlantic. The vertical Ekman supply of nitrate is found to be significant over the subpolar gyre, the tropics and eastern boundary, whereas the horizontal transfer is found to be dominant at the intergyre boundaries. On the northern flank of the subtropical gyre, the Ekman transfer provides a source of nitrate from 0.03 to 0.06 mol N m~2 yr~1, corresponding to a contribution to new production of between 0.4 and 0.8 mol C m~2 yr~1. This estimate represents a significant fraction of the total new production of typically 1 mol C m~2 yr~1 suggested by both remote chlorophyll and sediment trap observations. A simplified nitrogen cycle model is used to assess the role of the Ekman supply over the North Atlantic. In the model the Ekman supply of nitrate leads to a plume of nitrate and enhanced productivity extending up to 1000 km into the subtropical gyre from the intergyre boundaries. This lateral scale is controlled by the seasonal cycle of the mixed layer and the remineralisation of the particulate organic fallout. ( 1998 Elsevier Science Ltd. All rights reserved.

Meridional coherence of the North Atlantic meridional overturning circulation

The North Atlantic Meridional Overturning Circulation (MOC) is associated with deep water formation at high latitudes, and climatically-important ocean-atmosphere heat fluxes, hence the current substantial effort to monitor the MOC. While it is expected that, on sufficiently long time scales, variations in the MOC would be coherent across latitudes south of the deep water formation region, it is not clear whether coherence should be expected at shorter timescales. In this paper, we investigate the coherence of MOC variations in a range of ocean models. We find that, across a range of model physics, resolution, and forcing scenarios, there is a change in the character of the overturning north and south of about 40 degrees N. To the north the variability has a strong decadal component, while to the south higher frequencies dominate. This acts to significantly reduce the meridional coherence of the MOC, even on interannual timescales. A physical interpretation in terms of an underlying meridionally coherent mode, strongest at high latitudes, but swamped by higher frequency, more localised processes south of 40 degrees N is provided. Citation: Bingham, R. J., C. W. Hughes, V. Roussenov, and R. G. Williams ( 2007), Meridional coherence of the North Atlantic meridional overturning circulation

Does Stommel's Mixed Layer “Demon” Work?

Abstract Stommel argued that the seasonal cycle leads to a bias in the coupling between the surface mixed layer and the main thermocline of the ocean. He suggested that a “demon” operated that effectively only allowed fluid at the end of winter to pass from the mixed layer into the main thermocline. In this study, Stommels hypothesis is examined using diagnostics from a time-dependent coupled mixed layer-primitive equation model of the North Atlantic (CME). The influence of the seasonal cycle on the properties of the main thermocline is investigated using two methods. In the first, the rate and timing of subduction into the main thermocline is diagnosed using kinematic methods from the 1° resolution CME fields. In the second, tracer diagnostics of the CME and idealized experiments using a “date” tracer identifying the timing of subduction are performed. Over the subtropical gyre, both approaches generally support Stommels hypothesis that fluid is only transferred from the mixed layer into the main therm...

Distribution of dissolved organic nutrients and their effect on export production over the Atlantic Ocean

[1] A synthesis is provided of dissolved organic nitrogen (DON) and phosphorus (DOP) distributions over the Atlantic Ocean based upon field data from eight recent transects, six meridional between 50°N and 50°S and two zonal at 24° and 36°N. Over the entire tropical and subtropical Atlantic, DON and DOP provide the dominant contributions to total nitrogen and phosphorus pools for surface waters above the thermocline. Elevated DON and DOP concentrations (>5 and >0.2 μmol L−1, respectively) occur in surface waters on the eastern side of the North Atlantic subtropical gyre and equatorial sides of both the North and South Atlantic subtropical gyres, while particularly low concentrations of DOP (<0.05 μmol L−1) occur over the northern flank of the North Atlantic subtropical gyre along 36°N. This distribution is consistent with organic nutrients formed at the gyre margins supporting carbon export as they are redistributed via the gyre circulation. The effect of DON and DOP transport and cycling on export production is examined in an eddy-permitting, coupled physical and nutrient model integrated for 40 years: organic nutrients are produced in the upwelling zones off North Africa and transferred laterally into the gyre interior, facilitated in part by the mesoscale eddy circulation, as well as fluxed northward from the tropics as part of the overturning circulation. Inputs of semilabile DON and DOP to the tropical and subtropical Atlantic Ocean play an important role in sustaining up to typically 40 and 70% of the modeled particulate N and P export, particularly on the eastern and equatorward sides of the subtropical gyres.

The Spatial Pattern and Mechanisms of Heat-Content Change in the North Atlantic

The total heat gained by the North Atlantic Ocean over the past 50 years is equivalent to a basinwide increase in the flux of heat across the ocean surface of 0.4 ± 0.05 watts per square meter. We show, however, that this basin has not warmed uniformly: Although the tropics and subtropics have warmed, the subpolar ocean has cooled. These regional differences require local surface heat flux changes (±4 watts per square meter) much larger than the basinwide average. Model investigations show that these regional differences can be explained by large-scale, decadal variability in wind and buoyancy forcing as measured by the North Atlantic Oscillation index. Whether the overall heat gain is due to anthropogenic warming is difficult to confirm because strong natural variability in this ocean basin is potentially masking such input at the present time.

Physical Transport of Nutrients and the Maintenance of Biological Production

The oceanic distributions of nutrients and patterns of biological production are controlled by the interplay of biogeochemical and physical processes, and external sources. Biological and chemical processes lead to the transformation of nutrients between inorganic and organic forms, and also between dissolved and particulate forms. Physical processes redistribute nutrients within the water column through transport and mixing. The combined role of biogeochemical and physical processes is reflected in the observed distributions of nitrate, phosphate and silicate (macro-nutrients). These distributions broadly reflect those of classical water masses, as defined by temperature and salinity, highlighting the important role of physical transport. However, there are also significant differences between the nutrient and water-mass distributions, notably with nutrients showing stronger vertical and basin-to-basin contrasts. Biological production leads to these greater nutrient contrasts with inorganic nutrients consumed and converted to organic matter in the surface, sunlit ocean. A small fraction of the organic matter in this euphotic zone is exported to depth, driven by the gravitational sinking of particles and subduction of dissolved organic matter. This organic fallout is eventually remineralised leading to an accumulation of inorganic nutrients in deeper and older water masses.

A mixed-layer study of the formation of Levantine intermediate water

A mixed-layer model is used to investigate the formation of Levantine Intermediate Water (LIW) over the Eastern Mediterranean. The one-dimensional model is initialized with climatological hydrography and integrated over the Levantine basin with forcing by climatological surface fluxes. Realistic and repeated seasonal mixed-layer cycles are obtained if the annual surface heat input and water loss are offset by a parameterized horizontal advection. The model integrations show that LIW is formed during winter in the mixed layer of the Northwestern Levantine. The preferred formation region for LIW is found through idealized experiments to be controlled by the preconditioning of the hydrography, especially that of the cold, cyclonic Rhodes gyre, rather than by the pattern of the climatological fluxes. The annual-mean formation rate of LIW is estimated to be 1.0 Sv using the climatological surface fluxes. The magnitude of the annual surface fluxes alters the formation rate and modifies the formation region. An additional annual heat flux reduces the formation rate of LIW, whereas an extra cooling enhances it, as well as forming waters denser than LIW in the center of the Rhodes gyre.

Diagnosing Water Mass Formation from Air–Sea Fluxes and Surface Mixing

Abstract The formation rate of water masses and its relation to air–sea fluxes and interior mixing are examined in an isopycnic model of the North (and tropical) Atlantic that includes a mixed layer. The diagnostics follow Walin’s formulation, linking volume and potential density budgets for an isopycnal layer. The authors consider the balance between water mass production, mixing, and air–sea fluxes in the model in the context of two limit cases: (i) with no mixing, where air–sea fluxes drive water mass formation directly, and (ii) a steady state in a closed basin, where air–sea fluxes are balanced by diffusion. In such a steady state, since mixing always acts to reduce density contrast, surface forcing must act to increase it. Considered over the whole basin, including the Tropics, the model is in steady state apart from the densest layers. Most of the mixing is achieved by diapycnal diffusion in the strong density gradients within upwelling regions in the Tropics, and by entrainment into the tropical m...

On the eddy transfer of tracers: Advective or diffusive?

Geostrophic eddies have traditionally been viewed within oceanography as diffusing water masses and tracers in a down-gradient manner. However, eddies also have an advective role that may lead to an up-gradient transfer of tracers, as has been recognized in atmospheric tracer studies and recent eddy parameterizations developed for the ocean. Eddies provide an advective transfer or “bolus” velocity through the secondary circulation formed by the slumping of density surfaces in baroclinic instability. Here we use an eddy-resolving isopycnal ocean model to investigate the meridional transfer across a zonal jet. The jet undergoes baroclinic instability, forming a vibrant eddy field and inducing a meridional bolus velocity. The bolus velocity is found to be correlated with gradients of potential