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Anna Katavouta
,
Richard G. Williams
, and
Philip Goodwin

Abstract

The surface warming response to carbon emissions is affected by how the ocean sequesters excess heat and carbon supplied to the climate system. This ocean uptake involves the ventilation mechanism, where heat and carbon are taken up by the mixed layer and transferred to the thermocline and deep ocean. The effect of ocean ventilation on the surface warming response to carbon emissions is explored using simplified conceptual models of the atmosphere and ocean with and without explicit representation of the meridional overturning. Sensitivity experiments are conducted to investigate the effects of (i) mixed layer thickness, (ii) rate of ventilation of the ocean interior, (iii) strength of the meridional overturning, and (iv) extent of subduction in the Southern Ocean. Our diagnostics focus on a climate metric, the transient climate response to carbon emissions (TCRE), defined by the ratio of surface warming to the cumulative carbon emissions, which may be expressed in terms of separate thermal and carbon contributions. The variability in the thermal contribution due to changes in ocean ventilation dominates the variability in the TCRE on time scales from years to centuries, while that of the carbon contribution dominates on time scales from centuries to millennia. These ventilated controls are primarily from changes in the mixed layer thickness on decadal time scales, and in the rate of ventilated transfer from the mixed layer to the thermocline and deep ocean on centennial and millennial time scales, which is itself affected by the strength of the meridional overturning and extent of subduction in the Southern Ocean.

Open access
Richard G. Williams
,
Anna Katavouta
, and
Vassil Roussenov

Abstract

Projected changes in ocean heat and carbon storage are assessed in terms of the added and redistributed tracer using a transport-based framework, which is applied to an idealized climate model and a suite of six CMIP5 Earth system models following an annual 1% rise in atmospheric CO2. Heat and carbon budgets for the added and redistributed tracer are used to explain opposing regional patterns in the storage of ocean heat and carbon anomalies, such as in the tropics and subpolar North Atlantic, and the relatively reduced storage within the Southern Ocean. Here the added tracer takes account of the net tracer source and the advection of the added tracer by the circulation, while the redistributed tracer takes account of the time-varying circulation advecting the preindustrial tracer distribution. The added heat and carbon often have a similar sign to each other with the net source usually acting to supply the tracer. In contrast, the redistributed heat and carbon consistently have an opposing sign to each other due to the opposing gradients in the preindustrial temperature and carbon. These different signs in heat and carbon redistribution can lead to regional asymmetries in the climate-driven changes in ocean heat and carbon storage. For a weakening in the Atlantic overturning and strengthening in the Southern Ocean residual circulation, the high latitudes are expected to have heat anomalies of variable sign and carbon anomalies of a consistently positive sign, since added and redistributed tracers are opposing in sign for heat and the same sign for carbon there.

Open access
A. J. G. Nurser
,
Robert Marsh
, and
Richard G. Williams

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 mixed layer. Mixing from entrainment associated with the seasonal cycle of mixed layer depth in mid and high latitudes and lateral mixing of density within the mixed layer are less important than this tropical mixing. These model results as to the relative importance of the different mixing processes are consistent with a simple scaling analysis.

Outside the Tropics, the upwelling-linked mixing is no longer important, and a first-order estimate of water mass formation rates may be made from the surface fluxes. Lateral mixing of density within the mixed layer and seasonal entrainment mixing are as important as the remaining thermocline mixing within this domain.

An apparent vertical diffusivity is diagnosed over both the full and extratropical domain. It reaches 10−4 m2 s−1 for the denser waters, about four times as large as the explicit diapycnal diffusion within the thermocline.

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Rob A. Hall
,
John M. Huthnance
, and
Richard G. Williams

Abstract

Reflection of internal waves from sloping topography is simple to predict for uniform stratification and linear slope gradients. However, depth-varying stratification presents the complication that regions of the slope may be subcritical and other regions supercritical. Here, a numerical model is used to simulate a mode-1, M 2 internal tide approaching a shelf slope with both uniform and depth-varying stratifications. The fractions of incident internal wave energy reflected back offshore and transmitted onto the shelf are diagnosed by calculating the energy flux at the base of slope (with and without topography) and at the shelf break. For the stratifications/topographies considered in this study, the fraction of energy reflected for a given slope criticality is similar for both uniform and depth-varying stratifications. This suggests the fraction reflected is dependent only on maximum slope criticality and independent of the depth of the pycnocline. The majority of the reflected energy flux is in mode 1, with only minor contributions from higher modes due to topographic scattering. The fraction of energy transmitted is dependent on the depth-structure of the stratification and cannot be predicted from maximum slope criticality. If near-surface stratification is weak, transmitted internal waves may not reach the shelf break because of decreased horizontal wavelength and group velocity.

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Vassil Roussenov
,
Richard G. Williams
, and
Jane O'Dwyer

Abstract

Low potential vorticity extends over the deep waters of the North Pacific and, possibly, the bottom waters of the North Atlantic. Isopycnic model integrations are conducted to investigate how these potential vorticity distributions are controlled, first, for an idealized double-hemisphere and, second, for the Pacific with realistic topography. Dense water is released from a southern, high-latitude source and circulates over the domain with diapycnic mixing gradually reducing its stratification. The potential vorticity contrast is large over the Southern Hemisphere, but weak over the Northern Hemisphere where the meridional changes in planetary vorticity and layer thickness oppose each other. Including an active eddy field inhibits the grounding of dense water, which increases the potential vorticity contrast in the overlying layer. Incorporating realistic topography leads to the dense fluid spreading via deep channels with tight recirculations and jets bifurcating. The experiments suggest that extensive regions of low potential vorticity are formed whenever there is both enhanced bottom mixing and a basin is filled by a single water mass entering from across the equator.

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Gualtiero Badin
,
Richard G. Williams
,
Zhao Jing
, and
Lixin Wu

Abstract

Transformation and formation rates of water masses in the Southern Ocean are estimated in a neutral-surface framework using air–sea fluxes of heat and freshwater together with in situ estimates of diapycnal mixing. The air–sea fluxes are taken from two different climatologies and a reanalysis dataset, while the diapycnal mixing is estimated from a mixing parameterization applied to five years of Argo float data. Air–sea fluxes lead to a large transformation directed toward lighter waters, typically from −45 to −63 Sv (1 Sv ≡ 106 m3 s−1) centered at γ = 27.2, while interior diapycnal mixing leads to two weaker peaks in transformation, directed toward denser waters, 8 Sv centered at γ = 27.8, and directed toward lighter waters, −16 Sv centered at γ = 28.3. Hence, air–sea fluxes and interior diapycnal mixing are important in transforming different water masses within the Southern Ocean. The transformation of dense to lighter waters by diapycnal mixing within the Southern Ocean is slightly larger, though comparable in magnitude, to the transformation of lighter to dense waters by air–sea fluxes in the North Atlantic. However, there are significant uncertainties in the authors' estimates with errors of at least ±5 W m−2 in air–sea fluxes, a factor 4 uncertainty in diapycnal mixing and limited coverage of air–sea fluxes in the high latitudes and Argo data in the Pacific. These water mass transformations partly relate to the circulation in density space: air–sea fluxes provide a general lightening along the core of the Antarctic Circumpolar Current and diapycnal diffusivity is enhanced at middepths along the current.

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Laura Jackson
,
Chris W. Hughes
, and
Richard G. Williams

Abstract

The topographical control of western boundary currents within a basin and zonal jets in a channel is investigated in terms of the potential vorticity (PV) and barotropic vorticity (BV: the curl of the depth-integrated velocity) budgets using isopycnic, adiabatic wind–driven experiments. Along the western boundary, the wind-driven transport is returned across latitude lines by the bottom pressure torque, while friction is only important in altering the PV within an isopycnic layer and in allowing a closed circulation. These contrasting balances constrain the geometry of the flow through integral relationships for the BV and PV. For both homogenous and stratified basins with sloping sidewalls, the northward subtropical jet separates from the western wall and has opposing frictional torques on either side of the jet, which cancel in a zonal integral for BV but alter the PV within a layer streamline. In a channel with partial topographic barriers, the bottom pressure torque is again important in returning wind-driven flows along western boundaries and in transferring BV from neighboring wind-driven gyres into a zonal jet. The depth-integrated flow steered by topography controls where the bottom friction alters the PV, which can lead to different PV states being attained for separate subbasins along a channel.

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Richard G. Williams
,
Chris Wilson
, and
Chris W. Hughes

Abstract

Signatures of eddy variability and vorticity forcing are diagnosed in the atmosphere and ocean from weather center reanalysis and altimetric data broadly covering the same period, 1992–2002. In the atmosphere, there are localized regions of eddy variability referred to as storm tracks. At the entrance of the storm track the eddies grow, providing a downgradient heat flux and accelerating the mean flow eastward. At the exit and downstream of the storm track, the eddies decay and instead provide a westward acceleration. In the ocean, there are similar regions of enhanced eddy variability along the extension of midlatitude boundary currents and the Antarctic Circumpolar Current. Within these regions of high eddy kinetic energy, there are more localized signals of high Eady growth rate and downgradient eddy heat fluxes. As in the atmosphere, there are localized regions in the Southern Ocean where ocean eddies provide statistically significant vorticity forcing, which acts to accelerate the mean flow eastward, provide torques to shift the jet, or decelerate the mean flow. These regions of significant eddy vorticity forcing are often associated with gaps in the topography, suggesting that the ocean jets are being locally steered by topography. The eddy forcing may also act to assist in the separation of boundary currents, although the diagnostics of this study suggest that this contribution is relatively small when compared with the advection of planetary vorticity by the time-mean flow.

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John C. Marshall
,
Richard G. Williams
, and
A. J. George Nurser

Abstract

The annual rate at which mixed-layer fluid is transferred into the permanent thermocline—that is, the annual subduction rate S ann and the effective subduction period 𝒯eff—is inferred from climatological data in the North Atlantic. From its kinematic definition, S ann 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 S ann 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 period is that period of time during the shallowing of the mixed layer in which sufficient buoyancy is delivered to permit irreversible transfer of fluid into the main thermocline at the rate S ann. Typically 𝒯eff is found to be 1 to 2 months over the major part of the subtropical gyre, rising to 4 months in the tropics.

Finally, the heat budget of a column of fluid, extending from the surface down to the base of the seasonal thermocline is discussed, following it over an annual cycle. We are able to relate the buoyancy delivered to the mixed layer during the subduction period to the annual-mean buoyancy forcing through the sea surface plus the warming due to the convergence of Ekman heat fluxes. The relative importance of surface fluxes (heat and freshwater) and Ekman fluxes in supplying buoyancy to support subduction is examined using the climatologist observations of Isemer and Hasse, Schmitt et al., and Levitus. The pumping down of fluid from the warm summer Ekman layer into the thermocline makes a crucial contribution and, over the subtropical gyre, is the dominant term in the thermodynamics of subduction.

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Richard G. Williams
,
John C. Marshall
, and
Michael A. Spall

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, Stommel's 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 Stommel's hypothesis that fluid is only transferred from the mixed layer into the main thermocline over a short period, ∼1 month, in late winter/early spring. Tracer date experiments are also conducted using the eddy-resolving ⅓° CME fields. Eddy stirring is found to enhance the rate at which the tracer spreads into unventilated regions, but does not alter the seasonal bias of the Stommel demon mechanism.

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