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Richard G. Williams and Vassil Roussenov

Abstract

The role of sidewalls in determining the interior distribution of potential vorticity (PV) is investigated using eddy-resolving isopycnic experiments. The layer model is integrated at 1 16° resolution for a wind-driven double gyre with either vertical or sloping sidewalls. If there are vertical sidewalls, eddy stirring leads to PV homogenization within unforced, interior density layers. If there are sloping sidewalls, frictional torques lead to bands of low and high PV being formed along the western boundary of the subpolar and subtropical gyres, respectively. These regions of low and high PV are transferred into the interior by a separated jet at the intergyre boundary. Over a limited domain, this injection of the PV contrast can prevent eddy homogenization from occurring. However, over a larger-scale domain, eddies provide a downgradient transfer of PV, reducing the PV contrast downstream along the jet and enabling homogenization to occur for intermediate layers within the basin interior. Diabatic mixing along the slope can introduce low PV for intermediate layers and even mask the frictional contributions.

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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.

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Richard G. Williams, Vassil Roussenov, M. Susan Lozier, and Doug Smith

Abstract

In the North Atlantic, there are pronounced gyre-scale changes in ocean heat content on interannual-to-decadal time scales, which are associated with changes in both sea surface temperature and thermocline thickness; the subtropics are often warm with a thick thermocline when the subpolar gyre is cool with a thin thermocline, and vice versa. This climate variability is investigated using a semidiagnostic dynamical analysis of historical temperature and salinity data from 1962 to 2011 together with idealized isopycnic model experiments. On time scales of typically 5 yr, the tendencies in upper-ocean heat content are not simply explained by the area-averaged atmospheric forcing for each gyre but instead dominated by heat convergences associated with the meridional overturning circulation. In the subtropics, the most pronounced warming events are associated with an increased influx of tropical heat driven by stronger trade winds. In the subpolar gyre, the warming and cooling events are associated with changes in western boundary density, where increasing Labrador Sea density leads to an enhanced overturning and an influx of subtropical heat. Thus, upper-ocean heat content anomalies are formed in a different manner in the subtropical and subpolar gyres, with different components of the meridional overturning circulation probably excited by the local imprint of atmospheric forcing.

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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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Richard G. Williams, Vassil Roussenov, Doug Smith, and M. Susan Lozier

Abstract

Basin-scale thermal anomalies in the North Atlantic, extending to depths of 1–2 km, are more pronounced than the background warming over the last 60 years. A dynamical analysis based on reanalyses of historical data from 1965 to 2000 suggests that these thermal anomalies are formed by ocean heat convergences, augmented by the poorly known air–sea fluxes. The heat convergence is separated into contributions from the horizontal circulation and the meridional overturning circulation (MOC), the latter further separated into Ekman and MOC transport minus Ekman transport (MOC-Ekman) cells. The subtropical thermal anomalies are mainly controlled by wind-induced changes in the Ekman heat convergence, while the subpolar thermal anomalies are controlled by the MOC-Ekman heat convergence; the horizontal heat convergence is generally weaker, only becoming significant within the subpolar gyre. These thermal anomalies often have an opposing sign between the subtropical and subpolar gyres, associated with opposing changes in the meridional volume transport driving the Ekman and MOC-Ekman heat convergences. These changes in gyre-scale convergences in heat transport are probably induced by the winds, as they correlate with the zonal wind stress at gyre boundaries.

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Richard G. Williams, Vassil Roussenov, Philip Goodwin, Laure Resplandy, and Laurent Bopp

Abstract

Climate projections reveal global-mean surface warming increasing nearly linearly with cumulative carbon emissions. The sensitivity of surface warming to carbon emissions is interpreted in terms of a product of three terms: the dependence of surface warming on radiative forcing, the fractional radiative forcing from CO2, and the dependence of radiative forcing from CO2 on carbon emissions. Mechanistically each term varies, respectively, with climate sensitivity and ocean heat uptake, radiative forcing contributions, and ocean and terrestrial carbon uptake. The sensitivity of surface warming to fossil-fuel carbon emissions is examined using an ensemble of Earth system models, forced either by an annual increase in atmospheric CO2 or by RCPs until year 2100. The sensitivity of surface warming to carbon emissions is controlled by a temporal decrease in the dependence of radiative forcing from CO2 on carbon emissions, which is partly offset by a temporal increase in the dependence of surface warming on radiative forcing. The decrease in the dependence of radiative forcing from CO2 is due to a decline in the ratio of the global ocean carbon undersaturation to carbon emissions, while the increase in the dependence of surface warming is due to a decline in the ratio of ocean heat uptake to radiative forcing. At the present time, there are large intermodel differences in the sensitivity in surface warming to carbon emissions, which are mainly due to uncertainties in the climate sensitivity and ocean heat uptake. These uncertainties undermine the ability to predict how much carbon may be emitted before reaching a warming target.

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