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Andrew Weaver
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Andrew Weaver
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Bárbara Tencer
,
Andrew Weaver
, and
Francis Zwiers

Abstract

The occurrence of individual extremes such as temperature and precipitation extremes can have a great impact on the environment. Agriculture, energy demands, and human health, among other activities, can be affected by extremely high or low temperatures and by extremely dry or wet conditions. The simultaneous or proximate occurrence of both types of extremes could lead to even more profound consequences, however. For example, a dry period can have more negative consequences on agriculture if it is concomitant with or followed by a period of extremely high temperatures. This study analyzes the joint occurrence of very wet conditions and high/low temperature events at stations in Canada. More than one-half of the stations showed a significant positive relationship at the daily time scale between warm nights (daily minimum temperature greater than the 90th percentile) or warm days (daily maximum temperature above the 90th percentile) and heavy-precipitation events (daily precipitation exceeding the 75th percentile), with the greater frequencies found for the east and southwest coasts during autumn and winter. Cold days (daily maximum temperature below the 10th percentile) occur together with intense precipitation more frequently during spring and summer. Simulations by regional climate models show good agreement with observations in the seasonal and spatial variability of the joint distribution, especially when an ensemble of simulations was used.

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Augustus F. Fanning
and
Andrew J. Weaver

Abstract

An idealized coupled ocean–atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the poleward heat transport in the climate system. A series of experiments ranging from 4° to 0.25° resolution, with appropriate horizontal viscosities and diffusivities in each case, are performed. The coupled atmosphere–ocean model results contradict earlier studies, which showed that the heat transport associated with time-varying circulations counteracted increases in the time mean so that the total remained unchanged as resolution was increased. Even though the total oceanic heat transport has not converged, the net planetary heat transport has essentially converged owing to the strong constraint of energy balance at the top of the atmosphere. Consequently, the atmospheric heat transport is reduced to offset the increasing oceanic heat transport.

To interpret these results, the oceanic heat transport is decomposed into its baroclinic overturning (related to the meridional overturning and Ekman transports), barotropic gyre (that in the horizontal plane), and baroclinic gyre (associated with the jet core within the western boundary current) components. The increase in heat transport occurs in the steady currents. In particular, the baroclinic gyre transport increases by a factor of 5 from the coarsest- to the finest-resolution case, equaling the baroclinic overturning transport at mid- to high latitudes.

To further assess the results, a parallel series of experiments under restoring conditions are performed to elucidate the differences between heat transport in coupled versus uncoupled models and models driven by temperature and salinity or equivalent buoyancy. Although heat transport is more strongly constrained in the restoring experiments, results are similar to those in the coupled model. Again, the total heat transport is increased due to an increasing baroclinic gyre component.

These results point to the importance of higher resolution in the oceanic component of current coupled climate models. These results also stress the need to adequately represent the heat transport associated with the “warm core” region of the Gulf Stream (the baroclinic gyre transport) in order to adequately represent oceanic poleward heat transport.

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Augustus F. Fanning
and
Andrew J. Weaver

Abstract

An idealized coupled ocean–atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the thermohaline circulation. A series of experiments ranging from 4° to 0.25° resolution, with appropriate horizontal viscosities and diffusivities in each case, is performed for both coupled and ocean-only models. Spontaneous internal variability (primarily on the decadal timescale) is found to exist in the higher-resolution cases (with the exception of one of the restoring experiments). The decadal oscillation (whose period varies slightly between cases) is described as an advective–convective mechanism that is thermally driven and linked to the value of the horizontal diffusivity utilized in the model. Increasing the diffusivity in the high-resolution cases presented in this paper is enough to destroy the variability, whereas decreasing the diffusivity in the moderately coarse-resolution cases is capable of inducing decadal-scale variability. As the resolution is increased still further, baroclinic instability within the western boundary current adds a more stochastic component to the solution such that the variability is less regular and more chaotic (giving rise to intradecadal timescales). These results point to the importance of higher resolution in the ocean component of coupled models, revealing the existence of richer variability in models that require less parameterized diffusion.

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Andrew J. Weaver
and
Sophie Valcke

Abstract

The ocean component of the Geophysical Fluid Dynamics Laboratory coupled model is used to investigate whether or not the interdecadal variability found in Delworth et al. is an ocean-only mode or a mode of the full coupled system. In particular, it has been previously suggested that the variability in the full coupled model is either 1) an internal ocean mode driven by fixed fluxes (atmospheric annual mean plus flux adjustment), which is made less regular through forcing from atmospheric noise; 2) a consequence of the use of flux adjustments; or 3) an ocean-only mode that is excited by atmospheric noise. Through a series of experiments conducted under fixed-flux boundary conditions it is shown that none of these three hypotheses holds and therefore it is concluded that the interdecadal variability found in Delworth et al. is a mode of the full coupled system.

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Jeremy G. Fyke
and
Andrew J. Weaver

Abstract

The marine gas hydrate stability zone (GHSZ) is sensitive to temperature changes at the seafloor, which likely affected the GHSZ in the past and may do so in the future in response to anthropogenic greenhouse gas emissions. A series of climate sensitivity and potential future climate change experiments are undertaken using the University of Victoria Earth System Climate Model (UVic ESCM) with resulting seafloor temperature changes applied to a simple time-dependent methane hydrate stability model. The global GHSZ responds significantly to elevated atmospheric CO2 over time scales of 103 yr with initial decreases of the GHSZ occurring after 200 yr in shallow high-latitude seafloor areas that underlie regions of sea ice loss. The magnitude and rate of GHSZ change is dependent primarily upon the thermal diffusivity of the seafloor and the magnitude and duration of the seafloor temperature increase. Using a simple approximation of the amount of carbon stored as hydrate in the GHSZ, estimates of carbon mobilized due to hydrate dissociation are made for several potential climate change scenarios.

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Hannah Hickey
and
Andrew J. Weaver

Abstract

A coupled model of intermediate complexity is used to examine the effect of the Southern Ocean on tropical Atlantic variability. Model experiments with positive surface temperature, freshwater, and wind forcings in the Southern Ocean show an increased advection of polar water into the Atlantic basin, resulting in anomalously cold freshwater in the eastern Atlantic Ocean at 25°S. With time, these anomalies spread northward and away from the coast, into the tropical thermocline. The magnitude of the response increases linearly with the strength of the forcing. The results of these experiments show that southeastern Atlantic Ocean temperature and salinity are particularly sensitive to changes in the Southern Ocean. These findings suggest that this link could be the oceanic branch of a mode of variability linking the Southern and tropical Atlantic Oceans—a possible mechanism for the as-yet-unexplained decadal mode of tropical Atlantic variability.

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Paul G. Myers
and
Andrew J. Weaver

Abstract

The finite-element method possesses many advantages over more traditional numerical techniques used to solve systems of differential equations. These advantages include a number of conservation properties and a natural treatment of boundary conditions. The method's piecewise nature makes it useful when dealing with irregular domains and similarly when using variable horizontal resolution. To take advantage of these properties, a finite-element representation of the linearized, steady-state, barotropic potential vorticity equation is developed. The Stommel problem is used as an initial test for the model. A fourth-order eddy viscosity term is then added, and the resulting problem is solved in both simply and multiply connected domains under both slip and no-slip boundary conditions. The beta-plane assumption is then relaxed, and the model is reformulated in spherical coordinates. A realistic geography and topography version of this model is also used to examine the barotropic circulation in the North Atlantic Ocean. Results are found to agree very well with those of previous diagnostic calculations. In particular, the Gulf Stream separates at the correct latitude with the inclusion of the JEBAR (joint effect of baroclinicity and relief) term.

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Salil Kumar Das
and
Andrew J. Weaver

Abstract

The semi-Lagrangian adveation scheme is applied to the meridional-plane model of the thermohaline circulation developed by Marotzke et al., whose governing equations are solved under a variety of boundary conditions. To determine the extent to which the accuracy and efficiency of the calculations depends on the numerical integration scheme, the test problem is solved independently using an explicit finite-difference (leapfrog in time, centered difference in space) method and three implicit methods: a finite difference, a finite element, and an upwind scheme. Integrations of the model to several equilibria are performed to determine the accuracy, efficiency, and stability of each integration scheme as a function of time step. For the same level of accuracy the time step used in the semi-Lagrangian scheme is found to be at least five times greater than that employed in the case of the implicit methods. The time step used in the implicit methods in turn are at least six times greater than those needed in the explicit integration of the governing equations. It is further shown that Dirichlet, Neumann, and mixed boundary conditions can be handled efficiently with the semi-Lagrangian method.

The semi-Lagrangian method is applied in the usual three-time-level and two-time-level interpolating versions as well as in a noninterpolating, three-time-level version. The two-time-level scheme further doubles the speed of the time integration step for the same level of accuracy, beyond that which is achieved using the three-time-level scheme. The noninterpolating scheme does not eliminate the damping introduced by the interpolation, as pointed out by Ritchic. Hence, the two-time-level, semi-Lagrangian advection method stands out as a viable time integration scheme for climate models that are normally run for hundreds of years and is best suited for ocean climate studies.

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