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Michael Winton

Abstract

A technique is developed for diagnosing effective surface and atmospheric optical properties from climate model shortwave flux diagnostics. These properties can be used to distinguish the contributions of surface and atmospheric optical property changes to shortwave flux changes at the surface and top of the atmosphere. In addition to the four standard shortwave flux diagnostics (upward, downward, surface, and top of atmosphere), the technique makes use of surface-down and top-up fluxes over a zero-albedo surface obtained from an auxiliary online shortwave calculation. The simple model optical properties, when constructed from the time-mean fluxes, are effective optical properties, useful for predicting the time-mean response to optical property changes. The technique is tested against auxiliary online shortwave calculations at four validation albedos and shown to predict the monthly mean surface absorption with an rms error of less than 2% over the globe. The reasons for the accuracy of the technique are explored. Less accurate techniques that make use of existing shortwave diagnostics are presented and compared.

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Michael Winton

Abstract

A number of single-hemisphere ocean models, when forced with steady mixed boundary conditions, have produced self-sustaining oscillations with century to millennial scale periods. Here the energetics of these deep-decoupling oscillations are explored. It is found that in the deep-decoupled phase of the oscillation (when deep overturning and convection are absent) the sinks of potential energy (convection and conversion to kinetic energy) shut down, and potential energy is stored in the form of a basin warming. This potential energy becomes available when deep convection renews to begin the deep-coupled phase. During the deep-coupled phase the stored energy is drawn down as the potential energy sinks become larger than the diffusive source. This encourages one to consider the deep-coupled phase as a response to a pool of energy that grows in the absence of deep ventilation rather than a result of the specific mechanisms that are involved in breaking down the halocline in particular oscillations.

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Michael Winton

Abstract

By comparing the response of flat and bowl-shaped basins to fixed heat fluxes of various magnitudes, it is determined that coastal topography has a considerable damping influence upon internal decadal oscillations of the thermohaline circulation. It is proposed that this is because the adjustment of baroclinic currents to the no-normal-flow boundary condition at weakly stratified coasts is aided in the topography case by the generation of substantial barotropic flow.

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Michael Winton

Abstract

The sensitivity of North Atlantic Deep Water formation to variations in mean surface temperature is explored with a meridional-vertical plane ocean model coupled to an energy balance atmosphere. It is found that North Atlantic Deep Water formation is favored by a warm climate, while cold climates are more likely to produce Southern Ocean deep water or deep-decoupling oscillations (when the Southern sinking region is halocline covered). This behavior is traced to a cooling-induced convective instability near the North Atlantic sinking region, that is, to unstable horizontal spreading of a halocline that stratifies part of the region. Under the convective instability it is found that climate cooling is generally equivalent to increased freshwater forcing. This is because in a cold climate, high-latitude water masses approach the temperature of maximum density and the convection-driving, upward thermal buoyancy flux induced by surface cooling becomes insufficient to overcome the stratifying effect of surface freshening (a downward buoyancy flux). An extensive halocline is then formed and this halocline interferes with the heat loss necessary for the steady production of North Atlantic Deep Water.

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Michael Winton

Abstract

An expression is derived for the surface salt input needed to induce complete convective overturning of a polar water column consisting of 1) a layer of sea ice, 2) a freezing temperature mixed layer, 3) a pycnocline with linearly varying temperature and salinity, and 4) deep water with fixed temperature and salinity. This quantity has been termed the bulk stability by Martinson. The bulk stability is found to consist of three components. The first two make up Martinson’s salt deficit and are the salt input needed to increase the density of the mixed layer and the pycnocline layer to that of the deep water (the mixed layer stability and pycnocline layer stability, respectively). The third component is Martinson’s thermal barrier: the potential for pycnocline heat to melt ice, reducing the surface salinity. It is found that when the pycnocline density gradient due to temperature offsets more than one half of that due to salinity, the pycnocline layer stability is negative. Consequently, it is possible for a stably stratified water column to have zero or negative bulk stability.

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Michael Winton

Abstract

A technique for estimating surface albedo feedback (SAF) from standard monthly mean climate model diagnostics is applied to the 1% yr−1 carbon dioxide (CO2)-increase transient climate change integrations of 12 Intergovernmental Panel on Climate Change (IPCC) fourth assessment report (AR4) climate models. Over the 80-yr runs, the models produce a mean SAF at the surface of 0.3 W m−2 K−1 with a standard deviation of 0.09 W m−2 K−1. Relative to 2 × CO2 equilibrium run estimates from an earlier group of models, both the mean SAF and the standard deviation are reduced. Three-quarters of the model mean SAF comes from the Northern Hemisphere in roughly equal parts from the land and ocean areas. The remainder is due to Southern Hemisphere ocean areas. The SAF differences between the models are shown to stem mainly from the sensitivity of the surface albedo to surface temperature rather from the impact of a given surface albedo change on the shortwave budget.

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Michael Winton

Abstract

Coarse-resolution f-plane and β-plane frictional geostrophic models are used to study the response to restored surface buoyancies and fixed surface buoyancy fluxes. With restored surface buoyancies it is found that the overturning and meridional buoyancy transport generally follow the scaling from thermal wind and vertical advective–diffusive balance When maximum midbasin rather than maximum overall streamfunction is used as the metric, the sensitivity of overturning magnitude to vertical diffusivity agrees quite closely with that of a two-dimensional Rayleigh frictional model that follows an analogous scaling. This measure of meridional overturning, as well as the meridional buoyancy transport, also roughly follow the predicted f −1/3 scaling and are relatively insensitive to variations in β and horizontal viscosity.

The sensitivity experiments indicate that the coarse-resolution model overturning and thermodynamic structure are well characterized by the adjustment of a uniformly rotating viscous fluid to boundaries parallel to the surface forcing gradient. This adjustment is examined in more detail with linear and nonlinear models. In stratified regions, thermal wind currents normal to the coast initiate the adjustment by forcing pycnocline depth anomalies in the coastal (horizontal) Ekman layer. These anomalies propagate around the coast in the Kelvin wave direction, setting up geostrophic currents parallel to the coast. In a model initialized with a high-latitude baroclinic jet, a warm (cold) boundary signal spreads around the poleward (equatorward) part of the basin, initiating geostrophic currents connecting the flow onto the coast in the east with that away from the coast in the west to form two gyres. The warm coastal signal propagates slowly along the weakly stratified polar wall. In steady circulations, strong damping inhibits warm signal propagation, and the warm water on the poleward part of the eastern coast is forced to downwell.

Self-sustaining decadal-scale oscillation is a robust feature of the models when forced with fixed buoyancy fluxes. This variability is inherently three-dimensional (it does not occur in a two-dimensional frictional model) and involves the periodic growth and decay of a baroclinic jet in the poleward eastern corner. Decay occurs when a jetlike disturbance, normal to the coast, propagates cyclonically around the basin replacing the cold water along the boundary with warm. Forcing of thermal wind currents normal to weakly stratified coasts and weak damping of the resulting propagating boundary disturbances are found to be conducive to oscillations.

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Michael Winton

Abstract

The sensitivity of Northern Hemisphere sea ice cover to global temperature change is examined in a group of climate models and in the satellite-era observations. The models are found to have well-defined, distinguishable sensitivities in climate change experiments. The satellite-era observations show a larger sensitivity—a larger decline per degree of warming—than any of the models. To evaluate the role of natural variability in this discrepancy, the sensitivity probability density function is constructed based upon the observed trends and natural variability of multidecadal ice cover and global temperature trends in a long control run of the GFDL Climate Model, version 2.1 (CM2.1). This comparison shows that the model sensitivities range from about 1 to more than 2 pseudostandard deviations of the variability smaller than observations indicate. The impact of natural Atlantic multidecadal temperature trends (as simulated by the GFDL model) on the sensitivity distribution is examined and found to be minimal.

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Michael Winton

Abstract

A model is presented that provides an efficient approximation to sea ice thermodynamics for climate studies. Semtner’s three-layer framework is used, but the brine content of the upper ice is represented with a variable heat capacity as is done in more physically based models. A noniterative fully implicit time-stepping scheme is used for calculation of ice temperature. The results of the new model are compared to those of Semtner’s original model.

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Michael Winton

Abstract

Integrations of coupled climate models with mixed-layer and fixed-current ocean components are used to explore the climatic response to varying magnitudes of ocean circulation. Four mixed-layer ocean experiments without ocean heat transports are performed using two different atmosphere–land components—the new GFDL AM2 and the GFDL Manabe Climate Model (MCM)—and two different sea ice components, one dynamic and one thermodynamic. Both experiments employing the dynamic sea ice component develop unstable growth of sea ice while the experiments with a thermodynamic sea ice component develop very large but stable ice covers. The global cooling ranges from modest to extreme in the four experiments.

Using the fixed-current climate model, a trio of 100-yr integrations are made with control currents from a GFDL R30 ocean simulation, same currents reduced by 50%, and same currents increased by 50%. This suite is performed with two coupled models again employing the two atmosphere–land components, AM2 and MCM, for a total of six experiments. Both models show a large sensitivity of the sea ice extent to the magnitude of currents with increased currents reducing the extent and warming the high latitudes. Low cloud cover also responds to circulation changes in both models but in the opposite sense. In the AM2-based model, low cloudiness decreases as ocean circulation increases, reinforcing the sea ice changes in reducing the planetary reflectivity, and warming the climate. This cloudiness change is associated with a reduction in lower-atmospheric stability over the ocean. Because the AM2-based model is able to simulate the observed seasonal low cloud–stability relationship and the changes in these quantities with altered ocean circulation are consistent with this relationship, the AM2 interpretation of the cloud changes is favored.

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