On the causes of glacial inception at 116 kaBP.

Yoshimori, M., M.C. Reader, A.J. Weaver and N.A. McFarlane

To explore processes involved in glacial inception at 116 kaBP, the response of an AGCM to changes in lower boundary conditions is investigated. Two 116 kaBP experiments are conducted to examine the effect of sea surface conditions (sea surface temperature and sea ice distribution), one with the present-day sea surface conditions and the other with 116 kaBP sea surface conditions. These two different sea surface conditions are obtained from coupled climate model simulations. Two additional 116 kaBP experiments are conducted to examine the combined effect of sea surface conditions and land surface conditions (vegetation), one with the present-day sea surface and modified land surface conditions and the other with 116 kaBP sea surface and modified land surface conditions. Perennial snow cover occurred over northern Canada under 116 kaBP orbital and CO2 forcing with present-day ``warm'' sea surface conditions, and further expanded when 116 kaBP ``cool'' sea surface conditions were applied. Modifying vegetation based on cooling during summer induced by 116 kaBP sea surface conditions, lead to much larger areas of perennial snow cover. Our results suggest that the capturing of glacial inception at 116 kaBP requires the use of ``cooler'' sea surface conditions than the present.

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Improved representation of sea ice processes in climate models

Saenko, O. A., G.M. Flato and A.J. Weaver

The apparent sensitivity of high latitudes to climate pertubations has spurred the development of global climate model components with improved parameterisations of sea-ice related processes. We focus on two of these. The first involves the ocean component in which we generalize a recently developed parametrisation of brine rejection during sea ice formation for use in a multi-category sea ice model (i.e. one that resolve the thickness distribution function). It employs explicit subsurface mixing of brine-enriched surface waters, resulting from sea ice growth. The parameterisation is implemented in the UVic coupled model, and numerical experiments are performed to highlight the physical processes and feedbacks involved. It is shown that a better representation of brine rejection improves the simulation of intermediate and deep ocean waters. Over the Arctic Ocean it also improves the simulation of the warm Atlantic Layer and sharpens the halocline. The second part of this paper focusses on the sea-ice component. We perform a series of stand-alone sea-ice model experiments comparing a recently developed multi-layer energy-conserving thermodynamic scheme with the simplified scheme used in many existing climate models. Experiments are done with and without the inclusion of dynamic processes (ice motion and deformation). Of particular interest is the impact of changes in the representation of dynamic and thermodynamic processes on the response of sea ice to climate perturbations. This is accomplished by comparing results obtained with present-day and future climate forcing, the latter obtained from the CCCma coupled climate model. We find that the more sophisticated thermodynamic scheme increases the sensitivity of ice volume, but decreases the sensitivity of ice area. As in previous studies, the introduction of ice dynamics tends to reduce sensitivity relative to a thermodynamic-only model.

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Absence of deep-water formation in the Labrador Sea during the last interglacial period

Hillaire-Marcel, C., A. de Vernal, G. Bilodeau, and A.J. Weaver

The two main constituent water masses of the deep North Atlantic Ocean-North Atlantic Deep Water at the bottom and Labrador Sea Water at in intermediate level - are currently formed in the Nordic seas and the Labrador Sea, respectively. The rate of formation of these two water masses tightly governs the strength of the global ocean circulation and the associated heat transport across the North Atlantic Ocean. Numerical simulations have suggested a possible shut-down of Labrador Sea Water formation as a consequence of global warming. Here, we use micropaleontological data and stable isotope measurements in both planktonic and benthic foraminifera from deep Labrador Sea cores to investigate the density structure of the water column during the last interglacial period, which was thought to be about 2C warmer than present. Our results indicate that today's stratification between Labrador Sea Water and North Atlantic Deep Water never developed during the last interglacial period. Instead, a buoyant surface layer was present above a single water mass originating from the Nordic seas. Thus the present situation, with an active site of intermediate-water formation in the Labrador Sea, which settled some 7000 years ago, has no analogue throughout the last climatic cycle.

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The UVic Earth System Climate Model: Model Description, Climatology, and Applications to Past, Present and Future Climates.

Andrew J. Weaver, M. Eby, E. C. Wiebe, C. M. Bitz, P. B. Duffy, A. F. Fanning, M. M. Holland, A. MacFadyen, O. Saenko, A. Schmittner, H. Wang, M. Yoshimori

A new earth system climate model of intermediate complexity is developed and its climatology is compared against observations. The UVic Earth System Climate Model consists of a three-dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea ice model, an energy-moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrised through Fickian diffusion, and precipitation is assumed to occur when the relative humidity reaches greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to instantaneously return to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrisation of water vapour/planetary long wave feedbacks, although the radiative forcing associated with changes in atmospheric CO2 is prescribed as a modification of the planetary long wave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present day winds in its climatology although a dynamical wind feedback is included which exploits a latitudinally-varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the GFDL Modular Ocean Model 2.2, with a global resolution of a 3.6 degrees (zonal) by 1.8 degrees meridional and 19 vertical levels, that includes an option for a brine-reject ion parametrisation. The sea ice component incorporates an elastic-viscous-plastic rheology to represent sea ice dynamics and various options for the representation of sea ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection.

Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0 deg. C for a doubling of CO2, in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but eventually re-establishes to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere GCMs. This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning re-establishes to a strength that is greater than its initial condition.

When applied to the climate of the Last Glacial Maximum, the model obtains tropical cooling (30 deg. N - 30 deg. S), relative to the present, of about 2.1 deg. C over the ocean and 3.6 deg. C over the land. These are generally cooler than CLIMAP estimates, but not as cool as some other reconstructions. This moderate cooling is consistent with alkenone reconstructions and a low to mid climate sensitivity to perturbations in radiative forcing. An amplification of the cooling occurs in the North Atlantic due to the weakening of North Atlantic Deep Water formation. Concurrent with this weakening is a shallowing and a more northward penetration of Antarctic Bottom Water.

Climate models are usually evaluated by spinning them up under perpetual present-day forcing and comparing the model results with present-day observations. Implicit in this approach is the assumption that the present day observations are in equilibrium with the present day radiative forcing. The comparison of a long transient integration (starting at 6 KBP), forced by changing radiative forcing (solar, CO2 orbital), with an equilibrium integration reveals substantial differences. Relative to the climatology from the present-day equilibrium integration, the global mean surface air and sea surface temperatures (SSTs) are 0.74 deg. C and 0.55 deg. C colder, respectively, deep ocean temperatures are substantially cooler, and southern hemisphere sea ice cover is 22% larger, although the North Atlantic conveyor remains remarkably stable in all cases. The differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene. It is also demonstrated that a global warming simulation that starts from an equilibrium present-day climate (cold start) underestimates the global temperature increase at 2100 by 13% when compared to a transient simulation, under historical solar, CO2 and orbital forcing, that is also extended out to 2100. This is larger (13% compared to 9.8%) than the difference from an analogous transient experiment which does not include historical changes in solar forcing. These results suggest that those groups that do not account for solar forcing changes over the 20th century may slightly underestimate (~3% in our model) the projected warming by the year 2100.

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Simulations of Heinrich Events in a coupled ocean-atmosphere-sea ice model

Meissner, K.J., A. Schmittner, E. C. Wiebe and A. J. Weaver

Correlations between oxygen isotope measurements in Greenland ice and records of sea surface temperature from North Atlantic sediments have shown that between 20 and 80 kyr ago several cooling cycles occurred which culminated in a discharge of icebergs into the North Atlantic. These so called `Heinrich Events' (HEs) were followed by an abrupt shift to a warmer climate. Here we use a coupled ocean-atmosphere-sea ice model to study the response of the climate system under glacial conditions to a hypothetical HE. The HE is simulated by meltwater discharges and combined changes in land albedo and ice sheet topography mimicing a break-up of a considerable part of the Laurentide ice sheet. Despite very different initial strengths of the overturning circulation in the Northern Hemisphere, the model response to the HEs is qualitatively similar. A warming of ocean and atmosphere temperatures over the North Atlantic due to the topography/albedo changes is simulated after the iceberg discharge. North Atlantic Deep Water (NADW) formation drops and reestablishes due to the meltwater event.

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The structure of the upper water column in the northwest North Atlantic: modern vs. last Glacial maximum conditions.

de Vernal, A., C., Hillaire-Marcel, W.R. Peltier, and A.J. Weaver

During the last glacial maximum, the northwestern North Atlantic constituted a major conduit for Labrador and Greenland ice sheet meltwaters. Vertical density gradients in its upper water masses have been reconstructed by combining information from transfer functions based on dinocysts and from oxygen isotope measurements (d18O) in planktonic foraminifera. Transfer functions yield temperature and salinity, thus potential density (sq) for the warmest (August) and coldest (February) months in the photic zone. d18O-values in different size fractions of epipelagic (Globigerina bulloides) and mesopelagic (Neogloboquadrina pachyderma leftcoiled -Npl) foraminifera allow us to assess sq-gradients through the pycnocline between surface and intermediate waters, based on the calibration of a sq vs. d 18O relationship from transfer function reconstructions. The size and density of Npl shells provide further constraints on these sq-gradients. The results show the development of a very strong pycnocline during the LGM with a difference of about 3 (summer) to 1.5 (winter) sq-units between surface and underlying waters. They indicate conditions unfavorable for vertical convection and support the hypothesis of the spreading of a shallow, low salinity buoyant layer over the northern North Atlantic. This layer depicted a strong E-W gradient, with maximum seasonal contrast and minimum absolute sq-values westward.

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Importance of wind-driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model.

Oleg A. Saenko and Andrew J. Weaver

An ocean-atmosphere-sea ice model is used to show the importance of wind-driven sea ice motion in the formation of low salinity Antarctic Intermediate Water (AAIW). The model is still able to reasonably simulate a tongue of relatively low salinity AAIW even when the direct momentum transfer from wind to the ocean is neglected, provided that the wind stress is applied to sea ice. In contrast, when the wind stress exclusively drives the ocean, the model fails to capture the properties of AAIW. The large-scale wind-driven sea ice motion preconditions the growth of sea ice in locations different from regions of ice melt on the annual mean basis. Melting of sea ice then provides fresh water to feed AAIW, whereas its growth makes near-surface Antarctic waters saltier, contributing to the formation of AABW. That is, the growth and subsequent offshore transport of sea ice acts as a freshwater conduit from near-shore regions, where AABW is formed, to subpolar regions, where AAIW is formed. Sea ice dynamics are also shown to be important in the simulation of a local salinity minimum at intermediate depths in the southern Indian Ocean and a local salinity maximum in the western Weddell Sea. It is concluded that the proper representation of southern hemisphere ventilation processes in climate models requires the use of wind-driven sea ice dynamics

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North Atlantic Response to the Above-Normal Export of Sea Ice from the Arctic

Oleg A. Saenko, Edward C. Wiebe and Andrew J. Weaver

The response of the thermohaline circulation (THC), as well as the freshwater and heat budgets of the northern North Atlantic, to above-normal sea ice export from the Arctic are examined using a coupled model. Two cases are considered: a pulse-like and a persistent above-normal export of sea ice from the Arctic. In the pulse-like case, the export of ice is doubled and sustained at that level for a specified period of time, ranging from one to five years. We show that, depending on the cumulative ice flux, the strength of the THC and the heat transport from the subtropics to the subpolar North Atlantic decrease by 5-20\%. It takes 15-20 years for the extra sea ice to convert into the freshwater anomaly and propagate towards and then within the North Atlantic water column of deep water formation, from the surface to the depths below 1000 m. About the same time is needed for the THC to return to its normal (control) state.

In the case of a persistent above-normal export of sea ice from the Arctic, the THC does not collapse, at least within the range of the ice export increase (1.5 to 3 times) used here. Rather, after about 15-20 years the THC shows a tendency for returning back to its normal (control) state. Two factors are involved in this process. First, the internal (to the coupled system) redistribution of freshwater between the Arctic and North Atlantic, associated with the enhanced export of sea ice, makes the North Atlantic fresher and Arctic Ocean saltier. This, if persistent, decreases the amount of freshwater leaving the Arctic towards the North Atlantic in a liquid form. Second, because the THC does not collapse, the freshwater anomaly propagates downward in the North Atlantic, removing the excess of buoyancy from the surface.

It is suggested that the decadal time scale of 15-20 years for North Atlantic THC variability is linked to the variability of sea ice export on different time scales. The variability of sea ice export produces freshwater anomalies within the Arctic Ocean and North Atlantic of opposite sign. It then takes about the same time (15-20 years) for the freshwater anomalies to both propagate horizontally from the Arctic Ocean interior to the North Atlantic region of deep water formation and downward within the North Atlantic water column.

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On the role of wind-driven sea ice motion on ocean ventilation.

Oleg Saenko, A. Schmittner and A.J. Weaver

Simulations with a coupled ocean-atmosphere-sea ice model are used to investigate the role of wind-driven sea ice motion on ocean ventilation. Two model experiments are analyzed in detail: one including and the other excluding wind-driven sea ice transport. Model simulated concentrations of chlorofluorocarbons (CFCs) are compared with observations from the Weddell Sea, the southeastern Pacific and the North Atlantic. We show that the buoyancy fluxes associated with sea ice divergence control the sites and rates of deep and intermediate water formation in the Southern Ocean. Divergence of sea ice along the Antarctic perimeter facilitates bottom water formation in the Weddell and Ross Seas. Neglecting wind-driven sea ice transport results in unrealistic bottom water formation in the Drake Passage and too strong convection along the Southern Ocean sea ice margin, whereas convection in the Weddell and Ross Seas is suppressed. The freshwater fluxes implicitly associated with sea ice export also determine the intensity of the gyre circulation and the rate of downwelling in the Weddell Sea.

In the North Atlantic, the increased sea ice export from the Arctic weakens and shallows the meridional overturning cell. This results in a decreased surface flux of CFCs around 65 N by about a factor of two. At steady state, convection in the North Atlantic is found to be less affected by the buoyancy fluxes associated with sea ice divergence compared to that in the Southern Ocean.

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The Science of Climate Change What Do We Know?

Gordon McBean, Andrew Weaver and Nigel Roulet

The greenhouse effect is a natural process that keeps the earth at a temperature which makes it a livable planet. The combined evidence of increased atmospheric concentrations of greenhouse gases, observed changes in the climate itself such as increased global mean temperature, and modeling experiments has led to credible scientific assessments of climate change. Through these assessments climate change has managed to become an issue in policy agendas. Although there are uncertainties surrounding projections of how human activities will affect the climate in the future, increasingly competent computer models have convinced the scientific community that there will be not only higher temperatures to deal with, but also more intense precipitation events and magnified warming in countries in high latitudes such as Canada. Nevertheless, uncertainties need to be reduced before the detailed refinement of response strategies can be done. For example, we need a clearer understanding of the spatial and temporal variations in climate change, especially of extreme events, before being able to refine response strategies.

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Distinguishing the influences of heat, freshwater and momentum fluxes on ocean circulation and climate

Saenko, O.A., J.M. Gregory, A.J. Weaver and M. Eby

The separate and combined effects of windstress and freshwater forcing on the ocean circulation and on ocean transports of heat and freshwater are analyzed using a coupled model. Suppressing the freshwater flux weakens the north Atlantic meridional overturning by 15% of its control value. With thermal forcing (no freshwater or momentum fluxes), it falls by only 20%. Thermal forcing is therefore dominant, in contradiction of the suggestion that freshwater forcing (net evaporation in the Atlantic) is the major driving force of this circulation. In the north Pacific, the meridional overturning intensifies, resulting in the appearance of a deep western boundary current there. Supressing the momentum flux (windstress) eliminates the subtropical barotropic gyres and reduces the flow through the Drake Passage by 65%, but does not lead to a substantial weakening of the deep outflow from the Atlantic at 30 S. However, with thermal forcing only, the outflow is reduced by 75%, suggesting that in this model the outflow is controlled by thermohaline rather than windstress forcing. Ocean meridional heat transport is somewhat sensitive to the removal of freshwater and momentum forcing, but freshwater transport is not. We show that gyre transport cannot be attributed uniquely to windstress forcing, and argue that the question remains open of whether the thermohaline "conveyor" transports freshwater into or out of the Atlantic.

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Instability of glacial climate in a model of the ocean-atmosphere-cryosphere system

Schmittner, A., M. Yoshimori and A.J. Weaver

In contrast to the relatively stable climate of the last 10,000 years, during glacial times the North Atlantic region experienced large-amplitude transitions between cold (stadial) and warm (interstadial) states. Here, using an Earth System Climate Model, we demonstrate that hydrological interactions between the Atlantic thermohaline circulation (THC) and adjacent continental ice sheets can trigger abrupt warming events and also limit the lifetime of the interstadial circulation mode. These interactions have the potential to destabilise the THC, which is already more sensitive for glacial conditions than for the present day climate, thus providing an explanation for the increased variability of glacial climate.

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Diurnal temperature range trends in 20th and 21st century simulations of the CCCma coupled model.

Stone, D.A. and A.J. Weaver

Trends in the diurnal temperature range (DTR) are examined in the late twentieth and the twenty-first centuries in a coupled climate model representing the atmosphere, ocean, sea ice, and land surface systems. Consistent with past studies, the DTR decreases during this time. These decreases are concentrated in middle latitudes, with much smaller changes occurring in the low latitudes. Strong seasonal characteristics to this pattern exist, although these are different in either hemisphere. In the model integrations, variations in the DTR are much more sensitive to changes in feedbacks than in direct forcing. The DTR is found to be rather insensitive to the scattering of sunlight by sulphate aerosols and the increased mean temperature. Instead, variations in the DTR arise mostly from changes in clouds and in soil moisture. Consequently, the decreases arise from increases in the reflection of solar radiation by clouds moderated by decreases in the ground heat capacity due to decreasing soil moisture. Both factors contribute about equally to the DTR trend. The exception to this relation occurs in the middle latitudes during winter, when snow cover reduces the influence of changes in solar radiation and soil moisture. Decreases during this season are a consequence of the tendency in CGCM1 for the DTR to be very small when the mean temperature is near the freezing point. The importance of soil moisture found here implies that changes in the physiological response of vegetation and in land use could have important effects on the DTR.

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The ventilation of the North Atlantic Ocean during the Last Glacial Maximum - a comparison between simulated and observed radiocarbon ages

K. J. Meissner, A. Schmittner, and A. J. Weaver

The distribution of radiocarbon during simulations of the Last Glacial Maximum with a coupled ocean-atmosphere-sea ice model are compared with sediment core measurements from the Equatorial Atlantic Ceara Rise and from the Blake Ridge. During these simulations, we introduce a perturbation of North Atlantic freshwater fluxes leading to varying strengths of the Atlantic meridional overturning. The best fit with the observations is obtained for a weakened or shutdown overturning motion consistent with a recent study comparing sea surface properties with reconstructions. In one of the locations (Blake Ridge) we simulate the phenomenon of an 'age reversal' found in deep-sea corals.

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Daily maximum and minimum temperature trends in a climate model.

Stone, D.A., and A.J. Weaver

The recent observed global warming trend over land has been characterised by a faster warming at night, leading to a considerable decrease in the diurnal temperature range (DTR). Analysis of simulations of a climate model including observed increases in greenhouse gases and sulphate aerosols reveals a similar trend in the DTR of -0.2 degrees C per century, albeit of smaller magnitude than the observed -0.8 degrees C per century. This trend in the model simulations is related to changes in cloud cover and soil moisture. These results indicate that the observed decrease in the DTR could be a signal of anthropogenic forcing of recent climate change.

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The UVic Earth System Climate Model: Model description, climatology and application to past, present and future climates.

Weaver, A.J., M. Eby, E. C. Wiebe, C. M. Bitz, P. B. Duffy, T. L. Ewen, A. F. Fanning, M. M. Holland, A. MacFadyen, H.D. Matthews, K.J. Meissner, O. Saenko, A. Schmittner, H. Wang and M. Yoshimori

A new earth system climate model of intermediate complexity is developed and its climatology is compared against observations. The UVic Earth System Climate Model consists of a three-dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea ice model, an energy-moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrised through Fickian diffusion, and precipitation is assumed to occur when the relative humidity reaches greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to instantaneously return to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrisation of water vapour/planetary long wave feedbacks, although the radiative forcing associated with changes in atmospheric CO2 is prescribed as a modification of the planetary long wave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present day winds in its climatology although a dynamical wind feedback is included which exploits a latitudinally-varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the GFDL Modular Ocean Model 2.2, with a global resolution of 3.6 degrees (zonal) by 1.8 degrees (meridional) and 19 vertical levels, that includes an option for a brine-reject ion parametrisation. The sea ice component incorporates an elastic-viscous-plastic rheology to represent sea ice dynamics and various options for the representation of sea ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection. Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0 degrees C for a doubling of CO2, in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but eventually re-establishes to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere GCMs. This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning re-establishes to a strength that is greater than its initial condition. When applied to the climate of the Last Glacial Maximum, the model obtains tropical cooling (30 deg. N--30 deg. S), relative to the present, of about 2.1 deg. C over the ocean and 3.6 deg. C over the land. These are generally cooler than CLIMAP estimates, but not as cool as some other reconstructions. This moderate cooling is consistent with alkenone reconstructions and a low to mid climate sensitivity to perturbations in radiative forcing. An amplification of the cooling occurs in the North Atlantic due to the weakening of North Atlantic Deep Water formation. Concurrent with this weakening is a shallowing and a more northward penetration of Antarctic Bottom Water. Climate models are usually evaluated by spinning them up under perpetual present-day forcing and comparing the model results with present-day observations. Implicit in this approach is the assumption that the present day observations are in equilibrium with the present day radiative forcing. The comparison of a long transient integration (starting at 6 KBP), forced by changing radiative forcing (solar, CO2, orbital), with an equilibrium integration reveals substantial differences. Relative to the climatology from the present-day equilibrium integration, the global mean surface air and sea surface temperatures (SSTs) are 0.74 deg. C and 0.55 deg. C colder, respectively, deep ocean temperatures are substantially cooler, and southern hemisphere sea ice cover is 22% larger, although the North Atlantic conveyor remains remarkably stable in all cases. The differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene. It is also demonstrated that a global warming simulation that starts from an equilibrium present-day climate (cold start) underestimates the global temperature increase at 2100 by 13% when compared to a transient simulation, under historical solar, CO2 and orbital forcing, that is also extended out to 2100. This is larger (13% compared to 9.8%) than the difference from an analogous transient experiment which does not include historical changes in solar forcing. These results suggest that those groups that do not account for solar forcing changes over the 20th century may slightly underestimate (~3% in our model) the projected warming by the year 2100.

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Evidence of change in the Sea of Okhotsk: Implications for the North Pacific.

Hill, K.L., A.J. Weaver, H.J. Freeland and A. Bychkov

Russian data from 5 cruises during the period 1949 to 1952 are compared with observations taken during WOCE P1W in 1993 to examine changes which may have occurred in the Sea of Okhotsk during the latter half of the last century. A basinwide warming and freshening of the Sea of Okhotsk was found in the archived data. Since the Sea of Okhotsk is thought to be the major source region for North Pacific Intermediate water (NPIW), calculations were conducted to see whether or not this change in Sea of Okhotsk water properties is consistent with evidence of large-scale freshening of intermediate waters in the North Pacific. From several Okhotsk-to-Pacific salt flux calculations, we conclude that the Sea of Okhotsk was capable of causing the freshening noted in the NPIW over the past half century under certain assumed outflow conditions.

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The role of the thermohaline circulation in abrupt climate change

Clark, P.U., N.G. Pisias, T.F. Stocker, and A.J. Weaver

Many model simulations of the coupled atmosphere-ocean system show the potential for an abrupt reduction in the strength of the thermohaline circulation (THC) with future increases in concentrations of greenhouse gases. Assessing the likelihood of future abrupt change remains uncertain, however, as model sensitivity is dependent on parameterizations of important physics that are not otherwise treated explicitly. Evidence of abrupt climate changes during the last glaciation provides a critical source of information for testing the ability of models to simulate nonlinear behavior of the Earth's climate system. Here we discuss the evidence suggesting that past abrupt climate change originated through changes in the Atlantic THC. Atmospheric responses to these changes were transmitted globally through a number of feedbacks. Large changes in the THC caused an anti-phased response centered on regions of the Southern Hemisphere. The geologic data support model results in showing that the stability of the THC is dependent on the climate state, and that during glaciations the THC is sensitive to small changes in the hydrological cycle.

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A region of enhanced northward Antarctic Intermediate Water transport in a coupled climate model.

Saenko, O.A., A.J. Weaver and M.H. England

A region of enhanced northward transport of freshwater across 60 S is found in a coupled model. The fresh water escapes the subantarctic region between about 100 W and the Antarctic Peninsula, rather than being transported to the north in a circumpolar manner. A majority of this freshwater is not of local origin. It is transported from the north-west to the south-east in the Pacific. This freshwater accumulates to the west of the Antarctic Peninsula, which can be seen in both the model-simulated and observed salinities. It then moves to the north in a rather localized region, contributing to the formation of Antarctic Intermediate Water (AAIW). Observations of zonal salinity gradients west of the Antarctic Peninsula suggest that our model results are consistent with AAIW pathways in the real ocean.

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Tidally driven mixing in the Oceanic General Circulation

Simmons, H.L., S.R. Jayne, L.C. St. Laurent and A.J. Weaver

A parameterization of tidally-driven mixing that evolves spatially and temporally is developed and incorporated into a global ocean model. At equilibrium, globally-averaged mixing has a profile ranging from 0.3 cm2s-1 at thermocline depths to 7.7 cm2s-1 in the abyss, with globally averaged values of 0.9 cm2s-1, in close agreement with inferences from global balances (Munk, 1966). Water properties are strongly influenced by the combination of weak mixing in the upper ocean and enhanced mixing in the deep ocean. Climatological comparisons show substantial reduction of temperature/salinity bias, relative to a control run with a uniform vertical mixing rate of 0.9 cm2s-1. This suggests that bottom intensified mixing is an essential component of the balances required for maintenance of ocean stratification. We offer an energy-consistent and practical means of both improving the physical representation of ocean mixing processes in climate models and demonstrate the substantial improvements arising from this improved representation of ocean physics.

References:
Munk, W. H., Abyssal recipes, Deep Sea Res., 13, 707--730, 1966.

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Response of the inorganic carbon cycle to future climate warming in a coupled climate model

Ewen, T.L., A.J. Weaver and M. Eby

With increased anthropogenic carbon dioxide emitted into the atmosp here, climate feedbacks could potentially reduce further uptake of carbon by the oceans. The most significant feedbacks acting on the system to reduce carbon sequestration by the oceans are reductions in the thermohaline circultion (THC) and increased sea surface temperatures (SSTs). Although changes to the SSTs affect the solubility of atmospheric CO2 across the ocean-atmosphere interface, changes to the THC lead to more fundamental modifications to ocean circulation and further transport and storage of carbon to the deep ocean. Using a coupled model of intermediate complexity which incorporates a carbon solubility pump, we project atmospheric carbon dioxide levels under global warming scenarios. We find a weakening of the THC and increased SSTs in all simulations. Although these positive feedbacks are acting on the carbon system to reduce uptake, we find that the ocean has the capacity to take up an additional 65-75% of the atmospheric CO2 increase when anthropogenic forcing is stopped. This reduces by about 5% for each 50 year period that anthropogenic emissions are maintained at a stabilised and elevated atmospheric greenhouse CO2 level.

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The effect of land use change on 20th century climate as simulated by a climate model of intermediate complexity

Matthews, H.D., A.J. Weaver, M. Eby and K.J. Meissner

The effect of changing human land-use patterns on the climate of the past 300 years is discussed through analysis of a series of equilibrium and transient climate simulations using the UVic Earth System Climate Model. Land-surface changes are prescribed through varying land cover type, representing the replacement of natural vegetation by human agricultural systems from 1700 to 1992. First, equilibrium climate simulations are presented using (1) present-day vegetation, (2) year 1992 croplands superimposed onto a potential vegetation field and (3) year 1700 croplands superimposed onto potential vegetation. Second, a transient climate simulation forced by land-use changes alone is compared to a control and two other simulations, forced by: (1) changes in atmospheric CO2; and (2) changes in land-use and atmospheric CO2. All simulations show a cooling res ulting from land-use induced changes to surface albedo and evapotranspiration. The globally averaged cooling is in the range of 0.09 to 0.22 oC with larger regional changes caused by local positive feedbacks. Transient runs show that land-use cooling is in the range of 12 to 22% of the magnitude of greenhouse gas induced warming.

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Southern Ocean upwelling and eddies: Sensitivity of the global overturning to the surface density range.

Saenko, O.A., and A.J. Weaver

A simple interhemispheric ocean model is used to examine the sensitivity of water sinking in the northern hemisphere to the equator-to-pole density contrast. The model assumes that the sinking is compensated by upwelling in both the low latitude ocean and the Southern Ocean. We compare two vertical mixing schemes: one with a fixed vertical diffusivity and another with fixed mixing energy. When Southern Ocean upwelling is controlled only by northward Ekman transport, the rate of deep water formation has an opposite dependence on the equator-to-pole density contrast between the two vertical mixing schemes. However, when Southern Ocean upwelling is controlled by both Ekman transport and eddy-induced transport across the Antarctic Circumpolar Current, the two mixing schemes give qualitatively similar dependence: the rate of water sinking increases with the equator-to-pole density contrast, regardless of whether the diffusivity or the mixing energy is held fixed.

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Detecting anthropogenic influence with a multi model ensemble

Gillett, N.P., F.W. Zwiers, A.J. Weaver, G.C. Hegerl, M.R. Allen and P.J. Stott

Averaging results from multiple models has previously been found to improve estimates of the climatology and seasonal predictions of atmospheric variables. Several coupled models have been used individually to detect greenhouse gas and sulphate aerosol influence on surface temperature over the past fifty years. Here we describe how a multi-model mean of the simulated response to these forcings may be used to synthesize results from several models, and to better constrain uncertainties in the results. The scaling factor on a combined to greenhouse gas plus sulphate aerosol pattern was estimated using a five model ensemble, and this response was found to be close to the centre of the range of the scaling factors estimated using individual models, with similar uncertainties. When the method was applied to the simultaneous detection of separate greenhouse gas and sulphate aerosol responses, the multi-model method indicated a closer consistency between the simulated and observed response patterns, with reduced uncertainties. This improvement was at least in part due to the larger ensemble sizes and longer control available when data from multiple models are combined.

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Coupling of the hemispheres in observations and simulations of glacial climate change

Schmittner, A., O.A. Saenko and A.J. Weaver

We combine reconstructions, climate model simulations and a conceptual model of glacial climate change on millennial time scales to examine the relation between the high latitudes of both hemispheres. A lead-lag analysis of synchronised proxy records indicates that temperature changes in Greenland preceded changes of the opposite sign in Antarctica by 400-500 years. A composite record of the Dansgaard-Oeschger events suggests that rapid warming (cooling) in Greenland was followed by a slow cooling (warming) phase in Antarctica. The amplitudes, rates of change and time lag of the interhemispheric temperature changes found in the reconstructions are in excellent agreement with climate model simulations in which the formation of North Atlantic Deep Water is perturbed. The simulated time lag between high northern and southern latitudes is mainly determined by the slow meridional propagation of the signal in the Southern Ocean. Our climate model simulations also show that increased deep water formation in the North Atlantic leads to a reduction of the Antarctic Circumpolar Current through diminishing meridional density gradients in the Southern Ocean.
We construct a simple conceptual model of interhemispheric Dansgaard-Oeschger oscillations. This model explains major features of the recorded temperature changes in Antarctica as well as the general shape of the north-south phase relation found in the observations including a broad peak of positive correlations for a lead of Antarctica over Greenland by 1000-2000 years. The existence of this peak is due to the regularity of the oscillations and does not imply a southern hemisphere trigger mechanism, contrary to previous suggestions. Our findings thus further emphasises the role of the thermohaline circulation in millennial scale climate variability.

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Factors Contributing to Diurnal Temperature Range Trends in 20th and 21st Century Simulations of the CCCma Coupled Model.

Stone, D. A. and A. J. Weaver

Trends in the diurnal temperature range (DTR) are examined in the late twentieth and the twenty-first centuries in a coupled climate model representing the atmosphere, ocean, sea ice, and land surface systems. Consistent with past studies, the DTR decreases during this time. These decreases are concentrated in middle latitudes, with much smaller changes occurring in the low latitudes. Strong seasonal characteristics to this pattern exist, although these are different in either hemisphere. In the model integrations, variations in the DTR are much more sensitive to changes in feedbacks than in direct forcings. The DTR is found to be insensitive to the scattering of sunlight by sulphate aerosols and the increased mean temperature. Instead, variations in the DTR arise mostly from changes in clouds and in soil moisture. Consequently, the decreasing trends arise from increases in the reflection of solar radiation by clouds moderated by decreases in soil moisture, mostly through its effect on the ground heat capacity. Both factors contribute about equally to the DTR trend. The exception to this relation occurs in the middle latitudes during winter, when snow cover reduces the influence of changes in solar radiation and soil moisture. Decreases during this season are a consequence of the artificial tendency in the model for the DTR to be very small when the mean temperature is near the freezing point. While the accuracy of these conclusions depends upon the model's ability to represent the relevant processes, the results highlight the importance of clouds and land surface processes to the DTR and its long term change. The importance of soil moisture found here implies that changes in the physiological response of vegetation and in land use could have important effects on the DTR.

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Variation of Labrador Sea deep water formation over the last glacial cycle in a climate model of intermediate complexity

Cottet-Puinel, M., A.J. Weaver, C. Hillaire-Marcel, A. de Vernal and P.U. Clark

The variation of North Atlantic Deep Water (NADW) formation over the last glacial cycle, from isotopic substage 5e (the Eemian) through to future global warming projections, is investigated using the UVic Earth System Climate Model. The results are compared with available micropaleontological and stable isotope proxy paleo-reconstructions. Equilibrium simulations for the Eemian (125kyr BP) and the Last Glacial Maximum (LGM -- 21 kyr BP) reveal the absence of Labrador Sea Water (LSW) formation although NADW formation still occurs, albeit at a reduced rate relative to the modern times. For the Eemian, the location of convection in the eastern North Atlantic is similar to the present, although it is generally shallower and less extensive. In the case of the LGM, deep convection has moved southward to the western coast of Europe and is much more localised. The inferred inception of a modern-like circulation slightly before 7 kyr BP revealed by proxy reconstructions is not captured by the model unless the melt water forcing from the Laurentide ice sheet is applied in a long transient simulation. This raises questions concerning the applicability of equilibrium simulations in capturing the early Holocene climate. In all global warming projections, the LSW formation initially ceases as atmospheric CO2 rises, but recovers once the level is held fixed in the atmosphere. Convection in the north extends further into the Nordic Seas as the sea ice edge retreats. In all simulations convection remains active in the eastern North Atlantic, with its latitude depending on the position of the sea ice edge, suggesting that the formation of lower NADW is a robust feature of Late Quaternary climate. As the Labrador Sea is found to be very sensitive to the freshwater forcing, it suggests that this region represents an ideal location for the concentration of observational studies to monitor a possible oceanic response to anthropogenic climate change.

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The Neoproterozoic 'Snowball Earth': Dynamic ice over a quiescent ocean

Lewis, J.P., A.J. Weaver, S.T. Johnston and M. Eby

Low-latitude sea level glacial deposits suggest the existence of "snowball Earth" conditions in the Neoproterozoic. Previous modelling studies have offered conflicting support for the snowball hypothesis. We use a climate model of intermediate complexity, including an ocean GCM and a sophisticated thermodynamic/dynamic sea ice component, to conduct a suite of experiments with different orbital/paleogeographical configurations and atmospheric CO2 levels. In the snowball Earth environment, the global ocean heat transport is essentially zero, so that specified ocean heat transport estimates based on the present-day climate are inconsistent with the existence of a global sea ice cover. We show that depending on the orbital configuration and paleogeography, snowball conditions prevail even with atmospheric CO2 levels up to 1800 ppmv. Overall our modelling paradigm is consistent with the original snowball hypothesis (references in paper) in which an ice covered ocean surrounds a largely snow and ice free barren land.

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The role of land-surface dynamics in glacial inception: a study with the UVic Earth System Model

Meissner, K. J., A. J. Weaver, H. D. Matthews and P. M. Cox

The first results of the UVic Earth system model coupled to a land surface scheme and a dynamic global vegetation model are presented in this study. In the first part of the paper the present day climate simulation is discussed and compared to observations. We then compare a simulation of an ice age inception with a preindustrial run. Emphasis is placed on the vegetation's reaction to the combined changes in solar radiation and atmospheric CO2 level. A southward shift of the northern treeline as well as a global decrease in vegetation carbon is observed in the ice age inception run. In tropical regions, up to 85% of broadleaf trees are replaced by shrub and C4 grasses. These changes in vegetation cover have a remarkable effect on the global climate: land related feedbacks double the atmospheric cooling during the ice age inception as well as the reduction of the meridional overturning in the North Atlantic. The introduction of vegetation related feedbacks also increases the surface area with perennial snow significantly.

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On the link between the two modes of the ocean thermohaline circulation and the formation of global-scale water masses

Saenko, O. A. , Andrew J. Weaver, Jonathan M. Gregory

A close link between the formation of global-scale water masses, such as North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW), and two stable modes of the thermohaline circulation (THC) is investigated. In the upper 2-3 km of the Atlantic, the THC modes are characterized by meridional overturning circulations of opposite sign, with either a dominance of the AAIW cell over the NADW cell ("off" THC mode) or vice versa ("on" THC mode). A transition between the THC modes is controlled by the relationship between the densities in the source regions of formation of AAIW and NADW water masses. This is shown by forcing the hysteresis loops of the NADW and AAIW circulations with an externally-imposed freshwater perturbation. Unlike in previous studies, the freshwater perturbation is applied to the region of enhanced AAIW formation in the Southern Ocean around the southern tip of South America. The transitions between the two modes of the THC occurs when the densities in the source regions of AAIW formation and NADW formation become comparable to each other.

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Modelling carbon cycle feedbacks during abrupt climate change

Ewen, T., Weaver, A. J., Schmittner, A.

Past climate, both before and after the Last Glacial Maximum, was marked by a series of abrupt climate transitions from cold to warm states corresponding to significant changes in atmospheric CO2. Mechanisms which led to these transitions most likely include variability in the thermohaline circulation (THC) as inferred from deep sea sediment records. In this study, we investigate the changes in atmospheric CO2 concentration that arise during abrupt climate change events. This is accomplished through our use of meltwater pulse scenarios applied to an ocean-atmosphere-sea ice model coupled to an inorganic carbon component. We perform transient simulations with increased freshwater discharge to high latitude regions in both hemispheres from a glacial equilibrium climate to simulate meltwater episodes. We find that changes in ocean circulation and carbon solubility lead to significant increases in atmospheric CO2 concentrations when we simulate meltwater episodes in both hemispheres. The magnitude of increase in atmospheric CO2 is between 10-40 ppmv, which accounts for some of the changes in CO2 as recorded in the ice core records.

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Atlantic deep circulation controlled by freshening in the Southern Ocean

Oleg A. Saenko, Andrew J. Weaver and Andreas Schmittner

Numerical simulations with a climate model of intermediate complexity are used to illustrate the effect of meridional moisture transport in the Southern Hemisphere mid-latitudes on the meridional overturning circulation (MOC) and heat transport in the Atlantic. A novel feature of the model is a diapycnal mixing scheme in the ocean, which ensures low values of diffusivity (about 10-5 m2/s) in the pycnocline. It is shown that the Atlantic MOC, northward oceanic heat transport and the associated air-sea heat flux anomalies are all proportional to the southward moisture transport from subtropical to subpolar regions in the Southern Hemisphere. The effect of the intensified ocean circulation on sea surface temperature and salinity is also illustrated.

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The Neoproterozoic 'Snowball Earth': Dynamic ice over a quiescent ocean

Lewis, J.P., A.J. Weaver, S.T. Johnston and M. Eby

Low-latitude sea level glacial deposits suggest the existence of "snowball Earth" conditions in the Neoproterozoic. Previous modelling studies have offered conflicting support for the snowball hypothesis. We use a climate model of intermediate complexity, including an ocean GCM and a sophisticated thermodynamic/dynamic sea ice component, to conduct a suite of experiments with different orbital/paleogeographical configurations and atmospheric CO2 levels. In the snowball Earth environment, the global ocean heat transport is essentially zero, so that specified ocean heat transport estimates based on the present-day climate are inconsistent with the existence of a global sea ice cover. We show that depending on the orbital configuration and paleogeography, snowball conditions prevail even with atmospheric CO2 levels up to 1800 ppmv. Overall our modelling paradigm is consistent with the original snowball hypothesis (references in paper) in which an ice covered ocean surrounds a largely snow and ice free barren land.

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The impact of varying atmospheric forcing on the thickness of Arctic multi-year sea ice

Dumas, J.A., G.M. Flato, and A.J. Weaver

A 1-D thermodynamic sea ice model, forced with North Pole Drift Station observations from 1954-91, is used to study the effect of changing atmospheric forcing on multi-year Arctic sea ice. From 1954-70, most seasons show positive trends in calculated sea ice thickness over much of the Arctic. A dip in calculated ice thickness takes place between 1971-77 over most of the Arctic. Following the North Pacific regime shift in 1976-1977, the period 1978-91 reveals large negative trends in calculated sea ice thickness in all seasons. The results indicate that an important part of the variability and trends in Arctic sea ice thickness is thermodynamically-driven. Of the total variance in sea ice thickness, 10 to 20% is explained by variations in the Arctic Oscillation and Pacific North American patterns. The multi-year ice thickness response to a positive wintertime Arctic Oscillation anomaly occurs the following summer and persits for more than a year.

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J. M. Gregory, O. A. Saenko and A. J. Weaver

The role of the Atlantic freshwater balance in the hysteresis of the meridional overturning circulation

We have studied the response of the Atlantic meridional overturning circulation to surface freshwater forcing using an ocean GCM coupled to an energy-moisture-balance atmosphere model. The overturning collapses rapidly when a slowly increasing forcing applied to the North Atlantic passes a positive threshold, and spins up equally quickly when the forcing falls below a negative threshold. This well-known behaviour is referred to as hysteresis because the thresholds in forcing are different for the transitions in opposite directions. However, we argue that the behaviour of the Atlantic salinity is more fundamental than the forcing. Hysteresis as a function of freshwater forcing occurs because the states with North Atlantic overturning on and off each tend to reinforce their associated salinity distributions and inhibit the transition to the other state. During the collapse, the Atlantic becomes less saline because of the import of 80 Sv year of freshwater by ocean transports across 30S; during the spin-up this freshwater is exported again. We show that qualitatively similar hysteresis behaviour can be produced by perturbing the system without any net freshwater forcing. The salinity flip-flop is associated with the appearance and disappearance of a shallow reverse overturning circulation south of the Equator, which is present while the northern overturning is absent, and may provide the mechanism for the ocean freshwater influx during collapse.

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