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.
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.
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.
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.
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 2ºC 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.
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
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.
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.
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
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.
On the role of wind-driven sea ice motion on ocean ventilation.
Oleg Saenko, A. Schmittner and A.J. Weaver
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.