Download Land-Sea Warming Contrast

Land-Sea Warming Contrast
Chris Eldred
Primary Source Paper: The Ocean's Role in Continental Climate Variability and
Change, Dommenget 2009, J Climate
Outline
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Introduction
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Key Questions
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Data and Models
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Land-Sea Contrast
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Sensitivity Experiments
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Box Model
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Physical Mechanisms
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Summary
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References
Introduction
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The land sea warming contrast is a consistent feature of
global warming scenarios
Recent studies have found that this a feature of equilibrium
scenarios as well
This study investigates the land-sea warming contrast for :
– external forcing (anthropogenic climate change)
– natural internal climate variability
using:
– A coupled atmosphere-ocean GCM
– An uncoupled atmosphere-only GCM
Key Questions
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What is the land-sea warming contrast?
Why is it present in both equilibrium and climate
change scenarios?
What is the sensitivity of the land surface
temperature to changes in the ocean surface
temperature? What about the reverse?
What are the physical mechanisms that drive
the asymmetry between land-ocean and oceanland interaction?
Data
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Land surface observations taken
from Climatic Research Unit
Temperature dataset (CRUTEM3v)
land surface temperatures
SST's taken from Hadley Centre
Sea Ice and Sea Surface
Temperature dataset (HadISST)
Coverage gradually decreases as
we go back in time
Therefore, only data past 1950 will
be used
Model Runs
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Note that the ocean model used is a 1-D mixed layer model
Legend
Coupled Baseline
Uncoupled Baseline
Box Model
Sensitivity Study + Box Model
Land-Sea Warming Contrast
Land-Sea Variability Contrast
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Tsurf- observed surface
temperature at each grid
point
Tland- Average global
mean land surface
temperature
Tocean- Average global
mean ocean surface
temperature
Observed surface
temperature variability has
larger amplitude over land
than water in observations
and simulations with a
land-sea ratio of ~1.5
Sensitivity Experiments
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Compared a uniform 1K Tocean
forcing to a 1K Tland forcing
Tocean forcing leads to 1.3K rise
in Tland
Tland forcing leads to 0.2K rise in
Tocean
2xCO2 simulation shows that it is
not just heat capacity differences
that drive the land-sea warming
contrast
Box Model
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Use the model simulations to estimate the local feedback parameters
(cl, co), effective coupling coefficients (clo, col) and net external
forcings (Fl, Fo) based on T'land and T'ocean (deviations from
climatological global means)
T'land = 2.3943Fo + Fl
T'ocean = 1.69Fo + 0.38461Fl
(In Equilbrium)
Box Model
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Box Model Conclusions:
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Land surface temperature is 10x as sensitive to ocean temperatures
changes as the ocean is to land temperature changes
Oceanic forcing plays a much larger role than direct radiative forcing on
land surface temperatures
Tland is more strongly forced by Fo than Fl
Land sea warming ratio comes from coupling and feedback parameters,
NOT differences in Fo/Fl
Land Ocean Asymmetry
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Several characteristics of the
climate system may lead to the
asymmetry in land-ocean
interaction:
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Differences in areal coverage
between the oceans and land,
which means land points are on
average much closer to ocean
points that ocean points are to
land points
The greater heat capacity and
effective depth of the oceans
compared to the land surface
(applies only to transient
changes)
Land Ocean Asymmetry
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Increased moisture availability over the oceans → greater water vapor
content over the oceans →
latent heat release when condensation and precipitation
occurs
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increased longwave downwelling
(1.3Wm^-2 per Kg m^-2 of water vapor, comparable to direct
radiative forcing)
– Note that vapor content over land is more sensitive to Tocean than Tland
but vapor content over the oceans is independent of Tland
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Larger latent heat release over the oceans → reduced atmospheric lapse
rate → upper tropospheric warming → (via advection) surface heating
•
Tocean influences the free troposphere much more strongly
than Tland
Summary
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Land surface warming is driven primarily by forcing from the
oceans and not by direct radiative forcing
Land-sea warming contrast exists in both equilibrium and
climate change scenarios
Land-sea warming contrast is driven physically by the local
feedback and land-ocean coupling, not differences in
external forcing
Further studies are needed to investigate the model
dependencies of these results (especially a full 3-D ocean
model), the role of the three processes in producing the
land-ocean asymmetry and amplification over land, the
regional differences in land-ocean interactions, the role of
the tropics vs. the extratropics and the sensitivity of the
results to the model resolution used
References
•
Dommenget D. The ocean's role in continental climate variability and change. J Clim
2009, 22: 4939-4952. DOI: 10.1175/2009JCLI2778.1.
•
Compo, G. P. and P. D. Sardeshmukh, 2009: Oceanic influences on recent continental
warming. Climate Dyn., 32:333–342. doi:10.1007/s00382-008-0448-9.
•
Joshi, M. M., J. M. Gregory, M. J. Webb, D. M. H. Sexton, and T. C. Johns, 2007:
Mechanisms for the land/sea warming contrast exhibited by simulations of climate
change. Climate Dyn., 30:455–465. doi:10.1007/s00382-007-0306-1.
•
Lambert, F. H. and J. C. H. Chiang, 2007: Control of land-ocean temperature contrast by
ocean heat uptake. Geophys. Res. Lett., 34:L13704. doi:10.1029/2007GL029755.
•
Sutton, R., B. Dong, and J. M. Gregory, 2007: Land/sea warming ratio in response to
climate change: IPCC AR4 model results and comparison with observations. Geophys.
Res. Lett., 34:L02701. doi:10.1029/2006GL028164.
•
Zhang, R., T. L. Delworth, and I. M. Held, 2007: Can the Atlantic Ocean drive the
observed multidecadal variability in Northern Hemisphere mean temperature? Geophys.
Res. Lett., 34:L02709. doi:10.1029/2006GL028683.
•
http://data.giss.nasa.gov/gistemp/ (for surface temperature anomaly plot)
Other Studies
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Compo et. al. 2008:
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Sutton et. al. 2007:
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Found a warming ratio between 1.36-1.84 that was independent of global mean
temperature change. Latitudinal variations were present, with a minima in equatorial
latitudes and a maximum in the subtropics.
Lambert et. al. 2007:
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Land surface warming has occurred largely as a response to oceanic warming, rather
than direct radiative effects. Hydrodynamic-radiative teleconnections are invoked as the
explanation.
Found that the land-ocean temperature change contrast (ie warming ratio) remained
constant under a variety of different external climate forcings
Zhang et. al. 2007:
– A portion of the multidecadal variability in the Northern Hemisphere mean temperature
can be explained by variability in the Atlantic ocean
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Joshi et. al. 2007:
– The land/sea contrast in both equilibrium and transient simulations is associated with
local feedbacks and the hydrological cycle over land, rather than with externally imposed
radiative forcing