The Response of the Thermohaline
Circulation to Future Climate Change
Kimberly Chamales
GFD I - Fall 2011
Why is the Thermohaline Circulation (THC) important
when examining future climate change?
‣ Global warming can induce changes in the THC by altering ocean temperature and salinity
‣ Modeling studies indicate that significant changes in the THC may be possible in the future due to
increased CO2 (Cubasch et al. 2001)
‣ There are many uncertainties in the response of the THC to an increased freshwater flux and how
this response will affect the climate
Schematic of the general structure of the THC and the distribution of salinity (PSS). Red currents
represent warmer surface water and blue currents represent colder deep water. Figure from: http://
earthobservatory.nasa.gov/
CMIP water-hosing experiments
‣ Set of so-called “water-hosing experiments” developed for the Coupled Model
Intercomparison Project (CMIP) of the World Meteorological Organization/World Climate
Research Program/Program on Climate Variability and Predictability Working Group on
Coupled Models
The 0.1-Sv water-hosing experiment:
‣ An external freshwater flux of 0.1 Sv (105 m3s-1) is applied uniformly between 50° and 70° N for
100 years
‣ Flux is shut off after model year 100 and the integration continues
‣ Control integration with no external freshwater forcing is run in parallel to the water-hosing
integration
‣ A flux of 0.1 Sv was chosen to represent the predicted increase in freshwater flux into the Arctic
and Atlantic Oceans due to a large CO2-induced warming (Dixon et al. 1999, Church et al. 2001)
‣ Stouffer et al. (2006) comprise the results of the CMIP water-hosing experiments from 14 models to
investigate the response of the THC in the future climate.
‣ The 14 models consist of either coupled atmosphere-ocean general circulation models (AOGCMs) or
earth system models of intermediate complexity (EMICs)
Results: The response of the THC
‣ Stouffer et al. (2006) define THC intensity as the maximum meridional overturning
streamfunction value in the North Atlantic
‣ The THC weakens in all models
‣ Mean reduction of THC intensity at the
100th year is 5.6 Sv (a 30%
weakening)
‣ Individual models range from
reductions of 1.3 to 9.2 Sv (a 9% to
62% weakening)
‣ Freshwater input reduces the deep
convection in the northern North
Atlantic - ultimately reduces THC
intensity
Time series of the relative THC intensity (Sv) anomalies compared with
the long-term mean of the THC intensity in the control experiments
Results: Ocean temperature
‣ General decrease in sea surface temperature (SST) in the North Atlantic
‣ General increase in SST in the South Atlantic
‣ Robust cooling south of Greenland
‣ Robust warming in the Barents Sea - due to the design of the experiment
Geographical distribution of the ensemble mean SST (°C)
anomalies in the 0.1-Sv experiment. Years 81-100.
Results: Sea ice
‣ increase in sea ice thickness in the Labrador Sea
‣ decrease of sea ice thickness in Barents Sea because of the shift of the deep convection
sites
‣ positive sea ice anomalies in the Weddell Sea because of the deep convection in the area
Geographical distribution of the ensemble mean sea ice thickness (m)
anomalies in the 0.1-Sv experiment. Years 81-100.
Results: Surface air temperature
‣ Surface air temperature (SAT) distribution is similar to the SST distribution
‣ Enhanced SAT anomalies due to a positive sea ice feedback in the northern North Atlantic and the
Weddell Sea
Geographical distribution of the ensemble mean SAT (°C) anomalies in the
0.1-Sv experiment. Years 81-100.
‣ A slowing of the THC will offset the CO2-induced increase in SAT and sea ice melt in the Northern
Hemisphere!
Do we see this negative feedback in
simulations with a doubling of CO2?
‣ Yes, however... these models do not include freshwater input from ice sheets and glaciers as
the climate warms
‣ The decrease in THC intensity is a result of changes in precipitation and evaporation and in
buoyancy fluxes due to ocean warming (Cubasch et al. 2001)
Multi-model ensemble annual mean change of temperature (°C) for the CMIP2 scenarios at the time of CO2 doubling
Conclusions and Future Work
‣ An external freshwater flux of 0.1 Sv for 100 years results in an average
slowing of the THC by 30%, with a range of results for individual models
‣ The slowing of the THC results in a decrease in SAT of about 3°C, enough to
offset a warming due to the doubling of CO2
‣ Hopefully advancements of model capabilities will lead to the development of
model experiments that can capture the freshwater fluxes from ice sheets
and glaciers that result from increased in greenhouse gas emissions
‣ More work needs to be done to determine the reasons for the decrease in
SAT south of Greenland - compare a fully coupled atmosphere-ocean GCM
and model with a slab ocean
References
Church, J. A., J. M. Gregory, P. Huybrechts, M. Kuhn, K. Lam- beck, M. T. Nhuan, and P. L. Woodworth,
2001: Change in sea level. Climate Change 2001: The Scientific Basis, J. T. Houghton et al., Eds.,
Cambridge University Press, 640–693.
Cubasch, U., and Coauthors, 2001: Projections of future climate change. Climate Change 2001: The
Scientific Basis, J. T. Houghton et al., Eds., Cambridge University Press, 525–582.
Dixon, K. W., T. Delworth, M. Spelman, and R. J. Stouffer, 1999: The influence of transient surface fluxes on
North Atlantic overturning in a coupled GCM climate change experiment. Geophys. Res. Lett., 26, 2749–
2752.
Stouffer, R. J., and Coauthors, 2006: Investigating the causes of the response of the thermohaline
circulation to past and future climate changes. J. Climate, 19, 1365–1387.
Questions?