1. A. Laurian S. Drijfhout W. Hazeleger B. van den Hurk Response of the western European climate to a THC collapse Koninklijk Nederlands Meteorologisch Instituut, The Netherlands Climate and Global Dynamics Division Seminar – July 11, 2008

2. - Transports heat and salt poleward - A change in its strength can lead to global and regional climate changes (Vellinga and Wood 2002; Dong and Sutton 2002; Chang et al. 2008) The Thermohaline Circulation

3. - Transports heat and salt poleward - A change in its strength can lead to global and regional climate changes (Vellinga and Wood 2002; Dong and Sutton 2002; Chang et al. 2008)  What is the response of Western European climate to a THC collapse? The Thermohaline Circulation

4. Outline Introduction Model and experiment Response of the 2-m air temperature Mechanisms leading to a different response over the ocean and above land - Response of the net atmospheric fluxes - Change in clouds and associated feedbacks - Change in moisture transport Conclusions

5. Model and experiment Coupled model ECHAM5/MPI-OM - Good skills at reproducing the Atlantic MOC - Realistic atmospheric circulation over Europe

6. Model and experiment Coupled model ECHAM5/MPI-OM - Good skills at reproducing the Atlantic MOC - Realistic atmospheric circulation over Europe ECHAM5: horizontal resolution T63, 31 vertical levels MPI-OM: horizontal resolution ~ 1.4° x 1.4°, near the Equator: 1.4°x0.5°, near the poles: O(20-40 km), 40 vertical levels

7. Model and experiment Coupled model ECHAM5/MPI-OM - Good skills at reproducing the Atlantic MOC - Realistic atmospheric circulation over Europe ECHAM5: horizontal resolution T63, 31 vertical levels MPI-OM: horizontal resolution ~ 1.4° x 1.4°, near the Equator: 1.4°x0.5°, near the poles: O(20-40 km), 40 vertical levels ESSENCE project ( Sterl et al. 2008) 17-member ensemble of runs (1950-2100) - From 1950 to 2000: runs forced by the observed concentrations of GHG and tropospheric sulfate aerosols - From 2001 to 2100: runs followed the SRES A1b scenario

8. Model and experiment We investigate 2 five-member ensemble runs. In one ensemble, a freshwater anomaly of 1 Sv was uniformly applied between 50° and 70°N over the northern North Atlantic Ocean from 2001 onwards, starting from the five initial states of the other ensemble.

9. Model and experiment We investigate 2 five-member ensemble runs. In one ensemble, a freshwater anomaly of 1 Sv was uniformly applied between 50° and 70°N over the northern North Atlantic Ocean from 2001 onwards, starting from the five initial states of the other ensemble. The ensemble mean of the perturbed runs (HOSING) is compared to the ensemble mean of the associated control runs (ENSMALL) over the period 2091-2100 in western Europe (30°W-20°E, 30°N-60°N).  How does global warming evolve when a THC collapse occurs during the process?

10. ENSMALL HOSING In ENSMALL the THC gradually weakens by 20% after 100 years due to global warming. The freshwater supply leads to a collapse of THC within 50 years. Model and experiment

11. ENSMALL HOSING In ENSMALL, warming of 4°C in 150 years. THC collapse  Cooling of 0.7°C during 25 years which tempers global warming Model and experiment

12. Cooling everywhere, 6°C in the North Atlantic ocean, 2.5°C in Western Europe  Strong land/sea contrast: what maintains this pattern? Response of the 2-m air temperature

13. MSE in the whole column is transported eastward and advection from the ocean is a source of MSE between 36° and 52°N  One may expect that the prevailing westerlies reduce land/sea contrast Convergence of zonal MSE transport (MSE = gz + C p T + L v q) ENSMALL (m.s ), column-integrated

14. Outline Introduction Model and experiment Response of the 2-m air temperature Mechanisms leading to a different response over the ocean and above land - Response of the net atmospheric fluxes - Change in clouds and associated feedbacks - Change in moisture transport Conclusions

15. Response of the net atmospheric fluxes (SW-LW-SH-LH) : surface Positive values above sea indicate that the ocean cools the atmosphere and the atmosphere warms the ocean, to compensate for the absent ocean heat transport from the South.  The ocean is driving the net atmospheric flux response 85 W/m² in the North Atlantic Ocean  Latent heat release over the ocean

16. SW : surface Response of the net atmospheric fluxes SW at the surface clearly supports the suggestion of a cloud response. SW by about 6.5 W/m² over sea  Suggesting an of clouds there blocking the penetration of SW SW by about 4.5 W/m² over land  Suggesting a of clouds there allowing more SW to penetrate to the surface.

17.  Net effect of clouds in the mid latitudes = Cooling of the atmosphere (60 W/m² over the ocean, 25 W/m² over land) Radiative effect of clouds (SW+LW)toa, clouds - (SW+LW)toa, clear sky in ENSMALL

18. Over the ocean: (8 W/m²) consistent with an of clouds Change in radiative effect of clouds (SW+LW)toa, clouds - (SW+LW)toa, clear sky in HOSING-ENSMALL Over land: (4 W/m²) consistent with a of cloud cover

19. Change in liquid water (column integrated) liquid water over land (0.1g/m²) liquid water over the ocean (0.04g/m²) THC collapse 

20. Seasonality of the liquid water response over land in winter over the ocean in summer DJF JJA

21. Vertical distribution of the liquid water response (48°N) Over sea: low clouds in all seasons (below 900 hPa) Over land: mid-level clouds in winter (between 900 and 600 hPa) DJF JJA

22. Why is the liquid water increasing over the ocean? DJF JJA The cooler air temperature over the ocean  stability of the atmospheric BL, inhibiting convection and maintaining a wetter marine BL.  shallow clouds over the ocean

23. Why is the liquid water decreasing over land? DJF JJA  Other mechanism to explain the decrease of cloud cover over land

24. Change in specific humidity (q) at the surface q over the ocean by about 2 g/kg q over land by about 1.2 g/kg (Evaporation and the colder air is more easily saturated)  Similar zonal contrasts as the 2-m temperature response

25. Change in relative humidity (RH) at the surface RH above the ocean by about 5% RH above land by about 1%  This strong land/sea contrast in the RH response is consistent with the clouds over land and the clouds over sea

26. Change in relative humidity (RH) at the surface Over the ocean, in RH dominated by in T Over land, in RH dominated by in q

27. The change in moisture sink (P-E) is balanced by the change in moisture divergence by advection. Over the ocean the transport of moist air towards land by about 1 mm/day. The transport of moisture towards land is consistent with the strong in E over the ocean related to the strong in LH flux. Réponse du budget d’humidité Change in moisture transport

28. A decomposition of the anomalous advection of moisture into a dynamical term and a thermodynamical term shows that the reduction of moisture transport towards land is mainly driven by the in q Réponse du budget d’humidité Change in moisture transport

29. Réponse du budget d’humidité Change in moisture transport  This results in cloud cover above land which acts as a secondary response maintaining the land/sea contrast of the temperature response.

30. Summary

31. Conclusions The response to a THC collapse features a strong zonal gradient of surface air temperature between the ocean and the continent. Above the ocean, more low clouds are formed due to an increase in relative humidity and a more stable marine BL. These clouds have a cooling effect on the ocean which enhances the cooling due to the primary ocean-driven response. A decrease in convergence of moist static energy above land is responsible for a decrease of clouds above land. The effect of this is to warm the atmosphere over land, thereby weakening the cooling over land that results from advection of colder and dryer air from the sea. The secondary cloud response therefore acts to maintain the strong land/sea contrast in surface air temperature response.

32. Discussion 1. The temperature rise of the last decades is larger over land than over sea and a similar cloud feedback modulates the temperature response to a global warming (Joshi et al. 2008). 1. How would Western European climate respond to a “realistic” THC collapse?  Sensitivity experiments with different amplitude for the freshwater pulse 2. How sensitive is the response of the climate to a THC collapse?  Sensitivity experiments with different climate scenarios 3. If the THC variability is predictable, can we predict the impacts on European climate?  Useful for society

33. Thank you!

34. Temperature response due to a THC collapse