Response of the western European climate to a THC collapse
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Response of the western European climate to a THC collapse. A. Laurian S. Drijfhout W. Hazeleger B. van den Hurk. Koninklijk Nederlands Meteorologisch Instituut, The Netherlands. The T hermo h aline C irculation.

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Response of the western European climate to a THC collapse

A. Laurian

S. Drijfhout

W. Hazeleger

B. van den Hurk

Koninklijk Nederlands Meteorologisch Instituut, The Netherlands

Climate and Global Dynamics Division Seminar – July 11, 2008


The t hermo h aline c irculation l.jpg

The Thermohaline Circulation

  • 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 t hermo h aline c irculation3 l.jpg

    The Thermohaline Circulation

    • 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?


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    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


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    Model and experiment

    Coupled model ECHAM5/MPI-OM

    -Good skills at reproducing the Atlantic MOC

    - Realistic atmospheric circulation over Europe


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    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


    Model and experiment7 l.jpg

    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


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    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.


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    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?


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    Model and experiment

    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.


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    ENSMALL

    HOSING

    Model and experiment

    • In ENSMALL, warming of 4°C in 150 years.

    • THC collapse  Cooling of 0.7°C during 25 years which tempers global warming


    Response of the 2 m air temperature l.jpg

    Response of the 2-m air temperature

    Cooling everywhere, 6°C in the North Atlantic ocean, 2.5°C in

    Western Europe

     Strong land/sea contrast: what maintains this pattern?


    Convergence of zonal mse transport l.jpg

    (MSE = gz + CpT + Lvq)

    Convergence of zonal MSE transport

    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

    ENSMALL (m .s), column-integrated


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    Outline

    Introduction

    Model and experiment

    Response of the 2-m air temperature

    Mechanismsleading to a differentresponse over the ocean and above land

    - Response of the net atmospheric fluxes

    - Change in clouds and associated feedbacks

    - Change in moisture transport

    Conclusions


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    Response of the net atmospheric fluxes

    (SW-LW-SH-LH) : surface

    85 W/m² in the North Atlantic Ocean  Latent heat release over the ocean

    • 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


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    Response of the net atmospheric fluxes

    SW : surface

    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.


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    Radiative effect of clouds

    (SW+LW)toa, clouds - (SW+LW)toa, clear sky

    in ENSMALL

     Net effect of clouds in the mid latitudes = Cooling of the atmosphere (60 W/m² over the ocean, 25 W/m² over land)


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    Change in radiative effect of clouds

    (SW+LW)toa, clouds - (SW+LW)toa, clear sky

    in HOSING-ENSMALL

    • Over the ocean: (8 W/m²) consistent with an of clouds

    • Over land: (4 W/m²) consistent with a of cloud cover


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    Change in liquid water

    (column integrated)

    liquid water over land (0.1g/m²)

    liquid water over the ocean (0.04g/m²)

    THC collapse 


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    Seasonality of the liquid water response

    DJF JJA

    • over land in winter

    • over the ocean in summer


    Vertical distribution of the liquid water response 48 n l.jpg

    DJF JJA

    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)


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    DJF JJA

    Why is the liquid water increasing over the ocean?

    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


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    DJF JJA

    Why is the liquid water decreasing over land?

     Other mechanism to explain the decrease of cloud cover over land


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    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


    Change in relative humidity rh at the surface l.jpg

    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


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    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


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    Change in moisture transport

    Réponse du budget d’humidité

    • 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.


    Slide28 l.jpg

    Change in moisture transport

    Réponse du budget d’humidité

    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


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    Change in moisture transport

    Réponse du budget d’humidité

    • This results in cloud cover above land which acts as a

    • secondary response maintaining the land/sea contrast of the

    • temperature response.



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    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.


    Slide32 l.jpg

    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




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