The Role of Hadley Circulation and Lapse-Rate Changes for the Future European Summer Climate

By the end of the century, climate projections for southern Europe exhibit an enhanced near-surface summer warming in response to greenhouse gas emissions, which is known as the Mediterranean amplification. Possible causes for this amplified warming signal include a poleward Hadley cell expansion as...

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Bibliographic Details
Published in:Journal of climate 2019-01, Vol.32 (2), p.385-404
Main Authors: Brogli, Roman, Kröner, Nico, Sørland, Silje Lund, Lüthi, Daniel, Schär, Christoph
Format: Article
Language:English
Subjects:
Amplification
Atmosphere
Boundary conditions
Climate change
Climate models
Climate science
Computer simulation
Cyclones
Emissions
Feedback
Global climate
Global climate models
Global warming
Greenhouse effect
Greenhouse gases
Hadley circulation
Humidity
Precipitation
Regional climate models
Regional climates
Simulation
Summer
Summer climates
Troposphere
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Summary:By the end of the century, climate projections for southern Europe exhibit an enhanced near-surface summer warming in response to greenhouse gas emissions, which is known as the Mediterranean amplification. Possible causes for this amplified warming signal include a poleward Hadley cell expansion as well as tropospheric lapse-rate changes. In this work, regional climate model (RCM) simulations driven by three different global climate models (GCMs) are performed, representing the RCP8.5 emission scenario. For every downscaled GCM, the climate change signal over Europe is separated into five contributions by modifying the lateral boundary conditions of the RCM. This simulation strategy is related to the pseudo–global warming method. The results show that a poleward expansion of the Hadley cell is of minor importance for the Mediterranean amplification. During summer, the simulated Hadley circulation is weak, and projections show no distinct expansion in the European sector. The north–south contrast in lapse-rate changes is suggested as the most important factor causing the Mediterranean amplification. Lapse-rate changes are projected throughout Europe, but are weaker over the Mediterranean than over northern Europe (around 0.15 vs 0.3 K km−1 by the end of the century). The weaker lapse-rate changes result in a strong near-surface summer warming over the Mediterranean, since the upper-tropospheric warming is of similar magnitude throughout Europe. The differing lapse-rate changes can be understood as a thermodynamic response to lower-tropospheric humidity contrasts.
ISSN:0894-8755
1520-0442
DOI:10.1175/jcli-d-18-0431.1