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Abstract:
The occurrence and intensity of heatwaves is expected to increase with climate change. Early warnings of
hot summers have therefore a great socio-economical value. Previous studies have shown that hot summers are preceded
by a Southern European rainfall deficit during winter, and
higher spring temperatures. Changes in the surface energy
budget are believed to drive this evolution, in particular
changes in the latent and sensible heat fluxes. However these
have rarely been investigated due to the lack of long-term
reliable observation data. In this study, we analyzed several
data-derived gridded products of latent and sensible heat
fluxes, based on flux tower observations, together with reanalyses
and regional climate model simulations over Europe.
We find that warm summers are preceded by an increase
in latent heat flux in early spring. During warm summers, an
increase in available energy results in an excess of both latent
and sensible heat fluxes over most of Europe, but a latent heat
flux decrease over the Iberian Peninsula. This indicates that,
on average, a summertime soil-moisture limited evapotranspiration
regime only prevails in the Iberian Peninsula. In
general, the models that we analyzed overestimate latent heat
and underestimate sensible heat as compared to the flux
tower derived data-product. Most models show considerable
drying during warm seasons, leading to the establishment of
a soil-moisture limited regime across Europe in summer.
This over-estimation by the current generation of models of
latent heat and hence of soil moisture deficit over Europe in
summer has potential consequences for future summertime
climate projections and the projected frequency of heat
waves. We also show that a northward propagation of
drought during warm summers is found in model results, a
phenomenon which is also seen in the flux tower dataproduct.
Our results lead to a better understanding of the role of latent and sensible heat flux in summer heatwaves, and provide a framework for benchmark of modeling studies.
