Climatic Feedbacks and Desertification: The Mediterranean Model

Climatic Feedbacks and Desertification: The Mediterranean Model. Millán, M. M., Estrela, M. J., Sanz, M. J., Mantilla, E., Mart́ın, M., Pastor, F., Salvador, R., Vallejo, R., Alonso, L., Gangoiti, G., Ilardia, J. L., Navazo, M., Albizuri, A., Art́ıñano, B., Ciccioli, P., Kallos, G., Carvalho, R. A., Andrés, D., Hoff, A., Werhahn, J., Seufert, G., & Versino, B. 18(5):684–701.

Paper doi abstract bibtex

Mesometeorological information obtained in several research projects in southern Europe has been used to analyze perceived changes in the western Mediterranean summer storm regime. A procedure was developed to disaggregate daily precipitation data into three main components: frontal precipitation, summer storms, and Mediterranean cyclogenesis. Working hypotheses were derived on the likely processes involved. The results indicate that the precipitation regime in this Mediterranean region is very sensitive to variations in surface airmass temperature and moisture. Land-use perturbations that accumulated over historical time and greatly accelerated in the last 30 yr may have induced changes from an open, monsoon-type regime with frequent summer storms over the mountains inland to one dominated by closed vertical recirculations where feedback mechanisms favor the loss of storms over the coastal mountains and additional heating of the sea surface temperature during summer. This, in turn, favors Mediterranean cyclogenesis and torrential rains in autumn?winter. Because these intense rains and floods can occur anywhere in the basin, perturbations to the hydrological cycle in any part of the basin can propagate to the whole basin and adjacent regions. Furthermore, present levels of air pollutants can produce greenhouse heating, amplifying the perturbations and pushing the system over critical threshold levels. The questions raised are relevant for the new European Union (EU) water policies in southern Europe and for other regions dominated by monsoon-type weather systems. [Excerpt: Discussion] Available experimental results suggest that, because of the nature of the atmospheric circulations along its coasts, the hydrological cycle in the western Mediterranean basin has been drifting toward a critical threshold level, that is, when the cloud condensation levels of the surface air masses rise above the coastal mountain ridges, leading to the loss of summer storms. Any additional perturbations to the surface air masses, including increasing their temperature (by whatever means) or decreasing evaporation and evapotranspiration along their paths, or any combinations of these, could tip the present situation further toward the loss of mountain storms and desertification at the local-regional scale. [] The question of the end radiative effect of air pollution in the Mediterranean basin, that is, is it cooling or warming the sea and surface air mass, is still open, even though our results indicate that warming dominates in the western Mediterranean at this stage. The hypothesis that the radiative effects of particulates cool the Mediterranean (Lelieveld et al. 2002) would indeed lead to less evaporation and fewer summer storms, but it would also lead to fewer torrential rains in autumn-winter-spring, which is contrary to observations in the western basin. [] Our working hypotheses on the possible reasons for the observed precipitation changes in the western Mediterranean basin relate to 1) land-use changes, as the main factor drying the surface and 2) atmospheric composition changes, that is, an increase in tropospheric ozone and aerosols producing additional greenhouse heating. The hypotheses are as follows: [::] Land use that results in surface drying leads to increased heating of the surface air mass, lower evaporation, higher cloud condensation levels, and thus fewer summer storms in the mountains inland. [::] The radiative effects of air pollution could be further heating the surface air mass and lifting the CCL; this could be perturbing (present) local balances just enough to tip the equilibrium toward decreased summer storms. [::] In turn, the recirculatory nature and the long residence times of the air masses in this region lead to the accumulation of ozone and water vapor (both acting as greenhouse gases) in the lower-to-upper troposphere over the western Mediterranean, adding to the heating of the sea surface during the summer and to an increase in torrential rains in autumn and winter at the regional (basin) scale. [] In relation to climate-vegetation feedbacks we can consider the following: [::] long-term effects, that is, fewer storms in the mountains, positive feedback through vegetation losses (less evapotranspiration), and increased surface heating (drier soil) toward desertification; [::] delayed effects, that is, a warmer Mediterranean by the end of summer and early autumn leading to more frequent and intense torrential rains, by positive feedback. These rains can occur anywhere in the basin and result in flash floods over the coasts and nearby mountain slopes; and [::] if the slope vegetation response to these flash floods is increased soil erosion, then the positive feedback toward desertification would be further reinforced. [] Finally, another far-reaching conclusion is that any perturbation to the hydrological cycle in any part of the western basin can propagate to other parts of the basin and neighboring regions. This derives from the fact that the warmer water pools move (i.e., rotate) within the basin, and, thus, any increase in sea surface temperature can result in torrential rain and flash floods anywhere in the basin.

@article{millanClimaticFeedbacksDesertification2005,
title = {Climatic Feedbacks and Desertification: The {{Mediterranean}} Model},
author = {Millán, M. M. and Estrela, M. J. and Sanz, M. J. and Mantilla, E. and Mart́ın, M. and Pastor, F. and Salvador, R. and Vallejo, R. and Alonso, L. and Gangoiti, G. and Ilardia, J. L. and Navazo, M. and Albizuri, A. and Art́ıñano, B. and Ciccioli, P. and Kallos, G. and Carvalho, R. A. and Andrés, D. and Hoff, A. and Werhahn, J. and Seufert, G. and Versino, B.},
date = {2005-03},
journaltitle = {Journal of Climate},
volume = {18},
pages = {684--701},
issn = {1520-0442},
doi = {10.1175/jcli-3283.1},
url = {https://doi.org/10.1175/jcli-3283.1},
abstract = {Mesometeorological information obtained in several research projects in southern Europe has been used to analyze perceived changes in the western Mediterranean summer storm regime. A procedure was developed to disaggregate daily precipitation data into three main components: frontal precipitation, summer storms, and Mediterranean cyclogenesis. Working hypotheses were derived on the likely processes involved. The results indicate that the precipitation regime in this Mediterranean region is very sensitive to variations in surface airmass temperature and moisture. Land-use perturbations that accumulated over historical time and greatly accelerated in the last 30 yr may have induced changes from an open, monsoon-type regime with frequent summer storms over the mountains inland to one dominated by closed vertical recirculations where feedback mechanisms favor the loss of storms over the coastal mountains and additional heating of the sea surface temperature during summer. This, in turn, favors Mediterranean cyclogenesis and torrential rains in autumn?winter. Because these intense rains and floods can occur anywhere in the basin, perturbations to the hydrological cycle in any part of the basin can propagate to the whole basin and adjacent regions. Furthermore, present levels of air pollutants can produce greenhouse heating, amplifying the perturbations and pushing the system over critical threshold levels. The questions raised are relevant for the new European Union (EU) water policies in southern Europe and for other regions dominated by monsoon-type weather systems.

[Excerpt: Discussion]

Available experimental results suggest that, because of the nature of the atmospheric circulations along its coasts, the hydrological cycle in the western Mediterranean basin has been drifting toward a critical threshold level, that is, when the cloud condensation levels of the surface air masses rise above the coastal mountain ridges, leading to the loss of summer storms. Any additional perturbations to the surface air masses, including increasing their temperature (by whatever means) or decreasing evaporation and evapotranspiration along their paths, or any combinations of these, could tip the present situation further toward the loss of mountain storms and desertification at the local-regional scale.

[] The question of the end radiative effect of air pollution in the Mediterranean basin, that is, is it cooling or warming the sea and surface air mass, is still open, even though our results indicate that warming dominates in the western Mediterranean at this stage. The hypothesis that the radiative effects of particulates cool the Mediterranean (Lelieveld et al. 2002) would indeed lead to less evaporation and fewer summer storms, but it would also lead to fewer torrential rains in autumn-winter-spring, which is contrary to observations in the western basin.

[] Our working hypotheses on the possible reasons for the observed precipitation changes in the western Mediterranean basin relate to 1) land-use changes, as the main factor drying the surface and 2) atmospheric composition changes, that is, an increase in tropospheric ozone and aerosols producing additional greenhouse heating. The hypotheses are as follows:

[::] Land use that results in surface drying leads to increased heating of the surface air mass, lower evaporation, higher cloud condensation levels, and thus fewer summer storms in the mountains inland.

[::] The radiative effects of air pollution could be further heating the surface air mass and lifting the CCL; this could be perturbing (present) local balances just enough to tip the equilibrium toward decreased summer storms.

[::] In turn, the recirculatory nature and the long residence times of the air masses in this region lead to the accumulation of ozone and water vapor (both acting as greenhouse gases) in the lower-to-upper troposphere over the western Mediterranean, adding to the heating of the sea surface during the summer and to an increase in torrential rains in autumn and winter at the regional (basin) scale.

[] In relation to climate-vegetation feedbacks we can consider the following:

[::] long-term effects, that is, fewer storms in the mountains, positive feedback through vegetation losses (less evapotranspiration), and increased surface heating (drier soil) toward desertification;

[::] delayed effects, that is, a warmer Mediterranean by the end of summer and early autumn leading to more frequent and intense torrential rains, by positive feedback. These rains can occur anywhere in the basin and result in flash floods over the coasts and nearby mountain slopes; and

[::] if the slope vegetation response to these flash floods is increased soil erosion, then the positive feedback toward desertification would be further reinforced.

[] Finally, another far-reaching conclusion is that any perturbation to the hydrological cycle in any part of the western basin can propagate to other parts of the basin and neighboring regions. This derives from the fact that the warmer water pools move (i.e., rotate) within the basin, and, thus, any increase in sea surface temperature can result in torrential rain and flash floods anywhere in the basin.},
keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-7749100,climate,complexity,desertification,feedback,land-use,mediterranean-region,non-linearity},
number = {5}
}

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