Winter time long dry spells in the Mediterranean Basin and associated atmospheric dynamics: actual and future variability (1957-2100)

Defended on the 4th December 2017

Funding: ministerial research allocation

Supervisors: Pierre Camberlin and Albin Ullmann

Started in October 2014

Summary

In the context of climate change, as reflected by a dryer Mediterranean basin, this thesis focused on the study of the contemporary and future variability (1957-2100) of the wintertime (September to April) very long dry spells events (called VLDSe) in the Mediterranean basin. An original methodology was developed in this thesis in order to define VLDSe as singular climatic events, characterized by location, duration and spatial extent criteria.

76 VLDSe were detected in the Mediterranean basin on the contemporary period (1957-2013). These events are divided into 4 main geographical patterns: North-East, West, Scattered Localized and South-East. North-East and West configurations are associated with anticyclonic conditions located approximately 1 000 km northwestern to the areas affected by the VLDSe, favoring a clear sky and no precipitations. The Scattered Localized and South-East configurations are special: the first one is characterized as a residual class grouping VLDSe with small spatial extent and distributed throughout the entire basin, and the second one is characterized by seasonal VLDSe which are the continuation of the dry summer observed in the east of the Mediterranean basin.

Euro-Atlantic weather regimes have some control on the VLDSe. The positive phase of the north-atlantic oscillation regime (NAO+) is the only one that is clearly favorable to the development of VLDSe on almost the entire basin. The east-atlantic regime (EA) does not show any control on the VLDSe, and the atlantic ridge (AR) and the negative phase of the northatlantic oscillation (NAO-) regimes are generally detrimental to VLDSe. However, some VLDSe can sometimes be associated with AR, EA and NAO- regimes. This requires these three weather regimes to be associated with slightly higher atmospheric pressure northwest of the areas impacted by the VLDSe, compared to their respective climatology. Long duration of the AR, EA and NAO+ regimes, which are coupled with sustained atmospheric stability, are preferentially associated with VLDSe, in contrast to the short duration. Conversely, the long duration of the NAO- regime, reinforcing the low atmospheric pressure on Europe and the Mediterranean basin, are weakly associated with VLDSe.

Although the two climate models ALADIN52 and LMDZ4-NEMOMED8 differ in several respects, they agree in that VLDSe should be longer by 2100, especially in the RCP8.5 trajectory. A multi-model analysis with 12 CMIP5 simulations shows that wintertime sea-level pressure tends to increase in the Atlantic Ocean, off the French coast and in the central the Mediterranean basin for the RCP8.5 trajectory. Conversely, the frequency and duration of the 4 weather regimes do not show significant trends until the end of the 21st century.

Finally, a study is carried out to assess the impact of VLDSe on agricultural production in Spain. The number of VLDSe days has a larger impacts on the yields of barley, wheat and oats (winter species and cultivated through rainfed agriculture) than the simple ratio of dry days or seasonal rainfall amounts in Spain. A two-season case study, based on seasons with comparable rainfall amounts, shows that in addition to yields, a VLDSe causes a significant decrease in soil moisture and in the Ebro River flow.

Key words

Mediterranean Basin, winter droughts, atmospheric conditions, climate change, agro-environmental impacts

Steering committee

Sylvain Bigot (UJF, LTHE, Grenoble)
Philippe Drobinski (École pPolytechnique, LMD, Paris)
Nicolas Martin (UNS, Espace, Nice)

Jury

Martin Beniston, université de Genève – reviewer
Vincent Moron, université d’Aix-Marseille – reviewer
Philippe Drobinski, CNRS/École polytechnique – examiner
Sylvain Bigot, université Grenoble-Alpes – examiner
Yves Tramblay, IRD/HydroSciences Montpellier  – examiner
Pierre Camberlin, université de Bourgogne-Franche Comté – supervisor de thèse
Albin Ullmann, université de Bourgogne-Franche Comté – co-supervisor)

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Winter time long dry spells in the Mediterranean Basin and associated atmospheric dynamics: actual and future variability (1957-2100)

Defended on the 4th December 2017

Funding: ministerial research allocation

Supervisors: Pierre Camberlin and Albin Ullmann

Started in October 2014

 

Summary

In the context of climate change, as reflected by a dryer Mediterranean basin, this thesis focused on the study of the contemporary and future variability (1957-2100) of the wintertime (September to April) very long dry spells events (called VLDSe) in the Mediterranean basin. An original methodology was developed in this thesis in order to define VLDSe as singular climatic events, characterized by location, duration and spatial extent criteria.

76 VLDSe were detected in the Mediterranean basin on the contemporary period (1957-2013). These events are divided into 4 main geographical patterns: North-East, West, Scattered Localized and South-East. North-East and West configurations are associated with anticyclonic conditions located approximately 1 000 km northwestern to the areas affected by the VLDSe, favoring a clear sky and no precipitations. The Scattered Localized and South-East configurations are special: the first one is characterized as a residual class grouping VLDSe with small spatial extent and distributed throughout the entire basin, and the second one is characterized by seasonal VLDSe which are the continuation of the dry summer observed in the east of the Mediterranean basin.

Euro-Atlantic weather regimes have some control on the VLDSe. The positive phase of the north-atlantic oscillation regime (NAO+) is the only one that is clearly favorable to the development of VLDSe on almost the entire basin. The east-atlantic regime (EA) does not show any control on the VLDSe, and the atlantic ridge (AR) and the negative phase of the northatlantic oscillation (NAO-) regimes are generally detrimental to VLDSe. However, some VLDSe can sometimes be associated with AR, EA and NAO- regimes. This requires these three weather regimes to be associated with slightly higher atmospheric pressure northwest of the areas impacted by the VLDSe, compared to their respective climatology. Long duration of the AR, EA and NAO+ regimes, which are coupled with sustained atmospheric stability, are preferentially associated with VLDSe, in contrast to the short duration. Conversely, the long duration of the NAO- regime, reinforcing the low atmospheric pressure on Europe and the Mediterranean basin, are weakly associated with VLDSe.

Although the two climate models ALADIN52 and LMDZ4-NEMOMED8 differ in several respects, they agree in that VLDSe should be longer by 2100, especially in the RCP8.5 trajectory. A multi-model analysis with 12 CMIP5 simulations shows that wintertime sea-level pressure tends to increase in the Atlantic Ocean, off the French coast and in the central the Mediterranean basin for the RCP8.5 trajectory. Conversely, the frequency and duration of the 4 weather regimes do not show significant trends until the end of the 21st century.

Finally, a study is carried out to assess the impact of VLDSe on agricultural production in Spain. The number of VLDSe days has a larger impacts on the yields of barley, wheat and oats (winter species and cultivated through rainfed agriculture) than the simple ratio of dry days or seasonal rainfall amounts in Spain. A two-season case study, based on seasons with comparable rainfall amounts, shows that in addition to yields, a VLDSe causes a significant decrease in soil moisture and in the Ebro River flow.

Key words

Mediterranean Basin, winter droughts, atmospheric conditions, climate change, agro-environmental impacts

Steering committee

Sylvain Bigot (UJF, LTHE, Grenoble)
Philippe Drobinski (École pPolytechnique, LMD, Paris)
Nicolas Martin (UNS, Espace, Nice)

Jury

Martin Beniston, université de Genève – reviewer
Vincent Moron, université d'Aix-Marseille – reviewer
Philippe Drobinski, CNRS/École polytechnique – examiner
Sylvain Bigot, université Grenoble-Alpes – examiner
Yves Tramblay, IRD/HydroSciences Montpellier  – examiner
Pierre Camberlin, université de Bourgogne-Franche Comté – supervisor de thèse
Albin Ullmann, université de Bourgogne-Franche Comté – co-supervisor)