|TITLE||CHANGES IN PRECIPITATION REGIMES SINCE 850 A.D.: THE ROLE OF THE INTEGRATED WATER VAPOR TRANSPORT AND GLOBAL WARMING|
|AUTHORS||PM Sousa (1), AM Ramos (1), RM Trigo (1), CC Raible (2,3), M Messmer (2,3), JG Pinto (4)|
- Instituto Dom Luiz (IDL), FCUL, Lisboa, Portugal
- Oeschger Centre for Climate Change Research, Bern, Switzerland
- Climate and Environmental Physics, Bern, Switzerland
- Institute of Meteorology and Climate Research, Karlsruhe, Germany
|ABSTRACT||Changes in Atlantic moisture transport corridors control a large fraction of the precipitation regimes in Western Europe, particularly though the long corridors that transport large amounts of water vapor, in the form of an Atmospheric River. The vertically integrated water vapor transport (IVT) has been computed using a long-term climatic simulation spanning between 850-2100 A.D. to examine long-term variability in precipitation regimes in this region. The simulation was performed with the Community Earth System Model (CESM 1.0.1), using a modified reconstruction of total solar irradiance, volcanic forcing and the RCP8.5 forcing scenario for the 21st century.
We analyzed the variability of IVT fluxes along the Iberian Peninsula and UK, and compared them against: i) precipitation series (large scale, convective and total) in those regions; ii) mean surface temperature in oceanic boxes west of those coastlines. After the Industrial Period a steady increase in moisture transport towards Western Europe (superimposed to the inter-annual and inter-decadal variabilities) occurred, in line with the concurrent rise in surface temperatures. Nevertheless, while recent and projected rainfall regime changes follow this positive trend in the UK, a negative precipitation trend is foreseen through the 21st century for Iberia. In fact, the steady correlation between the IVT and precipitation (0.75) in the Pre-Industrial period is lost afterwards, especially towards the end of the simulation (21st century). As a consequence, further precipitation decreases in Southern Europe are projected, being this reduction substantial during autumn and spring transition months.
Our results show that dynamical features are driving precipitation regime changes towards drier conditions in Southern Europe, despite the observed and expected rises in evaporative processes and moisture availability in a warming climate. To illustrate this, a Weather Type Classification (WTs) approach was performed, showing a significant increase (decrease) in the frequency of anticyclonic (cyclonic and/or westerly flow) days over Iberia, and an opposite response in the UK. Results also suggest a shortening of the rainy season in Iberia, as typical summer-like WTs extend towards November by the end of the 21st century. All these dynamical changes are occurring at a magnitude unprecedented for the time-scales of this long-simulation period, stressing the long-term drying trends in Mediterranean-like climates.
This work was supported by project IMDROFLOOD – Improving Drought and Flood Early Warning, Forecasting and Mitigation using real-time hydroclimatic indicators (WaterJPI/0004/2014) funded by FCT. Alexandre M. Ramos was also supported by a FCT postdoctoral grant (FCT/DFRH/ SFRH/BPD/84328/2012).|