` Synoptic weather conditions and changing fire regimes in a Mediterranean environment

Analysis of global change effects on fire regimes requires evaluations of key processes explaining fire activity at the appropriate spatial and temporal scales. Classifications of the weather conditions prevailing at large continental scales (called Synoptic Weather Types, SWT) offer convenient potential proxies for integrating weather-related factors into our understanding of fire regime attributes at regional scales. Here we establish a methodology for identifying the major SWT that lead to wildfires and assessing their influence on fire regime in interaction with other global drivers such as drought events or fire suppression policies. Based on days with fires larger than 50 ha that occurred in Catalonia, a region located in the western Mediterranean Basin, we propose a clustering methodology using data of temperature at 850 hPa, sea level pressure and winds at 925 hPa from the NCEP/NCAR reanalysis dataset covering the whole of western Europe (25-70 degrees N and 20 degrees W-40 degrees E). Our classification method proposes 6 SWT: three that were characterized by synoptic conditions leading to strong winds in the region, two that led to 'hot and dry' environments, and one that was not characterized by any strong weather determinants. Fires under 'hot and dry' conditions, such as the South intrusion SWT, triggered the bulk of fires and burnt area in the region. Spatial analyses of fire distribution under each SWT revealed markedly different fire-prone locations, opening the possibility for strategic planning of fire management based on local fire regimes. Fires occurring during mild years (wet spring conditions) and under 'hot-and-dry' SWTs have been eradicated from the region thanks to enhanced firefighting capability, and fire sizes in dry years have strongly reduced. In contrast, fires occurring under windy situations have not followed the same course of change and have not diminished in incidence over time, and seem to be more difficult to control using current fire suppression strategies. The role of SWT on determining fire regimes and its interaction with fire suppression strategies has a huge potential to help researchers and managers develop better fire analyses based on sound physical grounds and serve to understand and eventually regulate the adverse impacts of fire regime changes in a global change context.