Apr 4, 2016

Disruption of the European climate seasonal clock in a warming world

Christophe Cassou, Julien Cattiaux
Nature Climate Change
  • Highlights how temperatures over Europe are largely driven by the strength and inland penetration of the oceanic westerly flow
  • Highlights that the wind influence depends on season: blocked westerlies, linked to high-pressure anomalies over Scandinavia, induce cold episodes in winter but warm conditions in summer
  • Proposes defining the onset of the two seasons as the calendar day on which the daily circulation/temperature relationship switches sign
  • Assesses this meteorologically based metric using several observational data sets and we provide evidence for an earlier onset of the summer date by ~10 days between the 1960s and 2000s
  • Climate model results show that internal variability alone cannot explain this calendar advance
  • Finds that the earlier onset can be partly attributed to anthropogenic forcings
  • Finds the modification of the zonal advection due to earlier disappearance of winter snow over Eastern Europe, which reduces the degree to which climate has continental properties, is mainly responsible for the present-day and near-future advance of the summer date in Western Europe
  • Holds that these findings are in line with phenological-based trends (earlier spring events) reported for many living species over Europe, for which the authors provide an alternative interpretation to the traditionally evoked local warming effect
  • Expects a summer advance of ~20 days compared with pre-industrial climate by 2100, whereas no clear signal arises for winter onset, based on the Representative Concentration Pathway (RCP) 8.5 scenario, which assumes that greenhouse gas emissions continue to rise throughout the twenty-first century