Heat and cold waves in mainland Spain: Origins, characteristics, and trends

Heat and cold waves are extreme temperature events with a high potential of causing negative impacts on human health, and natural and socioeconomic systems, depending on their duration and intensity. There is, however, no consensual approach to address their definition, which is critical to set priority action areas to prevent such risks. Mainland Spain experiences heat and cold waves every year with important impacts especially in the most populated areas with mild or transition climates. Here we used a high-resolution (5 x 5 km) gridded daily temperature dataset and employed a combination of threshold exceedances of maximum and minimum temperature in the same day to identify heat and cold wave events over 75 years (1940-2014). We further examined the duration and the seasonal/annual intensities to detect potential spatial and temporal patterns. Additionally, we used the days within the most widespread events to perform a synoptic classification to categorise the atmospheric conditions leading to high-risk situations. Our results show a similar historical duration of heat and cold waves (4-5 days) and a much higher seasonal intensity of cold ones (double than heat waves). We find a tipping point in the early 1980s from which heat waves became more frequent, longer, and more intense than cold waves. Finally, we discern between 9 historical weather types with a dominance of southern advections driving heat waves and cold continental north-northeast air masses causing cold waves. Understanding the patterns and trends of heat and cold waves, as well as the mechanisms of their genesis is key to assist in risk management in mainland Spain, especially in the context of a warming climate scenario.