Assessment of drought and heat coupling during summer using copulas

Droughts and hot extremes constitute key sources of risk to several socio-economic activities and human lives throughout the world, and their impacts can be exacerbated by their co-occurrence. Moreover, their occurrence is expected to increase under future global warming. Therefore, understanding the drought-heatwave feedback mechanisms is crucial for estimating the risk of impacts associated with their compound occurrence.

 

Several studies have examined individual extreme events or analyzed how intense certain events where. Nevertheless, limited research has explored drought-heatwave dependence, mostly focusing on the contribution of low antecedent soil moisture or preceding precipitation deficits to summer hot extremes. In the latest efforts in assessing the interactions between hot and dry extremes, the development of models describing the joint behaviour of climate extremes is still a challenge.

 

Here we propose to assess the probability of summer extremely hot days in the Iberian Peninsula (IP) being preceded by drought events in spring and early summer, based on their joint probability distribution through copula theory. Drought events were characterized by the Standardized Precipitation Evaporation Index (SPEI) for May, June and July for different timescales (3-, 6- and 9-months). The Number of Hot Days per month (NHD) summed over July and August were considered to characterize hot extremes.

 

Asymmetrical copulas with upper tail dependence were identified for the majority of the IP’s regions (except in northwestern regions), suggesting that compound hot and dry extremes are strongly associated. Moreover, the transition from previous wet to dry regimes increases substantially the probability of exceeding summer NHD extreme values. These results are region and time-scale dependent: 1) northeastern, western and central regions were found to be the regions more prone to summer hot extremes induced by dryness; 2) southwestern, northwestern and southeastern regions are less prone.

 

This assessment could be an important tool for responsible authorities to mitigate the impacts magnified by the interactions between the different hazards.

 

Acknowledgements: This work was supported by project IMPECAF (PTDC/CTA-CLI/28902/2017). Andreia Ribeiro thanks FCT for the grant PD/BD/114481/2016.