Climate Change: The Co-occurrence of Droughts and Heatwaves in the Pearl River Basin
Increases in air temperature and changes in precipitation regimes caused by global warming lead to considerable variations in heatwaves and droughts, two of the most costly natural hazards for society. Droughts and heatwaves are mainly caused by large-scale abnormal weather systems. Meanwhile, the regional land-atmosphere interactions between droughts and heatwaves can enhance each other. Such co-occurrences of droughts and heatwaves are self-perpetuating and often associated with record-breaking drought-heatwave events.
The project aimed to investigate the changes, interactions, and impacts of the co-occurrence of droughts and heatwaves across the Pearl River Basin under the changing climate. The objectives of the study were three-fold: (1) to characterise the spatio-temporal changes in the duration, intensity and joint return period of the co-occurrence during the 21st century; (2) to analyse the sensitivity of land-atmosphere interactions to the development and evolution of the co-occurrence events under the changing climate, and; (3) to evaluate the impact of the co-occurrence, and humidity, on society.
Our results show that soil moisture, an important factor in droughts and the key to land-atmosphere interactions, decreased in the past half-century. The warming climate played a crucial role in soil moisture drying. Global climate models project continuously decreasing soil moisture over the 21st century under the Representative Concentration Pathway (RCP)2.6, RCP4.5, RCP6.0, and RCP8.5 scenarios.
Our numerical experiments based on the high-resolution Weather and Research Forecasting model show that the soil moisture deficit of the drought amplifies the heatwave during a drought-heatwave co-occurrence event via soil moisture-temperature coupling. The latent cooling is suppressed due to the low soil moisture, resulting in higher sensible heat which then warms the troposphere. The drought also self-amplifies through soil moisture-precipitation coupling as precipitation decreases in simulations with low soil moisture. The heatwave also affects soil moisture variations by changing land surface fluxes and precipitation fields. The increases in temperature lead to larger latent heat which further reduces soil moisture. The precipitation fields also tend to be more extreme under a hotter atmosphere.
Apparent temperature, the human-perceived equivalent temperature, is affected by changes in air temperature, humidity, and wind speed. With consideration to the changes in humidity and wind speed, heatwaves based on apparent temperature increase faster than those based on air temperature under global warming. The faster increases in apparent temperature imply that heatwaves will have a more significant impact on society when compared with previous studies that were based on air temperature only, e.g. labour capacity, temperature-related discomfort, stress, stroke, morbidity, and even mortality. Effective climate change mitigation efforts to achieve low-emission scenarios can considerably alleviate the faster increase in apparent temperature. As a result, the research outcomes of this project can help provide scientific knowledge to water management and public health agencies, enabling them to formulate and implement adaptation measures for climate change.
Co-occurrence of droughts and heat waves under the changing climate in the Pearl River Basin
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