Recently, OUC’s research team from Frontier Science Center for Deep Ocean Multispheres and Earth System /Key Laboratory of Physical Oceanography has made significant progress in tropical climate change research. Their findings, titled “Hot season gets hotter due to rainfall delay over tropical land in a warming climate”, were published in Nature Communications. The research discovered the phenomenon of hot season gets hotter in a warming climate, and revealed the critical role of delayed tropical rainfall seasons in this process.
On an annual timescale, the tropics are primarily characterized by distinct wet and dry seasons. The hottest period of the year often occurs just prior to the onset of the rainy season; once the rainy season comes, the high temperatures are significantly alleviated. Previous studies have indicated that many tropical land monsoon regions are experiencing noticeable delays in the rainfall season due to global warming. The hypothesis that prolonged exposure to sunlight brings about intensified high temperatures was further put forward. This study, based on multi-model, multi-scenario simulations from CMIP5 and CMIP6, confirms the hypothesis, revealing that this phenomenon is particularly pronounced in the Amazon region (Figure 1). On the one hand, the delayed onset of the rainy season results in reduced cloud cover, leading to a significant increase in incoming shortwave solar radiation at the surface. On the other hand, the relative decrease in rainfall lowers soil moisture, which suppresses evaporative cooling effects and causes more energy to be released through sensible heat transport and longwave radiation, ultimately exacerbating warming. This phenomenon of hot season gets hotter is also aligned with the amplification of warming at higher percentiles of temperature.
There are two theoretical mechanisms for the delayed rainfall in tropical regions presented by previous research. The first suggests that under global warming, the effective specific humidity of the atmosphere increases, causing a slower response of the atmosphere to seasonal solar radiation. The second indicates that global warming leads to drier dry seasons and lower soil moisture levels before the onset of the rainy season. As a result, it takes more time for the atmosphere to accumulate energy. This study further investigated these two mechanisms by conducting sensitivity experiments that maintain constant soil moisture, in order to clarify how the two mechanisms contribute to the delay in tropical rainfall seasons. The findings revealed that the phenomenon of rainfall delay remains significantly pronounced under conditions of global warming, even when soil moisture is held constant, albeit with the intensity reduced by approximately half. This indicates that the increase in effective specific humidity is a triggering mechanism for the delay, while soil moisture plays a significant role in amplifying this delay through positive feedback. Finally, based on multiple observational datasets, the study suggests that over the past four decades, the hot season gets hotter phenomenon has emerged in the Amazon and southern Africa regions, with a significant rise in the risk of compound events involving drought and extreme heat. This will pose new challenges to the habitability of tropical climates.
