Unlike the well-known mid latitude regions, tropical regions typically have dry and wet seasons throughout the year, corresponding to drought, rainy season, or winter and summer. Individual areas near the equator experience two rainy and dry seasons a year. Research has shown that in the context of global warming, tropical regions generally show a trend of shortening the rainy season and prolonging the dry season.
Recently, Professor Hu Shujuan's team from the College of Atmospheric Sciences of Lanzhou University and researcher Guan Yuping from the State Key Laboratory of Tropical Marine Environment of the South China Sea Institute of Oceanography, Chinese Academy of Sciences jointly revealed the new characteristics of the transformation and change of rainy and dry seasons in tropical areas under the background of global warming, pointing out that the area with twice a year rainy and dry seasons is expanding at the rate of 31000 square kilometers every decade. The relevant research results are published in Environmental Research Letters. Guo Jinyuan, a postgraduate, is the lead author, and Hu Shujuan and Guan Yuping are the corresponding author.
Using historical data since 1935 and the model products of the 6th International Coupled Model Comparison Program (CMIP6), the study analyzed the long-term changes of rain and dry season frequency and time in tropical areas. The results show that during 1935-2014, the frequency changes of rain and dry season in tropical areas were mainly concentrated in the three latitudes near the equator, the Tropic of Cancer and the Tropic of Cancer (figure). Some areas have changed from an annual rain and dry season to a double yearly rain and dry season. In some areas, the change has resulted in precipitation being concentrated in one rainy season, thereby increasing the likelihood of flooding disasters. For example, the severe floods in Pakistan since June 2022 have caused significant casualties and social property losses. With the changes in the number of rainy and dry seasons, the area with two rainy and dry seasons per year gradually moves northward and shows an expanding trend, with an expansion rate of approximately 31000 square kilometers per decade (as shown in the figure below), further exacerbating the scope and extent of the impact of drought and flood disasters.
The changes in the length of rain and dry seasons not only lead to drought and flood disasters, but also affect agricultural production, environmental ecology, and pose a threat to human survival. The frequency changes of rain and dry seasons are even more so. This study introduces changes in the number of dry and wet seasons into the evaluation framework of tropical dry and wet season changes, providing a new perspective for accurately predicting future climate changes in tropical regions during rainy and dry seasons.
Figure (top) Compared with 1935-1954, the spatial distribution of rain and dry season frequency changes in tropical areas during 1995-2014;
(Lower) Spatial distribution of frequency changes in rain and dry seasons during 2075-2094 (CMIP6SSP5-8.5) compared with 1995-2014
This study was supported by the National Natural Science Foundation (Nos. 41975076,41830538), the National Key Research and Development Program (Nos. 2019 YFA0607104) and the Key Talent Project (Nos. GML2019ZD0306) of Guangdong Provincial Laboratory of Marine Science and Engineering (Guangzhou).