The Changes In Wet And Dry Seasons In Low-latitudes Under Global Warming

Unlike the four seasons in mid-latitudes,the distinct feature of seasons in low-latitudes is the transition between the dry and wet seasons which correspond to dry and rainy seasons or winter and summer.As a pivotal modulator of the phenology and human productive activities,wet and dry seasons are crucial for ecosystems,food security,and disease prevention.In the context of global warming,many studies have shown the phase delay and amplitude enhancement in the precipitation annual cycle.The amplitude enhancement increases the difference in precipitation between wet and dry seasons.The phase delay results in a delay of transition between wet and seasons.However,most previous research only focuses on the local scale,changes in wet and dry seasons in the whole tropical region remain unknown.Meanwhile,little attention has been given to regime transformation between wet and dry seasons.The regime shifts and variations in the timing of wet and dry seasons have profound implications for agriculture,ecology,and even human well-being.Therefore,in this paper,we base on the daily precipitation datasets and use cumulative rainfall anomaly to divide wet and dry seasons.Firstly,we quantify the precipitation variation trends during the dry and wet seasons.Then,we using the20CRv3 reanalysis data and the harmonic analysis method,divide the low-latitudes region into annual regime region and biannual regime region and explore the regime shifts of the seasonal regime under global warming.In addition,for regions where the seasonal regime is unchanged,we quantify the changes in onset dates,cessation dates,and durations of wet and dry seasons.What’s more,based on the output of CMIP6under historical and future simulation,we quantify the future evolution of the regime and timing of wet and dry seasons.In this study,we introduce variations of the seasonal regime into the framework of assessing the variation of wet and dry seasons to improve the examination of wet and dry seasons.The main results are as follows:(1)From 1982-2019,with increased annual precipitation at low-latitudes,there is a significant increase in the wet season precipitation.CHIRPS(GPCC)data shows that 71.1%(65.6%)of the region receives more wet season precipitation,with an increased rate of 1.483 mm/year(1.130 mm/year).With the increase in wet season precipitation,during the wet season,the number of wet days,intensity of precipitation increases,and extreme precipitation also increase.However,during the dry season,the precipitation doesn’t have a significant trend of becoming wetter or drier.For instance,based on CHIRPS(GPCC),55.2%(49.2%)of the region shows increased precipitation during the dry season at an average rate of 0.262 mm/year(0.062mm/year).There is however a clear trend of increased intensity of precipitation during the dry season,which leads to a significant increase in extreme precipitation.Even though the increased tendency in precipitation during the dry season is relatively small compared to wet season,the extreme precipitation during the dry season shows a stronger tendency to increase.(2)Changes in seasonal precipitation cause variation in the regime,onset,and duration of wet and dry seasons.Regimes of wet and dry seasons have changed from1935 to 2014,across the equator and the Tropic of Cancer and Capricorn.Some regions where there used to be one wet season have become two wet seasons per year;instead,other regions have shifted from two distinct wet seasons into one wet season per year.With seasonal regimes shifting,areas of the biannual regime have expanded at a rate of 31000 km~2/decade.Within regions where the seasonal regime is unchanged,more than half of the terrestrial areas experience a shorter and later wet season.In annual regime regions,wet seasons have shortened in 60.3%of regions by an average of 7 d,and the onset dates of wet seasons have been delayed in 64.8%of regions by an average of 6 d.Similarly,in biannual regime regions,shortened wet seasons have occurred in 83.7%of regions,with an average shortening of 8 d.(3)Further,based on CMIP6 model data,the regime and timing of wet and dry seasons are expected to continue to change by the end of the 21st century.Based on the CMIP6 projections,Regimes of wet and dry seasons will still change along the equator and near the Tropics of Cancer and Capricorn.Especially,in the SSP5-8.5scenario,the area near the equator with two wet and dry seasons a year will shift significantly northward.Near the Tropic of Cancer,there is a trend of two wet and dry seasons a year shifting southward.In areas where the seasonal regime is unchanged,the wet season will continue to be shortened and delayed in the future.At the annual regime region,the SSP5-8.5 scenario shows that the wet season will shorten in 61.1%of the region by an average of 8 days,during 2075-2094.The onset date of wet season will delay in 76.3%of the region by an average of 7 days.Even under the SSP2-4.5scenario,by the end of the 21st century,60.9%of the region will maintain a shorter wet season,and the onset date of wet season will delay in 69.8%of the region.Even under the SSP2-4.5 scenario,by the end of the 21st century,60.9%of the annual wet and dry seasons in the region are shortened,and 69.8%of the annual wet and dry seasons in the region are delayed.