On the Role of Aerosol Radiative Effect in the Wet Season Onset Timing over the Congo Rainforest during Boreal Autumn

The boreal summer dry season length is reported to have been increasing in the last three decades over the Congo rainforest, which is the second-largest rainforest in the world. In some years, the wet season in boreal autumn starts early while in others it arrives late. The mechanism behind such a change in wet season onset date has not been investigated yet. Using multi-satellite datasets, we discover that the variation of aerosols in dry season plays a major role in determining the subsequent wet season onset. Dry season aerosol optical depth (AOD) influences the strength of the southern African easterly jet (AEJ-S) and thus the onset of the wet season. Higher AOD associated with a higher dust mass flux reduces the net downward shortwave radiation and decreases the surface temperature over the Congo rainforest region, leading to a stronger meridional temperature gradient between the rainforest and the Kalahari Desert as early as in June. The latter, in turn, strengthens the AEJ-S, sets in an early and a stronger easterly flow, leads to a stronger equatorward convergence and an early onset of the wet season in late August to early September. The mean AOD in the dry season over the region is strongly correlated (r =0.7) with the timing of the subsequent wet season onset. Conversely, in low AOD years, the onset of the wet season over the Congo basin is delayed to mid-October.

据观测，过去三十年间，全球第二大热带雨林——刚果盆地热带雨林所在区域的北半球夏季旱季时长呈持续增长趋势。该区域北半球秋季雨季的起始时间存在年际波动：部分年份雨季启动偏早，部分年份则推迟到来，而这种雨季起始日期变化背后的物理机制尚未得到深入研究。本研究依托多卫星数据集展开分析，发现旱季气溶胶的变化是决定后续雨季起始时间的关键因子。旱季气溶胶光学厚度（aerosol optical depth, AOD）会通过影响南非东风急流（southern African easterly jet, AEJ-S）的强度，进而调控雨季的起始时间。当气溶胶光学厚度更高时，伴随而来的粉尘质量通量也更大，这会削弱刚果雨林区域的净向下短波辐射，降低地表温度，进而早在6月便加剧了该雨林与卡拉哈里沙漠之间的经向温度梯度。经向温度梯度的增强又会进一步强化南非东风急流，催生更早且更强的东风气流，带来更显著的向赤道辐合，最终使雨季在8月末至9月初提前到来。该区域旱季平均气溶胶光学厚度与后续雨季的起始时间呈现极强的相关性（相关系数r=0.7）。反之，在气溶胶光学厚度偏低的年份，刚果盆地的雨季起始时间会被推迟至10月中旬。