Preprints
https://doi.org/10.5194/acp-2020-1138
https://doi.org/10.5194/acp-2020-1138

  15 Mar 2021

15 Mar 2021

Review status: this preprint is currently under review for the journal ACP.

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

Sudip Chakraborty1, Jonathon H. Jiang1, Hui Su1, and Rong Fu2 Sudip Chakraborty et al.
  • 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 2Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles

Abstract. 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.

Sudip Chakraborty et al.

Status: open (until 10 May 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2020-1138', Anonymous Referee #2, 10 Apr 2021 reply
  • RC2: 'Comment on acp-2020-1138', Anonymous Referee #1, 12 Apr 2021 reply

Sudip Chakraborty et al.

Sudip Chakraborty et al.

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Short summary
Boreal autumn is the main wet season over the Congo basin. Thus, changes in its onset date have a significant impact on the rainforest. This study provides compelling evidence that the cooling effect of aerosols modifies the timing and strength of the Southern African Easterly Jet that is central to the boreal Autumn wet season over the Congo rainforest. A higher boreal summer aerosol concentration is positively correlated with the boreal autumn wet season onset timing.
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