Impact of biomass burning emission variability on precipitation over tropical oceans

Numerical simulations mostly constrain the total amount of biomass burning aerosols but rarely prescribe the realistic emission variability. Ignoring high heterogeneity of emission variability may lead to uncertainties in climate projections. Based on the Community Earth System Model version 2 Large Ensemble Community Project (CESM2-LE), we investigated the impact of interannual variability of biomass burning emissions on tropical precipitation and extremes. Our results revealed that global carbonaceous aerosol emission was 180-320 Tg over the period 1990-2020. Tropical regions (30°S-30°N) had the largest emission flux and variability. Higher interannual variability triggered increasing precipitation and extremes in tropics where spatial heterogeneity of precipitation anomalies can be detected. More precipitation and northward ITCZ shift occurred in central and western Pacific Oceans, while precipitation reduction together with southward ITCZ rain-belt over eastern Pacific and Atlantic Basins. The asymmetries were attributable to weakened Walker circulation and its uplifting branch tilted toward the Southern hemisphere. Correspondingly, nonlinear aerosol-cloud interactions increased (reduced) the total and high cloud cover over the southern central-western Pacific (eastern Pacific and Atlantic) Oceans. Convective activities were then strengthened (weakened) due to lower (higher) outgoing longwave radiation at top of atmosphere, which drove the cross-equatorial heat transport variations, and ultimately led to southward (northward) shift of ITCZ. Our results revealed the synergistic mechanisms between biomass burning emission variability, radiation and cloud characteristics, and large-scale circulation modes, thereby gaining new insights into the tropical hydrological cycle.