The intricacies of producing accurate diurnal circulations over the Amazon in CESM

A nocturnal Amazonian low-level jet (ALLJ) was recently diagnosed using reanalysis data (Anselmo et al. 2018). This work assesses the ability of coupled and uncoupled versions of the CESM1.2.2 to reproduce the jet and explores the mechanisms by which the ALLJ enhances convection throughout the Amazon basin. While the coupled CESM1.2.2 simulates the ALLJ and its diurnal cycle realistically, the uncoupled version of the model (CAM5) does not. The ALLJ is an extension of the northeast trade winds over the Atlantic Ocean and is impacted by the land/sea temperature contrast. A cold surface temperature bias exists over northeast Brazil in the land model, thus coastal winds and the ALLJ are weaker in CAM5 than observed. Because the coupled CESM1.2.2 has a cold SST bias over the equatorial North Atlantic, the cold land bias is offset and stronger onshore winds and a more realistic ALLJ are produced. We hence use the coupled CESM1.2.2 to investigate local and remote impacts of the ALLJ.

GCMs significantly underestimate MAM precipitation over the northeast coast of Brazil (Richter et al. 2014) and have problems correctly simulating the timing of convection and its growth from shallow to deep over much of the Amazon (e.g. Betts and Jakob 2002). We ran two sensitivity experiments using the coupled CESM1.2.2 by adding bottom-heavy diabatic heating at noon and midnight along the northeast coast of Brazil during MAM for 2.5 hours to mimic the occurrence of shallow convection. When low-level heating is added during the noon, convective growth and heating is elevated the following hours, matching the onset of the ALLJ. The ALLJ then transports moisture inland from the ocean, enhancing deep convection growth and rainfall across the Amazon basin during the ensuing evening hours. When low-level heating is added during the night, convection doesn't deepen and the ALLJ is already well inland, thus transporting much less moisture than in the noon case. Therefore, diurnal circulations, like low-level jets and the timing of shallow convection, matter to the organization of convection and overall rain across the Amazon and thus should be represented in GCMs. The noon case further shows remote impacts from the enhanced Amazonian rainfall including influences on the strength of the Walker circulation over the Atlantic and precipitation over the Indian Ocean.