Diurnal vagaries over the Amazon in CESM

Poster PDF

Description

GCMs routinely underestimate rainy season precipitation over the Amazon, especially over the northeast coast of Brazil. GCMs also have problems correctly simulating the timing of convection and its growth from shallow to deep over much of the Amazon. While there are many diurnal processes at play over the Amazon that may contribute to these model biases, this poster will focus on two: a nocturnal low-level jet and the timing of coastal convection. A nocturnal Amazonian low-level jet (ALLJ) was recently diagnosed using reanalysis data, corroborated by GoAmazon2014/5 observations. Based on coincident satellite tracking, the ALLJ appears to be linked to convective organization across the Amazon, starting at the coast and reaching well inland. This poster will introduce the observational basis of the ALLJ and use the CESM1.2.2 to assess the jet’s impact on Amazonian rainfall. It turns out that the coupled version simulates the ALLJ and its diurnal cycle realistically, while the uncoupled version (CAM5) does not – the reason lies in the land/ocean temperature differences in each model. We thus use the coupled CESM1.2.2 to investigate local and remote convective impacts of the nocturnal jet. We ran two sensitivity experiments by adding bottom-heavy latent heating at noon and midnight along the northeast coast of Brazil during MAM (the rainy season over the Amazon) for 2.5 hours to mimic the occurrence of shallow convection. When low-level heating is added at midday (representing realistic shallow convection associated with the coastal sea breeze), 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 convective growth and rainfall across the Amazon basin during the ensuing evening hours and priming convective growth the following day. When low-level heating is added during the night, convection doesn’t deepen as much 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. 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 Pacific and precipitation over the Indian Ocean.