On the Onset of the Rainy Season in Amazonia: WHAT the Observations Show, and Why the Biases in Climate Models?

Abstract:

The onset of the Amazon rainy season shows a large temporal and spatial variability, delays on the date of the onset will have strong impacts on local agriculture, hydroelectric power generation as well as on the hydrology of large rivers. Two “once-in-a-century” droughts occurred in 2005 and 2010, and it was shown that in those events the rainy season started later than normal, and also that on the last 10 years the dry season has increased in length by about one month. These events highlight the urgency for improving our understanding and capability to model onset of the rainy season and drought variability, for the present and future. Most studies have attributed the variability of the rainy season onset over Amazonia to the variability of the tropical oceans whether other factors, such as climate change, land use and aerosols also contribute to the variability are not clear..

Global climate models run on seasonal climate forecast mode still show large uncertainties on the prediction of onset of seasonal rains. As for climate change, the CMIP3 and CMIP5 appear to underestimate the past variability, and also project virtually no future change of the onset of rainy season over the Amazon even when they are forced by strong greenhouse forcing under the RCP8.5 emission scenario. Why these models underestimate the variability of the rainy season onset, and whether this bias implies an underestimate of sensitivity of their dry season length to anthropogenic radiative forcing remain unclear.

This FAPESP DOE grant 2013/50538 aims to explore use of the measurements provided by the Atmospheric Radiation Measurement (ARM) Mobile Facilities (AMF)-GoAmazon and the Cloud processes of the main precipitation systems in Brazil (CHUVA) Field Experiments, along with global and regional model experiments, to explore the sources of the above described uncertainty. The project will address several issues, i.e. the inadequate representation of the types of convection (i.e., maritime versus continental) and their relationships to aerosols, land surface and atmospheric circulation as represented in climate models We will present our initial results addressing the factors that control the variability of the wet season onset over Amazonia, the influence of convective types on atmospheric diabatic heating based on GoAmazon and CHUVA.