Speaker: Kathy Pegion, Associate Professor, School of Meteorology, University of Oklahoma
Most global land regions have little to no significant average subseasonal skill for precipitation. However, parts of South America, particularly Brazil, have modest but significant skill in most subseasonal prediction models. The skill indicates that predictability exists and the source of the skill represents a source predictability. Pegion investigates where this skill is coming from to better understand subseasonal precipitation predictability in South America.
Using subseasonal re-forecasts from the NCAR-CESM2 model, Pegion demonstrates that significant skill persists even when interannual variability is removed. The highest skill occurs during austral summer and spring. To isolate sources of predictability, Pegion analyzes a novel set of re-forecast experiments initialized with climatological atmosphere, land and ocean states. Results indicate that atmospheric initial conditions are essential for achieving skill, while ocean initializations contribute to skill containing interannual variability and land initializations contribute minimally to skill.
Canonical correlation analysis is used to identify the most skillful spatial patterns and associated time series during the high-skill seasons. The most skillful patterns in December–February contain additional skill beyond the South American dipole – the dominant pattern of precipitation variability. This additional skill is not associated with propagating tropical convection or sea surface temperature anomalies indicating that the Madden–Julian Oscillation (MJO), El Niño–Southern Oscillation (ENSO), or South Atlantic SST variability are not the sources of predictability.
Instead, a wave-train-like structure extends across the South Pacific, resembling a Rossby wave response. Idealized experiments using a simplified atmospheric general circulation model show that this pattern can be reproduced by stationary tropical heating over the Maritime Continent, suggesting a dynamical link between tropical heating and South American precipitation variability on subseasonal timescales independent of ENSO and the MJO.
This seminar can also be viewed via our live stream.
Hosted by the Climate, People and the Environment Program (CPEP).