Subseasonal development of past North American droughts: The role of stationary Rossby waves in a changing climate

Drought is one of the most economically expensive recurring natural disasters to affect North America. Accurate drought predictions from weeks to a season in advance require an advanced understanding of predictability of drought development on subseasonal time scales and an improvement in the representation of drought processes in model prediction systems. Here we propose to investigate mechanisms and predictability of North American drought development on subseasonal time scales by studying the role of stationary Rossby waves. Recent research by the PI and Co-Is has shown that these waves play a key role in atmospheric circulation and surface meteorological variability on subseasonal time scales and that they have been crucial in
the development of recent warm season droughts and heat waves over North America (e.g. the 1988, 1998 and 2012 summer droughts). This proposal, which builds on our prior work, will shed new light on the role of stationary Rossby waves in the development of North American droughts, the physical processes that initiate and sustain the stationary Rossby waves, and the predictability of these waves.

Our proposed work has three thrusts. First, through a comprehensive diagnosis of observations and reanalyses, we will investigate the physical mechanisms by which leading quasi-stationary subseasonal atmospheric circulation variability affected the development of past North American droughts. We will also diagnose initiation and maintenance of these circulation anomalies by determining the key regional forcing anomalies. Second, we will investigate the predictability of the stationary Rossby waves, using a set of idealized Atmospheric General Circulation Model (AGCM) experiments and a series of short-term GCM hindcasts for selected past North American droughts. The potential sources of predictability for the stationary Rossby waves will be explored by investigating the roles of subseasonal SST processes in tropics and subtropics, extratropical air-sea interaction and land feedbacks over North America in initiating and sustaining the stationary Rossby waves. Additionally, the influence of model climate drift on model hindcasts of stationary Rossby waves and our understanding of their predictability will be addressed. Third, we will investigate the modulating effects of El Niño – Southern Oscillation (ENSO) and climate variations on decadal and longer time scales (including the Pacific Decadal Variability [PDV], Atlantic Multi-decadal Variability [AMV], and the globally warming trend) on the characteristics and frequency of occurrence of the stationary Rossby waves that affect North America. This will proceed by examining the changes they exert on the optimal stationary wave forcing distribution for the leading subseasonal atmospheric circulation patterns using reanalyses, and the statistics of stationary Rossby waves in a set of idealized AGCM runs forced with Sea Surface Temperature (SST) anomaly patterns associated with these climate variations.

The proposed work directly targets the priority area “Research to Advance Understanding, Monitoring, and Prediction of Drought” solicited by the FY 2014 NOAA MAPP Program. The expected outcome of the proposed work is an improved understanding of the predictability of the development of North American droughts on subseasonal time scales. It is expected to contribute to NOAA’s long-term goal of climate adaptation and mitigation through “Improved scientific understanding of the changing climate system and its impacts”.