Extreme Temperature Regimes in the United States: Long-term Trends and Low Frequency Mode Modulation in Observations and CMIP5 Models

During the winter season, extreme temperature regimes (ETRs), including cold air outbreaks (CAOs) and warm waves (WWs), affect regional economies and human safety over large portions of the United States and have significant impacts on energy consumption, local agriculture and human health. The primary focus of this project is to investigate the trends and low frequency mode modulation of ETRs over the continental United States during recent decades and in a global warming scenario. Previous research on this topic has found connections between ETRs and low frequency modes, while other studies have found little evidence of ETR trends even with a warming global background temperature. Because of the aforementioned impacts of ETRs, it is important to fully investigate the representation of this behavior in both observations and CMIP5 simulations of historical and future climate. A better understanding of the representation of these relationships and trends in state of the art climate models has important implications for improved forecasts of the future behavior of ETR events.

Reanalysis data from the National Centers for Environmental Prediction – National Centers for Atmospheric Resarch (NNR) will be used to study observed ETRs from 1949-2011, while CMIP5 model simulations (both historical simulations and future simulations with the Representative Concentration Pathways 8.5 Scenario (RCP8.5)) will be used to examine past and possible future behavior of ETRs in association with global warming. The ETR events are identified in terms of large amplitude temperature anomalies using a number of days metric. Linear correlation and regression analysis is then used to identify associations between historic and future ETRs and the seasonal mean state of the Atlantic Oscillation (AO), North Atlantic Oscillation (NAO), Pacific-North American pattern (PNA), Pacific Decadal Oscillation (PDO) and the El-Nino Southern Oscillation (ENSO).

Preliminary research found that the trends in ETRs are largely non-existent over the continental United States when the NNR data is detrended using the annual December-February mean temperature. Robust interannual variability in ETRs is also evident and statistical analysis links this behavior to several natural modes of low frequency variability.