Remote Influences of Atmospheric and Oceanic Variability on Heat Waves and Cold Spells in a Regional Climate Model

Heat waves and cold spells are extreme events that have significant economic and societal impacts in the United States. The occurrence of these events is determined by local atmospheric flow conditions as well as the remote influence of upstream storm activity and teleconnections excited by oceanic anomalies. In this study, we analyze observational data and regional climate simulations (using WRF atmospheric model experiments) to understand the role of remote influences of atmospheric and oceanic variability on climate extremes.

The statistical relationship between observed mean state and variability of temperature and precipitation is analyzed over the continental US domain. As expected, the precipitation variability is positively correlated with the mean precipitation. Temperature variability is not correlated with the mean temperature in the summer but shows a negative correlation in the winter.

Two sets of 20-year regional WRF integrations were carried out, one with observed sea surface temperatures (SST) and lateral boundary conditions derived from NCEP reanalysis for the period 1981-2000, and another using climatological SST and a modified lateral boundary condition containing only low-frequency variability over the oceanic regions adjoining the US. Analysis of these experiments suggests that the occurrence of short timescale extreme events like heat waves is not very sensitive to the SST or the low-frequency lateral boundary condition but the occurrence of longer timescale extremes events is.

Additional sensitivity experiments were carried out where high frequency transient eddies propagating eastward over the US from the Pacific storm track were filtered out at the western boundary of the domain. The results indicate that synoptic atmospheric variability from the Pacific can impact the mean circulation over the US, and thus modulate the occurrence of heat waves.