A Modified Persistent Anomaly Index Applied to High-Resolution Model Simulations of Present and Future Climates

Persistent weather regimes such as droughts, pluvials, and prolonged anomalous temperatures are often associated with persistent anomalies (PAs) in tropospheric geopotential height. With a changing climate, it is critically important to identify how the frequency, duration, magnitude, and other aspects of PAs may change. Current climate models are known to underrepresent PAs (e.g., Hoskins & Woolings 2015) for the present-day climate. Previous studies have shown that increased resolution, in the vertical and horizontal, and better parametrizations improve model representations of PAs.

In this study, we use high-resolution simulations from the Model for Prediction Across Scales (MPAS) to analyze changes in PAs with climate change. We analyze twenty 14-month simulations (10 current climate runs and 10 future runs) on a 0.15° latitude-longitude grid. The future climate simulations use altered sea surface temperatures and increased carbon dioxide concentrations based on the IPCC AR5 RCP 8.5 emissions scenario. To evaluate the representation of PAs within the MPAS model simulations, present-day results are compared to the European Center Interim Reanalysis dataset (ERA-Interim).

A magnitude threshold that varies with calendar day derived from the standard deviation of the model 500-hPa geopotential height anomalies is applied to the model 500-hPa geopotential height to identify anomalies that persist for at least 5-days. This technique is a modification of that of Dole & Gordon (1983), who defined a persistent anomaly using a fixed magnitude threshold, which was applicable only to winter months. Using a magnitude threshold that varies daily allows for the detection of persistent anomalies throughout the year. Unlike most blocking indices, this approach does not require the identification of a gradient reversal in either the geopotential height or potential vorticity; thus, our PAs are not synonymous with atmospheric blocks, but blocks are always associated with positive PAs.

Previous studies reached inconsistent conclusions regarding future changes in PAs. Generally, the frequency of blocking is reported to decrease with climate warming in the Pacific and Atlantic blocking sectors, but some studies identified circulation changes that would favor an increase in blocking frequency. Using our current state-of-the-art, high-resolution, global simulations we test the hypothesis that contributions to the onset, maintenance, and amplitude of PAs will increase in a warmer climate, owing to stronger diabatic heating associated with transient disturbances that initiate and sustain PAs.