How Will Atmospheric Persistent Anomalies Change in a Warming Climate?

Extratropical persistent anomalies (PAs) are extreme dynamical events that can be associated with societally impactful weather; they are often associated with blocking, though the terms are not synonymous. How will these events change in a warming climate? Specifically, will factors such as increased atmospheric water vapor and drastic reductions in autumn Arctic sea ice lead to changes in PA frequency, duration, and intensity? Here, we introduce a modified PA definition and apply it to historical reanalyses and to high-resolution future projections, with emphasis on the Northern Hemisphere. Our variable-threshold index allows study of PAs in all seasons and reveals a summer maximum in the frequency of PAs in the Arctic. We find no strong trends in PA activity over the 38-year period of the ERA-Interim reanalysis (1979-2017).

To investigate future changes in PA activity, we first use the Model for Prediction Across Scales (MPAS) to simulate 10 years of present-day conditions spanning a range of ENSO states. These simulations are run on a 15-km grid mesh over the Northern Hemisphere, relaxing to a 60-km mesh in the Southern Hemisphere, and feature high-resolution analyzed sea-surface temperatures (SSTs). We next run a corresponding set of 10 years with future conditions for comparison. The future SSTs have the same spatial pattern and variability as for the 10 present-day runs, but include a GCM-derived “delta” added to account for warming. Sea ice is updated daily and is identical in each present-day and in each future simulation. From late summer through the autumn, very little Arctic sea ice is present in the future simulations.

The present-day simulations replicate observed PA activity over the North Pacific Ocean and Asia, but underestimate PA activity in the North Atlantic. Future simulations, representing warmed conditions (RCP 8.5), do not indicate significant changes in Northern Hemisphere PA activity. The absence of a change in PA activity extends through the autumn, when future reductions in sea-ice cover are most extreme.