Examining the Forecast Skill of the Synoptic-Scale Flow Associated with Arctic Cyclones

Arctic cyclones are synoptic-scale low pressure systems that may originate over the Arctic or move into the Arctic from lower latitudes. Arctic cyclones may be associated with the poleward advection of warm, moist air, contributing to reductions in Arctic sea-ice extent and alterations of the synoptic-scale flow over the Arctic. Forecast skill of the synoptic-scale flow over the Arctic may be low at times relative to climatology. It is anticipated that relatively low forecast skill of the synoptic-scale flow over the Arctic may be attributed in part to forecast error growth accompanying alterations of the synoptic-scale flow induced by Arctic cyclones. The purpose of this study is to better understand how forecast skill of the synoptic-scale flow over the Arctic varies as a function of the number, location, and intensity of Arctic cyclones, and as a function of the associated synoptic-scale flow patterns of Arctic cyclones.

Forecast skill of the synoptic-scale flow over the Arctic is evaluated by calculating a standardized anomaly of anomaly correlation coefficient (ACC) and an area-averaged standardized anomaly of ensemble spread for day-5 forecasts of 500-hPa geopotential height over the Arctic during 2007–2017 using the 11-member NOAA GEFS reforecast dataset version 2 and the 51-member ECMWF ensemble prediction system. Periods characterized by exceptionally low and high values of standardized anomaly of ACC, and exceptionally high and low values of area-averaged standardized anomaly of ensemble spread in both datasets are referred to as low and high Arctic forecast skill periods, respectively. Arctic cyclones occurring during low and high Arctic forecast skill periods are identified using an ERA-Interim Arctic cyclone climatology. Variations in the magnitude of standardized anomaly of ACC and in the location, magnitude, and spatial extent of standardized anomalies of ensemble spread as a function of the number, location, and intensity of Arctic cyclones, and as a function of the associated synoptic-scale flow patterns of Arctic cyclones, are examined for the low and high Arctic forecast skill periods.