Composite Analyses of Low-Skill Arctic Cyclones during Summer

Arctic cyclones (ACs) are synoptic-scale low pressure systems that frequently form over the Arctic or move into the Arctic from lower latitudes during summer. It is anticipated that forecast error growth associated with interactions between ACs and the synoptic-scale flow over the Arctic, baroclinic processes, and latent heating may contribute to relatively low forecast skill of ACs and the synoptic-scale flow over the Arctic. The purpose of this study is to examine various features and processes governing the evolution of ACs that are characterized by low forecast skill and that occur during periods of low forecast skill of the synoptic-scale flow over the Arctic during summer.

Forecast skill of the synoptic-scale flow over the Arctic is evaluated by calculating a standardized anomaly of area-averaged root mean square error (RMSE) of day-5 forecasts of 500-hPa geopotential height over the Arctic during summer (June–August) for the 2007–2017 period in the 11-member NOAA GEFS reforecast dataset v2. Periods characterized by exceptionally high values of standardized anomaly of area-averaged RMSE are referred to as low-skill periods. ACs occurring during low-skill periods are identified using an ERA-Interim AC climatology. Forecast skill of ACs occurring during low-skill periods is evaluated by calculating the RMSE of the intensity of the ACs for forecasts in the GEFS reforecast dataset v2. The forecasts are of 120-h lead time and valid at the time of the lowest sea level pressure of the ACs when located in the Arctic during low-skill periods. ACs characterized by relatively high values of RMSE are referred to as low-skill ACs. Composite analyses are performed using the ERA5 on intense low-skill ACs in order to examine various features and processes governing the evolution of intense low-skill ACs. The composite analyses indicate that intense low-skill ACs intensify downstream of a mid-to-upper-tropospheric vortex in a region of relatively strong lower-to-midtropospheric baroclinicity, lower-to-midtropospheric ascent, tropospheric-integrated vapor transport, and upper-tropospheric divergence. The composite analyses suggest that a combination of baroclinic processes and latent heating play important roles in the intensification of intense low-skill ACs.