On the relationship between North Atlantic baroclinic growth rate regimes and surface cyclogenesis

Baroclinic instability is the fundamental theory explaining midlatitude weather systems, particularly cyclogenesis, as discussed by Hoskins and Valdes in their 1990 work on storm-tracks. They included measurements of dry baroclinic growth rates in their analysis, though they theorized that moist baroclinic growth rates (σ_m) might provide a more appropriate measure. We incorporate this metric to examine the relationship between large areal extent of high σ_m and surface cyclogenesis in the North Atlantic. This areal extent is calculated over the North Atlantic basin (25-60N and 0-80W) and over the vertical depth of 850-600hPa, using the National Centers for Environmental Prediction Reanalysis 1 for 1950-2014. The time series consists of standardized anomalies, obtained from removing a single, smoothed climatology spanning the entire 65-year period from the smoothed daily mean values.

Concentrating solely on the cold season months (DJFM), we find 103 events with standardized anomalies greater than two. Most events last one day, with a maximum length of 10 days. Cyclone data are provided by the National Snow and Ice Data Center for the years 1958-2008. Each of the extreme DJFM σm areal coverage events within this time period are lag-correlated with both cyclone numbers and cyclone deepening rates in the basin on each day. The DJFM events lasting longer than 3 consecutive days (n=8) are characterized by above average occurrences of explosive cyclogenesis near the end of the events. For the duration of each event the number of surface cyclones within the basin was fewer than average, with the extant cyclones deeper than average and anomalously high frequencies of explosive cyclogeneses.