Analyzing Large-Scale, Wintertime Extreme Warm Events using Self-Organizing Maps

Recent research has revealed that the areal extent of the Northern Hemisphere, lower-tropospheric, wintertime cold pool has been systematically shrinking over the past 66 years (Martin 2015).  That analysis found that the cold surge phase of the East Asian Winter Monsoon was almost always present in conjunction with extreme cold days, days on which the hemispheric extent of the -5°C air at 850 hPa exceeded the calendar day average by at least two standard deviations (+2s).  Notably absent from the analysis was exploration of the hemispheric circulation anomalies associated with extreme warm days (-2σ) days, despite the fact that they have been the exclusive mode of 2s anomaly in Northern Hemisphere winter since late February 1994.

The present analysis employs the self-organizing maps (SOM) methodology in order to gain new insight into the synoptic- and large-scale evolution of the hemispheric circulation anomalies associated with such extreme events.  Through identification of the 3D atmospheric structures that are most often associated with these extreme days, the analysis provides fresh insight into the variability of large-scale circulation anomalies associated with Northern Hemisphere winter in a warming world.