Examining the Influence of East Asian Winter Monsoon Cold Surges on the Areal Extent of the Lower Tropospheric, Northern Hemisphere Wintertime Cold Pool

A recent analysis of wintertime lower tropospheric temperatures from reanalysis data has revealed a systematic shrinking of the areal extent of air colder than -5°C at 850 hPa over the Northern Hemisphere (Martin 2015).  A component of that analysis considered the relationship between the time series of the hemispheric extent of the cold pool and the 850 hPa temperature at all grid points in the Northern Hemisphere.  Throughout the winter (DJF), these correlations were largest in eastern China suggesting a relationship exists between cold surges of the East Asian Winter Monsoon (EAWM) and the hemispheric extent of the cold air.

In order to further investigate this hypothesized connection, the present analysis considers aspects of the time series of 850 hPa temperature in a 20° x 20° box (25°N, 102.5°E to 45°N, 122.5°E) over 65 winter seasons (NDJFM, 1948/49 – 2012/13) using the NCEP-NCAR Reanalysis data.  In particular, we constructed a time series of the daily perturbation 850 hPa temperature over the box by summing the daily departures at each grid point.  Separate standard deviations for colder and warmer than normal days were calculated allowing identification of 2s cold and warm days.  Composite differences in hemispheric circulation and 850 hPa temperature distribution between these extreme events are examined to reveal characteristic structures and synoptic evolutions associated with each.  The tendency for extreme warm events to last longer than extreme cold events is investigated in the context of these circulation differences.