Charge and Discharge of Polar Cold Air Mass in Northern Hemispheric Winter

In the Northern Hemisphere winter, large amount of polar cold air mass is accumulated in high-latitude regions and intermittently outflows to the mid-latitudes. This study shows that the variability of polar cold air mass amount below potential temperature of 280 K and north of 45N can be understood with a concept of charge and discharge, where discharge indicates intermittent occurrence of cold air outbreak. The isentropic threshold enables us to quantitatively estimate the polar cold air mass amount and the zonal-mean two-box model proposed by Kanno et al. (2015) enable us to directly present the conceptual charge and discharge theory.

Lagged correlation/regression analyses against normalized daily discharge (equatorward cold air mass flux across 45N) reveal the relationship between cold air outbreak and the day-to-day variability of polar cold air mass amount. The polar cold air mass amount north of 45N gradually charges up with diabatic cooling before cold air outbreak, and then dramatically discharges due to cold air outbreak with a pulse width of about five days. Cold air outbreaks tend to bring colder winter in East Asia and the east coast of North America, while warmer winter prevails on the northern side of these regions. After cold air outbreak, polar cold air mass amount north of 45N recovers to the normal level for about 20 days. On the other hand, mid-latitude cold air mass amount south of 45N temporarily increases as results of cold air outbreak, but returns to the normal level in a few days.

Interannual variability of polar cold air mass amount is also explained with the charge and discharge theory. The interannual variability of polar cold air mass amount north of 45N has statistically significant negative correlation with January mean discharge, whereas the mid-latitude cold air mass amount south of 45N has statistically significant positive correlation. It indicates that strong cold air outbreak years tend to have less cold air mass amount at high latitudes and more at mid-latitudes.