Isentropic Analyses of the Polar Cold Airmass and Its Outbreaks

Ｗｉｎｔｅｒ climate is studied from the viewpoint of large-scale dynamics by the diagnostic parameter of cold airmass amounts and their horizontal fluxes defined below a designated potential temperature. The cold airmass is localized in the extratropical lower troposphere and its hemispheric total value is conserved under adiabatic condition. The atmospheric general circulation diagnosed in the mass-weighted isentropic zonal means (MIM) has a strong extratropical tropospheric direct (ETD) circulation, which is driven by wave-zonal mean flow interactions. When we choose threshold potential temperature of 280K, the lower half of ETD circulation is composed of cold airmass. The cold airmass is generated due to diabatic cooling in high latitudes, transferred to mid-latitudes, and lost in the subtropics. The life time of cold airmass is roughly estimated at 24 days in NH and 16 days in SH.

Geographically, NH winter climate has two major cold airmass streams; one is the East Asian stream and another is the North American Stream (Iwasaki et al, 2014). Both streams have exits in the active regions of cold surges. The cold airmass accumulated in high latitudes outbreaks intermittently under favorable synoptic conditions, and brings serious cold waves in mid-latitudes (Shoji, et al., 2014).

The El Ni･ño significantly reduces the hemispheric total cold airmass, particularly over the northern North America. The enhanced Aleutian low effectively draws cold airmass from the northern North America, transfers it anticlockwise and releases it to the central North Pacific. The reduction of cold airmass results in anomalously warm surface air temperature in the northern North America (Abdillah et al., 2018).

East Asian cold air outbreaks are also significantly under control of tropical convections through wave-propagations, both in inter-annual (Abdillah, et al., 2017) and intra-seasonal time scales (Abdillah, et al., 2018).

The hemispheric total amount of cold airmass is a good indicator of global warming. In NH, its decreasing trend during recent 30 years is estimated at about 1%/decade (Kanno et al., 2016). Interestingly, the decreasing trend becomes greater with lower threshold potential temperature. This reflect the polar amplification of climate change (Kanno, et al., 2019).

References

Abdillah et al., J. Climate, 30, 2989-3007 (2017)

Abdillah et al., J. Climate, 31, 473-490 (2018)

Abdillah et al., GRL, 45, 5643-5652 (2018)

Iwasaki et al., JAS, 71, 2230-2243 (2014)

Kanno et al., JGR, 121, 10138-10152, (2016)

Kanno et al., ERL, 14, 025006, (2019)

Shoji et al., J climate 27, 9337-9348.