First of all, we show that the mass-weighted isentropic zonal mean has a strong extratropical direct (ETD) circulation in the troposphere. The lower half of the ETD circulation is regarded as the zonally integrated polar cold air outbreaks, where the threshold for the cold air mass is suggested to be about θ=280K.
The geographical distributions of cold air outbreaks are of great concern. In particular, the isentropic threshold makes it possible to quantitatively analyze the Lagrangian nature of polar cold air mass, such as the genesis, transport and loss. polar cold air mass generated over the Eurasian continents and Arctic seaice, is transferred equatorward around East Asia and North America, and disappears around the storm tracks over the North Pacific and North Atlantic Oceans.
The cold air outbreaks are important processes for mutual tropical-extratropical interactions. Actually, cold air outbreaks occur intermittently with a pulse width of several days under particular synoptic situations. Rossby wave propagations from the tropics sometimes make favorable conditions for cold air out breaks in the extratropics, and cold air outbreaks initiate active convections in the tropics.
The polar cold air mass amount below an isentropic surface is a good indicator of the polar warming. Its long-term analysis reveals a steadily decreasing trend, and relatively shorter interannual variability. The latter is mainly caused by the ENSO. The mechanism will be presented in the seminar.
Arranged date for the seminar talk: Sep 17, 2018
Place: BCCR lecture rom 4020, Jahnebakken 5