Climatological features of blocking highs from the perspective of air mass and mass transport

We investigated the spatiotemporal features of the air mass and its transport during the lifetime of the four main blocking highs (the Greenland, Atlantic-Europe, Ural and North Pacific blocking highs) observed in the northern winter using the daily European Centre for Medium-Range Weather Forecasts Re-Analysis Interim data set for the time period 1979-2016. The results showed that the positive mass anomalies corresponding to a blocking high only appear in the mid-troposphere, while negative mass anomalies persist in the lower layer during blocking events. Budget analysis based on the mass continuity equation under isentropic coordinates indicated that adiabatic mass transport is the main contributor to the build-up of blocking highs and is always convergent in the mid-troposphere before the peak day of blocking. By contrast, diabatic mass transport always moves the air mass from the mid- to lower troposphere and acts to destroy the blocking high. The meridional flow makes the strongest contribution to mass convergence in the Greenland and North Pacific blocking highs in the mid-troposphere, whereas zonal flow is the main contributor to mass convergence in the Atlantic-Europe blocking high. Both zonal and meridional flows contribute to mass convergence in the Ural blocking high, although meridional flow makes a greater contribution.