Formation Mechanism of North Pacific Blocking: Comparison between Winter and Summer

The formation mechanism of North Pacific blocking events is investigated by solving the quasi-geostrophic geopotential tendency equation applied to reanalysis data over the period of 1979 - 2018. In both winter and summer, the geopotential height tendencies at 500-hPa are dominated by vorticity fluxes with a minor contribution from heat fluxes, implying that blocking dynamics are largely barotropic in the mid-troposphere. It is further found that vorticity fluxes by high-frequency eddies contribute to raise geopotential heights upstream of developing blocking ridges, in all seasons. The cross-frequency vorticity fluxes, representing the scale interaction between high-frequency vorticity and slowly-varying background flow, largely cancel the low-frequency vorticity fluxes in winter. In contrast, blocking formation in summer is equally driven by low-frequency and cross-frequency vorticity fluxes with a negligible contribution from high-frequency eddies. This seasonal difference in blocking formation mechanism suggests that eddy-mean flow interactions over the North-Pacific sector strongly depend on the seasons and has implications for the predictability of blocking-related weather extremes over the North Pacific sector.