Mode-Decomposed Equation Diagnosis for Atmospheric Blocking Development

This paper proposed a new diagnosis to blocking development without any time filtering, different from previous studies. The mode-decomposed vorticity equation was formulated on the basis of the principal components (PCs) of 500-hPa geopotential height, by applying a new idea that any variables can be decomposed into the projection corresponding to each PC owing to its orthonormality. Sectorial blocking episodes in Northern Hemisphere winter were next identified with a Barriopedro’s way. A blocking index for each longitudinal range was defined as the linear combination of the gravest 10 PCs, by means of the composite for the blocking episodes. The blocking development was finally diagnosed, in terms of low modes of PC1-10 and high modes of PC11-50. The result suggested that the intensification of blocking over the North Pacific and Eurasia was associated with the nonlinear interaction among high modes, whereas the intensification (decay) of North Atlantic blockings was mainly related to the enhanced nonlinear interaction among low-frequency (high-frequency) eddies. This main result was insensitive to the choice of blocking definition.