Mass Footprints of the North Pacific Atmospheric Blocking Highs

Abstract

he mass footprints associated with atmospheric blocks over the North Pacific are evaluated by constructing daily tendencies of total mass over the blocking domain from three-dimensional mass fluxes throughout the life cycle of a composite blocking event. The results highlight the major role of mass convergence driven by low-frequency (with periods >1 week) atmospheric disturbances during both the development and decay stage of a block. Specifically, low-frequency eddies are responsible for the accelerated mass buildup 4 days prior to the peak intensity of a block, and they also account for the rapid mass loss afterward. High-frequency, subweekly scale disturbances have statistically significant positive contributions to the mass loss during the decay stage, and also show weak negative contributions to the development of the blocking high prior to the peak of the high. The majority of the mass convergence (divergence) responsible for the intensification (decay) of the blocking high occurs in the middle-to-lower troposphere and is largely attributed to mass flux driven by low-frequency meridional (zonal) winds. Also discussed are the implications of this new mass perspective of atmospheric blocks for understanding dynamics of blocking highs and for model bias detection and attribution.