Key role of western boundary currents in wintertime Euro-Atlantic blocking

Ayako Yamamoto, JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, Masami Nonaka, Application Laboratory, JAMSTEC, Yokohama Kanagawa, Japan, Akira Yamazaki, JAMSTEC, Application Laboratory, Yokohama, Japan, Patrick Martineau, University of Tokyo, Tokyo, Japan, Young-Oh Kwon, Woods Hole Oceanographic Institution, Physical Oceanography Department, Woods Hole, United States, Hisashi Nakamura, University of Tokyo, Research Center for Advanced Science and Technology, Tokyo, Japan and Bunmei Taguchi, University of Toyama, Faculty of Sustainable Design, Toyama, Japan
Atmospheric blocking events play a crucial role in modulating the mid-latitude sub-seasonal atmospheric variability. The detailed physical mechanisms responsible for the formation and maintenance of blocking events, however, are yet to be fully understood, and deficiencies remain in accurately simulating their realistic occurrence in state-of-the-art climate models. While atmospheric blocking has conventionally been considered in the dry dynamical framework, recent studies attributed some of the modelled blocking frequency deficit to biases in the modelled ocean, implying the importance of the air-sea exchange of heat and moisture in the formation of the blocking events. Furthermore, a crucial role of latent heating in the blocking formation and maintenance has recently been highlighted. The source of heat and moisture associated in this process, however, is unidentified.

In this study, we aim to identify the sources of the atmospheric heat and moisture involved in wintertime Euro-Atlantic blocking events using a Lagrangian approach based on NCEP CFSR. Using an atmospheric dispersion model, we track atmospheric particles backward in time from the blocking centres located at 500 hPa, and estimate the associated turbulent heat fluxes whenever the particles fall within the planetary boundary layer over the ocean. Our results indicate that approximately half of the particles released from the blocking centres has undergone moist processes over the course of 10 days before arrival at the blocking centres. These particles influenced by moist processes acquire lower potential vorticity compared to the dry counterpart, implying the central role of the moist processes in blocking events. We found that approximately 70% of these particles receive heat and moisture locally from the Atlantic basin and about 25% of them from the Pacific, before swiftly ascending and thus undergoing latent heating. In particular, our results point towards the importance of the western boundary currents and their extensions in both basins in providing substantial amount of heat and moisture to these particles that make up of the central part of the atmospheric blocking.