Oceanic origins for wintertime Euro-Atlantic blocking

Abstract. Although conventionally attributed to dry dynamics, increasing evidence points to a key role of moist dynamics in the formation and maintenance of blocking events. The source of moisture crucial for these processes, however, remains elusive. In this study, we identify the moisture sources responsible for latent heating associated with the wintertime Euro-Atlantic blocking events detected over 31 years (1979–2010). To this end, we track atmospheric particles backward in time from the blocking centres for a period of 10 days, using an offline Lagrangian dispersion model applied to an atmospheric reanalysis data. The analysis reveals that 36–55 % of particles gain a massive amount of heat and moisture from the ocean over the course of 10 days. Via large-scale ascent, these moist particles transport low potential vorticity (PV) air of low-altitude, low-latitude origins to the upper troposphere where the amplitude of blocking is the most prominent, consistent with the previous studies. PV of these moist particles remains significantly lower compared to their dry counterparts throughout the course of 10 days, preferentially constituting blocking cores. Further analysis reveals that approximately two-thirds of the moist particles source their moisture locally from the Atlantic, while the remaining one-third from the Pacific. The Gulf Stream and Kuroshio and their extensions, as well as the eastern Pacific northeast of Hawaii, not only provide heat and moisture to the particles but also act as springboards for their large-scale, cross-isentropic ascent. While the particles of the Atlantic origin swiftly ascend just before their arrival at the blocking, those of the Pacific origin ascend additional few days earlier, after which they carry low PV in the same manner as dry particles. Thus, our study reveals that what may appear to be a blocking maintenance mechanism governed by dry dynamics alone can, in fact, be of moist origin.