ALBERT

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Yamamoto, A., Nonaka, M., Martineau, P., Yamazaki, A., Kwon, Y.-O., Nakamura, H., & Taguchi, B. Oceanic moisture sources contributing to wintertime Euro-Atlantic blocking. Weather and Climate Dynamics, 2(3), (2021): 819–840, https://doi.org/10.5194/wcd-2-819-2021.

Description: 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 d using an offline Lagrangian dispersion model applied to atmospheric reanalysis data. The analysis reveals that 28 %–55 % of particles gain heat and moisture from the ocean over the course of 10 d, with higher percentages for the lower altitudes from which particles are released. Via large-scale ascent, these moist particles transport low-potential-vorticity (PV) air of low-altitude, low-latitude origins into the upper troposphere, where the amplitude of blocking is the most prominent, in agreement with previous studies. The PV of these moist particles remains significantly lower compared to their dry counterparts throughout the course of 10 d, 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 of moist particles source it from the Pacific. There is also a small fraction of moist particles that take up moisture from both the Pacific and Atlantic basins, which undergo a large-scale uplift over the Atlantic using moisture picked up over both basins. The Gulf Stream and Kuroshio and their extensions as well as the eastern Pacific northeast of Hawaii not only provide heat and moisture to moist particles but also act as “springboards” for their large-scale, cross-isentropic ascent, where its extent strongly depends on the humidity content at the time of the ascent. While the particles of Atlantic origin swiftly ascend just before their arrival at blocking, those of Pacific origin begin their ascent a few days earlier, after which they carry low-PV air in the upper troposphere while undergoing radiative cooling just as dry particles. A previous study identified a blocking maintenance mechanism, whereby low-PV air is selectively absorbed into blocking systems to prolong blocking lifetime. As they used an isentropic trajectory analysis, this mechanism was regarded as a dry process. We found that these moist particles that are fuelled over the Pacific can also act to maintain blocks in the same manner, revealing that what appears to be a blocking maintenance mechanism governed by dry dynamics alone can, in fact, be of moist origin.

Description: This research has been supported by the Japan Society for the Promotion of Science (JSPS) (grant nos. JP19H05701, JP19H05702, JP19H05703, JP19H05704 on Innovative Areas 6102, and JP20H01970), the Japan Science and Technology Agency through Belmont Forum CRA “InterDec” and COI-NEXT (grant no. JPMJPF2013), the Japanese Ministry of Environment through the Environment Research and Technology Development Fund (grant no. JPMEERF20192004), the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) through the Arctic Challenge for Sustainability (ArCS and ARCS II; grant nos. JPMXD1300000000 and JPMXD1420318865) programmes, the Japanese Ministry of Environment through the Environment Research and Technology Development Fund (grant no. 2-1904), the US NSF Climate & Large Scale Dynamics Program (grant no. AGS-2040073), and the US DOE CESD Regional and Gloabal Model Analysis programme (grant no. DE-SC0019492). Patrick Martineau was partly supported by Grant-in-Aid for JSPS Research Fellows.