Unusual Atmospheric-River-like structures coming from Africa induce extreme precipitation over western Mediterranean Sea

Long filaments of high integrated water vapor transport (IVT) content, widely known as atmospheric rivers (ARs), play a relevant role in the water cycle being also associated with many extreme flooding events worldwide. In this work, we inspect whether similar structures can be found over the western Mediterranean. The methodology used here to detect these AR-like structures is based on standard ARs detection methods but imposing a strong IVT advection westward component. The ERA5 global reanalysis and Spain02 high resolution gridded observational dataset are used, covering the period 1979-2017, to analyze the composites of mesoscale features and associated impacts on rainfall.

Results show that AR-like structures over the Mediterranean (abbreviated here Med-ARs) have relatively low incidence with an approximately once-per-year frequency. Nevertheless, these rare events are usually associated with extreme precipitation, often amplified by orographic features, contributing to more than 40% to the annual precipitation in some cases (Lorente-Plazas et al., 2020). During a typical Med‐AR, the value of IVT increases significantly due to high horizontal winds and water vapor contents. Med-ARs are always associated to the placement of a cutoff cyclone with the cold core over northwestern Africa and warmer air mass over northern Europe. The vertical structure of Med-ARs suggests an occluded front with a low-level jet in the warmer front where Med‐ARs reside and, moisture penetrating into high atmospheric levels where cold and warm front intersect leading to severe convection. To sum up, long filaments of IVT can be found over the western Mediterranean Sea, traveling in an east-west direction, playing a relevant role in hydrometeorological impacts. Although these structures share some features with ARs over the Pacific/Atlantic Ocean they present so many specific characteristics that can be also considered to constitute a variant of this well-established meteorological phenomenon.

 

Acknowledgments

The author would like to acknowledge the financial support by Fundação para a Ciência e Tecnologia (FCT) through project UIDB/50019/2020 – IDL. A. M. Ramos was supported by the Scientific Employment Stimulus 2017 from FCT (CEECIND/00027/2017).

 

References

Lorente-Plazas, R., Montavez, J. P., Ramos, A. M., Jerez, S., Trigo, R. M., & Jimenez-Guerrero, P. (2019). Unusual Atmospheric-River-like structures coming from Africa induce extreme precipitation over western Mediterranean Sea. Journal of Geophysical Research: Atmospheres, 124. doi: 10.1029/2019JD031280