A new study has presented for the first time a global and systematic climatological analysis of breaking waves and atmospheric moisture transport, in tandem and in isolation, for the occurrence of extreme precipitation events, which frequently cause catastrophic flooding with socioeconomic effects in many parts of the world.
Extreme precipitation events cause severe flooding that leads to fatalities and leaves socioeconomic impacts to many parts of the world every year. Different weather systems can drive such events, so it's crucial to have a detailed knowledge of the atmospheric processes behind their formation.
For the first time, a new study by Andries-Jan de Vries-- an atmospheric scientist at ETH Zürich and the Max Planck Institute for Chemistry in Mainz, Germany-- presented a global analysis that revealed two intertwined atmospheric processes behind the formation of many large-scale extreme precipitation events around the world, specifically in dry subtropical regions where such events can trigger catastrophic flooding.
For instance, severe flooding occurred in the Atacama Desert in March 2015. According to de Vries, it is precisely these dry subtropical regions, including deserts, "where these mysterious events are least expected but can cause devastating impacts."
The study highlighted the role of two atmospheric processes in driving the formation of such events: first, the breaking of Rossy waves, and second, intense atmospheric moisture transport.
Rossby waves are waves occurring in the ocean and atmosphere. In the latter, these waves determine the weather, to a large extent, in midlatitude regions. These waves can amplify and eventually break due to nonlinear processes-- the same as how ocean waves move onshore.
On the other hand, moisture transport refers to large masses of water vapor moving horizontally in the atmosphere, also known as an "atmospheric river" when it reaches several thousand kilometers in an elongated shape. This system is linked to flooding, usually along the western coasts of continents.
"When Rossby waves amplify and break, cold-air masses intrude from high latitudes into lower latitudes, and vice versa," de Vries explained.
"This atmospheric process can drive intense moisture transport, destabilize the troposphere, and force air masses to ascend, which together favor the formation of extreme precipitation. The stronger the wave breaking and the more intense the moisture transport, the larger the precipitation volumes."
De Vries analyzed extreme precipitation events around the world between 1979 and 2018, focusing on larger-scale events. He discovered that Rossby wave breaking can explain more than 90 percent of extreme precipitation events over the Mediterranean and central North America. However, over coastal zones, more than 95 percent of such events were driven by moisture transport.
"Importantly, the combined occurrence of these two atmospheric processes can explain up to 70 percent of extreme precipitation events in regions where one would expect them the least-- the dry subtropics," said de Vries. "Breaking waves that reach from the midlatitudes unusually far towards the equator can draw moisture from the humid tropics into the dry subtropics, which feeds the heavy rainfall."
Furthermore, the combined processes played a significant role in 12 historic extreme precipitation events that triggered catastrophic flooding and resulted in billion dollars in damage, thousands of fatalities, and sustained socioeconomic impacts. These floods included the 1987 Natal, South Africa flooding; 2000 Alpine floods; 2013 Uttarakhand, India flooding; 2013 Colorado floods, and 2015 Atacama Desert floods.
The results may help improve our understanding of atmospheric processes and weather systems that drive extreme precipitation events, which in turn, could help improve forecasts and the development of early warning systems. The analysis could also improve our knowledge of how such events will respond to climate change.
"A global climatological perspective on the importance of Rossby wave breaking and intense moisture transport for extreme precipitation events" - de Vries, A. J. - Weather and Climate Dynamics - DOI: 10.5194/wcd-2-129-2021
Extreme precipitation events (EPEs) frequently cause flooding with dramatic socioeconomic impacts in many parts of the world. Previous studies considered two synoptic-scale processes, Rossby wave breaking and intense moisture transport, typically in isolation, and their linkage to such EPEs in several regions. This study presents for the first time a global and systematic climatological analysis of these two synoptic-scale processes, in tandem and in isolation, for the occurrence of EPEs. To this end, we use 40-year ERA-Interim reanalysis data (1979–2018) and apply object-based identification methods for (i) daily EPEs, (ii) stratospheric potential vorticity (PV) streamers as indicators of Rossby wave breaking, and (iii) structures of high vertically integrated horizontal water vapour transport (IVT). First, the importance of these two synoptic-scale processes is demonstrated by case studies of previously documented flood events that inflicted catastrophic impacts in different parts of the world. Next, a climatological quantification shows that Rossby wave breaking is associated with >90 % of EPEs over central North America and the Mediterranean, whereas intense moisture transport is linked to >95 % of EPEs over many coastal zones, consistent with findings of atmospheric river-related studies. Combined Rossby wave breaking and intense moisture transport contributes up to 70 % of EPEs in several subtropical and extratropical regions, including (semi)arid desert regions where tropical–extratropical interactions are of key importance for (heavy) rainfall. Odds ratios of EPEs linked to the two synoptic-scale processes suggest that intense moisture transport has a stronger association with the occurrence of EPEs than Rossby wave breaking. Furthermore, the relationship between the PV and IVT characteristics and the precipitation volumes shows that the depth of the wave breaking and moisture transport intensity are intimately connected with the extreme precipitation severity. Finally, composites reveal that subtropical and extratropical EPEs, linked to Rossby wave breaking, go along with the formation of upper-level troughs and cyclogenetic processes near the surface downstream, reduced static stability beneath the upper-level forcing (only over water), and dynamical lifting ahead (over water and land). This study concludes with a concept that reconciles well-established meteorological principles with the importance of Rossby wave breaking and intense moisture transport for the formation of EPEs. Another conclusion with major implications is that different combinations of Rossby wave breaking and intense moisture transport can reflect a large range of EPE-related weather systems across climate zones and can thus form the basis for a new classification of EPE regimes. The findings of this study may contribute to an improved understanding of the atmospheric processes that lead to EPEs and may find application in climatic studies on extreme precipitation changes in a warming climate.
Featured image: Atacama Desert in Northern Chile. Credit: Manu Abad