Are we missing future extreme events by ignoring less persistent blocking?

Extreme weather events are often linked to atmospheric blocking. While a comprehensive theory of blocking is yet to be developed, the onset mechanism of these systems poses a major challenge. As a result, the representation of blocking in climate models is inadequate and consistently underestimated. Although successive improvements in the blocking representation are evident in climate models, they still underestimate the blocking frequency in the Northern Hemisphere (NH).

In this study, we observed an improvement in the representation of blocking in the Community Earth System Model Large Ensemble 2 (LENS2) during winter and a notable deficiency during summer. When compared with observations, the winter blocking bias (-4% to +2%) was found to be substantially reduced, while the summer blocking exhibited a significant bias (-12% to +12%) compared to previous studies. Under the SSP370 scenario, LENS2 suggests an overall decline in winter blocking (11%) and an increase in summer blocking (12%) by the year 2100 in the NH.

The underlying reason for the underestimation of blocking in climate models is often associated with the mean jet state and stationary waves. However, the lack of an onset theory causes challenges in identifying blocking systems. There is also a missing explanation of why temporal persistence is considered specifically as 5 days. Thus, we define less persistent blocking (LPB) as a blocking regime that satisfies the flow reversal criterion and persists for less than 5 days. We found a significant presence of the frequency of LPB (~15% to 25% maximum) in the NH in both models and observations. It means that we were ignoring the presence of these blocking systems, which may have a role in driving extreme events and can potentially emerge as stronger and more persistent blocking systems in the future.

Interestingly, the hotspot of LPB includes drought-prone regions such as the western coast of the United States and well-known blocking centers such as the Euro-Atlantic and Pacific regions. This leads us to propose a new potential avenue for studying the precursors of LPB dissipation, which will provide insights into the longstanding problem of the blocking onset mechanism.