Baroclinic Blocking

• Published in: Geophysical research letters 2022-08, Vol.49 (15), p.n/a
• Main Authors: Martineau, P., Nakamura, H., Yamamoto, A., Kosaka, Y.
• Format: Article
• Language: English
• Subjects: Anomalies; Anticyclones; Atmospheric blocking; Atmospheric circulation; Atmospheric circulation anomalies; Baroclinic flow; Barotropic mode; Blocking; Blocking events; Climatology; Cyclones; Dynamic height; Eddies; Energy; Energy conversion; Energy resources; Energy sources; Energy transfer; Extreme weather; Feedback; Geopotential; Geopotential height; Heat flux; Heat transfer; Kinetic energy; Northern Hemisphere; Potential energy; Temperature anomalies; Temperature gradients; Vertical profiles; Vortices; Weather; Weather conditions; Westerlies; Winds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL097791

Atmospheric blocking events, which cause abnormal weather conditions, have been studied from a viewpoint of quasi‐stationary barotropic circulation systems because of their apparent lack of vertical tilt. However, this work demonstrates that blocking events often have such structure that anomalies in geopotential height and temperature are horizontally out‐of‐phase, allowing blocking anomalies to produce fluxes of heat against the climatological‐mean gradients for amplifying and maintaining blocking‐related temperature anomalies. This process, which represents the baroclinic conversion of energy from the climatological‐mean flow to the blocking‐related anomalies, is shown to be one of the leading energy sources through an analysis of blocking energetics. In winter, the contribution of the baroclinic energy conversion, especially for North Pacific and Greenland blocks, is substantially larger than the contributions from barotropic energy conversion and feedback forcing by high‐frequency eddies. In summer, the baroclinic energy conversion and feedback from high‐frequency eddies are more comparable in magnitude. Plain Language Summary Blocking events feature persistent midlatitude atmospheric circulation anomalies that block the prevailing westerly winds, causing extreme weather events. On weather charts, the associated anticyclonic (clockwise if seen from above in the Northern Hemisphere) circulation at the surface and aloft appear vertically aligned over maritime regions. This vertical structure is typically considered to be suited for exchanges of kinetic energy. However, our detailed analysis of the structure of blocking events and associated energy exchanges reveals that their subtle vertical tilts allow efficient extraction of available potential energy from the background atmospheric flow via heat fluxes directed against the climatological temperature gradient. Depending on the location, this energy source is comparable to or even greater than the forcing by migratory cyclones and anticyclones that has been proposed as the main maintenance mechanism of blocking. Our results suggest that the frequency and amplitude of blocking events may be sensitive to future changes in horizontal temperature gradients. Key Points Temperature and geopotential height anomalies in blocking events are slightly out‐of‐phase, resulting in net horizontal fluxes of heat The resultant baroclinic energy conversion is one of the leading energy sources of blocking‐related subseas