Abstract
AbstractBlocking events are persistent weather systems that strongly impact daily weather and more importantly our societies. One reason behind their strong impact is their potential long duration, as blocking events can last from 5 days up to four‐five weeks. However, the mechanisms explaining this difference of duration have not been properly studied yet. Here, we investigate the differences between short blocks, which last 5 days, and long blocks, which last at least 10 days. We take a broad hemispheric and annual approach to this question, while recognizing that other specific factors may play a role in particular region and seasons. We show that long blocks often involve cyclonic Rossby wave breaking, while short blocks are equally associated with cyclonic and anticyclonic wave breaking. This main result is reproduced in a coupled climate model ensemble. The lower number of long anticyclonic blocks might be due to three main reasons: One/the anticyclone is reinforced on the downstream side during anticyclonic blocks which is less conducive to persistence; two/positive synoptic eddy feedback tends to force the mean zonal wind toward a more northward position during anticyclonic blocks, whereas it forces the mean zonal wind to the south of the block during cyclonic blocks, which has been previously shown to be associated with more persistent weather patterns; three/particularly sustained eddy feedback is needed to maintain long anticyclonic blocks.
Highlights
Blocks strongly impact regional weather due to their persistence
We show that long blocks often involve cyclonic Rossby wave breaking, while short blocks are associated with cyclonic and anticyclonic wave breaking
The lower number of long anticyclonic blocks might be due to three main reasons: One/the anticyclone is reinforced on the downstream side during anticyclonic blocks which is less conducive to persistence; two/positive synoptic eddy feedback tends to force the mean zonal wind toward a more northward position during anticyclonic blocks, whereas it forces the mean zonal wind to the south of the block during cyclonic blocks, which has been previously shown to be associated with more persistent weather patterns; three/ sustained eddy feedback is needed to maintain long anticyclonic blocks
Summary
Blocks strongly impact regional weather due to their persistence (up to 4–5 weeks) They correspond to a large-scale meridional reversal of the atmospheric circulation that blocks the westerly flow (Rex, 1950). No unique theory describing the blocking life cycle has emerged yet This is partly due to the nature of blocking event itself, as the term “blocking” covers a wide range of weather patterns: From the clear anticyclonic or cyclonic large-scale Rossby wave breaking (Davini et al, 2012; Masato et al, 2012; Rex, 1950) to the amplified ridge showing weaker reversal (Altenhoff et al, 2008; Drouard & Woollings, 2018; Sumner, 1954). Most blocks result from a large-scale Rossby wave breaking, which corresponds to a large-scale and irreversible overturning of the geopotential contours on a pressure level It results in the mixing of a low-latitude/high-geopotential air mass moving north and a high-latitude/low-geopotential air mass moving south.
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