AGU 2015: Jennifer Francis recent studies on Weather Blocking Patterns Recent Changes in Blocking Characteristics Assessed Using Self-Organizing Maps […]

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Climate State

Date Posted:

December 19, 2015
Recent Changes in Blocking Characteristics Assessed Using Self-Organizing Maps
Speaker: Jennifer Ann Francis

Blocking anticyclones are known to be associated with persistent weather patterns that often lead to extreme weather events. An outstanding question, however, is whether the frequency and/or intensity of these dynamical features are changing in response to human-caused climate change, and in particular, to a disproportionately warming Arctic.

In this presentation we describe a study using a pattern-recognition/clustering tool called Self-Organizing Maps (SOMs) to investigate the temporal behavior of blocks over recent decades, and attribute any changes to either frequency shifts in characteristic atmospheric patterns or to cluster-mean changes in a blocking characteristic for a given pattern. In this application, we use single contours of 500-hPa heights from reanalyses to identify characteristic ridge/trough patterns in the upper-level flow in the northern hemisphere.

By mapping daily assessments of blocking occurrence and intensity to the SOM-derived patterns, we investigate temporal and regional changes in blocking. We find that the relative frequency of blocking days – defined as the number of blocked days in a particular pattern relative to the total number of days in that pattern – has increased significantly ( larger than 95% confidence) in all regions: 60% of the patterns in the Atlantic, 80% in the Pacific, and 30% over continents. While the increases over oceans occur in all seasons, the higher occurrence of blocks over continents is confined to the warm season.

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Blocking intensity, however, has generally decreased, as expected with a weaker poleward temperature gradient. Blocking frequency and Arctic amplification are positively correlated over the Pacific and continental sectors, implying that as the Arctic continues to warm faster than mid-latitudes, the number of days with blocks should continue to increase. Better understanding the mechanisms for changes in blocks and other high-amplitude jet-stream patterns in a warming world will enhance predictability and help society prepare for unusual and extreme weather ahead.

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One Comment

  1. […] has a nice overview. Dr Francis spoke on this (again) in 2015: Abstract for the above is available here. She’s using self-organizing maps, a machine learning technique, to help identify blocking […]

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