An image from the NOAA National Weather Service illustrates an omega block. You can see the high pressure in the central part of the image with the low pressure on either side. The jet stream line traces a shape similar to the Greek letter omega. Photo: Weather.gov

The “Omega Block” – Torrential Rains Linked to Extreme Jet Stream Pattern

, senior climate scientist | June 3, 2016, 1:23 pm EDT
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As I make calls and send messages to friends who are in the path of torrential rains and dangerous flooding these past days, I know that even if they turn out to be out of harm’s way, many others are not. Flood protection along rivers and around bridge supports are straining as extreme precipitation falls on watersheds and over-runs cities and communities along these rivers.

In order to better protect lives and avoid economic disruptions during future events, we need to be vigilant and keep asking questions about what factors contribute to current and past disasters.

Science keeps improving the understanding of factors changing the weather and climate risks. Science keeps chipping away at the social and economic dimensions of historic decisions combined with current forces that contribute to exposure. Science keeps exploring the changing health risks and other vulnerabilities associated with some to a greater degree than others within the same exposed community. When these factors combine ferociously in a negative way, a disaster occurs.

The implications of amplified change in the Arctic

Today my thoughts turn to one important sliver of scientific understanding among the multiple factors contributing to the flooding these past days. This emerging science relates to the amplified change in the Arctic.

This change is so strong that it is what I refer to as the “Arctic tail that wags the global climate.”

It is quite clear how the shrinking Arctic glaciers contribute directly to global sea level rise. Yet the jury is still out on the evolving science of a wavier jet stream and potential links with mid-latitude extreme weather. For example, the conditions set up these past few days are what many refer to as an “omega block” pattern in the jet stream with the dots connected below:

  • A massive high-pressure ridge of the jet stream from east of Greenland to Scandinavia
  • Strong southward troughs of low pressure on either side to form the Greek letter omega shape
  • Low pressure that remained relative stationary for longer than normal, bringing rains to Europe
  • Moisture drawn from the Mediterranean that kept feeding the storm system

Europe is not the only region suffering from an unusually persistent low pressure system. An upper-level low has been “parked” over west Texas with catastrophic rains fed in part from the warm tropical air from the Gulf of Mexico and the Caribbean.

The evolving science of a wavier jet stream and potential links with mid-latitude extreme weather is worthy of support for further research. The potential links with arctic amplification are worthy of further investigation as well.  May 2016 shattered records for lowest Arctic sea ice extent.

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