Cold Winter Weather: The Omega Block in the Jet Stream


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An omega block

This winter’s omega block. Image courtesy of NOAA.

Different sources have blamed the cold winter in the midwest and eastern United States on the polar vortex, sunspots, and climate change.

All may have some part to play, but it’s a simple jet stream pattern that resembles the Greek letter omega that is the best explanation.

Four Jet Stream Patterns

The jet stream is a fast-moving river of air in the middle latitudes that circles the globe from west to east at an altitude of between 10,000 and 40,000 feet. It can take up a position along the U.S.-Canada border (high latitude zonal flow); or over the southwestern states, the northern Gulf of Mexico, and Florida (low latitude zonal flow).

Much of the time, the jet stream takes a middle path. Normally waves travel along the jet stream and produce weather at the ground: fair weather under the crests (ridges) and foul weather under the dips (troughs).

But there is one more jet stream pattern  that stops up the works and keeps the same weather in place for an extended period of time: the omega block.

The Omega Ω Block Needn’t be Greek to Anyone

When one of the ridges in the jet stream becomes so pronounced that it takes the shape of the Greek letter omega, (Ω) the overall pattern comes to a standstill. The ripples in the jet stream cannot penetrate the block, and everybody experiences much the same weather for an extended period of time. This winter’s pattern has featured an omega block in the eastern Pacific, sometimes encroaching on the Pacific coast of the U.S. The result has been warm and dry weather in the west under the ridge, exacerbating the California drought, and frigid temperatures over the eastern United States under the influence of an arctic flow from the northwest.

Omega Blocks Are Not Uncommon

From time to time the atmosphere seems to want to rest; an omega block is a convenient way for things to be in a steady state for a while. But the block this winter has had unusual persistence. Several times, computer forecasts have insisted (and beleaguered Midwesterners have hoped) that the omega block would break down. But wave after wave has been swallowed up by the block and the pattern persists.

Climate, Weather, and Cellular Blocking Patterns: Is This an Important Event in the Grand Scheme of Things?

Climatology is a tricky business, and extrapolating from a single event to a generality is fraught with peril. However, professional meteorologists are beginning to coalesce around the idea that weather patterns in the mid-latitudes are trending towards more cellular blocking patterns (the technical name for an omega block).

This conclusion is tentative because no one has suggested a mechanism that would cause it. But extended heat waves seem to be more common, and patterns such as the one that is causing the frigid conditions this winter persist for a longer time. There seems to be no clear pattern: A couple of winters ago, the midwest experienced record warmth for an extended period of time.

What Does This Mean for the Future?

There is no question that temperatures are rising worldwide. The Nobel prize-winning Intergovernmental Panel on Climate Change (IPCC) has put the probability that this temperature increase is manmade at 95%. The warming will have effects and there will be feedback loops. So far all we can say is that there are more omega blocks, but we don’t know where they will occur. It’s something worth watching.

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