What is this jet stream block over Greenland?

Answer 1

Quick background: Air pressure in the mid-levels of the atmosphere (about 3.5 miles up) is greater near the Equator and lower near the poles because of the differential heating from the sun. This pressure gradient induces wind as air tries to even out the pressure, moving from high pressure to low pressure - at least that's its intention.

The Earth's rotation spins the air so that it flows keeping lower pressure to its left and higher pressure to its right. In the simplest of situations, with low pressure over the poles and high pressure around the Equator in the mid-atmosphere, you end up with a broad band of wind encircling the (for me) northern hemisphere flowing in a west to east direction.

But things are never simple. The areas of high and low pressure on either side of the wind band move around and interact, inducing waves in this circular wind band - called the "jet stream" where the band is narrow and the wind strong. In these waves, the stream is diverted northward and/or southward from its general west-to-east orientation, making the jet stream look something like a sine wave. These waves drive surface weather systems and their evolution. They also progress generally eastward in the direction of the jet stream, just like if you help one end of a rope and moved it up then down sharply - you see a wave you created moving down the rope away from you. Same thing with waves in the jet stream or atmospheric circulations in general, though of course the real world isn't that simple.

Sometimes, like now, things get really screwy. Pressure systems and jet stream waves interact in ways that constantly morph the jet stream circulation, that's the planet for you. But sometimes those interactions result in an area of high pressure (in the mid-level atmosphere) up toward the poles where the lowest pressures would be in the most simplified case. As the jet tools along in the simple way west-to-east with lower pressure poleward, or to its left...suddenly it starts entering an environment where HIGHER pressure is to its left. This pretty much stops the jet stream in its tracks, weakening and diverting it in odd patterns, and sometimes even creating a closed circulation around the high pressure area where to its south air will flow east-to-west for a bit.

Such a northern latitude high pressure area in the middle atmosphere is called an atmospheric "block" because it disrupts the typical (more or less) west-to-east hemispheric circulation. The block not only interrupts the air movement through the jet stream, but also the eastward progression of the waves in the jet stream. Whatever there is of a jet stream will be diverted well south of the block, and whatever waves there are in the jet stream also are suppressed to the south.

This pattern with an atmospheric block over the Davis Strait keeps the jet stream well south of its typical position and prevents waves in the jet stream from riding northward, leading to cold and wet weather over the central and east U.S. This is the pattern that most often brings big snows to the middle and southern Atlantic states in the cold season = waves in a southward-suppressed jet stream are prevented from surging northward with warmer air AND from moving storm systems away very quickly.

it should be noted that this is most relevant during the colder half of the year. Jet streams are farther north and weaker during the warm half of the year; you can get odd-looking configurations pretty easily, but the flow is typically weak and diffuse in the warmer seasons, and almost always, whatever jet stream there is will be farther north and weaker. So these odd configurations aren't disrupting as strong a hemispheric circulation during the summer as opposed to the winter.