PUBLISHED SEPTEMBER 29, 2023

. . .how does the temperature of the surface of the tropical Pacific, thousands of miles away, change rain, snow, and temperature patterns over North America? The short answer: by shifting the location and strength of the jet stream.

The jet stream, a fast, narrow current of air flowing from west to east high up in the atmosphere, is the mechanism connecting ENSO to North American winter climate patterns. Although the jet itself is high up and can reach speeds over 200 mph, the lower edges can extend all the way to the surface, at much lower speeds. There are lots of jets in the atmosphere, but on the ENSO Blog, we often talk about the Pacific subtropical jet stream, which owes its existence to tropical heating and the rotation of the Earth.

The sun heats the tropical land and oceans, causing lots of warm, moist air to rise. As that air gets high up in the atmosphere, it has to go somewhere, so it flows north and south. Eventually, it cools and sinks back to near the surface around 30 ° latitude in both hemispheres. From there, it flows back toward the equator. This loop is called the Hadley circulation.

The subtropical jet forms near the northern edge of the Hadley cell, high up in the atmosphere. Let’s just talk about the Northern Hemisphere for the moment, for simplicity. Because the Earth is rotating from west to east, the path of a moving air parcel bends to the right (this Coriolis force is also why hurricanes and other storms rotate counterclockwise in the Northern Hemisphere). Around 30° north and south of the equator, the Coriolis force is strong enough bend the air flow so it’s west-east. So, the air traveling northward in the Hadley cell gradually turns toward the east, speeding up as it goes. It speeds up for the same reason a spinning figure skater speeds up when they pull in their arms—the air is closer to the axis of rotation and must accelerate to maintain the same relative speed. Voila, the jet stream!

Of course, this is Earth, so nothing ends up being quite as simple as all that—the jet certainly never flows in a straight west-to-east line, and it is stronger and weaker in various areas—but that’s the big picture of why the subtropical jet stream exists. (See footnote.)

The jet stream is a dividing line between cold northern air and warmer tropical air, and a track for storms. It also transports a lot of air around, so if the jet changes position, regions of higher and lower air pressure also shift. So, if you move the average position of the jet, you move what regions end up warmer or cooler than average, and where the rain and snow end up falling. El Niño and La Niña, through their impact on the jet stream, thus impact temperature and rain/snow patterns.