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Over the span of days or weeks, the strength of surface air pressure over the North Atlantic seesaws between Iceland and the Azores Islands. The shifting pressure reflects changes in atmospheric circulation that have a big impact on mid-latitude weather in the U.S. and Europe. 

The Arctic Oscillation (AO) refers to an atmospheric circulation pattern over the mid-to-high latitudes of the Northern Hemisphere. The most obvious reflection of the phase of this oscillation is the north-to-south location of the storm-steering, mid-latitude jet stream.

...but it's still a seasonal forecaster's best friend.

Guest blogger Dennis Hartmann makes the case that warm waters in the western tropical Pacific—part of the North Pacific Mode climate pattern—are behind the weird U.S. winter weather of the past two seasons.

Several times a year the MJO contributes to various extreme events in the United States, including Arctic air outbreaks during winter across the central and eastern portions of the country.

A first look at how we evaluate seasonal forecasts. How well do our eyes do?

How do changes in the equatorial Pacific Ocean impact places much farther away? The answer for the tropics, at least, lies in changes to the equator-wide atmospheric circulation called the Walker Circulation. 

Sea surface temperatures are up. So why haven't forecasters declared El Niño conditions? 

One of ENSO’s most important influences is to the Indian Monsoon—the large-scale circulation pattern that brings the Indian subcontinent the vast majority of its yearly rainfall. And while La Niñas tend to increase monsoon rainfall, the monsoon’s relationship with El Niño can be a little more complicated.  

How does El Niño affect U.S. winter temperature and precipitation?

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