National Weather Service Weather Forecast Office

The North Atlantic Oscillation (NAO) was discovered in the 1920s by Sir Gilbert Walker. Similar to the El Niño phenomenon in the Pacific Ocean, the NAO is one of the most important drivers of climate fluctuations in the North Atlantic and surrounding areas.

The NAO phase can greatly affect the temperatures across the eastern half of the United States.  This includes the Upper Mississippi River Valley.  When the NAO is in its positive phase, low pressure is located across Iceland and high pressure is located across the Azores.  In this phase, temperatures across the Upper Mississippi River Valley average above normal.  Meanwhile, when the NAO is in its negative phase, the high and low pressure systems are reversed and the Upper Mississippi River Valley experiences below normal temperatures. The images below from NOAA's Earth System Research Laboratory show what the typical surface pressure and temperature anomaly pattern looks like in each phase.

The Madden-Julian Oscillation (MJO) is a tropical disturbance that propagates eastward around the global tropics.  It typically originates over the Indian Ocean and then propagates around the world, taking anywhere from 30 to 60 days to do so.  As this occurs, it can affect temperatures and precipitation from the tropics into the mid latitudes.  For example, when the MJO is over Indonesia, the temperatures in the Upper Mississippi River Valley typically range from near to below normal. Meanwhile, when the oscillation is located over the equatorial western Pacific basin, the temperatures in the Upper Mississippi River Valley typically range from near to above normal. The temperature then returns to near to below normal as the oscillation moves into the central equatorial Pacific. With its time scale, it mainly affects the weather on a sub seasonal time scale and it is not useful in determining the average temperature for a 3-month season such as winter.  However, it can have a higher impact on periodic shifts in regional temperatures and circulations patterns during the entire season.