Teleconnections and Oscillations

Teleconnection: A recurring large-scale air pressure and circulation pattern that extends over a vast geographical area. (The El Niño/Southern Oscillation, or ENSO, is an example of a teleconnection.)

Oscillation: An air pressure pattern that changes back and forth so that each phase produces a unique, predictable pattern.

Madden Julian Oscillation (MJO): A 40- to 60-day period of alternately strong or weak trade winds that normally blow west. It is named after Roland Madden and Paul Julian, two scientists from the National Center for Atmospheric Research who in 1971 were studying wind patterns in the tropical Pacific. For unknown reasons, these tropical winds sometimes weaken, and the Sun-warmed pulse of ocean water that they usually cause to drift west drifts east. As this pulse of warm water, called a Kelvin wave, moves east-from the coast of Africa across the Indian and Pacific Oceans-it carries changed air patterns above it.

When the wave crashes into South America, the water stops, but the air pattern continues over land northeastward, into the Caribbean atmosphere and across the Atlantic Ocean. Before this cycle is complete, another pulse has already started in the Indian Ocean. Scientists are still studying the MJO. Most agree, however, that when the MJO cycle speeds up and warm Kelvin waves pile up in the Pacific Ocean, we have the start of an El Niño.

The MJO Effect

The MJO usually brings flooding rains (also known as the "Pineapple Express") to the Pacific Northwest and California. In an active hurricane season on the East Coast, it can mean several hurricanes within a few weeks, followed by a long spell with no hurricanes.

The Tropical Atlantic Variability (TAV): This is often called "the El Niño of the Atlantic." Like El Niño, the TAV is associated with trade winds. Unlike El Niño, which travels east, the TAV oscillation runs north and south. Depending on the strength of the southeast trade winds, it alternately warms the ocean water south of the equator, then north, then south again.

When the Atlantic sea-surface temperatures near the equator fluctuate, precipitation patterns change throughout the Atlantic Ocean. Like most oscillations, the TAV is affected by other global weather patterns.

The TAV Effect

When the TAV causes the water north of the equator to be unusually warm in summer, the warmed water acts as a channel for tropical storms and hurricanes striking the East Coast, especially in the Southeast. Much less is known about the effects of the TAV during winter.

The Pacific/North American pattern (PNA): This is a large teleconnection that dominates weather from Shanghai, China, to Atlanta, Georgia, every month except June and July. The air masses over the warm waters of Hawaii and the cool waters of Alaska's Aleutian Islands start a circulation pattern that sweeps east into North America. There the pattern interacts with a high ridge of air over the northern Rockies and a deep trough over the southern states.

When the difference in air pressure between two designated points within an atmospheric oscillation is great, conditions are described as "positive." When the difference is low, conditions are "negative."

As the PNA turns negative, the airflow becomes more directly west to east.

The PNA Effect

A positive PNA carries tropical moisture into British Columbia, and the United States ends up with cold and stormy weather in the Midwest and Southeast; cold in the East and warmth in the West; and tornadoes-and even snowstorms-along the Gulf Coast, as blasts of polar air meet warm, moist air in the South.

A negative PNA carries diminishing amounts of moisture eastward from California, resulting in cold and snowy weather in the West; rain and snow in the plains; and warm and relatively dry weather in the East.

The Pacific Decadal Oscillation (PDO): The PDO is a change in ocean currents that lasts for 20 to 30 years. It affects not only water temperatures but also air currents overhead. A warm PDO means a warm central Pacific Ocean with cool water in the west, north, and south. During the cool phase, the central waters are cool and the peripheral waters are warm. During the last three decades of the 20th century, we experienced a warm phase.

The PDO Effect

A warm PDO may result in more El Niños, more precipitation, and fewer typhoons. A cool PDO results in cool air and less precipitation in the western United States.

The North Atlantic Oscillation (NAO): The NAO affects the eastern United States. This teleconnection sits between the cool atmospheric low over Iceland and the warm high over the Azores.

The NAO Effect

In winter, a positive NAO causes Atlantic storms to veer north, and warm, wet winters to occur in the eastern United States and Europe. Northeastern Canada tends to be unusually cold. In summer, a positive NAO means fewer tropical storms and hurricanes striking the East Coast.

In winter, a negative NAO forces cold air into the eastern United States, and causes heavy rains in southern Europe. Northeastern Canada is generally warmer, and areas around the Great Lakes tend to have a cooler, wetter spring. Studies of a negative NAO in summer have proved inconclusive.