What’s measured: sea surface temperatures
Timescale: 50–80 years from peak to peak

Sea surface temperatures across the North Atlantic Ocean (between 0° and 70°N) reflect a multidecadal pattern of spikes and dips, with above-average values prevalent since about 1995. The long-term warming trend of the last century has been removed from this dataset, The long-term warming trend of the last century has been removed from this dataset, compiled by the NOAA Earth System Research Laboratory. (Illustration by Marsupilami Rosentod, courtesy Wikimedia Commons.)

What’s measured: winds, driven by atmospheric temperature contrasts
Timescale: a few weeks to many months

The Arctic Oscillation varies between a positive phase (left) and negative phase (right), with the negative phase more prone to outbreaks of snow and cold air across eastern North America and Europe. (Illustration courtesy John Wallace, University of Washington, and the National Snow and Ice Data Center.)

What’s measured: atmospheric pressure and sea surface temperatures
Cycle: 2–6 years from peak to peak

(Illustration courtesy IRI - International Research Institute for Climate and Society.)

This schematic shows typical conditions across the tropical Pacific during El Niño (top) and La Niña (bottom) for the northern-winter months of December through February. Depicted are sea surface temperatures (colder waters in blue, warmer waters in orange), the predominant upper-level winds, the most favored locations for showers and thunderstorms, and the location of the thermocline separating cold, deep waters from warmer surface waters. (Illustration courtesy NOAA Climate Prediction Center.)

What’s measured: winds, sea surface temperatures
Timescale: 2 or more years from peak to peak

The IOD modulates trade winds and the location of relatively warm water across the tropical Indian Ocean, in much the same way as El Niño and La Niña do across the tropical Pacific Ocean. It also influences India’s summer monsoon, making it stronger during the positive phase and weaker during the negative phase. (Illustration by E. Paul Oberlander, © Woods Hole Oceanographic Institution.)

What’s measured: sea surface temperatures
Timescale: 40–60 years from peak to peak

This graphic portrays typical conditions during the positive (warm) and negative (cool) phases of the PDO. Sea surface temperatures appear in color as anomalies, or amounts above or below normal, in degrees Celsius (center index). Also shown are typical departures of surface wind from their average values. (Image courtesy University of Washington.)

What’s measured: winds, driven by atmospheric temperature contrasts
Timescale: a few weeks to many months

The Southern Annular Mode has steadily trended positive in recent decades. Computer models indicate this trend is related to ozone depletion above Antarctica and increases in greenhouse gases. (Image courtesy Jianping Li, China Institute for Atmospheric Physics.)

What’s measured: atmospheric pressure
Timescale: a few weeks to many months

The NAO’s positive and negative modes shape weather across much of the Northern Hemisphere. The NAO can switch from positive to negative phase or vice versa in a matter of weeks, but one phase or the other often predominates in a given year, sometimes across several years. (Illustration courtesy Climate Change Research Center, University of New Hampshire.)