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Arctic Oscillation, Pineapple Express, monsoonal flow, El Niño, Madden-Julian Oscillation—weather and climate patterns with strange-sounding names appear and disappear from the news. This glossary helps decode the major patterns that wax and wane, stretching for thousands of miles across the atmosphere and shaping Earth's weather and climate. Jump right to the glossary, or read on to put these weather makers in context.
Earth's major circulation patterns arise because of heating contrasts between the poles and equator, modulated by the changing seasons, and because land and water absorb and release heat at different rates. The result is an ever-changing patchwork of warmer and cooler regions. The contrasts produce fixed patterns of global atmospheric circulation as well as transitory preferred patterns that can persist for weeks, months, or even years.
The patterns are often called modes or oscillations, although they lack the symmetry of a pendulum's motion. For example, El Niño occurs every three to seven years, and may be changing as the Earth warms. It alternates with La Niña. About half the time, neutral conditions prevail, with neither El Niño nor La Niña under way.
Like the other preferred patterns, El Niño and and La Niña are shaped by the ocean’s ability to retain heat or cold longer than land and air. This lag time in the oceans, along with Earth's topographic features like mountains, ice sheets, and the locations of continents, leads to contrasts in temperature and air pressure from which the patterns arise.
Because the patterns help drive the preferred tracks of cyclones and anticyclones, they can foster variations in how wet, dry, warm, or cold a particular spot on Earth will be at any time of year. Detecting and understanding the patterns helps forecasters predict next week's weather and how an entire season may unfold.
Studying these shapers of variability also gives scientists a better handle on future climate around the globe. These patterns and how they change have considerable effects on regional temperature and precipitation. Therefore, research questions include how the frequency of these patterns changes and whether the patterns themselves will change.
At the end you'll find links to many more patterns resources from across the research community.
Links for cross-referenced terms defined within the glossary take you to that definition within this page; links beyond the glossary open a new window.
You can search for keywords with your browser's "Find" feature (CTRL+F on PCs; CMD+F on Macs) or just jump to a section alphabetically:
A B C
A hemispheric-scale circulation pattern defined by changes in the westerly winds at midlatitudes. Those winds arise from the temperature contrast between the tropics and polar regions. The annular modes see-saw between positive and negative phases in what's called a dipole pattern for weeks or months and take a circular shape (annular means ring-shaped). The two major types are the Northern Annular Mode (also called the Arctic Oscillation and strongly related to the North Atlantic Oscillation) and the Southern Annular Mode (also called the Antarctic Oscillation).
See Southern Annular Mode (SAM)
(Also called the Northern Annular Mode, or NAM). In its positive phase, sea-level pressure is relatively low in the Arctic and high in the midlatitude region to the south. This is associated with a stronger and more northerly vortex encircling the pole and fewer intrusions of cold Arctic air into midlatitudes. The negative phase, with higher-than-usual pressure in the Arctic and lower-than-usual pressure in midlatitudes, features a weaker, more variable vortex and a greater risk of Arctic outbreaks of cold air into eastern North America and Europe. The AO is closely associated with the North Atlantic Oscillation.
The AO trended toward positive values from the 1960s until the early to mid-1990s, with more variability after that point. The winters of 2009–10 and 2010–11 experienced extreme negative values. Large ups and downs on shorter timescales may be in store, but the story looks different in the long run. Based on computer-model results and physical reasoning, scientists have expected the global increase in greenhouse gases to foster a slightly positive AO trend over the coming century. However, research continues on how factors such as melting sea ice might influence the AO's future, and natural variations will remain important.
A series of quasi-periodic variations, each lasting several decades, in sea-surface temperatures (SSTs) across the North Atlantic over the last 150 years. Over the last century, SSTs have risen across this ocean, as well as globally.
Scientists disagree on the role and importance of the AMO, depending on how they distinguish North Atlantic SSTs from long-term increases in SSTs globally. One statistical way to do this is to treat long-term global warming as a steady, linear process. When that linearity is removed from the Atlantic SST record, the AMO emerges as a cycle of warm and cool periods, each lasting 20–40 years.
However, the actual warming trend in global and Atlantic SSTs has not been linear. For instance, an increase in Sun-blocking pollutants (or aerosols) from the 1940s to the 1970s apparently contributed to a drop in global air temperature and SSTs during that time. When these and other nonlinear effects are taken into account, the AMO cycle appears to be less influential. Scientists at NCAR and elsewhere are exploring new approaches to account for this complex relationship.
In the last decade or so, hurricane analysts have cited the AMO as a major factor in enhancing or suppressing the intensity of Atlantic hurricanes. However, new studies are challenging the importance and/or existence of this link. See, for example, Global Warming Surpassed Natural Cycles in Fueling 2005 Hurricane Season (NCAR news release).
Energy from the Sun puts Earth's atmosphere and oceans in motion. One reason is that solar radiation is not evenly distributed: the Sun hits the equator more directly than the poles, resulting in more heating in the tropics than at higher latitudes. In addition, the tilt on its axis that gives Earth its seasons results in a twice-yearly shift in the zone of maximum radiation: northward from December to June and southward from June to December.
(Illustration courtesy NASA/JPL. Click here or on the image to enlarge.)
Heated air rises above the tropics and flows toward the higher latitudes, where it cools, sinks, and flows back toward the tropics in an easterly (east to west) surface pattern known as the trade winds. This constant, overturning flow in the tropics is called the Hadley circulation.
Farther from the equator, Earth's rotation combines with temperature contrasts between the tropics and polar regions to create midlatitude westerlies, along with cyclones and anticyclones, that move heat from the subtropics to higher latitudes. The warm fronts seen on weather maps are the leading edge of warm air pushing poleward, while cold fronts signal the flow of cold air toward the equator.
The oceans also redistribute heat via the thermohaline circulation, sometimes referred to as the ocean conveyor belt.
An introduction to atmospheric and ocean circulation, including explanations of pressure gradients and the Coriolis effect, for example, may be found on a University of Wisconsin Web site.
D E F
A pattern with two distinct and usually opposite behaviors, such as high vs. low pressure or warm vs. cool sea-surface temperatures. These contrasting behaviors are labeled the positive and negative phases of that dipole. The Southern Oscillation is one example. For dipole patterns based on the contrast in atmospheric pressure between two locations, such as the North Atlantic Oscillation, the positive phase indicates a large difference between the high- and low-pressure regions; the negative phase indicates the difference in pressure is relatively small. For those based on temperature, the positive phase indicates warming, the negative phase cooling.
The most dominant pattern responsible for interannual, or year-to-year, climate variability across the globe. During an El Niño event, warmer-than-normal sea-surface temperatures occur in the central and eastern equatorial Pacific, while cooler-than-normal temperatures are observed in the western part of the tropical Pacific. In addition, convection over the equatorial Pacific tends to be farther east than the climatological average, bringing more rain to the U.S. Pacific coast and drought to Australia, among other teleconnections. El Niño is the warm (or positive) phase of the El Niño–Southern Oscillation, or ENSO; its structural opposite is La Niña, the cool (negative) phase. Trade winds are weaker than usual during El Niño and stronger than usual during La Niña.
El Niño (Spanish for "The Baby Boy" of the Christmas story) was named more than 100 years ago by Peruvian fishers who noticed the warming water off their shores around Christmas time. El Niño events recur at intervals ranging from about two to seven years and typically last from one to three years. Researchers believe El Niño behavior could change with global warming, although the exact nature of this change remains uncertain.
The acronym for the El Niño–Southern Oscillation, which refers to the changing state of the ocean and atmosphere that produces El Niño (the warm phase) and La Niña (the cool phase), as well as neutral conditions.
Changes in sea-surface temperature in the central and eastern Pacific Ocean are normally accompanied by a see-saw in atmospheric pressure called the Southern Oscillation. The oscillation is between large areas of low and high pressure on either side of the Pacific. This dipole pattern is typically measured by determining the difference in atmospheric pressure between two points: Darwin, Australia, and a weather station on the island of Tahiti. The result is the Southern Oscillation Index. When pressures at Darwin are high, they are almost always low over Tahiti (as in El Niño), and vice versa (La Niña). The pattern, which reverses every few years, alters the trade winds, which in turn causes a change in the ocean's thermocline.
G H I
Also called the Indian Ocean Zonal Mode. An interannual (year-to-year) climate pattern across the tropical Indian Ocean first identified in 1999. In the positive phase of the IOD, trade winds are stronger than usual, and cooler-than-average sea-surface temperatures are prevalent across the eastern tropical Indian Ocean, near Indonesia and Australia. To the west, near Madagascar, waters are warmer than average and convection is intensified. These patterns are reversed during the IOD’s negative phase. The IOD also affects India’s summer monsoon, making it stronger during the positive phase and weaker during the negative IOD (the concurrent phase of ENSO can complicate this relationship).
Variations of weather or climate within a season.
Variations of weather or climate from year to year.
The zone where tropical trade winds from the Northern and Southern Hemispheres converge. It is associated with upward motion and active convection and the Hadley circulation. The oceanic ITCZ is located near latitudes 5–10º north on a yearly average. Early ocean explorers named this zone the doldrums. Over land it is associated with the monsoon trough of low surface pressure; its location shifts north and south with the seasons, generally toward the hemisphere where the warm season is under way.
J K L
Several channels of strong upper-level winds that encircle the globe, generally about three to seven miles above the surface, each flowing from west to east. The term most often refers to the two polar jet streams, whose average locations are between latitudes 40° and 60° north or south. Polar jet winds may exceed 100 miles per hour (160 kilometers per hour); they steer many of the fronts and cyclones affecting midlatitude areas.
Subtropical jet streams are typically located around 30° north and south and are more persistent and stronger on average than the polar jet streams. They are strongest in winter and can exceed 150 mph (240 km/hr).
The cool phase of the El Niño–Southern Oscillation, or ENSO. During a La Niña event, cooler-than-normal sea-surface temperatures (SSTs) occur in the central and eastern equatorial Pacific and warmer-than-normal SSTS are measured in the western part of the tropical Pacific. During La Niña, convection over the western Pacific tends to be farther west than the climatological average, bringing heavier-than-usual and more persistent rains to Indonesia and northern Australia, among other teleconnections. Researchers named La Niña ("The Baby Girl" in Spanish) to indicate the opposite phase of El Niño ("The Baby Boy" of the Christmas story); an alternate name is El Viejo ("The Old Man."). Like its brother, La Niña recurs at intervals ranging from about two to seven years, with events typically lasting from one to three years. Researchers believe La Niña behavior could change with global warming, although the exact nature of this change remains uncertain.
M N O
An irregular tropical disturbance that travels eastward around the globe and has a cycle of roughly 30 to 60 days. It is associated with regional westerly winds that replace the easterly trades, along with enhanced showers and thunderstorms, particularly over regions of high sea-surface temperatures in the Indian and western tropical Pacific oceans. The MJO propagates eastward most vigorously during northern winter/spring and can influence weather beyond the tropics. In the late summer and autumn, the MJO can enhance hurricane activity in the Gulf of Mexico. The MJO is an important source of intraseasonal variability in the tropics. In the fall of 2011, a six-month field campaign headed to the Indian Ocean to study the MJO with the goal of improving long-range weather forecasts and seasonal outlooks (more on the DYNAMO campaign here,)
A measure, often depicted as a graphical plot, of the comparative strength of ENSO events. It's derived by combining several different indices that separately measure weather variables in the tropical Pacific, such as sea-surface temperature, sea-level pressure (SOI), surface winds, surface air temperature, cloudiness, precipitation, and/or other variables. (View the CIRES MEI.)
A major tropical wind system that reverses direction twice yearly, creating distinct wet and dry seasons. Monsoons are produced by the temperature contrast between land and ocean regions that builds up during a season. Seasonal temperature changes are considerable over land, but smaller over oceans, where the warming and cooling process is slowed by the ocean’s great mass and high heat capacity (the amount of energy required to change its temperature). The resulting changes in surface air pressure create a flow from the colder area toward the warmer one. Topography and internal variability of the atmosphere also play a role. Most summer monsoons, which blow from the sea toward land, bring heavy rainfall. Most winter monsoons, which blow from land toward the sea, bring the dry season and perhaps drought. Major monsoonal circulations occur in southern Asia, Australia, Africa, northern South America, and southwestern North America, among other regions.
See Siberian Express.
(Illustration courtesy IRI - International Research Institute for Climate and Society.)
Several regions across the central and eastern tropical Pacific where sea-surface temperatures are monitored to track and analyze the evolution of El Niño and La Niña.
From east to west, the key regions are:
The most important mode of climate variability affecting the North Atlantic and nearby land areas. The NAO is typically measured through variations in the normal pattern of lower atmospheric pressure over Iceland and higher pressure near the Azores and Iberian Peninsula. A positive NAO index refers to an increased difference in pressure between these two regions and thus stronger westerly winds. This corresponds to a stronger storm track across the Atlantic, winter weather that is wetter and milder than usual across northern Europe and the eastern United States, and relatively warm, dry conditions across the Mediterranean. The negative mode of the NAO—when there is less difference than usual in pressure across the two regions—features a weakened Atlantic storm track, a greater risk for Arctic outbreaks of cold air across the northeastern United States and northern Europe, and wetter-than-usual conditions in southern Europe. 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.
The NAO trended toward more positive values from the 1960s to the mid-1990s, but has since returned to more normal values. However, based on computer-model results and physical reasoning, scientists think this positive trend is related to the global increase in greenhouse gases. Moreover the models suggest that this trend is apt to continue.
See Arctic Oscillation (AO).
P Q R
A multidecadal pattern of climate variability centered across the North Pacific Ocean. During the positive (warm) phase of the PDO, sea-surface temperatures tend to be above average along the west coast of North America and in the eastern tropical Pacific; while across the central North Pacific they are cooler than average. The opposite patterns occur during the negative (cool) phase. Each phase typically persists for 20 to 30 years, with a warm phase predominating since the late 1970s. The PDO may be related to ENSO, but differs mainly because the timescale for the PDO is much longer (several decades) and because the PDO more clearly involves the extratropical Pacific and the Aleutian Low pressure system.
The North Pacific Index (NPI) is a measure of the average sea-level pressure over the North Pacific. It is strongly related to the PDO and has associated teleconnections across North America, which are labeled the Pacific/North American Pattern (PNA, see next entry).
A prominent teleconnection pattern of climate variability affecting North America, especially in winter. The PNA is shaped by the location of the polar jet stream as it flows from eastern Asia across the Pacific to North America. During the PNA's positive phase, the jet stream flows more directly across the Pacific, increasing the odds of above-average temperatures across the U.S. West Coast and western Canada. This positive PNA pattern is more likely to occur when an El Niño event is in progress, while a negative PNA pattern is more likely during La Niña (see ENSO).
An informal name for a flow of low- and mid-level moist air, driven by the subtropical jet stream, that sometimes extends from the region around Hawaii (hence "pineapple") to the west coast of the U.S. mainland. This flow pattern often forms when a dip in the jet stream coincides with atmospheric moisture associated with the Madden-Julian Oscillation. The Pineapple Express can last several days, producing very heavy rains across coastal California, Oregon, and Washington. It is slightly more likely to occur when an El Niño is in progress.
S T U
An informal term for the pattern that results when the polar jet stream arcs from northern Siberia across western Canada, then dips southward across central and eastern North America. This flow tends to strengthen the domes of bitterly cold air that form across parts of Siberia and northern Canada, and it tends to push the cold air southward, creating so-called Arctic outbreaks across the central and eastern United States and Canada. Along the northeast coast of North America, a similar pattern in which the cold air originates over Canada is called the Montreal Express.
(Also known as the Antarctic Oscillation, or AAO.) In the SAM's positive phase, sea-level pressure is low over and near Antarctica and high in the midlatitude region to the north, resulting in stronger westerly winds. This is associated with a stronger and more southerly vortex encircling the pole and fewer intrusions of Antarctic air into the southern oceans. The SAM's negative phase, with higher-than-usual pressure at high latitudes and lower-than-usual pressure in midlatitudes, features a weaker, more variable vortex and a greater risk of Antarctic outbreaks of cold air heading north.
The SAM has trended toward positive values since the 1960s. Computer models indicate this trend is related to ozone depletion above Antarctica and increases in greenhouse gases. According to most of the models, the trend toward a positive SAM is projected to continue, although some modeling shows that the gradual recovery of the ozone layer later this century could help reverse the trend.
A measure that represents the strength of the Southern Oscillation. An SOI is typically created by comparing the sea-level pressures measured at Tahiti (in the South Pacific) to those at Darwin, Australia, and listing the anomalies (the departures from average). During El Niño, pressures tend to be below normal at Tahiti and above normal at Darwin, producing a negative SOI. The opposite is true during La Niña. (View NCAR's graphs and analysis of the SOI over time.)
A strong connection between weather events in distant parts of the globe, generally associated with large-scale atmospheric waves or circulation patterns. Because the relationships between cause and effect are often complex, scientists rely on statistical techniques to identify teleconnections.
There are four major patterns whose teleconnections account for much of the weather variability around the world:
The region separating the uppermost part of the ocean, whose waters are well mixed, from the colder deep ocean, where mixing is reduced and temperature changes are much slower to occur. Roughly 90% of seawater lies below the thermocline. The thermocline varies seasonally. It is much shallower in summer, when there is strong surface heating that limits mixing. In winter, stronger winds and surface cooling encourage mixing and convection. In the Pacific warm pool in the western tropical part of that ocean, the thermocline lies at a depth of 330 feet (100 meters) or more, while in cooler upwelling regions in the tropical eastern Pacific it may occur at depths of less than 165 feet (50 meters). Find a discussion of ocean temperatures and a thermocline illustration on the Windows to the Universe site.
ENSO produces major shifts in the thermocline's height across the tropical Pacific; the thermocline rises in the western Pacific and lowers to the east during El Niño, as the trade winds relax. The opposite occurs during La Niña. Because the thermocline is so shallow in the eastern Pacific, the difference in surface water temperatures between El Niño and La Niña can be quite large.
The easterly surface winds prevailing across the tropics. The trade winds, part of the Hadley circulation, got their name at the dawn of the age of oceanic sailing, when cargo traders learned to rely upon them. Trade winds typically blow from the northeast across the northern tropics and from the southeast across the southern tropics. Trade winds pile up water toward the western end of the tropical Pacific, creating a deep thermocline and making the average sea level in the Philippines around 23 inches (60 centimeters) higher than on the south coast of Panama. The Pacific trade winds are typically weaker than average during El Niño, allowing warm water to build eastward, and stronger than average during La Niña.
V W X Y Z
The circulation associated with a surface low-pressure area. Vortices can range in size from dust devils to tornadoes to hemispheric patterns. Due to the Coriolis effect created by Earth's rotation, large vortices are cyclonic: that is, they rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Smaller-scale vortices, especially those smaller than tornadoes (including bathroom drains), may rotate either clockwise or counterclockwise.
A vast, loop-shaped circulation pattern spanning the tropical Pacific, with rising air in the western tropical Pacific, sinking air in the eastern tropical Pacific, west-to-east winds a few miles high, and east-to-west winds at the surface. Spanning almost half Earth's circumference, the Walker Circulation pushes the Pacific Ocean's trade winds from east to west, generates massive rains near Indonesia, and nourishes marine life across the equatorial Pacific and off the South American coast. Changes in the circulation, which varies in tandem with El Niño and La Niña events, can have far–reaching effects. In a 2006 study, scientists found that the Walker circulation has weakened by 3.5% since the mid-1800s, and it may weaken another 10% by 2100, likely due to human-induced climate change (UCAR news release).
Science in Sixty Seconds (NCAR's Earth Observing Laboratory)
Atlantic Multidecadal Oscillation (AMO)
Global Warming Surpassed Natural Cycles in Fueling 2005 Hurricane Season (NCAR news release)
ENSO/El Nino/La Nina
ENSO Information - Science Background, Climate Associations, Forecasts, etc. (NOAA Earth System Research Laboratory)
ENSO Background and Current Information (IRI - International Research Institute for Climate and Society)
Children of the Tropics: El Niño and La Niña (NCAR fact sheet)
Indian Ocean Dipole
About the Indian Ocean Dipole (United Nations Atlas of the Oceans)
North Atlantic Oscillation (NAO)
Current conditions and outlooks (NOAA Climate Prediction Center)
NAO: Driving Climate Across the Atlantic (American Museum of Natural History)
Includes Forecasting the Unpredictable - Interview with NCAR scientist James Hurrell, including text, video, and interactive graphics on predicting the NAO.
NAO/NAM Climate Indices (NCAR Climate Analysis Section)
North Atlantic Oscillation Page (Lamont-Doherty Earth Observatory)
About the North Atlantic Oscillation (United Nations Atlas of the Oceans)
Discovering How the North Atlantic Oscillation Drives Climate Change (NCAR Education & Outreach)
Northern Annular Mode (NAM), Arctic Oscillation (AO)
Current conditions and outlooks (NOAA Climate Prediction Center)
Pacific Decadal Oscillation
About the Pacific Decadal Oscillation (United Nations Atlas of the Oceans)
The Pacific Decadal Oscillation (University of Washington)
Pacific/North American Pattern (PNA)
Current conditions and outlooks (NOAA Climate Prediction Center)
Southern Annular Mode (SAM), Antarctic Oscillation (AAO)
Current conditions and outlooks (NOAA Climate Prediction Center)
Simulation of natural Earth system variability: CLIVAR (Climate Variability & Predictability - World Climate Research Programme)
Arctic Climatology and Meteorology (National Snow & Ice Data Center)
Climate Glossary (Bureau of Meteorology, Australia)
Climate Glossary for Meteorologists (UCAR COMET Program)
Thunderstorm Glossary (NCAR News Backgrounders)
Weather Glossary (American Meteorological Society)
Scientific adviser: Kevin Trenberth
Writers: Bob Henson, Zhenya Gallon
Last updated : February 2011
Backgrounders provide supplementary information and should not be considered comprehensive sources.
The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.