El Niño Southern Oscillation (ENSO)

Glossary of Climate-Related Terms

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Absolute Humidity

The actual amount of moisture in a given quantity of air. More specifically, it is the ratio of the mass of water vapor to the total mass of dry and water vapor in a volume of the air/water vapor mixture. Usually has units of grams per cubic meter.

Adiabatic Lapse Rate

The temperature curve a parcel of air would follow if it were raised or lowered adiabatically (no heat added or subtracted) due to the change in ambient air pressure. This can be calculated from the ideal gas law and the hydrostatic equation and is 9.76K/kilometer on Earth (dry rate).

Air-Sea Interaction

Both the ocean and the atmosphere affect each other at the oceans surface. The ocean affects the atmosphere through heat and moisture fluxes and the atmosphere affects the ocean through wind, moisture and heat fluxes. Researchers try to determine precisely what these affects are using data, theory and numerical coupled-climate models.


The ratio of a specific band of radiation reflected by a body to that incident on it. Often expressed as a percentage.

Aleutian Low

An area of time-averaged low pressure in the North Pacific centered around 180°W and 50°N. It's strongest in winter; in summer it is displaced northward and is almost nonexistent. It reflects the daily highs and lows moving through the region and is not a stationary low. It's position in a particular year can be affected by ENSO events.

Angular Momentum

The product of mass times the rotation velocity times the perpendicular distance from the axis of rotation. A rotating object will conserve its angular momentum unless a force acts to change its rotation. To conserve angular momentum, skaters and divers spin faster as their arms and legs are moved closer to the axis of rotation. As a parcel of air moves north or south, its distance from the Earth's spin axis changes; the parcel's angular velocity must change to conserve angular momentum. This means that the air must travel faster as it moves poleward.

Annual cycle

Same as seasonal cycle. The typical course of a variable throughout the year.


The difference between the value of a variable (for example temperature) at a given location and its "normal" or long term time average at that location. The anomaly may vary depending on what is used to define the mean.


The direction of rotation about high pressure center. It is clockwise in the Northern Hemisphere and counterclockwise in the Southern.


Arctic Oscillation. A "mode" of variance in the northern hemisphere that is largely annular (circular) and somewhat zonally symmetric. It represents a negative relationship in variables (e.g. air temperature) between the polar region and the region just to the south. It has a strong relationship with the NAO.

Asian Monsoon

The monsoon that occurs over Southeast Asia and India.


Autocorrelation is the correlation (usually linear-squared correlation) between members of a time series of observations, and the same values at a fixed time interval later.


Band Pass Filter

A construct (mathematical or physical) that only allows for ("passes") only frequencies between a specified high and low value.


The atmospheric state in which there are temperature gradients (and hence density gradients) on constant pressure surfaces. It can be contrasted with a "barotropic" atmosphere which has no temperature gradients on constant pressure surfaces.

Bermuda high

The semi-permanent, subtropical anticyclone located over the western North Atlantic Ocean at about 30°N.


A phenomenon, most often associated with stationary high pressure systems in the mid-latitudes of the northern hemisphere, which produces periods of abnormal weather. The normal eastward movement of depressions, troughs, anticyclones and ridges is stalled for periods ranging from a few days to as long as over a month.


The upward force that a fluid exerts on an object less dense than itself.



The flow or motion of a fluid.

Clausius-Clapeyron Equation

This equation describes the relationship between the vapor pressure and the absolute temperature of a liquid. It is expressed as:
ln (P2/P1) = -DHvap/R * (1/T2 – 1/T1)
In other words, the relationship between vapor pressure and temperature is non-linear and warm air can "hold" much more water vapor than cold air.


Canonical Correlation Analysis. Statistical technique that returns paired time-series from two different fields that are maximally correlated (subject to orthogonality constraints).

Climate Drift

In a climate mode, the unforced trend away from some initial state, with the trend not being part of normally occurring variability about a constant mean state.

Climate Lag

The delay in climate response caused by a particular forcing factor.

Climate Model

A representation of climate. Models make certain assumptions. They can be statistical (relaying on past data), numerical or theoretical.

Climate Modeling

A representation of climate processes using a theoretical mode, statistical model or numerical model. Climate models can be simple or very complex.

Climate Variability

How climate deviates from normal or "mean" conditions. The variability is w/respect to a particular time-period (say, 1950-1979) and w/respect to a particular time-scale (e.g. weekly).


A quantitative description of climate showing the characteristic values of climate variables over a region. Climate refers to the statistical collection of weather conditions over a specified period of time. Note that the climate taken over different periods of time (30 years, 1000 years) may be different.

Cold Event

A common term used which is synonymous with a La Niña event. Cold refers to below average sea surface temperatures in the tropical Eastern Pacific.

Cold Tongue

Climatological region of cold surface water that extends westward from the eastern boundary of the tropical Pacific ocean. It is due to Ekman induced upwelling by the surface easterlies along the equator. This tongue can go away during El Niño events due to the decrease or even reversal of the surface winds to westerly.


An average that is done according to a specific criteria. For example, one could produce a composite of the rainfall at a station for all years where the temperature was much above average.

Confidence Interval

A confidence interval is a range of values that has a specified probability of containing the parameter being estimated. For example, a true mean value might have a 95% probability of being between X1 and X2 where X1 and X2 are determined from sampled values: P(X1 le X leX2)=95%


A line that connects equal values of a variable on as plot (or as a verb, the process of connecting points of equal value on a plot). For example, topography plots show elevation contours.


Mass motions in a field resulting in transport and mixing of the field. More specifically, it refers to motion associated with a rising current of air.


The convergence of the flux of a quantity expresses the time rate of accretion of the quantity per unit volume. Convergent air moves towards from a point.


Generally refers to a measure of the linear relationship between two populations. In climate, it most often refers to the relationship between two time-series.

Coriolis Effect

The apparent affect on parcels of moving air of the Earth's rotation. A parcel of air that moves poleward will be moving into a region with less angular momentum (the Earth spins fastest at the equator and has no spin at the pole). The greater angular momentum of the air will cause it to move faster eastward and hence it will appear to curve to the right relative to the ground. This apparent force is call the coriolis force.

Coupled model (or coupled atmosphere-ocean model)

In the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other.


A cloud type that is dense and vertically developed and is associated with rain (particularly of a convective nature).


Any development or strengthening of cyclonic circulation in the atmosphere.


The direction of rotation about low pressure center. It is counter-clockwise in the Norther Hemisphere and clockwise in the Southern.



A coastal city in north central Australia. The sea level pressure there is used alone or in conjunction with that from Tahiti to represent the strength of the Southern Oscillation. Both Darwin and Tahiti have long periods of record.

Delayed Oscillator

A possible mechanism for ENSO. Disturbances in the eastern tropical Pacific basin drive westward Rossby Waves, which reflect at the western boundary and become eastward traveling equatorial Kelvin Waves. These eastward waves act to produce upwelling in the eastern Pacific, which cool and damp the original perturbation.

Dependent Variable

As opposed to a independent variable. For a linear regression equation y=B +B1X1+B2X2+..., the X's are the independent variables and are commonly called the or "predicated" or "regressors". Y is the independent variable and is commonly called a "predictand" or "regressand".


From a time series, removing the part of the signal that represents the long-term movement. Often, the time series obtained from a linear fit of the data is removed but there are other methods including removing trend curves or trend segments.


The divergence of the flux of a quantity expresses the time rate of depletion of the quantity per unit volume. Divergent air moves away from a point.


Interpolating the results of large spatial scale fields, generally obtained from climate model to higher resolution spatial scales. Downscaling can be done statistically or through the use of high-resolution models that use the coarser data as a boundary condition.

Dynamical Climate Models

Climate Models that use explicit physics (mechanisms). The physics can be simple or complex. Sometimes it is possible to model processes exactly; more often parameterizations are needed to describe processes that are too detailed, not well understood or too small scale for the model to resolve.



In meteorology, winds that come from the east.

Ekman Pumping

Wind stress will drive a relatively shallow upper ocean flow and transport water to the left/right and the southern/northern hemisphere in a process known as Ekman Dynamics. Horizontal variations in the wind stress results in changes in convergence and divergence of the surface flow. This forces vertical water motion and is called Ekman pumping.

EL Niño

Spanish for "The Boy". The phase of ENSO which is associated with warmer than normal sea surface temperatures (SSTs) in the eastern Pacific and warmer than normal SSTs in the west. Convection in the western Pacific tends to be further west than the climatological average. It is opposite to La Niña.

Energy balance model**

A simple class of climate models (abbreviated as EBM) that focuses on the required balance between incoming and outgoing radiation at the top of the atmosphere. EBMs are solely thermodynamic and include no dynamics or moisture processes and as such calculate only one dependent variable, the temperature. Nonetheless, the temperature fields calculated using 2-D EBMs agree well with both observations and fields calculated using more complex general circulation model.

Ensemble Forecasts

Instead of running a numerical weather model once, they can be run multiple times using slightly different initial conditions and the average of the runs used as a forecast. This method tried to correct for the fact the initial conditions cannot be entirely known and that this relatively small uncertainty can grow into large forecast errors. By repeating the runs, it is hoped the most probable future state of the atmosphere can be determined.


Originally, it referred to El Niño/ Southern Oscillation or the combined atmosphere/ocean system during an warm event. Currently, it generally refers to both the La Niña and El Niño phases of the coupled atmosphere/ocean system though sometimes it's still used as originally defined.


Empirical Orthogonal Function. A mathematical technique that attempts to rearranges a set of predictors into a new set of predictors which are orthogonal with each other and which maximizes the variance of the full set that can be explained by each predictor, in turn. The technique is useful for reducing the size of a dataset since the EOFs retained can be truncated and only those explaining large portions of the variance kept. It is also useful for studying relationships within a dataset by, for example, showing characteristic spatial patterns that describe most of the variance.


Eastern Pacific Oscillation. A teleconnection pattern representing a N/S dipole of geopotential height anomalies in the eastern North Pacific. A positive phase represents a northeastward extension of the Jet Stream towards western North America while a negative phase indicates a split flow over the eastern North Pacific.

Equatorial low (trough)

A region of lower pressure located between the subtropical highs on each side of the equator. Within this zone the trade winds from either the Southern and Northern hemisphere meet causing upward motion and large amounts of precipitation. It constitutes the equatorward, ascending portions of the Hadley mean meridional circulation cells of both hemispheres.

Equatorial Undercurrent

A subsurface eastward flow centered approximately on the equator whose core lies just beneath the base of the mixed layer in the top of the equatorial thermocline. The flow generally ranges from 100-200 m thick and 200-300 km wide and varies seasonally.


A time of the year when the sun shines directly over the equator (generally Mar 21 and Sep 21).


Region of the globe away from the Tropics; that is 20°N to the North Pole and 20°S to the South Pole.



Statistical test that examines the standard deviation of two populations. The test statistic is
where sigma is a sample variance. The statistical significance is determined from an F distribution, which is the ratio of two chi-squared distributions with degrees of freedom eta1 and eta2.


Predictions. Forecasts can be made statistically (based entirely on past data), numerically (using computer models that represent the atmosphere ocean system) or a combination of the two. The resources available, timescales of interest and quantity being forecasted will determine how a forecast is made.


Gaussian Distribution

See Normal Distribution.


General Circulation Model. A numerical climate model that tries to incorporate as much of the atmosphere and ocean processes as possible. Used for both making forecasts and for analyzing the dynamics of the atmosphere. Generally a three-dimensional time-dependent model of the atmosphere and/or ocean circulation. The solution to a set of mathematical equations governing the motions of a layer of fluid on a spherical planet is numerically approximated on a three-dimensional discrete grid of points to obtain temperatures, velocities, rainfall, pressure and any of several other dependent variables that collectively comprise the state of the climate. Often abbreviated as GCM. See Washington and Parkinson (1986).


The amount of work required to raise a unit mass to a height z from sea level.

Geopotential Height

See geopotential. The geopotential of a parcel of air divided by the global average gravity at mean sea level (9.80665).

Geostrophic Balance

In the absence of atmospheric motion, the force of gravity is exactly balanced by the vertical pressure gradient force. Or, dp/dz=rho g

Gulf Stream

An Atlantic Ocean eastern boundary current that parallels the eastern coast of North America and turns eastward towards Europe at about 50-55N. It carries warm water and has a strong influence on the climate of Eurasia.


Hadley Cell

A north-south circulation consisting of upward air motion at the ITCZ, poleward motion to both 30N and 30S, sinking motion at 30 degrees and a equatorward flow to the ITCZ. It is driven by the strong solar heating in the tropics. The circulation of the Hadley cell can be affected by ENSO events.

High Pass Filter

A construct (mathematical or physical) that only allows for ("passes") only frequencies above a specified value.


Icelandic low

A semi-permanent center of low sea-level pressure near Iceland (mainly between Iceland and southern Greenland). It is a principal center of action in the atmosphere circulation of the Northern Hemisphere. Decrease Icelandic low intensity is associated with the negative phase of the NAO.

Ideal gas law

Relates the pressure, volume and temperature of an air parcel. Specifically, pV=rhoRT.

Independent variable

As opposed to a dependent variable. For a linear regression equation y=B +B1X1+B2X2+..., the X's are the independent variables an care commonly called the or "predicated" or "regressors". Y is the "predictand" or "regressand".


The incoming solar radiation received at the Earth's surface. It is what ultimately drives the Earth's climate and weather.


Intertropical Convergence Zone. It is associated with upward motion and subsequent convection where the equatorial trades from the two hemispheres collide and is the ascending branch of the Hadley Cell. It tends to be located about 5N but it's location varies.


Referring to variations from year to year.


Referring to variations from decade to decade.


Referring to variations within a year.


A contour of constant pressure.


A contour of constant temperature.


Jet Stream

Strong winds concentrated within a narrow zone in the atmosphere. Often used in reference to the axis of maximum mid-latitude westerlies located in the high troposphere.


Kelvin Waves

Oceanic waves of a certain type. An equatorially trapped wave similar in character to coastally trapped Kelvin waves. The motion is unidirectional and parallel to the equator everywhere, and in each vertical plane parallel to the equator the motion is the same as for a nonrotating fluid. A required geostrophic balance between the east-west velocity and the north-south pressure gradient leads to solutions that decay away from either side of the equator on a length scale called the equatorial radius of deformation. These dispersionless waves propagate eastward at the same speed as they would in a nonrotating fluid, with the dispersion relation being. The magnitude of c for the first baroclinic mode for typical ocean values is around 2.8 m/s, which would take a Kelvin wave across the Pacific in about 2 months. See Gill (1982). A type of coastally trapped wave motion where the velocity normal to the coast vanishes everywhere. The wave is nondispersive and propagates parallel to the shore with the speed of shallow water gravity waves, i.e. sqrt (gH). The profile perpendicular to shore either decays or grows exponentially seaward depending on whether the wave propagates with the coast to its right or left (in the northern hemisphere). For vanishing rotation, the decay or growth scale becomes infinite and the Kelvin wave reduces to an ordinary gravity wave propagating parallel to the coast. The dynamics of a Kelvin wave are such that it is exactly a linearized shallow water gravity wave in the longshore direction and exactly geostrophic in the cross-shore direction.


La Niña

Spanish for "The Girl". The phase of ENSO which is associated with colder than normal sea surface temperatures in the eastern Pacific and warmer than normal SST's in the west. Convection in the western Pacific tends to be further west than the climatological average. It is opposite to El Niño.

Lapse Rate

Generally refers to the change in temperature of the atmosphere(ocean) with height(depth). The lapse rate is related to the stability of the atmosphere(ocean). If the lapse rate is high meaning that the temperature decreases quickly with height above the surface, then an air parcel given an initial "push" will continue to move upwards as it will be warmer and therefore less dense than the surrounding air. The atmosphere would then be referred to as "unstable". In contrast, a low lapse rate is associated with greater stability.

Latent Heat Release

As water changes from gaseous to liquid form, there is a release in heat proportion to the latent heat of vaporization of water. This process can occur over large regions in the atmosphere when moist air rises and subsequently condenses at high altitudes. The heat released in the manner can be substantial and is one of the ways that the tropics can affect midlatitudes.


Functions that have the property f(x+y) =f(x)+f(y) and f(ax) = af(x). In other words, if precipitation were a linear function of temperature, then doubling the temperature would double the precipitation.

Linear Inverse Modeling (LIM)

A type of statistical forecast which makes the assumption that the atmosphere can be statistically modeled as a linear system that is stochastically forced with white noise. This type of forecasting can produce results comparable with GCM's in certain situations.

Low Frequency

Concerned with variations of climate over long timescales of seasons and longer.

Low Latitudes

Near the equator. Generally ~30S-30N.

Low Pass Filter

A construct (mathematical or physical) that only allows for ("passes") only low frequency/long wavelength "signals".


Madden-Julian Oscillation

A tropical wave of convection that travels eastward around the Tropical oceans (20°S–20°N) with a characteristic speed of 30-70 days. The wave is associated with the passage of downward moving air associated with surface easterlies and upward moving air associated with surface westerlies. The upward moving air forces increased convection. The wave speed changes as a function of longitude as does the intensity of the perturbation with a greater affect over the Indian Ocean and western Pacific and a weaker effect over the Amazon and Congo River Basins. The MJO is the main source of intraseasonal variability in the tropics.


Arithmetic average. Mean is one possible statistical measure of the "average" of a population. Others include the median (50% of values above the median and 50% are below) and mode (the most common value). The population for which the mean is calculated must be specified.

Medium Range Forecast

A forecast made for a time period of 2-6 weeks from the time of the forecast.


In a north/south direction or parallel to lines of longitude. The meridional wind (v-wind) is the component of the wind that blows north/south. Positive values refer to winds from the south.


The atmospheric region between the stratosphere and the thermosphere (from the stratopause and the thermopause). It tends to occur from 50 to 80km above the Earth's surface and has decreasing temperatures with height.


Region of the globe between 30°S–50°S or 30°N–50°N.

Mixed Layer Depth

In oceanography, a nearly isothermal surface layer of around 40 to 150 m depth caused by wind stirring and convection.


See "Madden-Julian Oscillation".


A representation of a process. Models make certain assumptions. They can be statistical (relaying on past data), numerical or theoretical.


Seasonal winds. They are caused primarily by the greater annual variation in air temperature over large land surfaces compared to ocean surfaces though other factors like land-relief are important. They are related to the seasonal cycle of rain. Some notable monsoons are the Asian monsoon and the Arizona monsoon.


See Medium Range Forecast.



See "North Atlantic Oscillation".

Niño 1+2

Extreme Eastern Tropical Pacific SST (0–10°S, 90°W–80°W)

Niño 3

Eastern Tropical Pacific SST (5°N–5°S, 150°W–90°W)

Niño 4

Central Tropical Pacific SST (5°N–5°S) (160°E–150°W)

Niño 3.4

East Central Tropical Pacific SST (5°N–5°S) (170–120°W)

Noise, Red

Red noise is often used to refer to any linear stochastic process in which power declines monotonically with increasing frequency. More commonly it refers specifically to a first-order auto-regressive, or AR(1), process whose value at a time t depends on the value at time t-1 only
where eta is a Gaussian-distributed white-noise process, for which each value is independent of all previous values, x0 is the process mean and gamma and alpha are constant coefficients. A value in a red noise time-series is correlated with values around it. It's spectrum shows increased power at lower frequencies, Many time-series in meteorology are "red" and statistical tests must be performed to see if a particular time-series has variability that isn't just due to red noise. Noise with relatively enhanced low frequency power that results simply from serial correlation. The resulting power spectrum will have a negative slope. This is usually a good model for the noise component in a variety of climatic time series including proxy records, historical sea and air surface temperatures, and precipitation records. This type of noise can be explained in terms of the slow-response components of the climate system, such as the thermal inertia of the oceans, providing a memory that effectively integrates the forcing of such fast-response and more white noise-like components such as the weather. The produces a temporal persistence that leads to great noise energy at lower frequencies. Contrast with white noise.

Noise, White

A time series in which the values are serially independent (gamma=0 for an aR(1) process.). It's spectrum will be flat with no difference in power at any time-scale.

Noise, Climate

The inherently unpredictable portion of the climate. Researchers try to separate the (potentially) predictable part of the climate signal from the noise.


Not linear. In other words, functions that lack either of the properties: f(x+y) = f(x)+f(y) and f(ax) = af(x).

Normal Modes

A decomposition solution procedure based on the eigenvectors of the linearized dynamical equations

North Atlantic Oscillation

It is an atmospheric pattern which consists of a north-south dipole of anomalies, with one center located over Greenland and the other center of opposite sign spanning the central latitudes of the North Atlantic between 35°N and 40°N. It is most prominent in the Norther Hemisphere winter months. Most indices of its strength are calculated from surface pressure records *long) though it is evident in air temperature, winds and heights.

Numerical Prediction Models

Models that represent atmospheric and/or ocean processes mathematically. They can be simple or complex and can be used for forecasting and studying weather and climate.



The term used to describe the height of the surface in climate and weather models.


Pacific North American Pattern

A teleconnection pattern related to ENSO in which surface pressures are anomalously low over the north Pacific and the southeastern US and high over northwestern America. The pattern extends upwards in the atmosphere. Positive values of the PNA index are associated with El Niño and a negative SOI. Its cause is related to the location of tropical heat sources, which moves westward during El Niño events.

Pacific Decadal Oscillation

The time series that represents the first mode of variability of SST in the North Pacific. It has a time scale of oscillation 10-20 years and is associated with climate anomalies along the west coast of North America.


Occurring at regular intervals.

Peru Current

One of the world's western boundary currents. It is a cold ocean current and flows north along the coasts of Chile and Peru.

Potential Temperature

The temperature a dry parcel of air would have it were compressed or expanded adiabatically (no heat added/subtracted) to a standard pressure (usually 1000mb).

Pressure gradient

The change in pressure over some distance (e.g., 5 millibar/kilometer). Pressure gradients drive winds and vertical motions in the atmosphere.

Pressure, Air

The weight of the air above a point location. Usually reported in units of millibars or Pascals.


See Pacific North American Pattern.



See "Quasi-Biennial Oscillation".

Quasi-Biennial Oscillation

The name for the regular alternation of the mean zonal winds of the equatorial stratosphere between easterlies and westerlies, which occurs every 24-30 months; that is, about every 2 years.



Ocean temperature (and salinity) anomalies that form at the surface and spread throughout the deep winter mixed layer are sequestered beneath the mixed layer when it shoals in spring and are then re-entrained into the surface layer in the following fall and winter

Relative Humidity

A ratio of the amount of water vapor in a volume of air to the amount that air could hold (at the same temperature and pressure). Expressed as a percentage.

Rossby Waves

Waves which have their restoring force due to the latitudinal variation of the Coriolis parameter. For example, if air is forced northward from the equator, it will have more "spin" than the ground beneath it and hence will tend to curve anticyclonically (clockwise in the Northern Hemisphere) to conserve momentum.


Seasonal Cycle

The typical or "mean" variation of a variable throughout a year.

Seasonal Thermocline

The ocean layer below the mixed layer, which is a zone of rapid transition in temperature/salinity. It generally starts between 50-150m below the ocean's surface and is shallow in spring and summer, deep in autumn, and disappears in winter in regions other than the tropics (where winter cooling isn't strong enough to destroy the layer).


Southern Oscillation Index. A time-series that represents the strength of Southern Oscillation. Typically, it is taken as the normalized SLP at Tahiti minus Darwin though other definitions can be used. It tends to be highly correlated with time-series of ENSO though not 100%.

Southern Oscillation

The Southern Oscillation is a seesaw of atmospheric mass (pressure) between the Pacific and Indo-Australian areas. The pressure difference result in circulation changes.

Specific Humidity

The amount of water vapor in unit volume of air. It is specified as the ratio of the weight of vapor/weight of air in a volume or as in grams/kilogram.


Sea Surface Temperature.


A measure of the tendency for a parcel of air or water to move when displaced. Unstable air (water)will continue to move in the direction of the displacement. Stable air(water). will remain in place. The stability of air and the ocean is related to the temperature lapse rate. Air stability is also affected by moisture content of the parcel and the surrounding air.

Staggered Grid

In a gridded model, one (or more) variable is moved a distance of d/2 from the others. This results in the model calculating for twice the number of grid points and a near doubled computation time. However, certain processes like geostrophic adjustment are represented more accurately.

Stationary Waves

Waves (flow patterns with periodicity in time and/or space) that are fixed relative to Earth.

Storm Track

The path followed by the left of a low (of atmospheric pressure). Or, paths over which vigorous mid-latitude cyclones are most frequently observed. ENSO events can affect the position of the Strom track due to the change in the location of convection in the tropics.


The stratosphere is the layer of the earth's atmosphere that exists between approximately 15 and 50 kilometers above the earth's surface. The stratosphere is between the troposphere and the mesosphere. It is very stable with a positive temperature lapse rate. There is little mixing between the troposphere and stratosphere.


The descending motion of air. Since air is a compressible fluid, sinking air becomes compressed and therefore warmer according to the ideal gas law PV=RT.

Subtropical High

Surface zone of atmospheric high pressure located at about 35 degrees north and south latitude. These high-pressure systems produced by vertically descending air currents from the Hadley cell. They move northward and are most intense in the summer season. Over the oceans, the subsiding air warmed by compression aloft and cooled at the surface by the cold ocean currents normally found off the west coasts of the continents, forms a pronounced temperature inversion (cold air over warm), called the trade-wind inversion. This inversion blocks convection is responsible for the frequent fog on the west coast during summer as well as the generally dry air.

Subtropical Jet

An area of strong winds concentrated within a narrow region around 30°N, which is more intense exiting out of Asia. The subtropical jet over the US is intensified during El Niño springs.


Singular Value Decomposition. Statistical technique that returns paired patterns that explain the maximum cross covariance (subject to orthogonality constraints).


In meteorology, the use of meteorological data to give a snapshot of the weather at different locations at the same time. Also, the study of weather on short (hour to daily) timescales.



A group of islands in the South Pacific near 20°N and 150°W. The time series of sea level pressure there is used in conjunction with that from Darwin to form the most common definition of the Southern Oscillation Index.


A strong statistical relationship between weather in different parts of the globe. More specifically, "teleconnection pattern" refers to a recurring and persistent, large-scale pattern of pressure and circulation anomalies that spans vast geographical areas. Teleconnection patterns are also referred to as preferred modes of low-frequency variability. These patterns can exist from time periods of several weeks to years and thus can explain large portions of the variability. For example, there appears to be a teleconnection between the tropics and North America during El Niño.

Temperature Gradient

The change in temperature over a distance (e.g. 5K/kilometer). The thermal wind is caused by horizontal temperature gradients.

Thermal Wind Equation

This says that the difference in wind velocity between two heights is proportional to horizontal temperature gradient perpendicular to the wind. In other words, the u component of the thermal wind is proportional to the meridional temperature gradient and the v to the zonal temperature gradient. The relationship is derived hydrostatic relationship, the ideal gas law and the geostrophic relationship.


Specifically the depth at which the temperature gradient is a maximum. As one descends from the surface of the ocean the temperature remains nearly the same as it was at the surface. Soon, however, one encounters a zone in which temperature starts decreasing rapidly with depth. This zone is called the thermocline. The thermocline is important because it can support large-scale waves which play a major role in ENSO. In studying the tropical Pacific Ocean, the depth of 20°C water ("the 20°C isotherm") is often used as a proxy for the depth of the thermocline. Along the equator, the 20°C isotherm is typically located at about 50m depth in the eastern pacific, sloping downwards to about 150m in the western Pacific.


Convective events occur with lightning. They are responsible for much of the latent heat release in the upper atmosphere of the tropics.

Time Series

An evenly spaced set of values of a variable at one location.

Trade Winds

Often call "trades". Climatological winds that blow from the subtropical highs (30-35degrees) towards the equatorial trough (slightly north of the equator). Winds are northeasterly in the northern hemisphere and southeasterly in the southern. The winds are the surface portion of the Hadley circulation.

Tropic of Cancer

The latitude line 23.5°N. It is the maximum latitude in the NH where the sun is directly overhead at noon (due to the 23.5° tilt of the Earth relative to the sun).

Tropic of Capricorn

The latitude line 23.5°S. It is the maximum latitude in the Southern Hemisphere where the sun is directly overhead at noon (due to the 23.5° tilt of the Earth relative to the sun).


The near-equatorial region between the Tropic of Cancer and the Tropic of Capricorn (23°N-23°S).


The boundary between the troposphere and the stratosphere. It is defined by the temperature minimum between the two layers. It varies between 16km near the equator and 9km near the poles.


The region from the Earth's surface to the tropopause boundary at the stratosphere. Almost all of the Earth's weather occurs here.


A t-test is a statistical test used to determine of the means of two populations are equal. The variances may be assumed to be equal or unequal. The general equation is:
The statistical significance is determined from values of the t distribution.



In ocean dynamics, the upward motion of sub-surface water toward the surface of the ocean. This is often a source of cold, nutrient-rich water. Strong upwelling occurs along the equator where easterly winds are present. Upwelling also can occur along coastlines, and is important to fisheries in California and Peru.



A measure of the spread about the mean. Mathematically it is the sum(Xi-Xm)**2, where Xm is the total mean and Xi are the observations.


The spread of observations about a mean.


A fluid property that is defined as twice the local rate of rotation of a fluid element or the curl of the velocity field. It is (generally) negligible except for the component in the horizontal plane.

Vorticity Equation

The equation that relates the rate of change of the vertical component of vorticity to the horizontal divergence.


Walker Cell

See Walker Circulation.

Walker Circulation

A name coined by Bjerknes for two circulation cells in the equatorial atmosphere, one over the Pacific and one over the Indian Ocean. Schematically these are longitudinal cells where, on one side of the ocean, convection and the associated release of latent heat in the air above lifts isobaric surfaces upward in the upper troposphere and creates a high pressure region there. The lack or lesser degree of the same process on the other side of the ocean results in lower pressure there, and a longitudinal pressure gradient is established which, being on the equator, cannot be balanced by the Coriolis force. Thus a direct zonal circulation is driven in the equatorial plane with countervailing winds at the surface and in the upper troposphere, with concomitant rising and sinking branches on the appropriate sides of the ocean.

Warm event

A common term used for to refer to El Niño events. Warm refers to above average sea surface temperatures in the Eastern Tropical Pacific.


A disturbance traveling through a medium by which energy is transferred from one particle of the medium to another without causing any permanent displacement of the medium itself.

Wave, Reflection

A change in direction that a wave experiences when it bounces off of a barrier between two kinds of media.

Wave, Refraction

The change in direction and change in velocity that a wave experiences when it leaves one medium and goes into another.


In meteorology, westerlies are winds that come from the west (oceanographers use the opposite convention).

Wind Stress

The force of the atmosphere per unit area on the ocean. Wind stress acts to transfer energy and momentum from the atmosphere to the ocean.



In an east/west direction (parallel to lines of latitude) Zonal winds (u-wind) can be easterly or westerly with positive values being from the west.