Glossary
This glossary was created in support of and borrowed from the Climate Variability Workshop held in the
COMET classroom during August 2002.
Absolute Vorticity
The vorticity of a fluid particle determined with
respect to an absolute coordinate system. The absolute vorticity vector is defined by two times the Earth's
angular velocity vector plus the curl of the three-dimensional relative velocity vector.
Atmospheric Angular Momentum (AAM)
The product of mass times the
rotational velocity times the perpendicular distance from the axis of rotation. A rotating object will
conserve its angular momentum unless a torque acts to change its rotation. The axial component is of
interest in climate and is determined by the distribution of atmospheric mass and zonal wind relative to the
earth’s rotation axis. Higher than normal surface pressure in the tropics or strong westerly flow
there contributes to greater AAM.
Annular Modes
Circulation patterns that display a high degree of zonal
symmetry. An example would be the Arctic Oscillation of the northern hemisphere.
Arctic Oscillation (AO)
A "mode" of variability in the northern
hemisphere that is largely annular (circular) and somewhat zonally symmetric. There is a negative
relationship in variables such as sea level pressure between the polar region and the mid-latitude region
just to the south. The pattern of the AO has a strong similarity to the NAO.
Baroclinic Atmosphere
An atmosphere in which horizontal temperature
gradients exist without any restrictions regarding their relationship to height contours at the same level.
In a baroclinic atmosphere, the geostrophic wind varies with height in direction as well as speed and its
shear is a function of the horizonal temperature gradient (the thermal wind equation).
Barotropic Atmosphere
An atmosphere characterized by total absence of
horizontal temperature gradients. The direction and speed of the geostrophic wind are independent of height,
i.e., no vertical shear.
Barotropic Vorticity Equation
The vorticity equation in the absence of
horizonal divergence and vertical motion, so that the absolute vorticity of an air parcel is materially
conserved.
Blocking
The situation when the normal zonal flow is interrupted by strong
and persistent meridional flow, often due to anomalous anticyclonic gyres/ridges.
Center of action
A region of high variability in a variable or one of the
centers describing a teleconnection pattern.
Climatology
A quantitative description of climate showing the
characteristic values of meteorological 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.
CLIMLOOP
A software interface used to animate *.gif images.
Composite
An average that is done according to a specific criterion. For
example, one could produce a composite of the rainfall for all years where the temperature was much above
normal.
Diabatic heating
The contribution to changing the temperature of the
atmosphere due to latent or radiative heating processes.
Dipole
A pattern that consists of two centers of opposite polarity (phase)
of some variable. For example, the North Atlantic Oscillation (NAO) can be thought of as a dipole.
Divergent Wind
The component of the horizontal wind, generally small, that
is linked with vertical motion. It can be computed from the velocity potential field.
Downscaling
The transformation from a large scale feature to a small scale
one, not necessarily of the same kind. For example, forecast and data analysis fields tend to be large-scale
compared to the small scale representative of a single station.
Eastern Pacific Oscillation (EPO)
A teleconnection pattern represented by
a north-south dipole of anomalies over the eastern North Pacific Ocean.
El Nino
Spanish for “the boy.” The phase of ENSO that is
associated with warmer than normal SSTs in the eastern equatorial Pacific and cooler than normal SSTs in the
west. Tropical convection over the equatorial Pacific tends to be further east than the climatological
average. It is opposite to La Nina.
Ensemble Prediction
The process of running a numerical weather prediction
model many times from the same initial time but using slightly different initial conditions. The average of
the runs can used as a forecast called the ensemble mean although the method seeks to estimate the full
distribution of the future state of the atmosphere.
ENSO-neutral
The atmosphere-ocean state when it is neither unusually warm
or cold in the ENSO SST regions (e.g., Nino 3.4).
Equations of Motion
A set of hydrodynamical equations representing the
application of Newton=s second law of motion to a fluid system, such as the Earth=s atmosphere (see AMS
Glossary 2000 for mathematical details).
Equivalent Barotropic
An atmosphere with horizontal temperature gradients
but with the constraint that thickness contours are everywhere parallel to the height contours. The speed of
the geostrophic wind can vary with height but not in direction.
Forecast Skill
The relative accuracy of a set of predictions (see Chapter
7 of Wilks (1995) for details).
Geostrophic Balance
For nearly straight and frictionless flow, this is a
balance between the Coriolis force and the pressure gradient force.
Gravity Wave
A wave disturbance in which buoyancy acts as the restoring
force on parcels displaced from hydrostatic equilibrium.
Group Velocity
The velocity at which an observable disturbance, and hence
its energy, propagates. For dispersive waves (phase speed is a function of wavenumber), the speed of the
wave group is generally different from the average phase speed of the individual wave components.
Hadley Cell (Circulation)
A north south (meridional) circulation
consisting of upward motion at the ITCZ, poleward motion to both 30N and 30S, sinking motion at 30 degrees
and an equatorward return flow to the ITCZ.
Hovmoller Charts
Usually plots of time (ordinate) versus longitude
(abscissa) for a single latitude or a latitude band. The isopleths can be any field variable, such as
geopotential height, anomalies of outgoing longwave radiation, temperature, etc.
Index Cycle
Temporal variations of the zonal index.
Intraseasonal
Referring to variations of weather or climate within a
season.
Interannual
Referring to variations of weather or climate from year to
year.
Intertropical Convergence Zone (ITCZ)
The zone where subtropical trade
winds from the northern and southern hemispheres converge. It is associated with upward motion and active
convection. It tends to be located at 5N, but its location varies.
Isentropic Potential Vorticity (IPV)
The potential vorticity on a surface
of constant entropy (or potential temperature). Potential Vorticity is a scalar quantity that involves
combining absolute vorticity and static stability.
Kelvin Wave
A type of low-frequency gravity wave trapped to a vertical
boundary (a coast or a mountain), or to the equator, which propagates counterclockwise around a basin in the
northern hemisphere. The eastward phase speed for the first baroclinic mode in the Pacific Ocean is around
2.8 m/s, which means it would take a Kelvin wave about 2 months to cross the Pacific. The phase speed for
the first baroclinic mode Kelvin wave in the atmosphere is about 40 m/s. Convectively-coupled Kelvin waves
are also observed in the atmosphere and these move eastward near the equator at about 15 m/s.
La Nina
Spanish for "The Girl." The phase of ENSO which is
associated with colder than normal SSTs in the eastern equatorial Pacific and warmer than normal SST's in
the west. Convection over the western Pacific tends to be further west than the climatological average. It
is opposite to El Nino.
LIM (linear inverse model)
A statistical model derived from multiple
regression analysis of observed anomalies. LIM can be used to predict the variable(s) being analyzed.
Low Frequency Variability (LFV)
A term applied to variations of the
atmospheric circulation having time scales longer than individual synoptic-scale systems (3, pg. 348).
Madden Julian Oscillation (MJO)
A tropical disturbance that travels
eastward around the globe with a recurrence interval of 30-60 days. It is associated with convection
anomalies particularly over regions of high SST over the Indian and western Pacific Oceans. The MJO is
baroclinic in structure, and is accompanied by large scale (zonal wave numbers 0-3) circulation anomalies
that can extend into mid-latitudes. Eastward propagation is most robust during northern winter/spring. The
MJO is an important source of intraseasonal variability in the tropics.
MEI (Multivariate ENSO Index)
An index of ENSO derived through cluster and
EOF analysis of surface meteorological fields.
Monsoon
A seasonally reversing wind system linked with the seasonal cycle
of rainfall. Monsoons are forced by seasonal temperature gradients that build up between land and ocean
regions, although other factors like land relief and internal variability of the atmosphere are also
important.
Monsoon high
The upper-level circulation associated with the surface
monsoonal low.
Nino 3.4
An index of east central tropical Pacific SST (5N-5S, 170W-120W).
Normal modes
The characteristic variations or fluctuations of a physical
system. They are generally defined by a spatial cycle and a decay time scale or oscillation period.
North Atlantic Oscillation (NAO)
A teleconnection pattern characterized by
a north-south dipole of geopotential height and sea level pressure anomalies over the North Atlantic Ocean
(2).
North Pacific Oscillation (NPO)
A north-south seesaw in sea level pressure
over the North Pacific Ocean.
Outgoing Longwave Radiation (OLR)
The longwave radiation at the top of the
atmosphere. OLR is used as a proxy for deep tropical thunderstorm activity, mainly within 20 degrees of the
equator.Negative (Positive) OLR anomalies are associated with enhanced (suppressed) tropical convection.
Optimal climate normal
A measure of the trend. The difference between the
seasonal mean temperature during the last 10 years and the 30-year climatology.
Pacific North American (PNA)
A teleconnection pattern with centers of
action over mid-latitude North Pacific, western Canada and the southeast U. S.
Pacific Decadal Oscillation (PDO)
Sometimes defined as the first empirical
orthogonal function (EOF) of North Pacific SST anomalies. Postulated to have inherent decadal variations.
Phase Speed
The speed of propagation of a mathematical surface of constant
phase of a time-harmonic wave. For example, the crest of a mid tropospheric ridge could be thought of as a
surface of constant phase, and its movement would be the phase speed of the ridge.
Quasi-Biennial Oscillation (QBO)
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.
Red spectrum
A spectrum where the variance in a fixed frequency interval
increases with decreasing frequency. Its shape is determined by the decay time scale of a time series.
“Red noise” has such a spectrum.
Re-emergence
When ocean temperature anomalies spread throughout the deep
winter mixed layer, are sequestered beneath the mixed layer as it shoals in the summer, only to be
re-entrained into the surface layer the following fall and winter.
Rossby Wave
Waves that have their restoring force due to the variation of
the Coriolis parameter with latitude. 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 NH) to conserve its absolute vorticity.
Rossby Wave Dispersion (RWD)
A property of Rossby waves because their
phase speed depends on wavenumber (i.e., they are dispersive). Thus, the shape of a group of Rossby waves
will not remain constant as the waves propagate since individual waves can reinforce or cancel each other.
The speed of the wave group is given by the group velocity.
Rossby wave source
A source of Rossby wave energy produced by the sum of
1) the divergent wind acting on an absolute vorticity gradient, and 2) the divergence scaled by the absolute
vorticity. The first term provides a direct link between the divergent outflow from tropical convection and
an energy source for Rossby waves.
Seasonal cycle
The normal seasonal evolution of a variable. It generally
includes all annual harmonics although sometimes only harmonics 1-3 are retained; these contain most of the
seasonal variance.
SST
Sea surface temperature
Soil Moisture
The total amount of water, including the water vapor, in
unsaturated soil.
South Atlantic Convergence Zone (SACZ)
A relatively persistent and
elongated area of enhanced lower tropospheric convergence that extends from central Brazil southeast toward
the mid-latitude South Atlantic Ocean. This zone, about 10 to 20 degrees wide, is often the result of
northward moving cold fronts from the extratropical southern hemisphere, which then become stationary and
eventually dissipate (a graveyard of fronts). The SACZ is best observed during the Australian summer.
South Pacific Convergence Zone (SPCZ)
Same as the SACZ, except that it
extends from about 0/140E to about 30S/120W (across the South Pacific Ocean). This zone is generally more
robust than the SACZ, and can be observed year-around. It is oriented east to west near the equator and
northwest-southeast at higher southern latitudes.
Southern Oscillation (SO)
The Southern Oscillation is a see saw of
atmospheric mass (pressure) between the Pacific and Indo Australian areas. The pressure difference can be
associated with planetary-scale circulation changes.
Southern Oscillation Index (SOI)
An index that represents the strength of
the Southern Oscillation, generally taken as the normalized SLP at Tahiti minus that at Darwin. It is highly
correlated with time series of ENSO. During a warm event, mean sea level pressures will tend to be below
normal at Tahiti and above normal at Darwin, Australia. Hence, the SOI would be negative.
Spherical Harmonics
An orthogonal set of basis functions for the sphere.
Longitude is represented by a sine/cosine wavenumber decomposition and latitude by Legendre polynomials.
Stationary Waves
Waves (flow patterns with periodicity in time and/or
space) that are fixed relative to Earth.
Stochastic process
A process of change governed by probabilities at each
step.
Storm Track
Paths over which vigorous midlatitude cyclones are most
frequently observed. For diagnostic purposes, the variance in the 2-6 day period band is often used to
define the storm track.
Streamfunction
A scalar function whose gradient is proportional to the
non-divergent horizontal wind vector. It is generally defined to be positive for clockwise flow and negative
for counterclockwise flow. Thus positive (negative) centers are anticyclones (cyclones) in the northern
hemisphere and cyclones (anticyclones) in the southern hemisphere. Mathematical details can be found in the
AMS Glossary, 2000 edition.
Subtropical Jet Stream (STJ)
An area of strong winds concentrated within a
narrow region around 30N, which is most intense exiting out of Asia. Prominent subtropical jets also occur
seasonally over North Africa, North America, and near Australia. The jet owes its existence partially to
conservation of atmospheric angular momentum within the Hadley cell.
Synoptic time scale
The time associated with growth of a synoptic-scale
storm, generally 1-2 days.
Synoptic eddy feedback
A process whereby synoptic scale eddies contribute
to the persistence of a large-scale anomalous circulation pattern or regime.
Teleconnection
A 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
(planetary-scale B greater than 10,000 km). Algebraic signs are often assigned to teleconnections. For
simplicity, a positive (negative) phase suggests stronger (weaker) westerly flow in regions of normal
climatological westerlies across the extratropics.
Thermocline
A region in the upper ocean where the vertical temperature
gradient is large. Generally, a top mixed layer with warm temperatures is separated from the colder waters
of the deep oceans by the thermocline. In the tropical Pacific Ocean, the depth of 20C water ("the 20C
isotherm") is often used as a proxy for the depth of the thermocline. Along the equator, the 20C
isotherm slopes from a depth of about 50m in the eastern Pacific, to about 150m in the western Pacific.
Tropical Northern Hemisphere (TNH)
A teleconnection pattern whose North
American features are shifted eastward with respect to those of the PNA pattern. The negative phase of the
TNH would favor positive middle and upper tropospheric heights across much of central Canada southward into
the north central U.S.and Great Lakes states, as is observed during the warm phase of ENSO.
Upwelling
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.
Velocity Potential
A scalar function whose gradient is proportional to the
horizontal divergent wind vector. Negative velocity potential centers represent regions of large-scale
diverging winds and positive centers correspond to converging winds.
Vertical Wind Shear
A change of the vector wind with height.
Vorticity
A measure of rotation in a fluid. Rotation can occur around any
axis, but in large-scale dynamic meteorology the vertical component of the vorticity is of most interest.
Vorticity Equation
A dynamic equation for the rate of change of the
vorticity of a parcel, obtained by taking the curl of the vector equation of motion.
Walker Cell (Circulation)
A name coined by Bjerknes for two circulation
cells in the equatorial atmosphere, one over the Pacific and one over the Indian Ocean. These longitudinal
cells are “direct” with upward motion over warm waters and downward motion over cold waters. The
vertical branches are linked by divergent zonal winds that are generally much weaker than the total zonal
flow.
Warm pool
Generally refers to the region of the Indian and west Pacific
Oceans where sea surface temperatures are > 28C. SST gradients are weak in this region and tropical
convection is frequent.
Wave dispersion
The spreading out of a wave group, normally in all
directions. The process occurs when the phase speed of individual waves within the wave group depend on the
wavelength.
Waveguide
A region of preferred wave propagation and activity.
Wavenumber
The number of waves in a certain distance. If the distance is
taken along a latitude circle, zonal wavenumber is used.
Wavenumber Transition
A postulated mode of planetary scale behavior that
involves a change, often quite abrupt, of the dominate zonal wavenumber governing the circulation state of
the westerlies. A typical zonal wavenumber transition would be from wavenumbers 2/3 to wavenumbers 5/7.
Wave Train
A super position of waves propagating in the same direction and
with almost equal phase speeds.
West Pacific Oscillation (WPO)
A teleconnection pattern that operates
across the North Pacific Ocean basin in all months. The positive (negative) phase involves an enhanced
(suppressed) east Asian jet stream.
White spectrum
A spectrum where each frequency contains the same amount of
variance. “White noise” would have such a spectrum.
Zonal Index
A measure of the strength of the middle latitude westerly
flow, usually expressed as the horizontal pressure difference between 35 and 55N, or as the corresponding
geostrophic wind. A high (low) index means strong (weak) westerly flow.
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