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 uppe- 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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