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.
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.
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).
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.
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.
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.
A software interface used to animate *.gif images.
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.
The contribution to changing the temperature of the atmosphere due to latent or radiative heating processes.
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.
The component of the horizontal wind, generally small, that is linked with vertical motion. It can be computed from the velocity potential field.
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.
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.
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.
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).
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.
The relative accuracy of a set of predictions (See Chapter 7 of Wilks (1995) for details).
For nearly straight and frictionless flow, this is a balance between the Coriolis force and the pressure gradient force.
A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium.
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.
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.
Temporal variations of the zonal index.
Referring to variations of weather or climate within a season.
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.
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.
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.
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.
The uppe- level circulation associated with the surface monsoonal low.
An index of east central tropical Pacific SST (5N-5S, 170W-120W).
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.
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.
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.
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.
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.
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.
Sea surface temperature
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.
An orthogonal set of basis functions for the sphere. Longitude is represented by a sine/cosine wavenumber decomposition and latitude by Legendre polynomials.
Waves (flow patterns with periodicity in time and/or space) that are fixed relative to Earth.
A process of change governed by probabilities at each step.
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
A region of preferred wave propagation and activity.
The number of waves in a certain distance. If the distance is taken along a latitude circle, zonal wavenumber is used.
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.
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.
A spectrum where each frequency contains the same amount of variance. “White noise” would have such a spectrum.
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.