Glossary

 

*all definitions are taken directly from the IPCC WGI report

 

Annular modes See Northern Annular Mode (NAM) and Southern Annular Mode (SAM).

 

Blocking Associated with persistent, slow-moving high-pressure sys­tems that obstruct the prevailing westerly winds in the middle and high latitudes and the normal eastward progress of extratropical transient storm systems. It is an important component of the intraseasonal climate variability in the extratropics and can cause long-lived weather conditions such as cold spells in winter and summer heat waves.

 

Climate index A time series constructed from climate variables that provides an aggregate summary of the state of the climate system. For example, the difference between sea level pressure in Iceland and the Azores provides a simple yet useful historical NAO index. Because of their optimal properties, climate indices are often defined using principal com­ponents—linear combinations of climate variables at different locations that have maximum variance subject to certain normalisation constraints (e.g., the NAM and SAM indices which are principal components of North­ern Hemisphere and Southern Hemisphere gridded pressure anomalies, respectively). See Box 2.5 for a summary of definitions for established observational indices. See also Climate pattern.

 

Climate pattern A set of spatially varying coefficients obtained by “projection” (regression) of climate variables onto a climate index time series. When the climate index is a principal component, the climate pat­tern is an eigenvector of the covariance matrix, referred to as an Empirical Orthogonal Function (EOF) in climate science.

 

Climate regime A state of the climate system that occurs more fre­quently than nearby states due to either more persistence or more frequent recurrence. In other words, a cluster in climate state space associated with a local maximum in the probability density function.

 

El Niño-Southern Oscillation (ENSO) The term El Niño was initially used to describe a warm-water current that periodically flows along the coast of Ecuador and Peru, disrupting the local fishery. It has since become identified with a basin-wide warming of the tropical Pacific Ocean east of the dateline. This oceanic event is associated with a fluctuation of a global-scale tropical and subtropical surface pressure pattern called the Southern Oscillation. This coupled atmosphere–ocean phenomenon, with preferred time scales of two to about seven years, is known as the El Niño-Southern Oscillation (ENSO). It is often measured by the surface pressure anomaly difference between Tahiti and Darwin or the sea surface temperatures in the central and eastern equatorial Pacific. During an ENSO event, the pre­vailing trade winds weaken, reducing upwelling and altering ocean cur­rents such that the sea surface temperatures warm, further weakening the trade winds. This event has a great impact on the wind, sea surface tem­perature and precipitation patterns in the tropical Pacific. It has climatic effects throughout the Pacific region and in many other parts of the world, through global teleconnections. The cold phase of ENSO is called La Niña. For the corresponding indices, see Box 2.5.

 

Extratropical cyclone A large-scale (of order 1000 km) storm in the middle or high latitudes having low central pressure and fronts with strong horizontal gradients in temperature and humidity. A major cause of extreme wind speeds and heavy precipitation especially in wintertime.

 

Indian Ocean Dipole (IOD) Large–scale mode of interannual variabil­ity of sea surface temperature in the Indian Ocean. This pattern manifests through a zonal gradient of tropical sea surface temperature, which in one extreme phase in boreal autumn shows cooling off Sumatra and warming off Somalia in the west, combined with anomalous easterlies along the equator.

 

Inter-Tropical Convergence Zone (ITCZ) The Inter-Tropical Conver­gence Zone is an equatorial zonal belt of low pressure, strong convection and heavy precipitation near the equator where the northeast trade winds meet the southeast trade winds. This band moves seasonally.

 

La Niña See El Niño-Southern Oscillation.

 

Madden–Julian Oscillation (MJO) The largest single component of tropical atmospheric intraseasonal variability (periods from 30 to 90 days). The MJO propagates eastwards at around 5 m s–1 in the form of a large-scale coupling between atmospheric circulation and deep convection. As it progresses, it is associated with large regions of both enhanced and sup­pressed rainfall, mainly over the Indian and western Pacific Oceans. Each MJO event lasts approximately 30 to 60 days, hence the MJO is also known as the 30- to 60-day wave, or the intraseasonal oscillation.

 

Mode of climate variability Underlying space–time structure with preferred spatial pattern and temporal variation that helps account for the gross features in variance and for teleconnections. A mode of variability is often considered to be the product of a spatial climate pattern and an associated climate index time series.

 

Monsoon A monsoon is a tropical and subtropical seasonal reversal in both the surface winds and associated precipitation, caused by differential heating between a continental-scale land mass and the adjacent ocean. Monsoon rains occur mainly over land in summer.

 

North Atlantic Oscillation (NAO) The North Atlantic Oscillation con­sists of opposing variations of surface pressure near Iceland and near the Azores. It therefore corresponds to fluctuations in the strength of the main westerly winds across the Atlantic into Europe, and thus to fluctuations in the embedded extratropical cyclones with their associated frontal systems. See NAO Index, Box 2.5.

 

Northern Annular Mode (NAM) A winter fluctuation in the ampli­tude of a pattern characterized by low surface pressure in the Arctic and strong mid-latitude westerlies. The NAM has links with the northern polar vortex into the stratosphere. Its pattern has a bias to the North Atlantic and its index has a large correlation with the North Atlantic Oscillation index. See NAM Index, Box 2.5.

 

Pacific Decadal Oscillation (PDO) The pattern and time series of the first empirical orthogonal function of sea surface temperature over the North Pacific north of 20°N. The PDO broadened to cover the whole Pacific Basin is known as the Inter-decadal Pacific Oscillation. The PDO and IPO exhibit similar temporal evolution. See also Pacific Decadal Variability.

 

Pacific decadal variability Coupled decadal-to-inter-decadal vari­ability of the atmospheric circulation and underlying ocean in the Pacific Basin. It is most prominent in the North Pacific, where fluctuations in the strength of the winter Aleutian Low pressure system co-vary with North Pacific sea surface temperatures, and are linked to decadal variations in atmospheric circulation, sea surface temperatures and ocean circulation throughout the whole Pacific Basin. Such fluctuations have the effect of modulating the El Niño-Southern Oscillation cycle. Key measures of Pacific decadal variability are the North Pacific Index (NPI), the Pacific Decadal Oscillation (PDO) index and the Inter-decadal Pacific Oscillation (IPO) index, all defined in Box 2.5.

 

Pacific–North American (PNA) pattern An atmospheric large-scale wave pattern featuring a sequence of tropospheric high and low pressure anomalies stretching from the subtropical west Pacific to the east coast of North America. See PNA pattern index, Box 2.5.

 

Representative Concentration Pathways (RCPs) Scenarios that include time series of emissions and concentrations of the full suite of greenhouse gases and aerosols and chemically active gases, as well as land use/land cover (Moss et al., 2008). The word representative signifies that each RCP provides only one of many possible scenarios that would lead to the specific radiative forcing characteristics. The term pathway emphasizes that not only the long-term concentration levels are of interest, but also the trajectory taken over time to reach that outcome. (Moss et al., 2010). RCPs usually refer to the portion of the concentration pathway extending up to 2100, for which Integrated Assessment Models produced corresponding emission scenarios. Extended Concentration Pathways (ECPs) describe extensions of the RCPs from 2100 to 2500 that were calculated using simple rules generated by stakeholder consultations, and do not represent fully consistent scenarios. 

 

Four RCPs produced from Integrated Assessment Models were selected from the published literature and are used in the present IPCC Assessment as a basis for the climate predictions and projections presented in Chapters 11 to 14: 

- RCP2.6 One pathway where radiative forcing peaks at approximately 3 W m–2 before 2100 and then declines (the corresponding ECP assuming constant     emissions after 2100) 

- RCP4.5 and RCP6.0 Two intermediate stabilization pathways in which radiative forcing is stabilized at approximately 4.5 W m–2 and 6.0 W m–2 after 2100 (the corresponding ECPs assuming constant concentrations after 2150) 

- RCP8.5 One high pathway for which radiative forcing reaches greater than 8.5 W m–2 by 2100 and continues to rise for some amount of time (the corresponding ECP assuming constant emissions after 2100 and constant concentrations after 2250) 

 

For further description of future scenarios, see Box 1.1. 

 

Sea surface temperature (SST) The sea surface temperature is the subsurface bulk temperature in the top few metres of the ocean, measured by ships, buoys and drifters. From ships, measurements of water samples in buckets were mostly switched in the 1940s to samples from engine intake water. Satellite measurements of skin temperature (uppermost layer; a fraction of a millimetre thick) in the infrared or the top centimetre or so in the microwave are also used, but must be adjusted to be compatible with the bulk temperature. 

 

Southern Annular Mode (SAM) The leading mode of variability of Southern Hemisphere geopotential height, which is associated with shifts in the latitude of the midlatitude jet. See SAM Index, Box 2.5.

 

Southern Oscillation See El Niño-Southern Oscillation (ENSO).

 

South Pacific Convergence Zone (SPCZ) A band of low-level con­vergence, cloudiness and precipitation ranging from the west Pacific warm pool south-eastwards towards French Polynesia, which is one of the most significant features of subtropical Southern Hemisphere climate. It shares some characteristics with the ITCZ, but is more extratropical in nature, especially east of the Dateline.

 

Teleconnection A statistical association between climate variables at widely separated, geographically-fixed spatial locations. Teleconnections are caused by large spatial structures such as basin-wide coupled modes of ocean–atmosphere variability, Rossby wave-trains, mid-latitude jets and storm tracks, etc. See also Teleconnection pattern.

 

Teleconnection pattern A correlation map obtained by calculating the correlation between variables at different spatial locations and a cli­mate index. It is the special case of a climate pattern obtained for stan­dardized variables and a standardized climate index, that is, the variables and index are each centred and scaled to have zero mean and unit vari­ance. One-point teleconnection maps are made by choosing a variable at one of the locations to be the climate index. See also Teleconnection.