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Analogue a method of forecasting weather by identifying climate records for places or times where there is a similar climate to that predicted for the place of interest. For example, if climate projections indicated stronger westerlies across New Zealand in future, then possible analogues of future South Island rainfall patterns could be drawn from a historical sample of warmer and stronger westerly days of the past record.

Average recurrence same as return period.

interval (ARI)

Catchment the land area drained by a river network.

Climate the ‘average weather’ over a period of time, ranging from months to thousands or millions of years. The classic period for calculating a ‘climate normal’ is 30 years.

Climate change a statistically significant variation in either the mean state of the climate or its variability, persisting for an extended period (typically decades or longer).

Climate model a numerical representation (typically a set of equations programmed into a computer) of the climate system. The most complex and complete climate models are known as general circulation models.

Climate projection a potential future evolution of the climate in response to an emission or concentration scenario of greenhouse gases and aerosols. A climate projection is often based on a simulation by a climate model.

Climate variability variations of the climate (eg, of the mean state, standard deviations and extremes) on all temporal and spatial scales beyond those of individual weather events.

CLIMPACTS an integrated assessment model for conducting analyses of the sensitivity of New Zealand’s managed environments to climate variability and change. Both spatial and temporal variations can be examined. For further information, see: (3 April 2008).

Consequence (or impact) the outcome (of an event), often expressed qualitatively in terms of the level of impact. Consequences can be measured in terms of economic, social, environmental or other impacts.

Design flood the flood that is expected to result from a hypothetical storm of specific storm duration and recurrence interval. For example, a 25-year/24-hour design storm means that the storm duration is 24 hours and the recurrence interval is 25 years. The total rainfall depth and its time distribution are two elements characterising a design storm. The total rainfall depth of a design storm is usually estimated by hydrological frequency analysis using historic rainfall records. For areas without historic rainfall records, design storm depths are often estimated using design storm data from adjacent sites.

Digital elevation model a topographic map expressed as elevations on a regular grid and available in electronic form.

Downscaling deriving estimates of local climate elements (eg, temperature, wind, rainfall) from the coarse resolution output of global climate models. Statistical downscaling uses present relationships between large-scale climate variables and local variables. Nested regional climate modelling uses the coarse resolution output from a global climate model to drive a high-resolution regional climate model.

El Niño a significant increase in sea surface temperature over the eastern and central equatorial Pacific that occurs at irregular intervals, generally ranging between two and seven years. Associated changes occur in atmospheric pressure patterns and wind systems across the Pacific. These can lead to changes in seasonal rainfall and temperature in parts of Australia and New Zealand.

El Niño–Southern a term coined in the early 1980s in recognition of the intimate

Oscillation (ENSO) link between El Niño events and the Southern Oscillation, which, prior to the late 1960s, had been viewed as two unrelated phenomena. The interactive global ocean–atmosphere cycle comprising El Niño and La Niña is often called the ‘ENSO cycle’.

Evapotranspiration the combined process of evaporation from the Earth’s surface and transpiration from vegetation.

Event in this context, an incident that is induced or significantly exacerbated by climate change and that occurs in a particular place during a particular interval of time. Examples of events are floods, very high winds and droughts.

Extra-tropics the mid-latitudes (~ 30°S–50°S).

Extreme weather event an event that is rare at a particular place. ‘Rare’ would normally be defined as being as rare as, or rarer than, the 10th or 90th percentile.

Flood run-off the amount of rainfall that runs into rivers.

Freeboard an allowance made to incorporate uncertainty in the water level due to one or more extreme meteorological, fluvial or oceanographic effects. Quoted design flood levels should include this safety factor.

General circulation a global, three-dimensional computer model of the climate

model (GCM) system, which can be used to simulate the general circulation and climate of the atmosphere and ocean, and particularly human-induced climate change. General circulation models are highly complex and represent the effects of such factors as the reflective and absorptive properties of atmospheric water vapour, greenhouse gas concentrations, clouds, annual and daily solar heating, ocean temperatures, and ice boundaries. General circulation models include global representations of the atmosphere, oceans and land surface.

Global climate model the same as a general circulation model.


Global warming generally used to refer to the rise in the Earth’s surface temperature predicted to occur as a result of increased emissions of greenhouse gases.

Greenhouse effect an increase in the temperature of the Earth’s surface and the lowest 8 kilometres or so of the atmosphere, caused by the trapping of heat by greenhouse gases. Naturally occurring greenhouse gases cause a greenhouse effect at the Earth’s surface of about 30 degrees Celsius. Further temperature increases caused by anthropogenic emissions are termed the ‘enhanced greenhouse effect’.

Greenhouse gases gases in the Earth’s atmosphere that absorb and re-emit infra-red (heat) radiation. Many greenhouse gases occur naturally in the atmosphere, but concentrations of some (such as carbon dioxide, methane and nitrous oxide) have increased above natural levels because of anthropogenic emissions.

Hazard a source of potential harm to people or property. Examples are erosion and inundation.

High Intensity Rainfall a software package that estimates the frequency of extreme

Design System (HIRDS) rainfall at a large number of sites across New Zealand. HIRDS is currently being updated (March 2010), with a new feature being added that will enable the climate change rainfall adjustment factor to be included automatically.

Hydrograph a chart showing how river flow changes with time. Inundation maps are based on a specific or implicit flood hydrograph.

Hyetograph a graphical representation of the distribution of rainfall over time.

Interdecadal Pacific a long time-scale oscillation in the Pacific Ocean–atmosphere

Oscillation (IPO) system that shifts climate every one to three decades. The IPO has positive (warm) and negative (cool) phases. Positive phases tend to be associated with an increase in El Niño events and negative phases with an increase in La Niña events.

Intergovernmental Panel the body established in 1988 by the World Meteorological

on Climate Change (IPCC) Organization and the United Nations Environment Programme to objectively assess scientific, technical and socio-economic information relevant to understanding the scientific basis of the risk of human-induced climate change, its potential impacts and options for adaptation and mitigation.

La Niña a significant decrease in sea surface temperature in the central and eastern equatorial Pacific that occurs at irregular intervals, generally ranging between two and seven years. La Niña is the cool counterpart to the El Niño warm event, and its spatial and temporal evolution in the equatorial Pacific is, to a considerable extent, the mirror image of El Niño. Like El Niño, there are associated changes in atmospheric pressures and wind systems across the Pacific, and related changes can occur in temperature and rainfall in parts of Australia and New Zealand.

Likelihood the probability or chance of something happening (can be a qualitative or quantitative measure).

Mesoscale weather model a numerical weather prediction model designed to simulate phenomena with horizontal scales between a few kilometres and several hundred kilometres. These include thunderstorms, squall lines, fronts, cyclone precipitation bands, and topographically generated weather systems such as mountain lee waves and sea breezes.

Monte Carlo a technique that uses a large number of simulations which are run using random quantities for uncertain variables to determine the sensitivity associated with particular variables and to find which outcomes are most likely.

Natural variability non-anthropogenic climate variability that may be irregular or quasi-cyclic. The El Niño–Southern Oscillation is probably the best-known example of a natural oscillation of the climate system, but there are many others. Changes caused by volcanic eruptions and solar variations can also be considered natural.

Precipitation rainfall, plus any contributions from snow (or other frozen forms). It is standard practice for climate models to further distinguish between convective and large-scale rainfall and/or snowfall.

Rating curve a relationship between river water levels and the corresponding river flows at a specific location.

Regional climate model a climate model that is run at high resolution over a ‘region’

(RCM) (eg, the eastern part of Australia, Tasman Sea plus New Zealand) to describe climate at the regional scale. RCMs are typically driven with data from global climate models, which run at lower resolution and therefore do not accurately simulate, for example, the effects of the Southern Alps on New Zealand’s climate.

Return period the average time between repetition of extreme weather events, such as heavy rainfall or flooding, in a stationary climate (that is, a climate without global warming or other trends). By definition, a 50-year return period event has one chance in 50 of occurring in any one year. In the case of rainfall, a return period is always related to a specific duration (eg, a 50-year return period of 24-hour extreme rainfall).

Risk the concept of the likely loss or gain from an event that may impact upon people or the things they value. Risk is measured in terms of consequence and likelihood. Climate change is likely to exacerbate flooding risk.

Run-off flow per unit catchment area, expressed as a depth for comparison with rainfall data.

Scenario a plausible and often simplified description of how the future may develop, based on a coherent and internally consistent set of assumptions about key driving forces.

Screening assessment an initial risk assessment that involves identifying current sensitivity to climate and possible future sensitivity to climate change, and the likely duration and extent of effects that may arise as a consequence of climate change.

Southern Oscillation a multi-year, low-latitude seesaw in sea-level pressure, with one pole in the eastern Pacific and the other in the western Pacific / Indian Ocean region. This pressure seesaw is associated with a global pattern of atmospheric anomalies in circulation, temperature, and precipitation. Its opposite extremes are the El Niño and La Niña events.

Southern Oscillation Index an index calculated from anomalies in the pressure difference

(SOI) between Tahiti and Darwin. Low negative values of this index correspond to El Niño conditions, and high positive SOI values coincide with La Niña episodes.

SRES scenarios a set of greenhouse gas and aerosol emissions scenarios developed in 2000 by Working Group III of the IPCC and used, among other things, as a basis for the climate projections in the IPCC’s Third Assessment Report.

Storm surge the excess above the sea level expected from tidal variation alone at a given time and place. The temporary increase in the height of the sea is caused by extreme meteorological conditions such as low atmospheric pressure and/or strong winds.

Stochastic model a tool for estimating probability distributions of potential outcomes by allowing for random variation in one or more inputs over time. The random variation is usually based on fluctuations observed in historical data for a selected period using standard time-series techniques. Distributions of potential outcomes are derived from a large number of simulations (stochastic projections) which reflect the random variation in the input(s).

Time-series a collection of measurements (eg, rainfall), arranged in order of time of occurrence.

Weather generator a computer programme that produces multiple time-series of numbers with statistical properties that resemble those of historical weather records. The most common weather generators produce outputs representing daily time-series of maximum and minimum temperature, rainfall and solar radiation. The numbers preserve observed characteristics such as persistence of temperature (eg, one hot day is often followed by another), as well as inter-relationships (eg, wet days tend to have lower solar radiation and lower maximum temperature, but higher minimum temperature).