Defining Climate Terms
Pressure (force per unit area) exerted by the atmosphere on any surface by virtue of its weight; it is equivalent to the weight of a vertical column of air extending above a surface of unit area to the outer limit of the atmosphere.
Bureau of Meteorology
The Bureau of Meteorology is Australia's national weather, climate and water agency. Its expertise and services assist Australians in dealing with the harsh realities of their natural environment, including drought, floods, fires, storms, tsunami and tropical cyclones. Through regular forecasts, warnings, monitoring and advice spanning the Australian region and Antarctic territory, the Bureau provides one of the most fundamental and widely used services of government.
Cloudiness refers to the amount of cloud in the sky compared to normal and is measured by satellites. Changes in cloudiness in tropical areas are often linked to warmer or cooler sea surface temperatures that alter the evaporation that forms cloud. Specifically, climatologists look for the presence or absence of cloud at the junction of the Equator with the International Date Line (an arbitrary line demarcating one calendar day from the next along 180 degrees latitude). Less cloud here is indicative of La Niña where greater cloud than normal usually forms with El Niño.
Global Circulation Model (GCM)
(Also known as General Circulation Model)
Coupled atmosphere-ocean global circulation models are mathematical models which simulate the planetary circulation of either the atmosphere of the oceans. Created using complex equations and computer programs the GCMs simulate both the variability which occurs within each component of the climate system, and the variability which arises from the interactions between components.
El Niño refers to the warming above normal of the central and eastern Pacific sea-surface temperatures that leads to a major shift in weather patterns across the Pacific. In Australia (particularly eastern Australia), El Niño events are associated with an increased probability of drier conditions. The inverse event (cold seas surface temperatures) is called a La Nina.
El Niño Southern Oscillation
The El Nino Southern Oscillation (ENSO) describes the variations of the Pacific sea surface temperatures as well as variations in surface pressures in the tropical Pacific region. The two most extreme phases of the oscillation are called La Niña and El Niño. (See glossary for definitions of La Niña and El Niño)
Ensemble modelling is when the predictions of several models are pooled in an attempt to improve prediction accuracy. This has often been used in the climate and atmospheric sciences. One of the key aims of the ensemble approach is to reduce uncertainty in the modelled predictions.
La Niña refers to the cooling below normal of the central and eastern Pacific Ocean seas surface temperatures. In Australia (particularly eastern Australia), La Niña events are associated with increased probability of wetter conditions. The inverse event (warm sea surface temperatures) is called an El Niño.
Climatologists and the Bureau of Meteorology use a “NINO” index to define the characteristics of the tropical Pacific Ocean. The NINO regions are divided based on position in the ocean. The most commonly used region is NINO3.4 which is between 5°N - 5°S and 170° - 120°W.
Queensland Government Patch Point Data
SILO is a daily meteorological data set which provides historical daily weather records from 1889 to the present. The patched point dataset provides data at any one of approximately 4700 Bureau of Meteorology recording stations around Australia, while the data drill provides data at any one point in Australia on an approximately 5km by 5km grid. There are 15 variables for this product and 15 common modelling formats. The dataset and data drill uses original Bureau of Meteorology measurements and interpolated station records.
Rainfall deciles are calculated by taking the observed rainfall record at a location and sorting the rainfall amounts into ten equal parts. Decile 1 would be the years with the lowest rainfall on record and decile 10 would be the years with the highest rainfall on record. Deciles tell us whether the rainfall is above average, average or below average.
Seasonal model forecasts
Seasonal model forecasts are complex computer programs that use complicated fluid dynamic and thermodynamic equations, initialised with observed weather and climate information, to determine how a forecast will play out in the coming months. Seasonal model forecasts predict the future temperature and rainfall for a location, as well as the oceans temperatures in the coming months.
Sea Surface Temperatures (SST)
Differences in the Sea Surface Temperature of the oceans are often associated with changes in the heat exchange between atmosphere and ocean, or changes in ocean currents and upwelling. These changes can drive large variations in rainfall and atmospheric circulation patterns. SSTs are therefore often strongly related to the development and maintenance of unusual climate patterns, such as ENSO.
Sea surface temperatures are shown here across the globe with red indicating warmer than average SST and blues indicating cooler than average SST.
Southern Oscillation Index (SOI)
The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. The SOI index measures the strength and direction of the Southern Oscillation. Sustained negative values of the SOI often indicate El Niño episodes. Positive values of the SOI are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia, known as La Niña.
The July/August SOI for many Victorian locations has skill in forecasting the rainfall outlook for spring. If the SOI is consistently positive, the chance of spring rainfall is higher. If the SOI is consistently negative, the chance of spring rainfall is lower.
This is a sample of what an SOI chart looks like, tracking the variations of the SOI (vertical axis) over time (horizontal axis).
A statistical model is a set of mathematical equations which describe the behaviour of an object of study in terms of random variables and their associated probability distributions. Statistical climate models provide use real data (not physical equations) to provide reasonable predictions of the climate, as long as things don’t change much.
Systematic sampling of the global oceans (at depth) provides information on oceanic flows and temperatures. In the Pacific Ocean sub-surface temperature measurements provide information about where warm and cool bodies of water lay. Understanding this helps forecast future surface temperatures and the likely occurrence of El Niño and La Niña.
This is a sample of a sub surface temperature chart, a cross section of the ocean with longitude across the bottom axis and depth on the vertical axis. Departures from normal are shown in red (above average) and blue (below average).
Sub Tropical Ridge - Position
The sub-tropical ridge is a belt of high pressure that encircles the globe in the middle latitudes. It is part of the global circulation of the atmosphere. During warmer months the ridge is positioned to the south and is often associated with stable high pressures over southern Australia. During cooler months the ridge position moves towards the equator. The equatorward movement of the subtropical ridge during the cold season is due to increasing north-south temperature differences between the poles and tropics.
East to southeasterly winds (in the southern hemisphere) which affect tropical and subtropical regions, including the northern areas of Australia. During the monsoon season in northern Australia, the easterly trade winds are replaced by moist north-westerly (monsoonal) winds from the Indian Ocean and southern Asian ocean waters.