SILO - Seasonal Climate Outlook (Definitions of terms)

analogues: in meteorology, a past situation which resembles the current situation is called an analogue. Analogues are usually selected from the same time of the year as the current situation. The sequence of weather, or seasonal climate pattern, which followed an analogue is sometimes used as the basis of a weather or climate forecast. In the former case, the analogue may be the synoptic weather chart. In the latter climate case, analogue may refer to monthly mean patterns or, for our seasonal outlooks, the evolving state of the SOI. We determine the ten closest analogues to the present season/year to examining SOI for the same period in previous years. The rainfall patterns which followed in the next few months in these analogues years are then provided as supporting information for the seasonal rainfall outlook.

cold episode: this refers to the existence of a pattern of negative anomalies of sea surface temperature over the eastern equatorial Pacific Ocean. It is the opposite of a warm episode and is associated with a La Niña and a positive phase of the Southern Oscillation (SO).

El Niño: originally this term referred specifically to a warming of the sea surface off the coast of Peru. It is now more generally used to describe the anomalous warming of the large area of the eastern equatorial Pacific Ocean. This warming is strongly linked to changes to the Walker circulation and to negative phases of the SO.

ENSO: El Niño - Southern Oscillation; ENSO is a composite term referring to the whole suite of events associated with these episodes.

graphs of raw SOI data and smoothed SOI data: these are graphs of the changes of the SOI with time (usually month-by-month). A graph of "raw" data plots the individual monthly values to show the progress with time. These graphs are usually quite "noisy", i.e. some ups and downs happen from month to month, although often a strong trend can still be seen. The "noisiness" is due to the natural variability of the atmosphere and the influence of daily weather patterns on the monthly SOI value. In the order to show the long-term trend more clearly, we also plot a "smoothed" curve of the SOI. The "smoothing" is done by applying a mathematical filter or averaging technique to five sequential monthly values of the SOI - the value being "smoothed", and two values on each side. The smoothed graph helps us to visualise the seasonal variation of the SOI - as opposed to the individual monthly values.

La Niña: this term is now used to refer to the opposite of an El Niño, i.e., to the opposite phase of the SO. La Niña thus refers to positive SOI values accompanied by cooler than average eastern Pacific temperatures and associated changes to the Walker circulation.

probabilities: the probability of an event happening can be given a percentage value if we have a knowledge of the processes causing the event, for example seasonal rainfall. Thus a probability of 30% means that there are 3 in 10 chances (30 in 100) of the event occurring. For example, what is the probability that the price of milk tomorrow will be the same as it was today? Given that it is a regulated commodity, and price changes occur infrequently, the answer would be higher than 90%. This may be contrasted with the price of petrol which fluctuates quiet a lot on a weekly and even daily basis. The probability that petrol will cost the same in a week as it does today would be less than 50% (particularly if you are a traveller!!!!!!). We know the strength of the long-term relationship between SOI and rainfall, and we know how that varies throughout the year and from location to location in eastern Australia. Because we know those details, we can assign a probability of receiving rainfall in any of the following categories: below average, average, or above average.

rainfall deciles: the word decile is derived from the same Latin word as decimal, i.e. the basic element is "ten". Deciles divide rainfall up into ten groups, according to the formula: the lowest 10% of falls belong to decile range 1, the next lowest 10% to decile range 2, and so on up to the highest 10% of recorded falls, which belong to decile range 10. As an example, the boundary between decile range 5 and decile range 6 separates rainfall into two equally likely groups; rainfall below that boundary occurs 50% of the time. For the purposes of seasonal outlooks, we are generally referring to seasonal falls.

The definition of below normal, normal and above normal used with outlook probabilities in this product is as follows:

below normal: below 33rd percentile

normal: between 33rd & 67th percentiles

above normal: above 67th percentile

Please note that in the analysis of actual rainfall we will use deciles as:

very much below average: decile 1 (within lowest 10%)
below average: deciles 2 and 3
average: deciles 4 to 7
above average: deciles 8 to 9
very much above average: decile 10 (within highest 10%)

rainfall districts and district rainfall: each rainfall district is a defined geographical area within which rainfall patterns are generally similar over the long-term. District rainfall is then the average rainfall received in that district, calculated by averaging the rainfall measured at a number of individual point locations. District rainfall is a useful quantity to describe rainfall received over longer periods of time - months or seasons. Over shorter periods, say a day or a week, spot falls or individual thunderstorms can produce locilised rainfall which is not at all representative of the district. District rainfall is generally speaking quite well suited to the seasonal outlook service because the SOI is a good predictor for broad areas and for months or seasons (the SOI is not such a good predictor for small areas and short time periods).

sea surface temperatures: play an important role in Australian rainfall variability, particularly the sea temperatures across the Pacific Ocean. We also know, however, that sea temperatures close to the Australian coastline influence rainfall as well. This influence is the subject of current research. Early results from the research suggest that, on some occasions, it may be as important as the El Niño phenomenon.

SO: abbreviation for Southern Oscillation.

SOI: abbreviation for Southern Oscillation Index, the index which measures the strength and sign (or phase) of the SO. The formal definition of the index (SOI) used by the Australian Bureau of Meteorology is

SOI = [standardised monthly anomaly in the Mean Sea Level (MSL) pressure difference between Tahiti and Darwin] multiplied by ten,

where the pressure differences are standardised throughout the year by dividing the monthly value by a measure of the variability in that month (the so-called standard deviation). The monthly pressure difference anomaly is the amount by which the difference in average pressure (Tahiti average minus Darwin average) for that month varies from the long-term average for that month.

Southern Oscillation: a major pressure shift across the Pacific Ocean, which is associated with changes in the Walker circulation -- i.e., with changes in strength, and/or major east-west shifts of the Walker Circulation features.

Walker circulation: this is the flow of air across the Pacific Ocean, forming a circulation in a vertical plane at the equator. The trade winds are the clearly identified part of that circulation at the surface. Aloft, the rising air is made visible on satellite photos by the clouds which form when air rises. Where air sinks at the opposite end of the circulation, the skies are cloud-free. Changes in the strength and position of the Walker circulation are associated with ENSO.

warm episode: this refers to the existence of a pattern of positive anomalies of sea surface temperature in the eastern Pacific; thus a warm episode corresponds to an El Niño and a negative phase of the Southern Oscillation.

This page contains a list of definitions of the main terms used regularly in the Seasonal Climate Outlook monthly issues.
	analogues	rainfall districts and district rainfall
	cold episode	sea surface temperatures
	El Niño	SO
	ENSO	SOI
	graphs of raw SOI data and smoothed SOI data	Southern Oscillation
	La Niña	Walker circulation
	probabilities	warm episode
	rainfall deciles