199899 annual weather

Mosquito abundance is dictated principally by rainfall patterns; the more rain, generally the greater the number of mosquitoes. Temperature, tidal flow and irrigation practices are also influential for mosquito breeding. Hence monitoring of environmental parameters, especially rainfall, is a critical component of the Program.

For 1998-99, rainfall across the state was mostly above average, however this was variable from region to region and some areas experienced patches of localised intense activity. Prior to the current season, much of the state was under drought conditions due to the series of El Ninó events over recent years. The drought was broken in mid-1998 with heavy rainfall across the state through winter and spring. In contrast, December was very dry, particularly in inland regions and along the south coast of the state. Except for some extremely wet conditions along the far north coast, the majority of the state had below average rainfall over the summer months. March heralded a return of very wet conditions, with around double the average rainfall, although the south coast remained dry. The remainder of autumn was moderately dry.

The major concern for the upcoming season is that the two environmental models for predicting MVE virus in southeastern Australia, suggest possible activity for the 1999-2000 summer. These models are known as the Forbes' hypothesis (Forbes, 1978) and the Nicholls' hypothesis (Nicholls, 1986), and will be briefly discussed in turn.

Forbes' Hypothesis: Forbes associated rainfall patterns with the 1974 and previous MVE outbreaks and discussed rainfall in terms of 'decile' values. A decile is a ranking based on historical values. The lowest 10% of all rainfall values constitute decile 1, the next 10% make up decile 2, and so on up to the highest 10% of rainfall constituting decile 10. Thus, the higher the decile value, the greater the amount of rainfall.

Forbes' hypothesis refers to rainfall patterns in the catchment basins of the main river systems of eastern Australia. These include: the Darling River system, the Lachlan/Murrumbidgee/Murray River systems, the Northern Rivers (that lead to the Gulf of Carpentaria), and the North Lake Eyre system.

The hypothesis states that if rainfall patterns in these four catchment basins are equal to or greater than decile 7 for either the last quarter of the previous year (eg. October-December 1998) or the first quarter of the current year (January-March 1999), and the last quarter of the current year (October-December 1999), then a MVE outbreak is probable. The quarterly decile figures (Bureau of Meteorology 1998,1999) covering the current relevant period are in the table below.

Catchment Basin	QUARTERLY DECILE
Catchment Basin	Oct-Nov-Dec 1998	Jan-Feb-Mar 1999	Oct-Nov-Dec 1999
Northern Rivers	9	7	?
North Lake Eyre	9	7	?
Darling	5	7	?
Lachlan/Murray/Murrumbidgee	7	7	?

As all the catchment basins had decile 7 rainfall in the first quarter of 1999, the first part of Forbes' hypothesis has been fulfilled. If rainfall patterns continue to be high in the last quarter of 1999, then a MVE outbreak is probable according to Forbes.

The main disadvantage with Forbes' hypothesis is that it is not totally predictive. The quarterly decile figures can only be obtained after the three-month interval. Thus, the complete decile figures for Oct-Nov-Dec 1999 will not be available until January 2000 and by this time, an MVE outbreak may well be under way. An alternative hypothesis, which is potentially more predictive, is known as the Nicholls' hypothesis.

Nicholls' Hypothesis: Nicholls (1986) suggested the Southern Oscillation (SO) as a tool to indicate possible MVE virus activity in southeastern Australia. The SO is a meteorological phenomenon, which influences rainfall across eastern Australia. It is usually represented by the Southern Oscillation Index (SOI), whereby consistently positive SOI values are generally associated with wetter than normal periods (and the converse is true).

Nicholls noted a correlation between past outbreaks of MVE and the SO for the autumn, winter and spring period prior to a disease outbreak. The autumn SOI value for 1999 is 10 (Bureau of Meteorology, 1999b), this is within the range of values for past MVE outbreaks. However, the autumn period is the least reliable indicator for MVE activity. The Nicholls MVE model becomes stronger later in the year with the winter and spring periods, and the higher the SOI values for these periods, the greater the probability of a MVE outbreak for 1999-2000 [note that the winter and spring values according to Nicholls hypothesis are no longer suggesting probable MVE activity - SD].