NSW Arbovirus Surveillance & Vector Monitoring Program

2000 - 2001 Annual Report

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WEATHER DATA

As noted previously, mosquito abundance is dictated principally by rainfall patterns. Temperature, tidal flow and irrigation practices are also influential for mosquito breeding. Hence, the monitoring of environmental parameters, especially rainfall, is a crucial component of the Program.

For 2000-2001, the La Niña conditions that developed through 1998 continued into 2000, with positive Southern Oscillation Index (SOI) values (which equate to wetter conditions) throughout most of 2000 and early 2001. Rainfall was especially intense in October 2000 with almost the entire inland region of the state experiencing above average rainfall, while the northeastern inland areas had rainfall very much above average. November saw even wetter conditions, with very much above average rainfall across the state and even record levels of rainfall in the northeastern inland region. During this time, some 14 rivers across NSW experienced flooding, and flooding was particularly intense upstream of the Peel and Gwydir River systems. In contrast, the summer was hot and dry, with rainfall below average for much of the inland. These dry inland conditions continued into the autumn months.

Along the coast, excluding November when the whole state was very wet, rainfall through spring and early summer was average to below average. The entire coast experienced above average rainfall in February and record levels of precipitation occurred in the mid-north coast during March, especially around the Kempsey region, which experienced extensive flooding.

Temperatures during October were close to average across the state, while both November and December had very cool maximum and warm minimum temperatures in the inland areas of the state. Summer saw very hot conditions, with 2-5^oC above the norm. Autumn had near average temperatures. The coastal region had relatively normal conditions, except for January, in which temperatures were 2-3^oC above average.

In the 1998-1999 Annual Report of the NSW Arbovirus Surveillance Program (Doggett et al., 1999), it was noted that there was a chance of Murray Valley Encephalitis (MVE) virus activity during 1999-2000, as both the Forbes’ (1978) and the Nicholls’ (1986) models were suggesting possible MVE virus in southeastern Australia. No MVE activity was recorded within NSW for that season, although there was some in Central Australia (Broom & Whelan, 2000). Possible MVE activity was again predicted for the recent 2000-2001 season as both Forbes’ and Nicholls’ models were once more indicating an MVE outbreak in southeastern Australia (Doggett et al., 2000).

To reiterate 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 rainfall.

Forbes' hypothesis refers to rainfall levels 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 levels 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 1999) or the first quarter of the current year (January-March 2000) and the last quarter of the current year (October-December 2000), then a MVE outbreak is probable. The quarterly decile figures (Bureau of Meteorology, 1998-2001) covering the current relevant period are in the table below.

Quarterly Decile figures for the Forbes (1978) catchments.

As all the catchment basins had decile 7 or greater rainfall in the last quarter of 1999 and the last quarter of 2000, Forbes’ hypothesis was satisfied for the 2000-2001 season.

[Note, Forbes calculates his deciles based on rainfall in millimetres, rather than the strict decile values as given above. However, his theory (like Nicholls’ below) has been developed on a very limited dataset. Since 1974, there have been significant ecological and cultural changes, which would impact on the severity of human MVE epidemics, including changes in wetland ecology, improvements in public health education campaigns and advancement in the knowledge of mosquito borne diseases. Thus, these weather models can not provide a ‘yes/no’ answer as to when likely MVE activity will occur, and should be used as a guide to possible activity only. Due to these obvious deficiencies and that decile figures are readily available, these have been used in preference].

Figure 2.  The SOI by seasons prior to MVE active years, according to Nicholls (1986). The black bars represent the pre-MVE active seasons.

Nicholls’ hypothesis uses the Southern Oscillation (SO) as a tool to indicate possible MVE virus activity. He noted a correlation between past outbreaks of MVE and the SO for the autumn, winter and spring period prior to a disease outbreak. Converting Nicholls’ model to use the SOI rather than the SO (Figure 2) leads to a better grouping of the pre-MVE seasonal values and thus possibly provides a better predictive model (S.Doggett pers.obs.) [Nicholls measures the SO by recording atmospheric pressures at Darwin, while the SOI is an index calculated from the differences in pressures between the eastern and western Pacific. Each column in Figure 2 represents the number of times a particular SOI value has occurred for that time of the year].

For the winter period of 2000, the SOI value of –1.3 was outside the range of values for the same period of past MVE active years (Figure 2, the June-August graph). Note that Forbes’ hypothesis is still suggesting possible activity for 2001-2002 as all catchment basins in the last quarter of 2000 had rainfall greater than decile 7. The autumn SOI 2001 period according to Nicholls’ hypothesis is again conforming to past MVE outbreaks, with a value of –0.7.

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