SPOTA-1 is a statistical analysis of summer rainfall probabilities for Queensland, based on 100 years of historical sea surface temperatures (SSTs) and rainfall data from 1900 to 1999. Sea surface temperatures across the Pacific Ocean from March through to October have, over the last 100 years, been related to rainfall in Queensland in the following summer (November to March). The SST information is brought together into a single index called the SPOTA-1 Index, described in detail below.
Calculation of the SPOTA-1 Index
The SPOTA-1 Index refers to summer (November to March) rainfall for the most highly stocked, grazing lands region of Queensland (Figure 1) and is based on two separate SST indices: 1) the Norfolk Hawaii Index (NHI); and 2) the South West Pacific Index (SWPI). The NHI and the SWPI are described in more detail in the following section. The SPOTA-1 Index ranges in value from 0 to 100. For any given month, the value of the index represents the percentile rank (0-100) of calculated rainfall values for next summer (based on the NHI and SWPI) from 1900 to 1999. For example:
- a SPOTA-1 Index value of zero means that the calculated rainfall value is lower than calculated values for every summer from 1900 to 1999;
- a SPOTA-1 Index value of 100 means that the calculated rainfall value is higher than calculated values for all summers from 1900 to 1999; and
- a SPOTA-1 Index value of 50 means that the calculated rainfall value is in the middle of the range of calculated values from 1900 to 1999.
The NHI is an index of the Interdecadal Pacific Oscillation (IPO). The IPO is a slow background change in Pacific Ocean SSTs, which affects the relationship between the El Niño-Southern Oscillation (ENSO) and Queensland summer rainfall. The NHI is calculated once a year in March, based on the difference in SST anomalies between a region of the south-western Pacific near Norfolk Island and a region of the North Pacific near Hawaii (see map of these regions).
South West Pacific Index (SWPI)
The SWPI is an index of the El Niño-Southern Oscillation (ENSO) that is comparable to the Southern Oscillation Index (SOI). The SWPI is calculated monthly from May through to October, based on the difference in SST anomalies between a region of the south-western Pacific (north of New Zealand) and a broad region of the central equatorial Pacific known as the South Pacific Dry Zone (SPDZ) (see map of these regions).
The relationship between Pacific Ocean SST and summer rainfall in Queensland becomes stronger from March through to October as the ENSO SST signal develops. Hence, the SPOTA-1 reports are first issued in early April and are then modified based on changes in monthly SSTs from May through to October.
Intended changes to SPOTA-1
SPOTA-1 now utilises a new data set (Reynolds 2) for near-real time SST information. Because Reynolds data is also available on a weekly basis, we intend to eventually be in a position to update SPOTA-1 on a weekly basis.
Last updated: 17 August 2010