Seasonal Climate Outlook Newsletter

The subtropical ridge is a belt of high pressure that encircles the globe in the middle latitudes and forms part of the global atmospheric circulation (Figure 2). Its position plays an important role in influencing Australia's weather patterns and how they vary throughout the seasons.

During the warmer half of the year (November to April), the subtropical ridge is generally located south of southern Australia. High-pressure systems, also known as anticyclones, typically move eastwards along the ridge. These systems are associated with descending air, which suppresses cloud formation and rainfall, bringing generally stable and dry conditions. The ridge also tends to restrict the movement of cold fronts across southern Australia. Any fronts that do penetrate the ridge are generally weaker, bringing less rainfall and less intense changes in temperature and wind than those experienced during winter.

During autumn (March to May), the subtropical ridge gradually moves northwards, reaching central Australia as winter (June to August) approaches. For much of the colder half of the year (May to October), the ridge is positioned over the Australian continent. This northward movement allows cold fronts and associated low-pressure systems to extend further into southern Australia. These winter systems typically bring cooler south-westerly winds, increased cloud cover and showery rainfall.

Figure 2. The subtropical ridge in the southern hemisphere is located across a belt of high-pressure systems over Australia. In winter, it moves north of the SWLD, allowing cold fronts to move across the region and bring rainfall. In summer, it shifts south of the SWLD, restricting the movement of cold fronts and resulting in generally drier conditions. Source: Bureau of Meteorology.

The Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperatures (SSTs) between 2 regions, or poles, of the Indian Ocean: a western pole in the Arabian Sea and western Indian Ocean, and an eastern pole in the eastern Indian Ocean, south of Indonesia. The IOD influences the climate of Australia and other countries surrounding the Indian Ocean and is an important contributor to rainfall variability across the region.

The IOD is one of the key drivers of Australia's climate and can have a significant impact on agriculture, as IOD events generally coincide with the winter cropping season. The IOD is typically active between May and November and has 3 phases: neutral, positive and negative. Events usually develop around May or June, peak between August and October, and rapidly decay with the arrival of the southern hemisphere monsoon, usually around November or December.

During a positive IOD (Figure 3), SSTs around Indonesia are cooler than average, while those in the western Indian Ocean are warmer than average. This temperature difference strengthens easterly winds across the tropical Indian Ocean and reduces cloud formation and convection near Australia. As a result, rainfall is generally suppressed across southern and central Australia, increasing the likelihood of below-average rainfall in the SWLD. Rainfall during positive IOD events is often in the lower deciles (deciles 3–4) across the SWLD.

Figure 3. Diagram showing the positive phase of the Indian Ocean Dipole. The winter-spring (June to November) mean rainfall decile for the SWLD in a positive IOD phase is 2-3. Source: Bureau of Meteorology.

Conversely, during a negative IOD (Figure 4), SSTs are warmer than average near Indonesia and cooler than average in the western Indian Ocean. This temperature pattern strengthens westerly winds across the tropical Indian Ocean, promotes increased cloud formation and convection near Australia, and generally favours above-average rainfall across parts of southern Australia. In particular, the eastern Wheatbelt may experience higher than average rainfall due to an increased frequency of rain-bearing cloud bands.

Figure 4. Diagram showing the negative phase of the Indian Ocean Dipole. The winter-spring (June to November) mean rainfall decile for the SWLD in a negative IOD phase is 4-6. Source: Bureau of Meteorology.

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation, is a pattern of climate variability that influences weather across southern Australia. It describes the north–south movement of the belt of strong westerly winds that circles Antarctica. These winds are associated with storm systems and cold fronts that generally move from west to east across southern Australia. Changes in the position of this wind belt can influence rainfall, wind and temperature.

During a positive SAM event (Figure 5), the belt of strong westerly winds contracts towards the South Pole. This results in weaker than normal westerly winds, easterly wind anomalies and higher-than-normal pressure across much of southern Australia. In summer, a positive SAM event has no influence on SWLD rainfall. During winter, these conditions tend to reduce the frequency of cold fronts reaching the SWLD, resulting in reduced rainfall for the majority of the SWLD, but more rain for Esperance.

Figure 5. A positive Southern Annular Mode (SAM) causes the belt of strong westerly winds and associated cold fronts to shift southwards towards Antarctica. During summer, this generally has little influence on rainfall across the SWLD. However, during winter, a positive SAM typically reduces rainfall across much of the SWLD, although parts of the south coast, including Esperance, can experience increased rainfall. Source: Weather Zone.

Conversely, during a negative SAM event (Figure 6), the belt of strong westerly winds expands northwards towards the equator. This brings the westerly winds and associated storm systems further north, generally resulting in more frequent cold fronts and lower pressure over the SWLD during winter. This tends to favour increased rainfall.

Figure 6. A negative Southern Annular Mode causes westerly winds and cold fronts expand towards Australia. In summer, this has no influence on SWLD rainfall, but in winter a negative SAM means more rainfall for the majority of the SWLD and less rain for Esperance. Source: Weather Zone.

Since 1979, positive SAM events have been more frequent than negative events during winter. SAM has been estimated to account for around 10–15% of weekly rainfall variability in south-west Western Australia, making its influence comparable to, or greater than, that of the El Niño – Southern Oscillation (ENSO). However, its influence varies depending on the season and location.

SAM generally has little influence on summer rainfall across south-west Western Australia. During spring, a positive SAM can contribute to wetter conditions along the south coast, including around Esperance. In contrast, a negative SAM generally has little influence on spring and summer rainfall across the SWLD.