Like the Australian climate, the climate of NSW varies greatly among different regions and from year to year.
The north-east of the state is dominated by summer rainfall, with relatively dry winters. Agriculture in the south, which depends on regular rainfall from cold fronts traversing Australia’s south-east in winter, receives less rain in summer. Outside these two regions, in large parts of NSW rainfall occurs evenly throughout the year.
The coast is influenced by the warm waters of the Tasman Sea, which moderate the temperature and provide moisture for abundant rain. The Great Dividing Range enhances rainfall near the coast, but contributes to a progressive decline in rainfall from east to west across the state.
The major drivers that influence the NSW climate, and most of which also affect weather in the rest of Australia, are:
- Sub-tropical ridge. The position of the ridge varies with the seasons, allowing cold fronts to pass over southern NSW in winter, but pushing them to the south in summer.
- East-coast lows (ECLs). These are intense low-pressure systems that occur off the eastern coast of Australia. ECLs are generally associated with strong and gusty winds, sustained heavy rainfall and high seas. They can cause widespread damage over a very short period of time, but they are also important for water security.
- El Niño Southern Oscillation (ENSO). ENSO has a major influence on the climate of eastern Australia, including almost all of NSW. El Niño is associated with warmer seas in the central and eastern tropical Pacific, leading to less rain in the state. La Niña is associated with a cooler sea surface and often with above average winter/spring rainfall over much of eastern Australia. El Niño and La Niña are natural variations in the climate system that occur on average every 3 to 8 years.
- Indian Ocean Dipole (IOD). IOD is a measure of changes in sea-surface temperature patterns in the northern Indian Ocean. Positive IOD conditions result in less rainfall, whereas negative conditions bring more, with the greatest impact being experienced west of the Great Dividing Range. Positive IOD events are more likely during El Niño years and negative ones are more likely during La Niña.
- Southern Annular Mode (SAM). SAM is a north–south shift in the belt of strong westerly winds across the south of Australia, affecting cold fronts, storm activity and rainfall. A positive SAM results in less rain, whereas a negative one results in more rainfall, stronger westerly winds and more intense low-pressure systems. The SAM system has its greatest influence on winter rainfall in southern NSW.
Large-scale natural climate drivers such as the ENSO, SAM and IOD influence annual variations in the state’s climate, including droughts and floods, and they lead to extreme conditions when they combine. ENSO and IOD have the largest impact on rainfall, and when both are in an extreme phase rainfall in NSW can be strongly affected.
La Niña combined with a negative IOD phase generally results in wetter than average conditions, whereas El Niño with a positive IOD phase is typically associated with drier periods during the cooler months.