Winter 2016, why was it so wet?
Issued
How wet was winter 2016?
Winter 2016 was the second wettest winter since records began in 1900. Furthermore, the four months from May to August was the wettest such period on record. Relative to all years (since 1900), over 55% of Australia received decile 10 rainfall (rainfall in the highest 10% records), and 7% saw highest on record totals.
High to record rainfall occurred in many regions, stretching from the west of Tasmania, through the interior and to the northern tropics of Queensland and the Northern Territory. The area-averaged rainfall was in decile 10 for all States and Territories except Victoria. The only region to experience below-average rainfall was southwest Australia, where the long-term rainfall decline is the dominant factor.
| Region | 2016 area-average (mm) | Rank (wettest to driest of 117 years) | Percentage of average | Comment |
|---|---|---|---|---|
| Australia | 163.3 | 1 | 177% | Previous record was 159.1 mm in 1978 |
| Queensland | 174.2 | 2 | 213% | Record is 180.0 mm in 1920 |
| New South Wales (including ACT) | 275.2 | 2 | 169% | Record is 292.1 mm in 1950 |
| Victoria | 340.0 | 15 | 127% | Highest since record of 419.6 mm in 1981 |
| Tasmania | 924.5 | 1 | 160% | Previous record was 866.8 mm in 1931 |
| South Australia | 132.7 | 6 | 173% | Highest since 1978 |
| Western Australia | 134.6 | 7 | 156% | Record is 158.1 mm in 1968 |
| Northern Territory | 88.9 | 7 | 280% | Record is 156.1 mm in 1968 |
| Murray–Darling Basin | 247.6 | 4 | 161% | Highest since 1983 |
| South-West WA | 373.0 | 85 | 92% | 2 years since 1996 above the long-term mean |
The May to August period stands in marked contrast to the first four months of the year. These were generally dry, with April the eighth driest on record. While an abrupt flip from 'drought' to 'flooding rains' is not unusual for Australia’s climate, the scale of the shift in 2016 has been remarkable.
What has influenced the rainfall?
The key modes of climate variability for Australian rainfall are the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). ENSO is a shift in the weather and ocean patterns across the Pacific Ocean, driven by changes in the trade winds and ocean temperatures, which are associated with El Niño and La Niña.
Historically, La Niña has generally led to wetter than average conditions for Australia in winter and spring. This is caused by cooler waters in the central and eastern tropical Pacific, and warmer waters around Australia. The pattern of the equatorial Pacific trade winds, described by the Southern Oscillation Index (SOI), is a good indicator of the above-average eastern Australian rainfall in La Niña years.
In contrast, the classical El Niño pattern is for warmer waters in the central and eastern Pacific, with cooler waters around Australia. This leads to winter-spring rainfall which is typically below-average across much of northern and eastern Australia. As measured by central Pacific sea surface temperatures, 2015 and the start of 2016 witnessed one of the strongest El Niños on record. It is not surprising that many parts of Australia started the year with dry conditions in place.
Instead, it is the period during which an El Niño declines which leads to above-average Australian rainfall. As El Niño breaks down, the waters around Australia tend to warm, replacing the cooler water from the previous year. Since December 2015, the waters around Australia, particularly the tropical waters, have been the warmest on record.
A composite (average) rainfall map for years following the breakdown of moderate-to-strong El Niños shows that the May–August period tends to be wetter than average. In other words, historically strong El Niño events have tended to end in above-average rainfall across large parts of Australia.
The Indian Ocean Dipole (IOD) is a counterpart to the Pacific Ocean's ENSO, describing a shift in sea surface temperature gradients across the equatorial Indian Ocean. It is a significant mode of climate variability that influences Australian rainfall, particularly across the southeast. In the negative IOD phase, westerly winds intensify along the equator, allowing warmer waters to concentrate to Australia’s northwest. The typical rainfall pattern associated with the negative IOD phase is for above-average rainfall across most of eastern Australia. The negative IOD phase has also contributed to the 2016 Australia winter rainfall. In particular, July 2016 saw the largest negative value in the IOD index since reliable records started in 1960.
The impact of either ENSO or IOD on Australian rainfall tends to be reinforced when the two modes of climate variability are in phase. A negative IOD phase in the Indian Ocean coincident with El Niño breakdown in the Pacific, as happened in 2016, is therefore likely to result in more Australian winter rainfall than either event separately.
Prediction of the winter rainfall
The switch from dry conditions early in 2016 to the wet winter was captured by climate models. The Bureau's climate outlooks showed above average-rainfall was likely across most of Australia, starting with the seasonal climate outlook for May–July issued in April.
The prospects for spring
A strong negative IOD remains in place while the Pacific continues to be ENSO-neutral, but near the threshold for La Niña. This pattern is reflected in the August seasonal climate outlook which favours above-average spring rainfall for much of Australia, particularly in northern parts and the inland east. In other words, we expect that spring may bring a continuation of above average rainfall to areas already affected by heavy winter rainfall, further boosting water supplies and river flows, but raising the prospects of flooding due to already wet soils.
Banner photo by Nicholas Browne
