The 2015–16 wet season has been substantially drier than normal in many parts of northern Australia. Monsoon bursts have been infrequent and mostly weak (except for one event which brought heavy rain in late December and early January) There has been a notable lack of tropical cyclone activity, with the Australian region currently on track for its least active tropical cyclone season since reliable records began in the 1950s.

The most significant dry conditions have been in the northern tropics, with the far north of Western Australia, the Top End of the Northern Territory, Cape York Peninsula and the Torres Strait Islands in Queensland all being affected. During the 2015–16 northern wet season (October to April), rainfall across many areas of coastal far northern Australia (particularly north of 14°S) has been within the driest ten per cent of all wet season periods since 1900, with seasonal totals typically 20 to 40 per cent below average. These dry conditions have also affected some areas further south, particularly the Pilbara in Western Australia and much of Queensland.

The total seasonal rainfall hides the severity of the dry in the second half of the season. Rainfall during the January–April period was very much below average across a broader region, extending further south into Northern Territory and the Gulf Country and parts of eastern Cape York Peninsula.

Over the region north of 15°S as a whole, October to April rainfall was 20 per cent below normal and the lowest since 1991-92, while January-April rainfall was 41 per cent below normal and the lowest since 1961, ranking third-lowest on record overall for this four month period.

Rainfall deciles for the period from October 2015 to April 2016
Rainfall deciles for the period from October 2015 to April 2016
Percentage of normal rainfall received for the period from October 2015 to April 2016
Percentage of normal rainfall received for the period from October 2015 to April 2016

The overall dry period has been interspersed with relatively short periods of heavy rainfall. The most significant of these occurred in late December, when a monsoon trough delivered rainfall across the northeast, resulting in very much above rainfall in most of the eastern Northern Territory and adjacent Queensland border regions and throughout parts of the central Cape York Peninsula. The system continued to bring rain in western Queensland into the opening days of January. The December system was sufficient to give the Northern Territory its wettest December on record, although the most extreme rainfall anomalies were south of 15°S, with much of the eastern half of the Territory south of that latitude receiving more than four times their average December rainfall.

During March, an active monsoon trough and associated tropical low brought rain to the eastern Gulf Country and parts of Cape York Peninsula in Queensland, whilst moisture associated with the remnants of Tropical Cyclone Winston (which caused great destruction in Tonga and Fiji but had weakened below cyclone intensity by the time it reached the Australian region) brought widespread, although somewhat patchy, heavy rains in March in western Queensland, contributing to that region having a near-normal wet season after three very dry years in succession. There was also regular rain in March in coastal north Queensland, which had hitherto had a dry season. More localised systems also brought heavy rains in places; one notable example was on the morning of 3 February at Urandangi, in the far west of Queensland, which received 162.2 millimetres (most of it in three hours), more than half their average annual rainfall. Rain in the first few days of May (after the formal end of the wet season) mostly fell south of 15°S and had no significant impact on seasonal totals or rankings in the northern tropics.

Some of the most significant dry areas

One of the most notable dry areas has been the Top End of the Northern Territory. The total rainfall at Darwin Airport for the October to April period was 1118.6 millimetres, and for January to April, 482.4 millimetres. The overall wet season at Darwin Airport was the driest since 1991–92 and the sixth-driest since records began there in 1942, whilst the January to April period is the airport site’s driest on record; when assessed over the city as a whole, it is the driest in more than a century, ranking only behind 405.8 millimetres observed at the Post Office in 1906. Numerous shorter-term sites in the Darwin region and on the Tiwi Islands also had their driest January to April on record.

It was also notably dry in areas further east in the Top End. In the Kakadu region, Jabiru had its driest wet season since records began there in the 1970s, whilst at Oenpelli, it was the driest wet season since 1951–52. Long-term rainfall data sets in Arnhem Land are few and far between, but Gove Airport had its driest October to April and January to April in 38 years of records.

The Kimberley region had rainfall 14 per cent below normal for the October to April period, making it the driest wet season since 2004–05. The northwest of Australia has seen a strong increase in rainfall in recent decades, and below-normal wet seasons which were a reasonably regular occurrence prior to 1970 have become much rarer since. Until 1970, regional averages below 500 millimetres occurred on average about once every four years, and there were some very dry seasons (such as 1951–52, which was 54 per cent below average), but there has only been one sub-500 millimetre wet season in the 24 years since 1991–92.

Wet season (October to April) rainfall for the Kimberley
Wet season (October to April) rainfall for the Kimberley

Seasonal rainfall in the Pilbara was further below normal than it was in the Kimberley; averaged over the Pilbara, October to April rainfall was about 50 per cent below normal, and a number of sites had less than 10 millimetres for the entire season, including Thevenard Island (4.4 millimetres), Learmonth (6.6 millimetres) and Exmouth (7.2 millimetres). However, rainfall in the Pilbara, which is heavily dependent on tropical cyclones and lows, is much more erratic than that in the Kimberley – for example, in 1971–72, Exmouth was completely rainless between October and April – and regional rainfalls on a par with or lower than those of 2015–16 occur about once a decade on average.

The final notably dry area was northern Cape York Peninsula and the Torres Strait Islands. Whilst heavy rains in the last week of April lifted seasonal rainfall in the Torres Strait above near-record levels, it was still the driest October to April since 1960–61 in the combined Horn Island/Thursday Island data set, and the third-driest on record. On the Cape York Peninsula, Weipa also had its driest October to April since 1960–61. The far north of Cape York Peninsula is the Australian region with wet season most strongly affected by El Niño; an average El Niño year has October to April rainfall which would rank in the bottom 20 per cent of all years.

As well as being dry, it was also hot

Temperatures in the later part of the tropical wet season have been well above normal. Averaged over the region north of 15°S, mean temperatures for the period from January to April were 1.32 °C above the average for the 1961–90 reference period, 0.34 °C warmer than the previous record, set in 1988. The heat was significant both by day and by night, with regional averages setting records for both maximum and minimum temperature. Every month from January to April was at least 1 °C above average, with February and March setting monthly records, and January and April both ranking second.

Darwin’s monthly average maximum temperature was the highest on record for November, February and April, with January and March both ranking in the top three. There have been 31 days above 35 °C since the start of October, more than four times the long-term average and ahead of the previous record for the equivalent period (27 in 2002–03).

The record heat also extended further south, with the 15–20°S latitude zone also setting a record for January to April mean temperature (1.39 °C above the 1961–90 average), although by a smaller margin – 0.08 °C ahead of 1992. For Australia as a whole this four month period with an anomaly of +1.28 °C set a new record (ahead of 2005 with 1.16 °C)

Mean temperature deciles for January to April 2016
Mean temperature deciles for January to April 2016

While background warming is clearly increasing the average temperature over Australia, the dry conditions contributed to the abnormal warmth. This occurred through reduced cloud cover and moisture, and indirectly because of the weakening of the usual cooling influence of green vegetation. There is also a strong connection between tropical warmth at this time of year and El Niño; whilst not every El Niño summer is hot, the six warmest January to April periods for the region north of 15 °S have all occurred during El Niño years. (Conversely, El Niño often brings below-normal temperatures to the northern tropics during the 'build-up' season before the main wet season starts; 2015 followed this script, with August, September and October all slightly cooler than normal).

Due to the increasing trend in wet season rainfall in northwest Australia, mean temperatures in the January to April period in the northern tropics show modest trend since the 1980s (in marked contrast to the situation outside the tropics). There has been, however, a strong increase in the number of extreme hot days, with the number of days above 35 °C at Darwin more than doubling since 2000.

The 2015–16 Australian region tropical cyclone season

Only three tropical cyclones (Stan, Uriah and Tatiana) have occurred in the Australian region during the season to date, and none have reached severe cyclone intensity (category 3 or above). Stan was the only one of these which crossed the coast, making landfall east of Port Hedland as a category 2 system in late January.

The 2015–16 season is on track to set the lowest seasonal total since comprehensive satellite records began in 1970 (before then, some systems, especially those a long way from the coast, are likely to have been missed). The existing record is five, set in 1987–88 and 2006–07. Whilst the tropical cyclone season is conventionally considered to run from November to April, cyclones outside those months, whilst rare, are not unheard of, and the final seasonal count will not be known until the end of June.

During a strong El Niño fewer tropical cyclones are expected in the Australian region. The record low seasons in 1987–88 and 2006–07 both occurred during El Niño years.

What’s normal in this region, and how was it affected by El Niño?

For much of Australia north of 18°S (approximately north of a line extending eastwards from Broome to the Great Dividing Range, inland of Mission Beach), on average more than 90% of annual total rainfall occurs between October and April, with the greatest proportion generally occurring between December to March. Reduced rainfall during this period, along with excessive warm daytime temperatures can impact considerably on water availability across the region. For far north and east Australia over the latter part of the northern wet season, both of these impacts have occurred more strongly during El Niño events, and the two driest wet seasons on record for the region north of 15°S, 1905–06 and 1951–52, both occurred during El Niño events.

The 2015–16 El Niño, although now in decline, peaked in November 2015. The impacts on rainfall during the decline of past El Niño events do vary between events and are less significant than what occurs across Australia’s south during the winter-spring. On average from past events, below average rainfall occurs during October–December rainfall across northern and eastern Australia and throughout the Top End of the Northern Territory. As the northern wet season progresses into March, the impacts are potentially less severe, but extend further south into the Northern Territory. For maximum temperatures, the El Niño impacts are largely contained to northern Queensland; particularly Cape York Peninsula. Slightly cooler than normal temperatures occur from October to December, then reversing from January to March, with northern Queensland much warmer than normal from January to March. Combined with reduction in rainfall across northern Queensland, above average daytime temperatures increases evaporation and further reduces available water. Over the wet season as a whole, the dry signal during El Niño events is strongest in northern parts of Queensland and the Northern Territory, especially Cape York Peninsula.

Rainfall composite for 12 historical moderate or strong El Niño events
Rainfall composite for 12 historical moderate or strong El Niño events