2020–21 Australian tropical cyclone outlook
Average to slightly more than average cyclones likely for Australia
Climate Driver Update
Climate drivers in the Pacific, Indian and Southern oceans and the Tropics
Sea surface temperature maps
Sea surface temperature maps are not available for forecasts before Spring 2018
SST outlooks for the next 3 months
ENSO is the oscillation between El Niño and La Niña states in the Pacific region. El Niños typically produce drier seasons, and La Niñas drive wetter years, but the influence of each event varies, particularly in conjunction with other climate influences.
International climate model outlooks
The graphs are based on the ensemble mean for the most recent model run.
These graphs show the average forecast value of NINO3.4 for each international model surveyed for the selected calendar month. If the bars on the graph are approaching or exceeding the blue dashed line, there is an increased risk of La Niña. Similarly, if the bars on the graph are approaching or exceeding the red dashed line, there is an increased chance of El Niño.
Weekly sea surface temperatures
Graphs of the table values
Monthly sea surface temperatures
Graphs of the table values
5-day sub-surface temperatures
- See also: Links open in new window
- Animation of recent sub-surface temperature changes
- Archive of sub-surface temperature charts
Southern Oscillation Index
- Data Source: Links open in new window
- TAO/TRITON data
Cloudiness near the Date Line
About El Niño and La Niña (ENSO)
ENSO is the oscillation between El Niño and La Niña conditions.
The term El Niño refers to the extensive warming of the central and eastern tropical Pacific Ocean which leads to a major shift in weather patterns across the Pacific. This occurs every three to eight years and is associated with a weaker Walker Circulation (see diagram below) and drier conditions in eastern Australia. El Niño Southern Oscillation(ENSO) is the term used to describe the oscillation between the El Niño phase and the La Niña, or opposite, phase.
In the eastern Pacific, the northward flowing Humbolt current brings cooler water from the Southern Ocean to the tropics. Furthermore, along the equator, strong east to southeasterly Trade winds cause the ocean currents in the eastern Pacific to draw water from the deeper ocean towards the surface, helping to keep the surface cool. However in the far western Pacific there is no cool current, and weaker Trades mean that this "upwelling" effect is reduced. Hence waters in the western equatorial Pacific are able to warm more effectively under the influence of the tropical sun. This means that under "normal" conditions the western tropical Pacific is 8 to 10°C warmer than the eastern tropical Pacific. While the ocean surface north and northeast of Australia is typically 28 to 30°C or warmer, near South America the Pacific Ocean is close to 20°C. This warmer area of ocean is a source for convection and is associated with cloudiness and rainfall.
However, during El Niño years, the trade winds weaken and the central and eastern tropical Pacific warms up. This change in ocean temperature sees a shift in cloudiness and rainfall from the western to the central tropical Pacific Ocean.
The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niño or La Niña events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin. The following figure demonstrates the typical fluctuations in SOI over a period of 11 years. Positive SOI values are shown in blue, with negative in orange. Sustained positive values are indicative of La Niña conditions, and sustained negative values indicative of El Niño conditions.
This graph shows the values of the SOI between 1991 and mid-2015. Monthly SOI data.
Each phase of the ENSO has a very different effect on the Australian climate. Events generally have an autumn to autumn pattern of evolution and decay. That is, they typically begin to develop during autumn, strengthen in winter/spring, then decay during summer and autumn of the following year. These effects are described in further detail on the following pages: El Niño and La Niña.
The following links provide further information regarding the El Niño Southern Oscillation and its impact on the Australian Climate.
El Niño, La Niña and Australia's Climate (pdf) provides further detail on ENSO and its impact on Australia.
The Latest Climate Driver Update provides the latest information on the state of ENSO and the likely effect this will have on Australia.
Australian Rainfall Patterns during El Niño and La Niña Events gives further case studies for the Australian region.
- Past La Niña events
- Past El Niño events
- Rainfall during El Niño
- Rainfall during La Niña
- 2010–11 and 2011–12 La Niña events
Timeline of monthly Southern Oscillation Index (SOI) values since 1876
The Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperatures between the eastern and western tropical Indian Ocean. A negative phase typically sees above average winter-spring rainfall in Australia, while a positive phase brings drier than average seasons.
International climate model outlooks
The graphs are based on the ensemble mean for the most recent model run.
Thse graphs show the average forecast value of the IOD index for each international model surveyed for the selected calendar month. If the majority of models are approaching or exceeding the blue dashed line, then there is an increased risk of a negative IOD event. If the majority of models are approaching or exceeding the red dashed line, then there is an increased risk of a positive IOD event.
About the Indian Ocean Dipole (IOD)
Indian Ocean sea surface temperatures impact rainfall and temperature patterns over Australia. Warmer than average sea surface temperatures can provide more moisture for frontal systems and lows crossing Australia.
Indian Ocean Dipole
Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole or IOD. The IOD is one of the key drivers of Australia's climate and can have a significant impact on agriculture. This is because events generally coincide with the winter crop growing season. The IOD has three phases: neutral, positive and negative. Events usually start around May or June, peak between August and October and then rapidly decay when the monsoon arrives in the southern hemisphere around the end of spring.
Indian Ocean Dipole years
The Southern Annular Mode, or SAM, refers to the north-south shift of rain-bearing westerly winds and weather systems in the Southern Ocean compared to the usual position.
Southern Annular Mode (SAM) history
About the Southern Annular Mode (SAM) outlook
The Southern Annular Mode can result in enhanced rainfall in regions of southern Australia.
The Southern Annular Mode, or SAM, also known as the Antarctic Oscillation (AAO), is a mode of variability which can affect rainfall in southern Australia. The SAM refers to the north/south movement of the strong westerly winds that dominate the middle to higher latitudes of the Southern Hemisphere. The belt of strong westerly winds in the Southern Hemisphere is also associated with the storm systems and cold fronts that move from west to east.
During the summer and autumn months (December through to May) the SAM is showing an increasing tendency to remain in a positive phase, with westerly winds contracted towards the south pole.
The contribution that the SAM makes to the climate variability in Australia and the apparent positive trend in the SAM are relatively recent discoveries and as such are still active areas of research.
The diagram above shows the area affected by the Southern Annular Mode, when it occurs and how long it may last.
In terms of mean sea level pressure, the SAM affects the coastal regions of southern Australia throughout the year. Extreme negative phases of the SAM can cause increased rainfall and cold air outbreaks in southern Australia.
Each SAM event, both positive and negative, tends to last for around ten days to two weeks. The time frame between positive and negative events however is quite random, but is typically in the range of a week to a few months.
The impact that the SAM has on rainfall varies greatly depending on season and region. If Australia were a few degrees further south, then the impact of changes in SAM would be much more pronounced. The diagram below describes the average impact on rainfall during a "positive" (westerly winds further south) SAM event.
The SAM also has an impact on temperatures. In general, in areas where rainfall is increased, temperature is decreased whilst where rainfall is decreased, temperature is increased.
The diagram above shows the impact that a "positive" SAM event (decreased westerly winds) has on Australian rainfall. Shading indicates daily rainfall anomaly in mm/day for each of the seasons. (Source: Hendon et al. 2007)
During July 2007, the SAM was in a strong negative phase. This was reflected in rainfall patterns across southern Australia.
- Have a look at the latest atmospheric circulation patterns.
- Current values of the Antarctic Oscillation Index (an index related to the strength and phase of the SAM) are available from the US National Weather Service (NOAA)
- The relationship between the SAM and Australian rainfall is discussed in detail in this research paper.
The Madden–Julian Oscillation (MJO) is the major fluctuation in tropical weather on weekly to monthly timescales. It can be characterised as an eastward moving 'pulse' of cloud and rainfall near the equator that typically recurs every 30 to 60 days.
Weekly tropical climate note
MJO phase diagram
*Note: There are missing satellite observations from 16/3/1978 to 31/12/1978.
The MJO phase diagram illustrates the progression of the MJO through different phases, which generally coincide with locations along the equator around the globe. RMM1 and RMM2 are mathematical methods that combine cloud amount and winds at upper and lower levels of the atmosphere to provide a measure of the strength and location of the MJO. When the index is within the centre circle the MJO is considered weak, meaning it is difficult to discern using the RMM methods. Outside of this circle the index is stronger and will usually move in an anti-clockwise direction as the MJO moves from west to east. For convenience, we define 8 different MJO phases in this diagram.
Daily averaged OLR anomalies
Westerly wind anomalies
Time-longitude plots of daily averaged OLR anomalies (left) and 850 hPa (approximately 1.5 km above sea level) westerly wind anomalies (right) are useful for indicating the movement of the MJO.
How to read the Time-Longitude plots
The vertical axis represents time with the most distant past on the top and becoming more recent as you move down the chart. The Horizontal axis represents longitude.
Eastward movement of a strong MJO event would be seen as a diagonal line of violet (downward from left to right) in the OLR diagram, and a corresponding diagonal line of purple in the wind diagram. These diagonal lines would most likely fall between 60°E and 150°E and they would be repeated nearly every 1 to 2 months.
About the Madden–Julian Oscillian (MJO) outlook
The Madden-Julian Oscillation is associated with weekly to monthly periods of enhanced and suppressed rainfall over parts of Australia.
The Madden-Julian Oscillation (MJO) is a global-scale feature of the tropical atmosphere.
The MJO is the major fluctuation in tropical weather on weekly to monthly timescales. The MJO can be characterized as an eastward moving "pulse" of cloud and rainfall near the equator that typically recurs every 30 to 60 days. However, the signal of the MJO in the tropical atmosphere is not always present.
MJO effects are most evident over the Indian Ocean and western equatorial Pacific. It influences the timing, development and strength of the major global monsoon patterns, including the Indian and Australian monsoons.
Tropical cyclones are also more likely to develop in association with certain phases of a strong MJO event.
The MJO is associated with variations in wind, cloudiness, and rainfall. Most tropical rainfall comes from tall thunderstorms which have very cold tops. Thunderstorms that have cold tops emit only low levels of longwave radiation. Therefore, the MJO can be monitored by using satellite measurements of outgoing longwave radiation (OLR) to identify areas of cloudiness (low OLR) within the tropics.
The diagram above shows the area most affected by the Madden-Julian Oscillation (MJO), the seasons during which the MJO's influence on Australia is greatest, and for how long each active phase of the MJO typically lasts.
The MJO has its greatest effect on the tropical areas of Australia during summer. It may have some effect on parts of southern Australia, however this impact appears small when compared to the effect on northern regions, and remains the subject of research.
The MJO can have an effect on the timing and intensity of "active" monsoon periods in northern Australia. This can lead to enhanced rainfall - in both the intensity of the rainfall and the duration of the rainfall.
During late January 2006, an active phase of the MJO coincided with an active monsoon period, resulting in enhanced rainfall over northern Australia.
The following links provide further information on the MJO.
- The Weekly Tropical Climate Note provides information on the current phase of the MJO.
- Technical information and maps relating to the Real-time Multivariate MJO Index, which is a way of monitoring the climate and weather variations caused by the MJO. Please note that this product is a research product, and as such is not always updated and may be under-going continual changes as it is developed.
The SST map for September shows cooler than average SSTs extending along the equator in the central and eastern tropical Pacific Ocean, and into the tropics south of the equator in the east of the basin. Warmer than average SSTs were evident in the far western equatorial Pacific and in the Tasman Sea.
The September values of the three key NINO indices were: NINO3 −0.8 °C, NINO3.4 −0.7 °C, and NINO4 −0.3 °C.
The sea surface temperature (SST) map for the tropical Pacific Ocean for the week ending 11 October show cool anomalies extending across the tropical Pacific, covering areas east of 160°E and to the south of the equator in the eastern Pacific. Cool SST anomalies have again strengthened compared to last fortnight. Warm anomalies remain in the Maritime Continent and waters close to much of northern and eastern Australia.
The latest values of the three NINO indices in the tropical Pacific for the week ending 11 October were: NINO3 −0.9 °C, NINO3.4 −0.8 °C, NINO4 −0.5 °C.
Persistent NINO3 or NINO3.4 values warmer than +0.8 °C are typical of El Niño, while persistent values cooler than −0.8 °C typically indicate La Niña.
The 30-day Southern Oscillation Index (SOI) for the 30 days ending 11 October was +11.7. The 90-day SOI value is also above La Niña thresholds at +9.3.
Sustained negative values of the SOI below −7 typically indicate El Niño while sustained positive values above +7 typically indicate La Niña. Values between +7 and −7 generally indicate neutral conditions.
Trade winds for the 5 days ending 11 October were stronger than average over the western half of the tropical Pacific. More generally, Pacific trade winds have been stronger than average in recent months.
During El Niño there is a sustained weakening, or even reversal, of the trade winds across much of the tropical Pacific. Conversely, during La Niña, there is a sustained strengthening of the trade winds.
The Madden-Julian Oscillation (MJO) is in the Maritime Continent, and is expected to increase in strength as it moves into the Western Pacific Ocean. At this time of the year, the MJO may increase the likelihood of above average rainfall over south-east Australia as it passes through the western Pacific.
Large parts of the central and eastern Indian Ocean are warmer than average. Compared to two weeks ago, warm sea surface temperature anomalies have strengthened in the west of the basin. The latest weekly value of the Indian Ocean Dipole (IOD) index to 11 October was +0.39 °C, a strong jump almost to positive IOD threshold values.
Three of the six surveyed climate models indicate values indicate the IOD will be met in November, however, a negative IOD event is now looking less likely compared to previous outlooks, following significant warming in the western pole.
Cloudiness near the Date Line was below average over the past fortnight and has generally been below average since early to mid-March.
Equatorial cloudiness near the Date Line typically increases during El Niño (negative OLR anomalies) and decreases during La Niña (positive OLR anomalies).
The four-month sequence of equatorial Pacific sub-surface temperature anomalies (to September) shows cooler than average water extending across the top 200 m of the sub-surface of the equatorial Pacific around and east of the Date Line. The strength and extent of cooler than average water has increased month-on-month compared to both August and July.
Weak warm anomalies persist across large parts of the column depth in the western equatorial Pacific.
For the five days ending 11 October, sub-surface temperatures were cooler than average in parts of the central to eastern equatorial Pacific, reaching more than 3 degrees cooler than average in a small region around 140°W between 100 and 150 m depth. These cool anomalies have weakened slightly compared to two weeks ago. Temperatures are close to average in most of the sub-surface of the equatorial Pacific Ocean.
A La Niña is underway in the tropical Pacific. All surveyed international climate models indicate this La Niña will persist through the southern hemisphere summer 2020–21.
Most models suggest the La Niña will strengthen, peaking in December. Around half the models anticipate a strong event, meaning there is a possibility it could reach similar strength to the La Niña of 2010–12. However, models forecast this event will be shorter, possibly ending in the first quarter of 2021. The strength of La Niña impacts on Australia are often related to the strength of the event.
Central and eastern tropical Pacific Ocean sea surface temperatures remain around La Niña thresholds (0.8 °C below average) and atmospheric indicators, including the Southern Oscillation Index (SOI), trade winds and cloud, are also at La Niña levels.
La Niña typically increases the chance of above average rainfall across much of Australia during spring. Above average summer rainfall is also typical across eastern Australia. Current climate outlooks indicate November 2020 to January 2021 will be wetter than average for much of the country.
In the Indian Ocean, there has been significant warming of sea surface temperatures in the west of the Basin over the past fortnight. Models have reduced the likelihood of a negative IOD event in 2020, noting that the IOD typically breaks down in late spring or early summer.
The Southern Annular Mode (SAM) is expected to be positive for the remainder of October. La Niña tends to favour positive SAM during spring and summer, further enhancing the wet signal in the east.
The Madden–Julian Oscillation (MJO) is in the Maritime Continent and is expected to increase in strength as it moves into the Western Pacific Ocean.
Climate change is also influencing the Australian climate. Australia's climate has warmed by around 1.4 °C since 1910, while southern Australia has seen a 10–20% reduction in cool season (April–October) rainfall in recent decades.
The Southern Annual Mode (SAM) is expected to remain positive over the remainder of October. La Niña tends to favour positive SAM during the spring to summer months, which typically enhances the La Niña wet signal in eastern Australia.
All of the international climate models surveyed by the Bureau indicate the current La Niña will persist until at least January 2021. Most models reach their peak in December, and all but one indicate thresholds will still be met in February.
La Niña increases the likelihood of above average rainfall across much of Australia during spring, and across much of eastern Australia during summer. La Niña increases the chance of cooler than average daytime temperatures for large areas. It also increases the chance of tropical cyclones, and earlier first rains of the northern wet season.
Product code: IDCKGEWW00