The latest on La Nina/El Nino, IOD, SAM and MJO - all things that drive our weather

In the Pacific Ocean, summer/autumn were 'La Nina' - ie the Pacific pushed extra moisture towards Australia. Winter is weak La Nina, and there are signs of further La Nina on the horizon. In the Indian Ocean, winter/spring are 'negative' (no fancy name but similar to La Nina) - ie the Indian pushes extra moisture towards Australia. Whenever that moisture meets up with low pressure, it brings heavier than usual rain.

The latest guidance using key indicators and forecasts to tell us if it is more likely to be wetter or drier in the weeks and months ahead.

In order for it to rain, we need two things: a feed of tropical moisture (from the Pacific or Indian Ocean), and low pressure (from troughs, lows, fronts). Where they meet up it rains. Conversely, if there is a lack of tropical moisture, it doesn't matter how many low pressure systems move through, they can't produce much rain.

The latest:

In the Pacific Ocean, summer/autumn were 'La Nina'. Winter is weak La Nina, and there are signs of further La Nina on the horizon. This means the Pacific Ocean continues to push more tropical moisture towards Australia than usual.
In the Indian Ocean, winter/spring are 'negative' (no fancy name but similar to La Nina). This means the Indian Ocean pushes more tropical moisture towards Australia than usual.
Whenever that moisture meets up with low pressure, it brings heavier than usual rain. Whenever there is high pressure (and no onshore winds), there is a dry spell.
The Southern Annular Mode (SAM) tells us how that pressure can look.

Follow the maps below to see the latest. Automatically updated daily to weekly.

Starting with the next week...

CURRENT WEATHER MAP

Updates four times daily.

8 DAY OUTLOOK

Updates twice daily, around 8am/8pm AEDT

See day by day maps here.

The indices we look at...

SEA SURFACE TEMPERATURE ANOMALY

Sea Surface Temperature, difference from average

Updates daily (showing how much warmer or cooler than usual the top of the oceans are)

Warm water in the boxes pushes tropical moisture away from Australia.

Cool water in the boxes pushes tropical moisture towards Australia.

Warm water around the top half of Australia helps push tropical moisture across Australia. Cool water means less moisture.

PACIFIC OCEAN INDEX (EL NINO/LA NINA)

Pacific Ocean Index showing La Nina/El Nino

Updates weekly on Monday evenings

The temperature of the Pacific Ocean box, and forecasts for a range of models over the next six months.

If the forecast enters the green area, the Pacific Ocean should push tropical moisture towards Australia (known as La Nina). When low pressure moves through it helps bring flooding rain, most felt in northern and eastern AUS.

If the forecast enters the brown area, the Pacific Ocean should push tropical moisture away from Australia (known as El Nino). This leads to drier than average conditions, even if low pressure is nearby, as there is less moisture available to feed them.

INDIAN OCEAN INDEX (IOD)

Indian Ocean Index, Negative/Positive IOD

Updates weekly on Monday evenings

The temperature of the Indian Ocean, and forecasts for a range of models over the next six months.

If the forecast enters the green area, the Indian Ocean should push tropical moisture towards Australia (known as a negative Indian Ocean Dipole [IOD]). When low pressure moves through it helps bring soaking rain to southern AUS - this area has chances of a wetter Winter and Spring.

If the forecast enters the brown area, the Pacific Ocean should push tropical moisture away from Australia (known as a positive Indian Ocean Dipole [IOD]). This leads to drier than average conditions, even if low pressure is nearby, as there is less moisture available to feed them.

SOUTHERN ANNULAR MODE (SAM)

Updates daily in the early afternoon

SAM controls our weather systems.

Positive encourages lows and troughs over inland parts, and weaker cold fronts. Makes it drier than average in southern Australia as there are no cold fronts, unless a low can wander down. Encourages lows and troughs to linger in NSW and QLD, and if there is extra tropical moisture from the Pacific Ocean (see the index above) then that leads to flooding rain when a low is nearby.

Negative encourages stronger cold fronts. This helps southern Australia see rain, but it only spreads inland if there is a connection to tropical moisture from the Pacific or Indian Ocean (see indexes above). If these aren't pushing moisture, then the cold fronts can't bring much rain away from the coast.

But...

High pressure can 'get in the way' no matter what the signal is. We can't all be covered in low pressure, there are highs in between.
Cold fronts only hit one area. If a strong front hits Perth, it won't do much for Melbourne... if a strong front hits Melbourne, it missed Perth.
Inland troughs, and lows, are the ones that produce the big wet weather. But you have to be on or east of the trough to get the rain, those to the west miss out... and south of the low if it is off the coast, those to the north miss out.

MADDEN JULIAN OSCILLATION (MJO)

Updates daily in the early afternoon

Weird graph, but bear with me! This shows the path of a pulse of tropical energy that helps push that tropical moisture around.

When in the green zone tropical moisture is pushed towards Australia. If you are affected by low pressure it helps intensify the rainfall even further. In the tropics it encourages periods of wet weather.

When in the white zone there is no push either way.

When in the brown zone tropical moisture is pushed away from Australia. If you are affected by low pressure the falls can be lighter than usual. The tropics generally enter a quiet period even if it is in the wet season.