Indian Ocean Water temperatures impact on SE Australian rainfall

February 9, 2009
By: David Walker, Executive Officer, LPLM

It seems that it is not only the temperature of Pacific Ocean waters that has an impact on rainfall in SE Australia. We have known for some time that the warming of the central and eastern tropical Pacific Ocean, known as El Niño, often means below average rainfall in eastern Australia. 

The converse of this is known as La Niña, refers to the extensive cooling of the central and eastern Pacific Ocean. In Australia (particularly eastern Australia), La Niña events are associated with increased probability of wetter conditions. 

Changes to the atmosphere and ocean circulation during La Niña events include:

• Cooler than normal ocean temperatures across the central and eastern tropical Pacific Ocean.
• Increased convection or cloudiness over tropical Australia, Papua New-Guinea, and Indonesia.
• Stronger than normal (easterly) trade winds across the Pacific Ocean (but not necessarily in the Australian region).
• High (positive) values of the SOI (Southern Oscillation Index).

However, we seem to regularly have La Niña conditions, but don’t get the wet seasons. Through the last 8 years or so there have been several La Niñas reported, but rainfall, particularly in the south, hasn’t materialised.

Recent research by a team from UNSW Climate Change Research Centre, led by Dr Caroline Ummenhofer and Professor Matthew England has hopefully shed some light on why La Niña sometimes lets us down. 

They have found that the phenomenon known as the Indian Ocean Dipole (IOD) - a variable and irregular cycle of warming and cooling of ocean water - dictates whether moisture-bearing winds are carried across the southern half of Australia.

This is probably not surprising – most of who watch the weather look to the rain systems from the NW of Australia to bring some of our best summer rainfall events. However, this study explains why the Indian Ocean has a major impact on all – year rainfall, particularly for the south.

When the IOD is in its negative phase, a pattern occurs with cool Indian Ocean water west of Australia and warm Timor Sea water to the north. This generates winds that pick up moisture from the ocean and then sweep down towards southern Australia to deliver wet conditions.

In its positive phase, the pattern of ocean temperatures is reversed, weakening the winds and reducing the amount of moisture picked up and transported across Australia. So the south-east misses out on its usual quota of rain.

Figure 1. How the Indian Ocean drives Australia's worst droughts.

The Indian Ocean Dipole has been in positive or neutral territory since 1992, a period coinciding with the record series of low rainfall years in southern NSW, Victoria and South Australia.

This is the longest period of positive since the measurements were first recorded in the 1880s.

But other long periods of cool Indian Ocean temperature are associated with earlier severe droughts, notably the Federation Drought of 1895-1902, and the WWII drought from 1937-1945.

According to Dr Ummenhofer,

"The state of the Indian Ocean is highly important for rainfall and droughts in south-east Australia. More than the variability associated with the El Nino/La Nina cycle in the Pacific Ocean, the Indian Ocean Dipole is the key factor for driving major south-east Australian droughts over the past 120 years.”

While we know that no one factor controls our weather, that it is a combination of cycles and systems over different time and spatial scales, there is no doubt that this improved knowledge will help with our seasonal and longer-range rainfall forecasting. Better information should mean better decisions by our land and water managers.

Click here for the full study ...