THE CLIMATE KELPIE BLOG: Indian Ocean in the climate ‘driver’s seat’ for 2019

Posted by BCG on 23rd September 2019

Some parts of southern Australia have received enough winter rainfall to make a reasonable start to the season. But for many, dry conditions are persisting and it’s the cool water in the Indian Ocean to our north west that is a major driver of these conditions.

“Where you get warmer than average water is where you get more cloud and rainfall,” says Dr Andrew Watkins, long-range forecaster with the Bureau of Meteorology. “This winter the warm water is on the other side of the Indian Ocean, near Africa, so that’s where the cloud is (Figure 1).”

Figure 1. Cloud and rainfall follow warm water. This season the water in the Indian Ocean is warmer off the coast of Africa leading to more cloud (blue shading highlighted by blue circle) and cooler with less cloud near Indonesia (highlighted brown shading). Cloudiness is measured by the Outgoing Longwave Radiation (OLR). Source: NOAA Earth System Research Laboratory’s Physical Sciences Division.

This scenario is referred to as a positive Indian Ocean Dipole, or +IOD. Essentially, the IOD is a measure of the difference between water temperature anomalies at either side of the Indian Ocean. “By anomaly, we mean whether the waters are warmer or cooler than normal,” says Dr Watkins.

“At the moment the IOD is positive, which means more rain over the African continent and less for much of Australia. While this may help farmers in Africa, it’s not so good for Australian farmers.”

Important source of rain

The Indian Ocean Dipole is similar to the El Niño Southern Oscillation (ENSO) in the Pacific Ocean. Together the two are the most important year-to-year climate drivers for Australia.

In a neutral IOD – neither positive or negative – the equatorial trade winds blow from the west, pushing warm water towards Indonesia where clouds and moisture will form off the north west coast of Australia. This moisture can be a source for frontal rain systems for southern Australia.  

If the westerly trade winds intensify, the IOD moves into its negative phase. More warm water is pushed towards Indonesia, generating more moist air and cloud and a much better chance of good rainfall to much of Australia. 

“Unfortunately, in our current IOD positive phase the trade winds reverse, taking that warm water away from Australia,” Dr Watkins says.

Figure 2. In a neutral IOD is trade winds bring warm water towards Indonesia generating clouds and moisture to the north west of Australia. In a negative IOD (-IOD) these winds increase often leading to wetter than average conditions. In a positive IOD (+IOD) the winds reverse taking the warm water and moist air towards Africa, leaving Australia with a lower chance of rainfall. Source: The Bureau of Meteorology.

The Indian Ocean Dipole tends to form around May-June and lasts through to the end of spring. It breaks down quickly once the tropical monsoon season gets going in November-December.

A positive IOD typically means higher pressures, less cloud and less rain over central and south eastern Australia. Depending on the season, these effects can be felt more broadly. “The positive IOD generally has a stronger impact on southern New South Wales, but this season we are really feeling the impact up into northern NSW and south-west Queensland,” Dr Watkins says.

“Less cloud also means we get warmer daytime temperatures across southern Australia in winter and spring, and sometimes cooler nights in the north. For example, this year we’ve seen cooler nighttime temperatures in the Northern Territory, which is great for the mango industry.”

A positive IOD is normally defined when temperatures exceed a +0.4°C threshold for eight weeks. However, the Bureau’s measurements show that while the IOD moved into positive territory in May, it has hovered either side of that official +0.4°C threshold throughout June and July, before strengthening in August.

“Basically, we measure the difference in temperature anomalies in two ‘boxes’ (or locations) in the ocean,” Dr Watkins says. “There have been some local weather effects near Africa that have seen the temperature in that specific location fluctuate. In reality, all the evidence is consistent with a positive IOD. Waters are cool near Indonesia and warm near Africa, and the clouds and winds have shifted to the west, away from Australia.”

“Regardless of the numbers, our weather patterns in 2019 are clearly consistent with a positive IOD.”

It’s all about probabilities

“While the official thresholds haven’t been reached, we are definitely seeing positive IOD-like behaviour with the wind and cloud patterns,” says Dale Grey, seasonal risk agronomist with Agriculture Victoria.

“But, as with all climate drivers, it is still all about probabilities.”

“In a neutral IOD year we see about one third of the seasons being wet, one third dry and one third average (Figure 3 left),” he says. But in a positive IOD year the scales are tipped in favour of a drier than average year. It doesn’t mean we can’t get an average or wet year, just that it is less likely.”

Figure 3. In a neutral IOD year rainfall probabilities are unaffected (left): one third of seasons are average, one third dry and one third wet. In a positive IOD year (right), drier conditions are more likely but average or wet are still possible. Source: Dale Grey, Agriculture Victoria.

For example, Elmore in Victoria has experienced dry springs in two thirds of positive IOD years since 1900 (Figure 3 right), but only about a quarter of these were decile 1. Some of these years were about average (14 per cent) and some were even wetter than average (19 per cent), although none have been wetter since 1983.

“Climate models expect the IOD to stay in positive territory through spring,” Mr Grey says. “This means there is definitely a higher risk of a dry spring and anyone relying on spring rainfall might want to play it on the safe side.”

“Every season is different. In 2019 we have seen some large high-pressure systems, typical of a positive IOD, but they have been sitting far enough north to let some cold fronts through coastal South Australia, Victoria, Southern NSW and Tasmania. This is where we see other climate drivers, like the Southern Annular Mode, coming into play and helping frontal activity out. Frontal activity won’t cut the mustard in spring through, where significant rainfall is needed from tropical sources.”

Serious impact

“Unfortunately, NSW and southern QLD have not been so lucky and are now very dry,” says Dr Watkins. “In fact, NSW is so dry that the Bush Fire Danger Period has been declared two months earlier than usual for some regions. Increased bushfire risk is very closely associated with a positive IOD, more so than with El Niño, and we are already seeing an increased incidence of fires in Indonesia.”

Recent positive IOD years include 2006, 2012 and 2015, although it is worth noting that in 2006 and 2015 a positive IOD coincided with an El Niño, amplifying the dry conditions.

When a positive IOD combines with an El Niño there is an increased likelihood of dry conditions for much of eastern and central Australia.

“At least in 2019 the positive IOD is not coupled with an El Niño” says Dr Watkins. “In 1982 this double whammy led to one of our worst droughts, with the Melbourne dust storm and the Ash Wednesday fires in February 1983.”

“The same goes for the opposite situation. When a negative IOD combines with La Niña we can get very wet conditions like we had 1974 and 2010.”

“Without the impact of El Niño we can be confident that the positive IOD will break down once the monsoon season kicks in around November-December,” says Dr Watkins. “It may be of some consolation to know that the positive IOD won’t have any impact on the potential for summer rainfall.”

Contact

Dr Andrew Watkins, 03 9669 4000, andrew.watkins@bom.gov.au

Dale Grey, 03 5430 4395, dale.grey@agriculture.vic.gov.au

Find out more information about the current state of the Pacific and Indian oceans with the Bureau of Meteorology’s ENSO Wrap-Up.