Australia Drought, The Indian Ocean Dipole & Sudden Stratospheric Warming

By Paul Homewood

One factor that seems to have received no attention at all in the media’s reporting of the Australian fires is the role of the Indian Ocean Dipole, or IOD.

The IOD is, in very simplistic terms, a similar phenomenon to ENSO. When it is in positive mode, the Indian Ocean is warmer than normal to the west, and cooler in the east.

This effect is driven by easterly winds, as we also see during La Ninas. In both cases, these winds drive the warm surface waters to the west, allowing cold, upwelling water to replace it.

This is how the Australian BOM describe it:

http://www.bom.gov.au/climate/enso/history/ln-2010-12/IOD-what.shtml

But what is important about the IOD is that, during positive phases, rainfall is reduced over southern Australia. As the winds cool the east and warm the west, the pattern of tropical convection changes. Tropical convection favours warmer waters, so we see lower pressure, more rain and storms in the west , where we have warmer waters. And we see the opposite in the east parts, where we have higher pressure, less rain and storms and drier conditions across the region over Indonesia and Australia.

But not only is Australia affected, as the same phenomenon brings greater rainfall to India and East Africa.

Although the IOD has not been monitored for long, scientists agree that it is a naturally reoccurring event, with strong events occurring roughly every ten years.

So far, so good.

It won’t come as any surprise then that this years Australian drought has coincided with just such a strong positive IOD. In fact, arguable one of the strongest on record:

http://www.bom.gov.au/climate/enso/indices.shtml?bookmark=iod

Indian researchers have looked at the IOD closely as well, given its importance to the Indian monsoon. Quite obviously the latest positive IOD is far more powerful than anything else on record, at least since 1999:

http://enformtk.u-aizu.ac.jp/blog/2019/09/03/how-strong-is-the-2019-iod/

Indian climatologist, Saji N. Hameed, saw this coming months ago:

“The strength of this IOD is enormous. We were anticipating it to exceed the 2006 event, based on the DMI comparisons at http://enformtk.u-aizu.ac.jp/blog/2019/09/03/how-strong-is-the-2019-iod/

But, this seems destined to break some records. Look at the wind and OLR anomalies. They are well matched with SST anomalies and will further cool the SST at the eastern Indian Ocean, pushing the IOD to greater strength by the end of October. Perhaps, this IOD is similar to the 1961 event as my good friend Prof. Ashok Karumuri pointed out a few days ago.”

http://enformtk.u-aizu.ac.jp/blog/2019/10/07/a-monster-of-an-iod/

And as we know, India has a very wet monsoon last summer.

He also stated in an interview in September:

“The dry spells over Indonesia, Australia and Singapore are strongly tied to the ongoing IOD. In fact, the Australian weather agency has been alerting Australians to adverse climate associated with IOD since early spring of this year”

https://weather.com/en-IN/india/monsoon/news/2019-09-16-2019-to-be-one-of-strongest-iod-years-on-record-expert

As Hameed states, the IOD is a naturally occurring event, which has nothing to do with climate change. Inevitably attempts have been made to link the strength of this event with global warming, but there is simply is not enough data to make such a deduction. What we do know is that Australia has been wetter since 1970 than it was before, which would suggest otherwise.

But there is one other factor to plug into the mix – a sudden stratospheric warming over the Antarctic. This is what the BOM were flagging up last September:

Record warm temperatures above Antarctica over the coming weeks are likely to bring above-average spring temperatures and below-average rainfall across large parts of New South Wales and southern Queensland.

The warming began in the last week of August, when temperatures in the stratosphere high above the South Pole began rapidly heating in a phenomenon called "sudden stratospheric warming".

In the coming weeks the warming is forecast to intensify, and its effects will extend downward to Earth’s surface, affecting much of eastern Australia over the coming months.

The Bureau of Meteorology is predicting the strongest Antarctic warming on record, likely to exceed the previous record of September 2002.

(Left) Observation of September 2002 stratospheric warming compared to (right) 2019 forecast for September. The forecast for 2019 was provided by the Australian Bureau of Meteorology and was initialised on August 30, 2019.

What’s going on?

Every winter, westerly winds – often up to 200 km per hour – develop in the stratosphere high above the South Pole and circle the polar region. The winds develop as a result of the difference in temperature over the pole (where there is no sunlight) and the Southern Ocean (where the sun still shines).

As the sun shifts southward during spring, the polar region starts to warm. This warming causes the stratospheric vortex and associated westerly winds to gradually weaken over the period of a few months.

However, in some years this breakdown can happen faster than usual. Waves of air from the lower atmosphere (from large weather systems or flow over mountains) warm the stratosphere above the South Pole, and weaken or "mix" the high-speed westerly winds.

Very rarely, if the waves are strong enough they can rapidly break down the polar vortex, actually reversing the direction of the winds so they become easterly. This is the technical definition of "sudden stratospheric warming."

Although we have seen plenty of weak or moderate variations in the polar vortex over the past 60 years, the only other true sudden stratospheric warming event in the Southern Hemisphere was in September 2002.

In contrast, their northern counterpart occurs every other year or so during late winter of the Northern Hemisphere because of stronger and more variable tropospheric wave activity.

What can Australia expect?

Impacts from this stratospheric warming are likely to reach Earth’s surface in the next month and possibly extend through to January.

Apart from warming the Antarctic region, the most notable effect will be a shift of the Southern Ocean westerly winds towards the Equator.

For regions directly in the path of the strongest westerlies, which includes western Tasmania, New Zealand’s South Island, and Patagonia in South America, this generally results in more storminess and rainfall, and colder temperatures.

But for subtropical Australia, which largely sits north of the main belt of westerlies, the shift results in reduced rainfall, clearer skies, and warmer temperatures.

Past stratospheric warming events and associated wind changes have had their strongest effects in NSW and southern Queensland, where springtime temperatures increased, rainfall decreased and heatwaves and fire risk rose.

The influence of the stratospheric warming has been captured by the Bureau’s climate outlooks, along with the influence of other major climate drivers such as the current positive Indian Ocean Dipole, leading to a hot and dry outlook for spring.

Anomalous Australian climate conditions during the nine most significant polar vortex weakening years (1979, 1988, 2000, 2002, 2004, 2005, 2012, 2013, 2016) on both maximum and minimum temperatures, and rainfall for October–November, as compared to all other years between 1979–2016. Bureau of Meteorology.

http://www.bom.gov.au/climate/updates/articles/a035.shtml

So, given the conjunction of an unusually strong IOD with an SSW event, is it at all surprising that large areas of Australia have just gone through one of the severest droughts on record?