Climate variability Australia & NZL

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Natural climate variability – A major risk factor

Extreme droughts, floods and tropical cyclones: atmospheric natural hazards and heavy losses seem to go hand in hand. In many cases, these weather events can be attributed to natural climate variability.

The deviations of natural climate variability can last for weeks, months or even years, and can present the insurance industry with immense challenges in terms of risk management. Knowing about important climate variabilities and their predictability is essential in mastering these challenges.

Australia and New Zealand are exposed to natural climate oscillations, which have a strong influence on the activity levels of atmospheric natural hazards. The ENSO cycle (El Niño/Southern Oscillation), in particular, impacts the entire region, causing variations that extend over many months.

The phenomenon is caused by two components of the climate system in the equatorial Pacific region – the ocean and the atmosphere – and the complex interaction between them. The way in which these components influence one another and the extent of this influence cause climate conditions to oscillate between El Niño, La Niña and the neutral state that lies between the two in a pattern referred to as the ENSO cycle.

El Niño and La Niña

In the El Niño phase, the trade winds, which usually blow from a southeasterly direction, drop or even reverse direction and begin to blow from the west. This moves the pool of warm surface water located to the northeast of Australia eastwards in the direction of the central and eastern equatorial Pacific – thereby shifting the associated regions of convective precipitation. Dryness and warmth in the northeast and east of Australia and in eastern parts of the New Zealand islands are the result.

In a La Niña phase, the effect is reversed: the trade winds grow stronger than the long-term mean, shifting the warm surface water to just off the coast of Indonesia and northeastern Australia. Large quantities of the warm surface water evaporate here, greatly increasing precipitation in the northeast and east of Australia.

Shift in activity patterns

El Niño and Thunderstorms
Severe thunderstorms can only occur under specific atmospheric conditions. Only in smaller sub-regions does the ENSO cycle cause a marked change in the frequency with which conditions conducive to severe thunderstorms develop in Australia, for instance, in a region in eastern Australia encompassing the southern half of Queensland, New South Wales and Victoria. During a strong El Niño event, conditions conducive to severe thunderstorms occur much less frequently here than under La Niña or neutral conditions.

However, the ENSO phases generally do have an effect on the geographical distribution of severe thunderstorms. This is because humidity decreases in southeastern Australia during El Niño episodes. As a result, severe thunderstorm activity decreases considerably in the coastal regions of New South Wales and along the Great Dividing Range and increases in southeast Queensland. Activity also increases in extensive parts of Western Australia, including the region around Perth.

During La Niña episodes, severe thunderstorm activity in eastern Australia shifts much further southwards and inland: this increases the frequency of severe thunderstorms in the south of New South Wales and the north of Victoria, including the area around Melbourne. Meanwhile, severe thunderstorm activity drops in the southeast of Queensland and the northeast of New South Wales.

El Niño and Cyclones 
During an El Niño event, cyclones tend to develop further to the east and closer to the equator. As a result, fewer storms hit the Australian coast than under La Niña conditions, when the storms develop much closer to the continent. By contrast the cyclone hazard increases for the islands of Oceania to the far northeast of Australia during El Niño phases. More, and above all stronger, cyclones then develop in this region. Their high wind speeds and storm surges can wreak untold damage on the islands.

The second force – The Indian Ocean Dipole

Apart from El Niño and La Niña(ENSO) a further natural climate oscillation also has a strong influence on extratropical and tropical Australia: the Indian Ocean Dipole (IOD). Its impact can combine with ENSO effects, diminishing or increasing their intensity.

The Indian Ocean Dipole (IOD) is characterised by an oscillation of mean annual sea surface temperatures. These natural oscillations show a dipole pattern between the east of the Indian Ocean off the coast of Sumatra and the west of the ocean basin – particularly in the period from May to November. The positive IOD phase (pIOD) causes low temperatures in the east and high temperatures in the west of the Indian Ocean. In a negative IOD phase (nIOD) the pattern is reversed.

The Big Dry and other droughts

Long periods of severe drought in Australia, in particular, are associated with Indian Ocean Dipole oscillations. The Federation Drought (1895–1902), the World War II Drought (1937–1945) and The Big Dry (1995–2010) are examples of such events. Recent research has revealed the causes of these prolonged droughts in southeastern Australia, which can extend over several years and affect other regions in the southwest and south of Australia: the drought episodes are primarily associated with positive or neutral Indian Ocean Dipole phases.

Negative Indian Ocean Dipole phases transport large quantities of moisture from the tropical eastern Indian Ocean in a southeasterly direction towards western, southern and southeastern Australia. The drop in overall precipitation in the cool season recorded over the course of the last 50 years is unlikely to be the result of natural climate oscillations alone. It is probable that climate change was already starting to take effect, reducing the frequency of the cut-off lows which bring rains to the southwest, south and southeast of Australia and to Tasmania.

A different situation can be observed in the eastern parts of the New Zealand islands. Here, drought episodes primarily occur during summer El Niño events. Winds from the west prevail during these periods, bringing precipitation to the western coasts and dry weather to eastern parts. There is a greater risk of forest fires during these phases.

Sources
Allen, J.T., and D.J. Karoly, 2013: A climatology of Australian severe thunderstorm environments 1979–2011: inter-annual variability and ENSO influence. Int. J. Climatol., DOI: 10.1002/joc.3667.

Australian Government, Bureau of Meteorology, 2015: http://www.bom.gov.au/climate/enso/, http://www.bom.gov.au/climate/enso/#tabs=Indian-Ocean.

CSIRO and Bureau of Meteorology, 2015: Climate Change in Australia. Information for Australia’s Natural Resource Management Regions: Technical Report, CSIRO and Bureau of Meteorology, Australia, 216 pages.

National Institute of Water and Atmospheric Research (NIWA), New Zealand, https://www.niwa.co.nz/our-science/climate/informationand-resources/clivar/elnino.

Ummenhofer, C.C., et al., 2009: What causes southeast Australia’s worst droughts? Geophysical Research Letters, 36, doi:10.1029/2008GL036801.

Ummenhofer, C.C., et al., 2011: Indian and Pacific Ocean influences on southeast Australian drought and soil moisture. Journal of Climate, 24, 1313-1336.

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