Historical teleconnection changes in southeastern Australia
Australia’s relationship with the El Niño–Southern Oscillation (ENSO) is a strange, turbulent one. The see-saw pattern of sea surface temperatures and atmospheric pressure that occurs around the tropical Pacific and Indian Oceans is the dominant driver of our year-to-year climate and rainfall variability. Yet there is still a lot we don’t know about ENSO’s long-term behaviour and influence. By looking back in time, we can get a better understanding of how ENSO has behaved, helping us predict what it might do in a warmer world.
ENSO: the global see-saw
We do know that when the Pacific trade winds weaken, warm water that is normally blown from east to west towards Indonesia sloshes back towards South America, making Australia’s tropical seas a bit cooler. This is also associated with an increase in pressure, leading to drier and warmer conditions over tropical Australia.
This pattern of cooler ocean temperatures near Australia is an El Niño event. The opposite occurs during a La Niña event, when lower pressure, warmer water in the western Pacific and stronger trade winds dominate.
ENSO events have a direct impact in the tropics, with El Niño events for example associated with floods in Peru and surrounding countries, and dry conditions over Queensland and Indonesia. But there is also an indirect impact, or teleconnection, that can be felt in many places around the world. The atmospheric and oceanic patterns cause what are essentially atmospheric ripples, or wave trains, suppressing or enhancing particular climate conditions in areas that are far away from the Pacific centre of action. El Niño events aremonitored very closely across Australia, for example, because they are closely linked to hot and dry weather in the southern part of the country, even though this area is 2,000 km away.
Now these teleconnections don’t always behave the same. The strength of La Niña events are more linearly related to wet conditions in Australia than the strength of El Niño events are related to drought, for example. Different flavours of ENSO leave different fingerprints across Australia and the globe. Other climate features, like what is happening around the Southern Ocean, or the Indian Ocean, can also affect how ENSO teleconnections are delivered (although there is much debate about whether these other features are really independent of ENSO at all).
Sometimes, these patterns seem to break down completely for a few years or decades. These times of weakened teleconnections are fascinating, abd have a big impact on long-term regional forecasts, yet no-one is really sure why they happen. Some people think that the Interdecadal Pacific Oscillation, a slower pattern in the Pacific Ocean interferes with the ENSO signal. Others find that the internal behaviour of ENSO itself changes the way in which its remote influence is felt.
Different researchers have suggested that changes in the global climate can change teleconnection patterns, for example global cooling due to volcanic eruptions. This theory has interesting implications for climate change, as the behaviour of ENSO in a warmer world is a big unknown. And there are some who argue that these changes are just random, noisy patterns of the natural world.
One period of low teleconnection strength, when ENSO’s remote ripple just did not get through to Australia was the 1920s to 1950s. During this time, ENSO events just did not pack the same punch as they normally do. But this is the only real time of weak ENSO teleconnections in the 20th century, when modern instrumental weather observations are available. To look at other instances, we need to go further back in time, which of course makes everything a little murkier.
Palaeoclimate data from the US and the Australian region suggest that the early part of the 1800s was another time when ENSO events did not register in regions away from the centre of action. Tree rings and ice cores from distant spots that are normally affected by ENSO do not show much of a corresponding temperature or rainfall pattern. However, there are very few instrumental records for our region that can look into this strange time with much detail. Until now.
What Australia’s historical rainfall data can tell us
As part of the South Eastern Australian Recent Climate History project (SEARCH), we recovered historical rainfall and rainday observations for southeastern Australia, which can now offer some insight into the rain variability experienced in the region. Rainfall data and counts of the number of rain days were recovered from more than 20 sources in southeastern Australia, going as far back as 1821.
Some data came from a farmer’s diary in New South Wales, other observations were taken by a doctor in northwestern Tasmania. Rain day records were published in local Sydney newspapers from the 1820s, while from 1841 the New South Wales Government set up official meteorological observatories in Melbourne, Sydney and Port Macquarie.
To explore southeastern Australia’s long-term rainfall teleconnections, we first put all of this rainfall information together, and compared it against other related data (like air pressure and temperature) as well as written records of the time, to check that the historical data were relatively reliable. They were. Next, we combined the historical data with modern rainfall observations to create a record that went from 1788 to 2012 (there are some gaps before about 1832).
Periods of wet and dry conditions in eastern and southern southeastern Australia (SEA) from 1832 to 2012. Image from Ashcroft et al. 2015.
From here we compared the rainfall data to a range of different databases (or reconstructions) of ENSO behaviour. One ENSO chronology gave a category for each year (weak, strong, extreme etc), based on palaeoclimatic and documentary records of the past. Two others used information from trees and coral to determine a numerical value of El Niño or La Niña strength. Comparing these three ENSO records to our rainfall data, we found some evidence that the ENSO signal was absent during the first part of the 1800s, about 1835–1850, especially in the data from Tasmania and Victoria. Or rather, we did not find strong evidence that it was around.
This was of course our next question. What is going on in this early period? To answer this, we looked at several different possible answers, including the reliability of the historical data, the state of the Interdecadal Pacific Oscillation, the role of the Indian Ocean, and the behaviour of the Southern Annular Mode during the 1830s to 1850s.
In the end, we found no clear answer. All of these things could have influenced our results, with the possible exception of the Indian Ocean. This is an issue with looking further back in time, particularly in the Southern Hemisphere: there are few reliable sources of temperature and rainfall information. The two reconstructions of the Southern Annular Mode that we looked at, for example, did not agree over the early part of the 1800s. We now need more historical information from the Southern Hemisphere to be certain of exactly went on in this fascinating time in our climate history.
Ashcroft, L., Gergis, J. and Karoly, D.J., 2015. Long-term stationarity of El Niño–Southern Oscillation teleconnections in southeastern Australia.Climate Dynamics, online view, DOI:10.1007/s00382-015-2746-3(paywalled but accepted manuscript available here).
This article was adapted from a post on Linden’s blog, www.lindenashcroft.com.