Thursday, October 31, 2013

El Niño Southern Oscillation activity and intensity increasing with Global Warming

A study by Australian and US researchers has found robust signs that global warming is having an increasing impact on the El Niño/La Nina (ENSO) cycle which drives a significant amount of changes to global weather. The El Nino cycle has been unusually active and intense in the 30 year period from 1979 to 2009, more than any time in the last 600 years, researchers found.

"Our research suggests in a warming world we are likely to see more extreme El Niño and La Nina events, which over the past decade in Australia have been related to extreme flooding, persistent droughts and dangerous fire seasons,” said lead author Dr Shayne McGregor from the University of NSW ARC Centre of Excellence for Climate System Science.

“Importantly, this study not only tells us how ENSO activity has behaved in the past in relation to global average temperature, it also opens the window for climate models to be able to estimate more accurately how this activity will change in the future." said Dr McGregor.

Related: El Niño intensification means stronger droughts for Australia, storms for Kiribati


The El Niño Southern Oscillation (ENSO) results from variations in sea surface temperatures (SST) in the eastern tropical Pacific ocean. These sea surface temperature changes cause changes in ocean currents and atmospheric circulation on a global level. Wind and rainfall patterns change causing extreme weather events such as flooding, droughts and changes in tropical cyclone activity.

There has been substantial multi-decadal variability in ENSO over the time when reliable records have been kept, approximately the last 150 years. So researchers sought methods to extend their data records of ENSO back substantially further, so that any long term trend outside the multi-decadal variability could be discerned.


The research analysed variance in ENSO using paleo proxy reconstructions going back several centuries, including from monthly to annually resolved tree rings, ice cores, lake sediments and coral records. Further robustness in the results was gained using multi-site reconstruction of ENSO variance, reducing the impact of variability of weather at single sites affecting the data.

They found that variance in the most recent 30 year period is much greater than the period 1590 to 1880, and probably larger going back over 600 years. The earlier period before 1590 has higher error bars so the researchers "cannot rule out that the levels of ENSO variance that occurred between 1400 and 1590 may be comparable to that of 1979 - 2009."

The research was published in Climate of the Past, an Open Access Journal of the European Geosciences Union.

The paper's authors conclude in the abstract:

"synthesizing existing ENSO reconstructions to arrive at a better estimate of past ENSO variance changes, we find robust evidence that the ENSO variance for any 30 yr period during the interval 1590–1880 was considerably lower than that observed during 1979–2009."

While ENSO cycles appear to track global mean temperature changes, scientists are still grappling with understanding the mechanism and process why this occurrs. "We still don't know why. Understanding this relationship will be vital to help us get a clear idea of the future changes to global climate," concluded Dr McGregor.

Another study recently published argued that El Niño is likely to become more intense with climate change, and produce drier conditions for Australia and the Western Pacific, with increases in rainfall in the central and eastern equatorial Pacific in the mid to late twenty first century. (El Niño intensification means stronger droughts for Australia, storms for Kiribati). See Power et al (2013) (abstract)


Sources:
  • McGregor, S., Timmermann, A., England, M. H., Elison Timm, O., and Wittenberg, A. T.: Inferred changes in El Niño–Southern Oscillation variance over the past six centuries, (abstract), Clim. Past, 9, 2269-2284, doi:10.5194/cp-9-2269-2013, 2013
  • ARC Centre of Excellence for Climate System Science News, 28 October 2013, El Nino events get more extreme as globe warms
  • Nature World News, 28 October 2013 - El Niño Activity Increased Over Last Century, New Research Suggests
  • Image: S. McGregor et al. Fig. 7. The running variance (grey dots) of each of the 14 ENSO reconstructions, overlaid with the MRV (thick black line).