We're going through changes - El Nino the Southern Oscillation

Variability of ENSO 

As I introduced in my most recent post, ENSO is a hugely important driver of climate across not only Australia, but the Pacific region and the wider world. Through the later part of the 20th century and into the 21st century, the nature of ENSO has changed, although there has always been variability between ENSO events, the goal posts have now moved and we’re playing in a different game.

ENSO is not one beast, but is made of different flavours, which have been increasing distinguished from one another, whilst increasing in amplitude, with a change in system, comes a change in broad teleconnections, so the way in which people prepare and adapt to ENSO must also change. 

The largest change in ENSO, is demonstrated by the shift in the focal point of the sea surface temperature (SST) anomaly in the equatorial Pacific. Traditionally the SST anomaly was focused in the Eastern Pacific (EP) and spread across the equator in a linear manor, increasingly however, the SST anomaly, is now focused in the central Pacific (CP) and spreads into a 'horse shoe' shape, with a central positive SST anomaly, flanked by negative SST anomalies (Fig. 1).
Figure 1. Altered from Karnauskas (2013).  Displaying the contrasting SST anomaly from both Eastern Pacific (left) averaged SST anomalies from 27th April 1997 to 7th June 1998 and Central Pacific (right) average SST anomaly from 8th September 2002 to 2nd February 2003.


Studies using both fossilized and living corals from the equatorial Pacific can show variations in ENSO through the last 130,000 years, with more focused projections giving higher resolution over the last 7000 years. Studies of this type, use d18O of the coral as a proxy of ocean SST, with U/Th radiometric dating to give precise ages of deposition.  Resulting data sets have shown that the 20th century had the highest magnitude of ENSO events over the last 130,000 years. The frequency of the ENSO events is also increasing, with 23 events having occurred from 1950 - 1997, as opposed to an average of 12 events per century from 7600 years ago - 7100 years ago and 8 events per century from 6100 years ago - 5400 years ago. 

On more recent time scales, the East Pacific El Nino occurrence ratio has increased from 0.01 events per year before 1990, to 0.29 events per year, post 1990, with the increasing amount of El Nino events with the SST anomaly in the central Pacific. The primary driver of this increase in CP ENSO events is a flattening of the thermocline through the equatorial Pacific, the cause of this and associated change in SST anomaly patterns is increases in Pacific ocean SST, caused by anthropogenic induced warming.

The changing effects upon Australia

So, there has been an anthropologically induced change in the type of ENSO events, how does this effect Australia? The typical El Nino (canonical El Nino) where the SST anomaly is centered over the East Pacific, is associated with large spread drought through the Eastern portion of Australia, during September - November (Austral spring) as a result of the Walker circulation shifting Eastward. The following La Nina will typically bring increased rainfall events, the magnitude of the La Nina event has a linear relationship with the intensity of the annual-mean rainfall across Australia. With a shift to the central Pacific of the SST anomaly comes a change in the tropical Pacific circulation patterns and so a number of teleconnetions for Australian weather patterns.

As a case study, the El Nino event of 2002 had a SST anomaly in the central Pacific (Fig. 1), in 2002 Australia had close to record drought levels (Fig. 2), which resulted in reduction of ~25% of gross farm product across the country. With a SST anomaly in the central Pacific, the Walker circulation is not moved as far Eastward as in EP El Nino, giving increased precipitation towards the dateline and suppressed rainfall over Australia. The correlation between Australian rainfall and the centre of SST anomalism within the Pacific can also be quantified, which is coherent with the mechanism described above.


Figure 2. From the Bureau of Meteorology. Showing rainfall percentile ranking across Australia during the El Nino event of 2002 - 2003.

Summary

 As I've outlined, the nature of ENSO is changing, primarily the location of SST anomaly of El Nino, this is due to the change in coupled ocean and atmosphere feedbacks, resulting from anthropogenic induced warming of the atmosphere and consequential flattening of the thermocline in the Pacific. The main implication from this is that Australia will receive stronger drought conditions during EL Nino. With the expected continued warming of global temperatures, ENSO may continue to change along with other coupled atmospheric phenomena, again changing the climate over Australia, reducing predictability and causing havoc across the country.

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