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John McLean – ENSO drives sea surface temperatures on the Great Barrier Reef

Ove Hoegh-Guldberg wants us to consider putting sun shades over the Great Barrier Reef, but it begs the question — how much is the reef heating up, and how sure are we that it’s man-made and not natural?

John McLean digs into the data and finds that temperature variations on the reef appear to be closely tied to the ENSO cycle, and that there is little reason to think our SUVs and coal fired plants have anything to do with the rises and falls.

We wonder, as usual, why those paid by taxpayers can’t do the same basic calculations and graphs that the volunteers do online.


Great Barrier Reef sea temperatures – What the data says

John McLean


Inspired by the absurdity of putting shades on the Great Barrier Reef (GBR),  I studied the observational data.

We can extract data  for the grid cells that cover the reef from NOAA’s “Optimal Interpolation” sea surface temperature data (see here).  When that data is averaged across the entire reef we find that the average sea surface temperature along the Great Barrier Reef has an annual cycle very similar to that of Willis Island, a Bureau of Meteorology observation station on an island near the middle of the reef.  Sometimes the sea surface temperature is slightly higher than Willis Island and sometimes it’s slightly lower. The trend since 1982 for both is around one degree/century, but if we look at the Willis Island trend since 1940 it’s almost flat, amounting to around 0.1C/century. The rise in the trend since 1982 is interesting but there’s more to it than you might imagine.

Using the average temperatures across the entire reef we can establish a 25-year average for each calendar month (1982-2006) and from that calculate the anomaly in each month of each year.  That monthly anomaly is shown in Figure 1.


Figure 1- Monthly sea surface temperature (SST) anomalies for the GBR since 1982

At first glance that graph suggests a warming in recent years but before we rush to claim it is due to human activity, as Hoegh-Guldberg did, it’s worth comparing to the major climate force in that part of the world, the El Nino Southern Oscillation (aka the ENSO).  It’s a force that’s existed for more than 125,000 years and it as a known influence on temperatures around much of the world, so maybe it’s the cause of the variation in sea temperatures on the Great Barrier Reef.

The drivers of the ENSO are still in dispute – the latest CSIRO marine climate report lists six candidates and I know of at least two others – but the situation is easily characterised. During neutral conditions easterly winds blow across the Pacific and warm water is found at the west side.  During El Nino conditions the winds decrease or even cease and the warm water is found in the centre of the Pacific, typically at the intersection of the equator and international dateline, and incidentally very close to the Pacific Warm Pool mentioned above.  During La Nina conditions the winds are stronger than normal and temperatures in the west are above normal.  It’s no wonder that El Nino events are often followed quickly by La Nina conditions; the warm water from an El Nino shift west with the wind.

This is a slightly simplistic description because the ENSO doesn’t switch between three distinct states but is a continuous range of conditions over which arbitrary thresholds have been applied to divide the range into three states.

We measure the ENSO using the Southern Oscillation Index, with a sustained period (typically 3 months) above 8 regarded as a “La Nina” event and the same length of period below -8 being regarded as an “El Nino” event.

What’s ENSO got to do with the Great Barrier Reef?

The reef is west-southwest of the Pacific’s centre and that means under normal conditions the reef water will be warm and the winds predominantly easterly. Under El Nino conditions the reef water will generally be cooler because there’s little inflow of warm water and the water had will cool by evaporation and convection.  With La Nina conditions the heat from the east is greater than usual and Great Barrier Reef sea surface temperatures rise.

Figure 2a – Monthly SST anomaly and SOI 1982-1996. Note the correlation between changes in SOI and temperature at the major shifts

Figure 2b  – Monthly SST anomaly and SOI 1996-2012. Note the correlation between changes in SOI and temperature at the major shifts. The relationship between SST anomaly and the SOI (the measure of the state of the El Nino-Southern Oscillation).


Figures 2(a) and 2(b) show the Great Barrier Reef monthly average sea surface temperature anomaly plotted with the SOI for that month.  (Why plot the anomaly and SOI?  Because the SOI is already an anomaly of sorts, and therefore this will compare apples with apples.)

It’s not an easy graph to follow because of short term irregularities caused by such things as variation in sea breezes and because the impact of some ENSO events were suppressed by volcanic eruptions.  We can however see cooling during abrupt shifts towards El Nino conditions (1982, 1991, 1997, 2006, 2008 and 2011) and the warming during shifts towards La Nina conditions (1983, 1988, 1990, 1998, 2005, 2009 and 2010).

The unusually strong El Nino in late 1997 spoilt this pattern somewhat because the warm pool of water that’s normally in the central Pacific expanded so much that it encompassed at least part of the Great Barrier Reef.   As it collapsed in 1998, the abrupt shift towards La Nina conditions meant the reef sea surface temperatures went even higher. Figure 2(b) shows that the ocean around the reef took about two years to cool.

It’s not human activity that’s to blame for the sea surface temperatures on the Great Barrier Reef but the ENSO, a very natural and well recognised climate phenomenon.

The analysis shown here took about thirty minutes to complete.  I just wish silencing the unfounded claims by alarmists could be done in a similar amount of time.



More on Great Barrier Reef temperatures, albeit only to July 2011, at http://mclean.ch/climate/GBR_sea_temperature.htm

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