THE CLIMATE KELPIE BLOG: How to read an SST Anomaly map: El Niño and La Niña, Indian Ocean Dipole

Posted by BCG on 10th December 2017

Australia, a country of drought and flooding rain, Dorothea Mackellar got that right in her famous ‘My Country’ poem.

In recent years, Wimmera and Mallee farmers have experienced the full gamut of climate phenomena including the 2010 La Niña and negative Indian Ocean Dipole (IOD), a 2011 La Niña, a 2012 positive IOD, a 2015 El Niño and positive IOD, a negative IOD in 2016 and now a neutral growing season with possibility of a late La Niña in 2017.

While there is plenty of information out there, one of the most trusted resources is the monthly newsletter ‘The Fast Break’ produced by the knowledgeable Dale Grey and Graeme Anderson from Agriculture Victoria.

The Fast Break contains up-to-date maps and graphs depicting what the indicators of climate drivers are doing.

One such map is the SST Anomaly map produced by NOAA/NESDIS, which is the National Oceanic and Atmospheric Administration of the US (the equivalent of our BoM).

These maps provide an understanding of the difference in sea temperature at the surface every three days, compared to the normal temperature for that time of the year. This data is compiled from multiple data sources including satellites, fixed and drifting buoys, ships and underwater drones.

Reading a SST map can be easy once you understand what you’re looking at.

Figure 1 is a map produced on the 30 December 2010, with 2010 being remembered for the significant rainfall and flooding in large parts of Victoria. 2010 was not only a La Niña through spring and summer, but was also a negative IOD during spring.

The huge mass of colder (blue) water in the Pacific Ocean is characteristic of a La Niña, which usually results in much warmer (orange) water to reside around the oceans to the north of Australia. Warmer northern tropical waters usually means more evaporation and a better moisture source for rainfall triggers to draw from.

Figure 1. SST Anomaly map from 30 December 2010 showing a La Niña.

Figure 2 from 30 August 2012 shows warm water off Africa and a much colder patch of water off Indonesia, this is a textbook pattern for a positive IOD. This phenomena is the best known killer of NW cloud band activity to Victoria.

Figure 2. SST Anomaly map from 13 November 2012 showing a positive IOD.

Figure 3 is a map from 14 September 2015 and shows the classic warming along the Equator which is an El Niño and a corresponding cooling to our north. This usually results in a reduced moisture feed into weaker rainfall triggers.

Figure 3. SST Anomaly map from 14 September 2015 showing an El Niño.

Figure 4 is a map for 5 September 2016. A negative IOD can be seen in the cooler/average water temperature off the African coast and the distinct warmer water off the Indonesian coast. This phenomena is the best-known bringer of north west (NW) cloud band activity to Victoria.

Figure 4. SST Anomaly map from 5 September 2016 showing a negative IOD.

Figure 5 is the most current SST Anomaly map and your chance to test your new-found map reading skills.

Figure 5. SST Anomaly map from 13 November 2017.

The Indian Ocean is consistently warm along the African and Indonesian coast line, and therefore a neutral IOD. The Indian Ocean Dipole is mainly active between May to October.

The Pacific Ocean has a section of cool water present along the equator, which is very close to La Niña coldness. If all other aspects play ball (like stronger trade winds along the Equator and a positive SOI) a weak to moderate La Niña is likely.

The Bureau of Meteorology are currently at La Niña watch, you can get the last BoM information on their website. Summer forming La Niña’s are rare and any La Niña over summer has historically had a variable effect on western Victorian rainfall.

With this new found understanding you can look at the next The Fast Break newsletter with a more in depth understanding. If you don’t currently subscribe to The Fast Break, go to The Fast Break website and do so.