Climate Model Summary

Climate Model Summary Archive: September 2015

El Niño set to persist at strongest values since 1997-98

El Niño continues to strengthen with sea surface temperatures in the central tropical Pacific now at levels not seen since the 1997-98 El Niño. Model outlooks, as well as all atmospheric and oceanic indicators, indicate further intensification is likely.

Temperatures in the NINO3.4 area of the central tropical Pacific are now at +2.0 °C. The all-model average forecast suggests this area could warm to 2.7 °C above average in December before weakening in early 2016.

The Indian Ocean Dipole (IOD) index has been at or above the positive threshold (+0.4 °C) for six weeks. If the index stays at this level for another fortnight, this will be considered a positive IOD event. Most models indicate this is likely. Positive IOD events tend to reduce rainfall in southern and central Australia during the winter and spring months, and can exacerbate rainfall deficiencies caused by El Niño.

While El Niño and the IOD are considered to dominate the Australian climate at specific times of year, other factors may have influence on current climate outlooks—see the Climate Outlooks page for the latest rainfall and temperature outlooks.

NINO3.4 outlook

The latest NINO3.4 forecasts (initialised in September) indicate that sea surface temperatures across the central tropical Pacific Ocean are likely to peak towards the end of the year, followed by a rapid weakening heading into autumn 2016. The all-model average NINO3.4 outlook for December is just above +2.7 °C, but drops to +2.1 °C by February. Individual model values range between +2.3 °C and +3.2 °C for December; all well above the El Niño threshold and indicative of a substantial El Niño.

The most recent NINO3.4 value is +2.0 °C for the week ending 13 September 2015. NINO3.4 values above +0.8 °C typically occur during El Niño events.

Indian Ocean Dipole (IOD) outlook

The Indian Ocean Dipole (IOD) index has been above the positive IOD threshold (+0.4 °C) for six weeks; the most recent weekly index value is +0.9 °C (13 September 2015). Should values persist above the threshold for another fortnight, it will be declared a positive IOD event. Four out of five surveyed models indicate this is likely, with forecasts suggesting the IOD index will remain above the positive IOD threshold until at least November. A positive IOD typically reduces winter–spring rainfall in central and southern Australia, and can therefore exacerbate El Niño driven rainfall deficiencies.

The following graph shows the average forecast value of the IOD index for each international model surveyed for the selected calendar month. If the majority of models are approaching or exceeding the blue dashed line, then there is an increased risk of a negative IOD event. If the majority of models are approaching or exceeding the red dashed line, then there is an increased risk of a positive IOD event.

Predictive Ocean Atmosphere Model for Australia (POAMA)

POAMA, run at the Bureau of Meteorology, generates an eight-month forecast each fortnight. The most recent model run (started on 15 September) suggests the central tropical Pacific Ocean will continue to warm, peaking towards the end of the year at values close to, or just exceeding, the peak NINO3.4 value of the 1997-98 El Niño. The following forecast values (at the top of the table) are for POAMA's NINO3.4 ensemble mean.

POAMA archive data is available here.

ENSO Dials

The arrows on the dials below indicate the combined average ofmonthly NINO3.4 outlooks from a survey of international global climate models. Note that the individual model runs vary around the average.

ENSO dial month+1 ENSO dial month+3 ENSO dial month+5

NINO3.4 outlook bar-graphs

The following graphs shows the average forecast value of NINO3.4 for each international model surveyed for the selected calendar month. If the bars on the graph are approaching or exceeding the blue dashed line, there is an increased risk of La Niña. Similarly, if the bars on the graph are approaching or exceeding the red dashed line, there is an increased chance of El Niño.

NINO3.4 bar graph month+1 NINO3.4 bar graph month+3 NINO3.4 bar graph month+5

IOD outlook bar-graphs

The following graph shows the average forecast value of the IOD index for each international model surveyed for the selected calendar month. If the majority of models are approaching or exceeding the blue dashed line, then there is an increased risk of a negative IOD event. If the majority of models are approaching or exceeding the red dashed line, then there is an increased risk of a positive IOD event.

IOD bar graph month+1 IOD bar graph month+3 IOD bar graph month+5

Past climate model summaries