Climate Model Summary

Climate Model Summary Archive: April 2015

Models maintain outlooks for El Niño though winter and spring

The tropical Pacific Ocean has warmed over the last month, with surface temperatures now just shy of typical El Niño values. All surveyed models suggest further warming will occur, potentially exceeding El Niño thresholds in May. Current wind and sub-surface patterns also support further warming of sea surface temperatures in the central and eastern tropical Pacific.

Model forecasts spanning the traditional ENSO transition period, February to May, generally have lower accuracy than forecasts made at other times of year. However, as we are nearing the end of this period, and with all eight models consistent in their outlooks for further warming in the Pacific Ocean, these outlooks cannot be discounted.

The Indian Ocean Dipole (IOD) generally has little impact on Australian climate from December to April due to the natural position of the monsoon trough at this time of year. The latest model outlooks for the IOD index from May are mixed with little to suggest a transition to either a positive or negative phase of the IOD during the current forecast period.

NINO3.4 outlook

The latest NINO3.4 forecasts (initialised in April) indicate the tropical Pacific Ocean is likely to warm over the coming months, with all models indicating El Niño thresholds are likely to be reached or exceeded in May. All models suggest the central Pacific Ocean will continue to warm past mid-year, with the average September (ensemble mean) forecasts for NINO3.4 more than 1.6 °C above normal.

Model forecasts spanning the February to May period tend to have reduced accuracy, and hence greater spread. This period is known as the "predictability barrier" as the temperature gradients across the tropical Pacific Ocean naturally weaken at this time of year, meaning the Pacific can undergo rapid change. Model outlooks for predictions made during this time should generally be used carefully, though a degree of confidence can be placed in the current outlooks as there is strong consensus among the models that warming above El Niño thresholds will occur by June.

The most recent NINO3.4 value is +0.7 °C for the week ending 12 April 2015. NINO3.4 values between –0.8 °C and +0.8 °C typically indicate neutral ENSO conditions.

Indian Ocean Dipole (IOD) outlook

The IOD does not affect Australian climate from December to April. This is due to the monsoon trough moving southwards over the regions of ocean where the IOD occurs. This alters the wind patterns, and results in negative or positive IOD patterns being unable to form during these months.

Current forecasts beyond the December to April low influence period suggest the IOD will remain neutral until at least spring. The state of the tropical Indian Ocean will be monitored closely as the influence of the monsoon weakens during the austral autumn, and the IOD starts to have more influence on Australian climate.

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 12 April) predicts NINO3.4 is likely to continue to warm throughout the forecast period. All ensemble members (forecast scenarios) anticipate El Niño thresholds to be met in May and remain above El Niño thresholds until the end of the year. This agreement amongst ensemble members suggests a higher degree of predictability, despite being in a period where model skill is generally low. The following forecast values (at the top of the table) are for POAMA's 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