Climate Model Summary

Climate Model Summary Archive: March 2015

Model consensus indicates El Niño by winter 2015

The tropical Pacific Ocean remains in a neutral ENSO state, but all surveyed models suggest temperatures in the central tropical Pacific Ocean are likely to increase above El Niño thresholds by June 2015, with further warming by August.

As a result of severe tropical cyclone Pam in the southern hemisphere, and tropical storm Bavi (category 1 tropical cyclone on the Australian scale) in the northern hemisphere, a strong reversal of the trade winds occurred in the equatorial Pacific just west of the Date Line. This is likely to initiate a downwelling Kelvin Wave and subsequent warming in the ocean sub-surface and potentially surface.

Model forecasts spanning the traditional ENSO transition period, February to May, generally have lower accuracy than forecasts made at other times of year—these forecasts should be treated with caution. However, on this occasion, all eight models are consistent in their outlooks for a warming to occur over the coming months in the tropical Pacific Ocean.

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. However, the IOD index has just recovered from a period of consistently negative values. The likelihood of an IOD event developing in the 2015 winter will be monitored closely as the influence of the monsoon weakens during the austral autumn.

NINO3.4 outlook

The latest NINO3.4 forecasts (initialised in March) 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 around June. All models suggest the central Pacific Ocean will continue to warm past mid-year, with all the August (ensemble mean) forecasts for NINO3.4 more than 1 °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 and the Pacific can undergo rapid change. Model outlooks for predictions made during this time should be used carefully.

The most recent NINO3.4 value is +0.3 °C for the week ending 15 March 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.

Although the Indian Ocean Dipole (IOD) remains neutral, over the last six weeks the IOD index has dropped below the negative IOD threshold (–0.4 °C) for an extended period. This week, the IOD index has returned to neutral values. 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 1 March) predicts NINO3.4 is likely to warm throughout the austral autumn and winter months, with El Niño thresholds likely to be reached by June. POAMA's forecast skill is lower at this time of year than at other times, due to the autumn predictability barrier. 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