Section 8.3.4.1 Nino Plumes

Niño Plumes

Nino plume charts show bias corrected ensemble member forecasts of the anomaly of sea-surface temperatures within defined El Niño regions of the Pacific Ocean. This allows forecasters to assess consistency of the forecasts for the upcoming months, to see the strength of a forecast El Niño (or ENSO) event or La Niña event, and to use the spread of results as an indicator of confidence.

Fig8.3.4.1-1: Location of the Niño regions for measuring sea-surface temperature in the eastern and central tropical Pacific Ocean.

Anomaly Plumes

Nino plumes are provided for forecasts starting on the first of every month. The chart provides information on the predicted evolution of the sea surface temperature (SST) anomalies for four different pre-defined “El Niño Regions” over the coming seven months to give an indication of potential ENSO (El Niño - Southern oscillation).

Fig8.3.4.1-2: El Niño plume from 7 month SEAS5 ensemble forecast. The plume shows bias-corrected ensemble member sea-surface temperature forecasts as an anomaly from climatological values (produced by ERA5) within the Niño Region 3.4 in the Pacific Ocean (red lines). In this case almost all ENS members show a steady increase in the sea surface temperature anomaly. Just two or three members show a weaker rise. A confident forecast of the onset of an El Nino event can be made. Blue dots are previously observed values.

Nino annual plumes are provided for forecasts starting on 1 February, May, August and November. For these dates, some ensemble members are continued to 13 months to give an outlook for ENSO (El Niño - Southern oscillation).

The chart provides information on the predicted evolution of the sea surface temperature (SST) anomalies for four different pre-defined “El Niño Regions” over the coming thirteen months.

Fig8.3.4.1-3: Outlook El Niño plume from 13 month SEAS5 ensemble forecast. The plume shows bias-corrected ensemble member sea-surface temperature forecasts as an anomaly from climatological values (produced by ERA5) within the Niño Region 3.4 in the Pacific Ocean (red lines). In this case almost all ENS members show a steady increase in the sea surface temperature anomaly followed by steady fall. Just two or three members show a weaker rise and subsequent fall in the anomaly while three members maintain the higher temperature anomaly with no indication of a fall . A confident forecast of the onset of an El Nino event can be made followed fairly confidently of a weakening of El Nino later. Blue dots are previously observed values.

Verification Plots

Verification plots for the Nino plumes and Nino annual plumes are also provided. They are updated with the forecast every month and show verification statistics for forecasts from 1993 to the previous year, starting in the specified month.

Sea surface temperature root mean square errors

The diagram shows:

the Root Mean Square (RMS)error of the ensemble (red line).
the spread of the RMS error of the ensemble (red dashed line).
the error obtained by persisting the initial anomaly (black dot-dashed line). This allows an assessment of the reliability of the width of the Nino plumes.

Comparison of the size of the spread with the forecast error shows the extent to which the forecast plume tends to be over- or under-dispersive.

Fig8.3.4.1-4:

Fig8.3.4.1-5:

The mean square skill score

The mean square skill score measured against climatology gives an overall measure of the skill of the forecast (1 = perfect forecast , 0 = no better than climatology).

The diagram shows the skill score:

of SEAS5 forecast of sea surface temperature (red line).
by persisting the initial anomaly (black dot-dashed line).

Inspection of the diagram:

indicates how much of the variation of observed sea surface temperature is being correctly forecast.
gives a sense of the lead time over which the forecast retains useful skill.