What an ensemble-mean anomaly shows

The ensemble mean anomaly is the average departure of all forecast members from the model climate. What it can and cannot tell you about the coming season.

Members, mean and anomaly

A seasonal forecast is not one run of a model but an ensemble: a few dozen runs started from slightly different initial states. The ensemble mean is the average of all of them. The anomaly is that mean minus the model’s own climatological average for the same start month and lead, taken from its hindcasts. The ensemble-mean anomaly map on this site is therefore “how far, on average, this model expects the month to depart from its normal”.

What the mean is good at

Averaging cancels the noise that individual members carry and leaves the part of the signal that many members share. Large-scale patterns such as a warm anomaly across a whole continent or a tropical Pacific SST anomaly show up clearly. Because every model on this site is expressed relative to its own hindcast climate, the maps of different centres can be compared directly even though their absolute climatologies differ.

What the mean hides

The mean says nothing about spread. A +0.5 °C anomaly can come from fifty members that all sit near +0.5, or from half the members at +2 and half at −1. Both are plausible seasonal forecasts and they mean very different things for planning. Anomalies also shrink at longer leads, because members disagree more and their departures cancel. A weak anomaly at lead 5 is usually low confidence, not a confident forecast of normal conditions.

Units and conversions

Temperature and SST anomalies are published in kelvin and shown in °C without any offset, because a difference of one kelvin is a difference of one degree Celsius. Precipitation is an anomalous accumulation rate, converted to mm/day. Pressure is converted from Pa to hPa. No further smoothing, masking or bias correction is applied to the C3S ensemble means.

For a view of the distribution rather than its centre, switch the forecast type to a tercile or quintile probability.