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19 October 2011

Note on interpreting model forecasts

The following graph and table show forecasts made by dynamical and statistical models for SST in the Nino 3.4 region for nine overlapping 3-month periods. Note that the expected skills of the models, based on historical performance, are not equal to one another. The skills also generally decrease as the lead time increases. Thirdly, forecasts made at some times of the year generally have higher skill than forecasts made at other times of the year--namely, they are better when made between June and December than when they are made between February and May. Differences among the forecasts of the models reflect both differences in model design, and actual uncertainty in the forecast of the possible future SST scenario.

Table 1. Forecast SST Anomalies (deg C) in the Nino 3.4 Region

	Seasons (2011-2012)
Model	OND	NDJ	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA
Dynamical models
NASA GMAO model	-1	-1.2	-1.3	-1.3	-1.2	-1	-0.9
NCEP Coupled Fcst Sys model	-1.3	-1.5	-1.5	-1.4	-1.2	-1.2	-1.2	-1.3
Japan Met. Agency model	-0.5	-0.4	-0.2	-0	0.1
Scripps Inst. HCM	-0.5	-0.7	-0.8	-0.9	-0.8	-0.7	-0.6	-0.4	-0.2
Lamont-Doherty model	-0.4	-0.3	-0.2	-0	0.1	0.3	0.5	0.7	0.8
POAMA (Austr) model	-0.6	-0.4	-0.3	-0.1	0.1	0.3
ECMWF model	-0.8	-0.9	-0.9	-0.8
UKMO model	-1.2	-1.2	-1.2	-1.1
KMA (Korea) SNU model	-0.4	-0.3	-0.1	0	0.1	0.2	0.2	0.3	0.3
ESSIC Intermed. Coupled model	-1.1	-1	-0.8	-0.6	-0.4	-0.3	-0.2	-0.1	-0
ECHAM/MOM	-0.6	-0.5	-0.4	-0.3	-0.2
COLA ANOM	-0	0.2	0.3	0.3	0.1	-0.1	-0.3	-0.5	-0.6
Japan Frontier Coupled model	-1	-1	-1	-1	-1	-1	-0.9	-0.8	-0.8
MÉTÉO FRANCE model	-1	-0.8	-0.7	-0.5	-0.4
COLA CCSM3 model	0.3	0.6	0.8	0.8	0.8	0.5	0.2	-0.2	-0.6
NCEP CFS version 2	-1.5	-2	-2.2	-2	-1.6	-1.3	-1	-0.7
CSIR-IRI 3-model MME	-1	-1.2	-1.4	-1.4	-1.1
Average, dynamical models	-0.7	-0.7	-0.7	-0.6	-0.4	-0.4	-0.4	-0.3
Statistical models
NCEP/CPC Markov model	-1	-0.9	-0.8	-0.6	-0.5	-0.3	-0.1	0	0.1
NOAA/CDC Linear Inverse	-0.4	-0.4	-0.4	-0.4	-0.4	-0.4	-0.4	-0.3	-0.3
NCEP/CPC Constructed Analog	-0.6	-0.6	-0.6	-0.6	-0.5	-0.4	-0.3	-0.2	-0.1
NCEP/CPC Can Cor Anal	-0.1	-0.2	-0.4	-0.4	-0.4	-0.3	-0.3	-0.2	-0.2
Landsea/Knaff CLIPER	-0.6	-0.5	-0.4	-0.4	-0.2	-0.1	0.1	0.2	0.3
Univ. BC Neural Network	-0.7	-0.6	-0.5	-0.5	-0.4	-0.3	-0.1	0	0.1
FSU Regression	-0.7	-0.6	-0.6	-0.5	-0.4	-0.3	-0.1	0	0.1
TDC - UCLA	-0.6	-0.7	-0.7	-0.7	-0.5	-0.3	-0.1	0	0.1
Average, statistical models	-0.6	-0.6	-0.5	-0.5	-0.4	-0.3	-0.2	-0.1	0
Average, all models	-0.7	-0.7	-0.6	-0.6	-0.4	-0.3	-0.3	-0.2	-0.1

Discussion of current forecasts

Approximately half of the set of dynamical and statistical model predictions issued during late September and early October 2011 predict neutral ENSO conditions from the October-December 2011 season currently in progress, through the early part of 2012, with most of the other half predicting La Nina conditions. However, when considering the latest observations, including particularly the negative heat content anomalies beneath the sea surface in the eastern tropical Pacific, for the October-December season the probabilities for La Nina, neutral and El Nino conditions are estimated at 78%, 22% and close to 0%, respectively. At the time of preparing this, the SST observations in the NINO3.4 region were in the weak La Nina range, with an area-averaged weekly anomaly of -0.8 C in the most recent week. Current predictions and observations indicate probabilities of 75% and 70% for La Nina conditions during the November-January and December-February periods, respectively, with corresponding probabilities for ENSO-neutral conditions of 25% and 29%. Probabilities for El Nino conditions are 5% or less through Feb-Apr 2012. After the first quarter of 2012, probabilities for each ENSO condition return toward their climatological values of 25%, 50% and 25% for La Nina, neutral and El Nino, respectively.

Summary of forecasts issued over last 22 months

The following plots show the model forecasts issued not only from the current month (as in the plot above), but also from the 21 months previous to this month. The observations are also shown up to the most recently completed 3-month period. The plots allow comparison of plumes from the previous start times, or examination of the forecast behavior of a given model over time. The first plot shows forecasts for dynamical models, the second for statistical models, and the third for all models. For less difficult readability, forecasts are shown to a maximum of only the first five lead times. Below the third plot, we provide a mechanism for highlighting the forecasts of one model at a time against a background of more lightly colored lines for all other models.

Individual Model View over last 22 months

View individual models here.

Additional information on Models

Notes on the data 

Only models producing forecasts on a monthly basis are included. This means that some models whose forecasts appear in the Experimental Long-Lead Forecast Bulletin (produced by COLA) do not appear in the table.

The SST anomaly forecasts are for the 3-month periods shown, and are for the Nino 3.4 region (120-170W, 5N-5S). Often, the anomalies are provided directly in a graph or a table by the respective forecasting centers for the Nino 3.4 region. In some cases, however, they are given for 1-month periods, for 3-month periods that skip some of the periods in the above table, and/or only for a region (or regions) other than Nino 3.4. In these cases, the following means are used to obtain the needed anomalies for the table:

• Temporal averaging, 
• Linear temporal interpolation 
• Visual averaging of values on a contoured map 

The anomalies shown are those with respect to the base period used to define the normals, which vary among the groups producing model forecasts. They have not been adjusted to anomalies with respect to a common base period. Discrepancies among the climatological SST resulting from differing base periods may be as high as a quarter of a degree C in the worst cases. Forecasters are encouraged to use the standard 1971-2000 period as the base period, or a period not very different from it.