Current Information
Summary of ENSO Model Forecasts
17 September 2002
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 eight 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 October than when they are made
between January and April. 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.
Discussion of current forecasts
The set of dynamical and statistical model forecasts issued during late
August and early September shows a range of possible sea surface temperature
conditions for the coming 3 to 8 months (October-November-December 2002
through May-June-July 2003). Most models are indicating a continuation
of warm conditions. Most of the models forecast warming sufficient to be
called an El Nino (e.g., warming to 0.6 degrees C or more above average
in the Nino 3.4 region for the November-December-January seasonal average).
A small number are forecasting ENSO conditions in the neutral category--less
than 0.6 degrees C away from normal. The warmest forecast for the November-December-January
period comes from the Lamont-Doherty (LDEO) simple coupled model from Columbia
University, U.S. (1.65 degrees C above normal), and the coldest one is
from the NOAA CDC's statistical Linear Inverse Model, calling for SST anomalies
0.1 to 0.2 degrees C below normal. The SST anomalies forecast by the models
for November-December-January tend to be forecast also for slightly later
periods, such as January-February-March 2003, but a decline toward normal
begins to set in around that time. For this later time, 12 of the 14 models
that forecast to that long a lead time still suggest El Nino conditions:
the NASA/NSIPP, the NCEP, JMA, Scripps, LDEO, BMRC, CSIRO, Korea SNU, CPC
Markov, CPC-CCA, Colorado State CLIPER, and the UBC nonlinear CCA. The
CPC Constructed Analogue and CDC Linear Inverse models predict SSTs that
fall short of El Nino levels for January-February-March, although the Constructed
Analogue indicates El Nino SST levels for the earlier November-December-January
period.
Table 1. Forecast SST Anomalies (deg C) in the
Nino 3.4 Region
| |
Season (2002-2003) |
| Model |
SON |
OND |
NDJ |
DJF |
JFM |
FMA |
MAM |
AMJ |
| Dynamical models |
| NASA/NSIPP model |
1.1 |
1.1
|
1.0
|
1.0
|
1.0
|
1.1
|
1.1
|
1.1
|
| NCEP Coupled model |
0.8
|
1.0
|
1.0
|
0.9
|
0.7
|
0.6
|
0.6
|
|
| Japanese Met. Agcy. model |
1.1
|
1.1
|
0.9
|
0.7
|
0.6
|
|
|
|
|
Scripps Inst. model
|
1.1
|
1.0
|
1.0
|
0.9
|
0.8
|
0.7
|
0.6
|
0.4
|
| Lamont-Doherty model |
1.5
|
1.6
|
1.7
|
1.5
|
1.3
|
1.0
|
0.8
|
|
| BMRC intermed. model |
1.0
|
1.0
|
1.1
|
1.2
|
1.2
|
1.1
|
1.0
|
|
| CSIRO model |
0.7
|
0.8
|
0.9
|
0.9
|
0.9
|
0.8
|
0.6
|
0.5
|
| ECMWF model |
1.0
|
1.1
|
1.1
|
|
|
|
|
|
| KMA SNU (Korea) model |
1.1
|
1.1
|
1.0
|
1.0
|
0.9
|
0.8
|
0.7
|
0.6
|
| Average, dynamical models |
1.1
|
1.1
|
1.1
|
1.0
|
0.9
|
0.8
|
0.8
|
0.7
|
|
|
| Statistical models |
| NCEP/CPC Markov model |
1.2
|
1.3
|
1.4
|
1.3
|
1.2
|
1.2
|
1.1
|
1.1
|
| NOAA/CDC Linear Inverse |
0.0
|
-0.2
|
-0.3
|
-0.3
|
-0.3
|
-0.4
|
-0.4
|
-0.3
|
| Dool Constructed Analog |
0.7
|
0.6
|
0.6
|
0.5
|
0.3
|
0.2
|
0.1
|
-0.1
|
| NCEP/CPC Can Cor Anal |
1.1
|
1.2
|
1.2
|
1.2
|
1.0
|
0.9
|
0.8
|
0.7
|
| Landsea/Knaff CLIPER |
1.1
|
1.1
|
1.1
|
0.9
|
0.8
|
0.6
|
0.4
|
0.2
|
| Univ. BC nonlinear Can Cor |
1.6
|
1.5
|
1.4
|
1.3
|
1.2
|
1.1
|
1.0
|
1.0
|
| Average, statistical models |
0.9
|
0.9
|
0.9
|
0.8
|
0.7
|
0.6
|
0.5
|
0.4
|
| Average, all models |
1.0
|
1.0
|
1.0
|
0.9
|
0.8
|
0.7
|
0.6
|
0.5
|
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
-
Regional SST anomaly adjustment using the climatological variances of one
region versus that of another
As an example of the last case, suppose only the Nino 3 anomaly is provided.
The Nino 3.4 anomaly is then obtained by decreasing the Nino 3 anomaly
by the factor defined by the ratio of the year-to-year variance of Nino
3.4 to the year-to-year variance of Nino 3 SST, for the 3-month season
in question.
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.
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