Current Information
Summary of ENSO Model Forecasts
16 October 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
September and early October shows a range of possible sea surface temperature
conditions for the coming 3 to 8 months (November-December-January 2002-03
through June-July-August 2003). Nearly all 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 0.7 degrees C or more above average in the Nino 3.4 region for
the December-January-February seasonal average). A small number are forecasting
ENSO conditions in the neutral category--less than 0.7 degrees C away from
normal. The warmest forecast for the December-January-March period comes
from the Lamont-Doherty (LDEO) simple coupled model from Columbia University,
U.S. (1.78 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
December-January-February tend to be forecast also for slightly later periods,
such as February-March-April 2003, although a decline toward normal in
indicated by many models by that time. For this later time, 12 of the 14
models that forecast to that long a lead time still suggest El Nino conditions
(0.6 degrees or more for that season): 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 February-March-April.
Table 1. Forecast SST Anomalies (deg C) in the
Nino 3.4 Region
| |
Season (2002-2003) |
| Model |
NDJ |
DJF |
JFM |
FMA |
JFM |
AMJ |
MJJ |
JJA |
| Dynamical models |
| NASA/NSIPP model |
0.8 |
0.7
|
0.6
|
0.7
|
0.9
|
1.1
|
1.1
|
1.0
|
| NCEP Coupled model |
0.8
|
1.0
|
1.0
|
0.9
|
0.7
|
0.6
|
0.6
|
|
| Japanese Met. Agcy. model |
1.3
|
1.2
|
1.0
|
0.9
|
0.9
|
|
|
|
|
Scripps Inst. model
|
0.8
|
0.8
|
0.8
|
0.7
|
0.6
|
0.5
|
0.4
|
0.3
|
| Lamont-Doherty model |
1.7
|
1.8
|
1.5
|
1.1
|
0.7
|
0.4
|
0.2
|
|
| BMRC intermed. model |
1.1
|
1.2
|
1.2
|
1.0
|
0.8
|
0.6
|
0.6
|
|
| CSIRO model |
0.9
|
0.8
|
0.8
|
0.8
|
0.7
|
0.6
|
0.5
|
|
| ECMWF model |
1.2
|
1.1
|
1.0
|
|
|
|
|
|
| KMA SNU (Korea) model |
1.2
|
1.2
|
1.1
|
1.0
|
0.9
|
0.8
|
0.6
|
0.5
|
| Average, dynamical models |
1.1
|
1.1
|
1.0
|
0.9
|
0.8
|
0.7
|
0.6
|
|
| Statistical models |
| NCEP/CPC Markov model |
1.5
|
1.6
|
1.5
|
1.4
|
1.3
|
1.3
|
1.2
|
1.1
|
| NOAA/CDC Linear Inverse |
0.0
|
-0.1
|
-0.3
|
-0.3
|
-0.4
|
-0.4
|
-0.4
|
-0.3
|
| Dool Constructed Analog |
0.9
|
0.9
|
0.7
|
0.5
|
0.3
|
0.1
|
-0.1
|
-0.4
|
| NCEP/CPC Can Cor Anal |
1.1
|
1.2
|
1.1
|
1.0
|
1.0
|
1.0
|
0.9
|
0.9
|
| Landsea/Knaff CLIPER |
1.2
|
1.0
|
0.9
|
0.8
|
0.6
|
0.4
|
0.2
|
-0.1
|
| Univ. BC nonlinear Can Cor |
1.5
|
1.4
|
1.4
|
1.3
|
1.1
|
1.0
|
1.0
|
1.0
|
| Average, statistical models |
1.0
|
1.0
|
0.9
|
0.8
|
0.7
|
0.6
|
0.5
|
0.4
|
| Average, all models |
1.1
|
1.0
|
1.0
|
0.8
|
0.7
|
0.6
|
0.5
|
0.4
|
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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