Forecast Descriptions

Dynamical Seasonal Climate Forecasts -

The dynamical climate predictions are currently made with 4 different atmospheric general circulation models: CCM3.2, ECHAM3.6, NCEP-MRF9, and NSIPP. Each month, each of these models is run 10 times (forming an ensemble) with one or both of two possible scenarios for the global sea surface temperature (SST), one to three seasons (three to nine months) into the future. In each set of ensemble runs for a given SST scenario, the only difference between the individual runs is the initial atmospheric state (the current weather) at the beginning of the run.

On the web pages for the various dynamical prediction maps/tools a table is presented in which each entry represents a particular model forced by a particular SST scenario. For example, "ECHAM(psst)" refers to the ECHAM model (described below) forced with the PSST (persisted SST described below) scenario.

Predicted climate anomalies, such as anomalies of temperature or precipitation, are calculated as the departure of the model predictions from average conditions for that season. The average conditions are computed for the climatological base period of 1969-1998 (Note: The climatological base period was updated in early 2001. This is close to the 1971-2000 climatology period used by IRI's Monitoring Group and many National Meteorological Services, but still incorporating the higher quality, higher resolution observed data available from the Climate Research Unit of the University of East Anglia). The climate anomalies for each model are taken with respect to each model's average climatology.

The IRI climate prediction system is part of an ongoing experimental research effort. The atmospheric models and the methods for producing SST scenarios (SST forecasts) have changed during the years since IRI began producing seasonal climate predictions in 1994 (predictions since 1997 are available on our web site) and will continue to evolve. The current approach is described below. There is also a link to a page that describes previous approaches to forecasting SSTs, as these older forecasts of SST, and of the climate anomalies resulting from them, are available on our web site.

Atmospheric General Circulation Models (AGCMs) -

CCM3.2 -
This model was developed at the National Centers for Atmospheric Research (NCAR) in the United States. It is freely available to the community. The resolution of the model is T42 (approximately 2.8x2.8 degrees horizontal resolution) with 18 vertical layers.

Initial atmospheric conditions are supplied by restart files from an integration in which CCM3 has been forced with observed SSTA for many years up through the forecast start date. At the beginning of the forecast for the first ensemble member, nine sets of restart files are generated, each for a successive model day, to yield nine additional forecast initial conditions.

ECHAM3.6 -
The European Community - HAMburg (ECHAM) model was developed at the Max Plank Institut fur Meteorolgie in Germany. The resolution of the model is T42 with 19 vertical layers. (Barnett et al, 1994, Tellus, 46A, 381-397; Bengtsson et al, 1993, Science, 261,1026-29).

Initial atmospheric conditions for each forecast ensemble member are supplied by restart files from separate ensemble members of a simulation in which ECHAM has been forced with observed SSTA up through the forecast start date.

NCEP-MRF9 -
The NCEP-MRF9 atmospheric climate model was developed at the National Centers for Environmental Prediction (NCEP) in the United States, based on a version of the medium range forecast model used by the National Weather Service. The resolution of the model is T40 with 18 vertical layers.

Initial atmospheric conditions are derived as with CCM3 from restart files from an integration in which NCEP has been forced with observed SSTA for many years up through the forecast start date. At the beginning of the forecast for the first ensemble member, nine sets of restart files are generated, each for a successive model day, to yield nine additional forecast initial atmospheric conditions.

NSIPP -
This model was developed at the NASA's Seasonal to Interannual Prediction Project (NSIPP) at Goddard Space Flight Center. The resolution of the model is 2.5 degrees longitude by 2.5 degrees latitude with 34 vertical layers. Each forecast consists of a 9 member ensemble.

Initial atmospheric conditions for the nine ensemble members are supplied by restart files from nine integrations in which NSIPP-1 has been forced with observed SSTs for many years up to the start date.

More details on the model and its performance can be obtained at: http://nsipp.gsfc.nasa.gov/atmos/atmosdescrip.html

Sea Surface Temperature Scenarios (SST Forecasts) -

PSST = Persisted SST anomalies :
The global SST anomalies are predicted as unchanging from the latest observed field. The prescribed SST field is thus persisted (held fixed). The observed SST anomalies are taken from the month previous to when the forecast was made and added to the climatological monthly average SSTs to obtain the total SST values that are used as boundary conditions to force the AGCM. For example, for the forcasts made in August, the observed SST anomalies are taken from July, and the forecast season is Sep-Oct-Nov. Dynamical predictions using persisted SST forcing are run only 1 season into the future.

ASST:
The predicted SST field contains forecast SST anomalies for the tropical oceans and damped-persisted observed SST anomalies for the mid-latitude oceans. At this time, the SST predictions are made separately for each of the tropical ocean basins.

In the tropical Pacific Ocean (15S-15N) the forecast SSTs are produced at NCEP using their coupled climate model, CMP12 (Pacific basin ocean/global atmosphere), where the ocean has been initialized with assimilated observed ocean temperature data as measured by the TAO buoy array.

For the tropical Atlantic Ocean (18S-30N) SST anomalies are forecast at CPTEC (Brazil) using the statistical method of CCA (canonical correlation analysis). The predictors for the tropical Atlantic CCA prediction of SST is the recent observed SST anomalies in the tropical Pacific and Atlantic Oceans.

The Indian Ocean SST anomalies (15S-15N) are also forecast using a CCA technique, which is run at the IRI. The predictors for the tropical Indian Ocean SST are the recently observed SST anomalies in the tropical Pacific and Indian Oceans and also the NCEP forecasts for the tropical Pacific.

The tropical oceans where forecast SST anomalies are specified are smoothed into the mid-latitude observed SST anomalies over approximately 8 degress of latitude.

In the mid-latitudes, the SST field is damped from observed initial SST anomalies to climatology (1969-1998 reconstructed SST), with an e-folding time of approximately 90 days.

Previous approaches to global SST prediction:

BSST = blended SST anomalies:
The prescribed SST field is a blend of forecast SST anomalies for the tropical Pacific anomalies based on the tropical Pacific SST forecasts from the NCEP coupled GCM and the Scripps Hybrid Coupled Model (HCM). The observed SSTs for the initial time of forecast are also used in forming the blended SST product for the first season. Weights assigned to the observations and various forecasts change spatially, and with season and forecast lead-time (weights for the observations are zero after the first season). The tropical Indian Ocean SSTs are statistically predicted based on the final forecast for the tropical Pacific.

NSST:
The nsst forecast uses a unique blend of SST anomalies forcing the specified atmospheric GCM. Over the Pacific from 25S to 30N, monthly forecast SSTAs from NCEP are used. The NCEP SSTA forecast is then used to make a statistical estimate of SSTAs in the Indian Ocean as far south as 25S. In the other global oceans, the latest observed SSTAs are persisted. As these data sets are joined together month by month, the SSTAs are smoothed where the different elements come together in order to avoid abrupt SSTA changes.

HCM = Hybrid Coupled Model Predicted SST anomalies:
The prescribed SST field is predicted from the integration of a hybrid coupled ocean-atmosphere model -- the atmospheric component is statistical, and the ocean component is a GCM covering the tropical Pacific. (See Barnett et al., J. Clim., 6 (1993), p. 1545-1566). Over the rest of the ocean basins time weighted values of the current observed SSTA are used. Time weights are 100% for the first four months of a forecast and then decrease to 90% in the fifth month and then linearly to 0% in the ninth month. The initial condition for the integration is the monthly mean fields of the previous month taken from the ocean model driven with statistically-derived (i.e. based on historical SST-surface wind stress relationships) winds. The forecasts presented are the ensemble averages of 10 runs.

FSST = forecast SST anomalies:
(These only exist prior to March 1997) The prescribed SST field is forecast for the tropical Pacific ocean (15S-15N) using the NCEP coupled climate model, CMP12 (Pacific basin ocean/global atmosphere), where the ocean has been initialized with assimilated observational sea temperature data. Elsewhere, the SST field is damped from observed initial SST anomalies to climatology (using 1961-1990 reconstructed SST data); tropical oceans are damped with an e-folding time of 90 days, all extra-tropical oceans are damped to climatology with an e-folding time of 30 days.

MSST = mixed SST anomalies:
The prescribed SST field is a combination of forecast and persisted observed SST in the tropical Pacific. Elsewhere, the SST field is damped from observed initial SST anomalies to climatology (using 1961-1990 reconstructed SST data); tropical oceans are damped with an e-folding time of 90 days; all extra-tropical oceans are damped to climatology with an e-folding time of 30 days.

For example, a forecast made in March, through the upcoming Apr-May-Jun-Jul would use the following weights for the SST fields to obtain the prescribed SST during the integration. Anomalies are added to observed climatology.

Tropical Pacific sst (15S-15N)
             osst      fsst
    FEB-MAR  1.0       0.
    APR      0.75      0.25
    MAY      0.5       0.5
    JUN      0.25      0.75
    Jul-     0.        1.00
 where,  osst: observed sst anomalies
         fsst: forecast sst anomalies

Extratropical sst (poleward of 25N/S):  

    observed sst damped to climatology with e-folding time 30days
    Feb15  1
    Mar15  1/e ~ 0.37
    Apr15  1/(e*e)~ 0.14

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