Dynamically Based Forecasts
for Tropical Pacific SST through Winter 1997-98
Using an Improved Hybrid Coupled Ocean-Atmospheric Model
contributed by Tim Barnett1, David Pierce1,
Nicholas Graham1 and Mojib Latif 2
1Scripps Institution of Oceanography, La Jolla, California
2Max Planck Institut fur Meteorologie, Hamburg, Germany
In past issues of this Bulletin (e.g. March and June of 1994, September
1995), forecasts of the tropical Pacific SST were presented using a hybrid
coupled ocean-atmosphere model (HCM) developed jointly at Scripps Institution
and the Max Planck Institute for Meteorology (MPI) (Barnett et al. 1993).
In the September 1996 issue, the forecast of an improved version of the
HCM (called HCM-3) was presented and compared with the forecast of the
original HCM (called HCM-1). Starting with this issue, only forecasts of
the new version of the HCM will be shown.
The original HCM-1 ocean model, created at MPI (Latif 1987), is
a fully nonlinear GCM bounded by 30oN and 30oS latitude and by Asia and
South America. It has 13 vertical levels, 10 of which are within the top
300 m. The seasonal cycle is governed by a Newtonian heat flux and observed
wind stress (Goldenberg and O'Brien 1981). The vertical mixing scheme is
dependent upon the Richardson number (Pacanowski and Philander 1981). The
atmospheric model is statistical, deriving the wind stress forcing for
the ocean GCM using the GCM's SST. This is done with a CCA-like regression
model, using historical observed data fields of anomalous SST and the corresponding
wind stress. The coupling process includes a MOS-like statistical correction
of the SST fields produced by the ocean GCM. The hybrid coupled model is
initialized with wind stress fields derived from observed SST data; thus,
it is indirectly "spun up" with SST information. Over the 1965-93
period the model demonstrated statistically significant predictive skill
out to 12 to 18 months, with best performance for the central equatorial
Pacific and for winter forecasts. (The skill distribution is shown in Barnett
et al. 1993 and in the March 1994 issue of this Bulletin.) The model was
developed using data from 1965-85, leaving 1986 and later for independent
forecasting.
The improved HCM-3 (Pierce et al. 1997) is based on the same strategy
used in the original HCM-1 described above. The main difference is in the
ocean GCM used. The ocean model is the HOPE2 from the Max Planck Institute
in Hamburg (Wolff and Maier-Reimer, 1992). The model resolution is approximately
the same as before. However, the numerical scheme has been improved to
significantly reduce the numerical diffusion, especially in the vertical.
The result is a much better representation of the main thermocline across
the tropical Pacific. A MOS corrector is still used, but in most cases
and areas the magnitude of the correction is 1C and generally less--a distinct
improvement over the old model. Statistical atmospheres were constructed
using both the FSU and the da Silva (da Silva et al. 1994) data sets. Model
performance was independent of which set was used, as long as a 3 to 5
month smoother was applied to the wind stress prior to model construction.
The final model used the da Silva data for the wind field.
The HCM-3 model performs much better in the hindcast mode than
did its predecessor. Hindcast correlational skill scores exceeding 0.8
for 3 to 6 month lead times now cover virtually the entire tropical Pacific,
dropping to 0.6 in the far western Pacific where the old model had negative
skills. The skill also remains high almost to the South American coast,
a region that also had negative skill in HCM-1. Preliminary evaluation
of independent sample forecast skills show they are about comparable to
those of the Lamont and NCEP (formerly NMC) models. As was found with the
Lamont model, the forecast skills for the 1980s and early 1990s in HCM-3
are much higher (exceeding 0.8 over a large region) than were the skills
during the 1970s.
Regular readers recall that from around spring 1995 to spring 1996,
HCM-1 forecasted a warm event for winter 1996-97. During mid-1996 these
forecasts diminished in magnitude, and the model also began initializing
less accurately.
The final HCM-1 forecast, presented in the September issue of this
Bulletin, still called for mild warm conditions for the tropical Pacific
for the present winter. In that issue, the first HCM-3 forecast was presented,
also showing slight warming for this winter, but to a still lesser extent.
In fact, the values were not significantly different from "normal"
conditions. In the current forecast, made the last week of November, this
has changed to a prediction of mild cooling in the region for Dec-Jan-Feb
1996-97.
Looking at the longer lead times, the HCM-3 is predicting steady
warming throughout 1997, peaking in early 1998 with anomalies in the range
of 1.0 to 1.2C. The model's warm anomalies start at the coast of South
America, and subsequently stretch west past the date line.
Caveat: The forecasts shown above are experimental in nature. The
reader is forewarned that the methods/forecasts are new and subject to
future change and improvement.
Acknowledgment: This work is supported by NOAA and the National
Science Foundation's Climate Dynamics Division.
Barnett, T.P., M. Latif, N. Graham, M. Flugel, S. Pazan and W.
White, 1993: ENSO and ENSO-related predictability: Part 1 - Prediction
of equatorial Pacific sea surface temperatures with a hybrid coupled ocean-atmosphere
model. J. Climate, 6, 1545-1566.
Da Silva, A.M., C.C.Young and S. Levitus, 1994: Atlas of surface
marine data 1994, Vol. 1: Algorithms and procedures. NOAA Atlas NESDIS
6, U.S. Department of Commerce, 83 pp.
Goldenberg, S.D. and J.J. O'Brien, 1981: Time and space variability
of tropical Pacific wind stress. Mon. Wea. Rev., 109, 1190-1207.
Latif, M., 1987: Tropical ocean circulation experiments. J.
Phys. Oceanogr., 17, 246-263.
Pacanowski, R.C. and S.G.H. Philander, 1981: Parameterization of
vertical mixing in numerical models of tropical oceans. J. Phys. Oceanogr.,
11, 1443-1451.
Pierce, D., J. Ritchie and T.P. Barnett, 1997: An improved hybrid
coupled model for tropical SST prediction. In preparation.
Wolff, J.-O. And E Maier-Reimer, 1992: HOPE, the Hamburg ocean
primitive equation model. 81 pp. Available from Max Planck Institut fur
Meteorologie, Hamburg, Germany.
Fig. 1. Scripps/MPI hybrid coupled model
(HCM-3) forecast of the field of tropical Pacific SST anomaly (oC) for
DJF 1996-97, JJA and SON 1997, and DJF 1997-98. Observed data up to November
23 1996 are used. Contour interval 0.25oC.