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Forecast of Pacific­Indian Ocean SSTs Using

Linear Inverse Modeling

contributed by Cecile Penland1, Klaus Weickmann2 and Catherine Smith1

1CIRES, University of Colorado, Boulder, Colorado

2Climate Diagnostics Center (CDC), ERL/NOAA, Boulder, Colorado

Using the methods described in Penland and Magorian (1993) and in previous issues of this Bulletin (particularly the December 1992 and June 1993 issues), the pattern of IndoPacific sea-surface temperature anomalies (SSTAs), the SSTA in the Nino 3 region (6oN­6oS, 90 o­150 oW), and the SSTA in the Nino 4 region (6oN­6oS, 150 oW-160 oE) are predicted. A prediction at lead time t is made by applying a statistically-obtained Green function G(t) to an observed initial condition consisting of SST anomalies (SSTAs) in the IndoPacific basin. Three-month running means of the temperature anomalies are used, the seasonal cycle has been removed, and the data have been projected onto the 20 leading empirical orthogonal functions (EOFs) explaining about 73% of the variance. The Nino 3 region has an RMS temperature anomaly of about 0.7oC; the inverse modeling prediction method has an RMS error of about 0.5oC at a lead time of nine months and approaches the RMS value at lead times of 18 months to two years. The COADS 1950-79 climatological annual cycle has been removed.

The predicted IndoPacific SSTA patterns based on the Mar-Apr-May 1996 initial condition for the following Jun-Jul-Aug, Sep-Oct-Nov, Dec-Jan-Feb 1996-97 and Mar-Apr-May 1997 are shown in Fig. 1 (contour interval is 0.2oC). Figure 2a shows the predictions (light solid or dotted lines) of the NiZo 3 anomaly for initial conditions Dec-Jan-Feb 1995-96, Jan-Feb-Mar, Feb-Mar-Apr and Mar-Apr-May 1996. Dashed lines indicate the 1 standard deviation expected error for the prediction based on the Dec-Jan-Feb initial conditions. Predictions based on Dec-Jan-Feb 1995-96 and Mar-Apr-May 1996 initial conditions are indicated by arrows in the graphs. Figure 2b is the same, but for the NiZo 4 region. Verifications consisting of contributions to the appropriate region by the 20 leading EOFs (heavy dashed line) and those including the truncation error (heavy solid line) are also shown.

The initial condition in Fig. 1 is not much different from the 3-month forecast published in the March issue of this Bulletin with one major exception: SSTAs in the far eastern south tropical Pacific (70-90EW, 2-26ES) are colder than predicted. We believe this to be a consequence of a transition to strong southeasterly trades there which occurred in April. This event appears to have affected a key area of the predictor region so that the forecast for the rest of this year is for continued cool, or even colder, SSTAs in the eastern tropical Pacific rather than the warm anomalies previously forecast.

Penland, C. and T. Magorian, 1993: Prediction of NiZo 3 sea­surface temperatures using linear inverse­ modeling. J. Climate, 6, 1067­1076.

Figures

Fig. 1. Linear inverse modeling forecasts of SST anomalies, relative to the standard 1950-79 COADS climatology both for the training period (1950-84) and for these forecasts. Forecast anomalies are projected onto 20 leading EOFs, based on Mar-Apr-May 1996 initial conditions (top panel). Contour interval is 0.2oC. Positive anomalies are represented by heavy solid lines, negative anomalies by dashed lines. SST anomaly data have been provided by NCEP, courtesy of R.W. Reynolds. Prediction by linear inverse modeling is described in Penland and Magorian (1993).

Fig. 2 (a): Inverse modeling predictions (light solid or dotted lines) of NiZo 3 SSTA for initial conditions Dec-Jan-Feb 1995-96, Jan-Feb-Mar, Feb-Mar-Apr and Mar-Apr-May 1996. Dashed lines indicate the 1 standard deviation expected error for the Dec-Jan-Feb prediction. Heavy dashed line indicates the contribution to the verification from the 20 leading EOFs; the heavy solid line indicates the verification including the truncation error. (b): As in (a) except for Nino 4. Prediction by linear inverse modeling is described in Penland and Magorian (1993).

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