Prediction of Niño 3 SST Anomaly in a Hybrid Coupled Model

with Piggy-back Data Assimilation

contributed by Hsin-Hsin Syu¹ and J. David Neelin²

¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

²Department of Atmospheric Sciences, University of California, Los Angeles, California

A hybrid coupled model (HCM) similar to the one used in Syu et al. (1995), Waliser et el. (1994) and Blanke et al. (1997), is used to predict the Niño 3 SST anomaly (SSTA). The atmospheric model is estimated from observations using a singular value decomposition (SVD) technique; hence it is empirical. It contains the first seven SVD modes of the covariance matrix calculated from the time series of pairs of observed monthly mean Reynolds SST and FSU pseudo-stress fields for the 19-year period of 1970-88. Atmospheric spin-up time, which was neglected in the previous version, is parameterized, albeit crudely, in the current version within the coupling procedures. A 60-day spin-up time scale is chosen for all ENSO simulations and predictions. Heat flux is parameterized according to Oberhuber's (1988) formulation using climatological data, with the negative feedback on SST estimated following Seager et al. (1988). The OGCM is a version of the GFDL Modular Ocean Model (Pacanowski, Dixon and Rosati, 1991, personal communication) for the Pacific basin. The vertical resolution is 27 levels, with 10 levels in the upper 100 meters. A Richardson-number-dependent vertical mixing scheme is combined with a surface mixed layer parameterization, as em-ployed in Latif et al. (1994).

The HCM reasonably simulates ENSO spatially and temporally, with ENSO periods of 3 to 4 years. Model performance in "retroactive real-time forecasts" (hindcasts hereafter) was shown in Figs. 1 and 2 of the September 1997 issue of this Bulletin. The ocean clima-tology used in the hindcast experiments is specified as the averaged model SST, forced by FSU wind stress over 1978-93 without modification by the data assimilation scheme. The climatological wind stress used in the hindcast experiment is also specified as the average of the FSU wind stress over the same period. The hindcast /forecast results are verified against the observations from Reynolds' (1988) SST data set.

The initialization scheme makes use of both the wind information (FSU converted wind stress) and the ocean model data assimilation product from the Climate Prediction Center (CPC) (Ji et al., 1995). In addition to the specified FSU wind stress forcing, the CPC reanalyzed anomalous ocean temperature field is "injected" into the ocean model (27 layers) every month from 1980 up to the start of the hindcast (injection scheme hereafter). Because our ocean model (GFDL MOM) is a version reasonably close to that used by CPC, approximate consistency is assumed in injecting the CPC reanalyzed data. To make a distinction between this procedure and raw-data injection, we refer to it as a "piggy-back" data assimilation scheme, because it makes use of the effort from a CPC data assimilation product.

With the injection scheme, both the model initial field as well as the hindcast correlation skills are competitive with other prediction results, as seen in the review paper by Latif et al. (1997). (Further details are provided in the September 1997 issue of this Bulletin.) We note that no Model Output Statistics (MOS) correc-tion, as used in, e.g., Barnett et al. (1993), is used in this forecast scheme. The "piggy-back" data assimilation scheme gives a substantial improvement in hindcast skill, as shown in the September 1997 issue, and thus appears to be a viable and economical forecast method.

Figures 1 and 2 present forecasts for the Niño 3 region from 1994 to the present. Observations through October 1997 are used. Figure 1 shows Niño 3 SSTAs of 3-month running means of the observations (thick black curves) and forecasts (gray curves) at 3-, 6- and 9-month leads. Averages of each lead month based on forecast verification over the 1980-1992 time span are removed before plotting the curves. Vertical bars represent plus and minus one RMS error, over the same forecast verification time span. All of the 3-, 6- and 9-month leads predict a warming for fall/winter 1997-98, but the warm event starts to decay after winter 1998 for lead months 3 and 6. The 3-month lead prediction suggests the observed warming trend should begin to level off soon. The rapidity of the warming over recent months was not predicted.

Figure 2 shows the latest two forecast results (starts from September and December of 1997, respect-ively, for 12 months), with the mean over the forecast verification time span (1980-1992) removed. The observation and model control run since 1994 are also displayed. Both forecasts predict a decaying of the warm event through the first half of 1998.

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Fig. 1: Forecasts of Niño 3 SST anomalies from 1994 to present. The solid line indicates observations. The latest forecast starts from October 1997. The mean for each lead month over the forecast verification time span (1980-92) is removed before plotting. Vertical bars represent plus and minus one RMS error for the same time span. Shown for (a) 3-month, (b) 6-month and (c) 9-month lead.

Fig. 2. The latest two forecasts (dotted lines) of Niño 3 SST anomalies up to 12 months lead time starting from the latest two initialization times--September 1997 and December 1997. Observations (solid line) and the model control run (dashed line) from 1994 to present are also shown. The mean for each lead month is removed as in Fig. 1. Vertical bars indicate the same plus and minus one RMS error used in Fig. 1.

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