while the results accumulated over the entire period 197 1-94 were unambiguously positive. The results of the hindcast experiments using the severely degraded subsurface data base over the period 1980-94 showed that
(6) A reduction of around 70% of the available information did not lead to a proportionate fall off in hindcast skill, though there was a consistent but small negative impact.
The simplicity of the models and the "skill" score adopted for this study, as well as the relatively short climate record, prevents strong recommendations being drawn from this study. However, the complementarity between the TAO and VOS XBT subsurface observing systems emerges quite strongly. The other theme that is beginning to emerge, at least for the initialisation of coupled forecast models with dominant large-scale, low-frequency modes, is that the subsurface data does have an important positive impact but is not very sensitive to significant changes in the thermal data base. It suggests this same impact might be obtained with a subset of the available data. Of course, this is not a denial of the utility of such data for many other applications, as is well illustrated in McPhaden et al. [1996], but that perhaps when the applications and questions asked are rudimentary, such as we have done here, then the demand for subsurface information is likewise much lower. This hypothesis can of course be tested by repeating experiments similar to these in more complex and complete coupled ocean-atmosphere general circulation models, but even this strategy will remain limited by the shortness of the climate record and quality, length and density of the input data.
References
Balmaseda, M.A., M.K. Davey, and D.L.T. Anderson, 1995. Decadal and seasonal dependence of ENSO prediction skill. J. Climate, 8, 2705-2715.
Bengtsson, L., M. Ghil and E. Kallen (eds.), 1981. Dynamic Meteorology: Data Assimilation Methods. Springer-Verlag Press.
Bourke, W., R. Seaman, and K. Puri, 1981. Data assimilation, Adv. Geophys., 28B, 123-155.
Busalacchi, A.J., and J.J. O'Brien, 1981. Interannual variability of the equatorial Pacific in the 1960's. J. Geophys. Res., 86, 1929-1952.
Kessler, W.S., and M.J. McPhaden, 1995. The 1991-93 El Nino in the central Pacific, Deep-Sea Res. II, 42, 295-334.
Kessler, W.S., M.C. Spillane, M.J. McPhaden, and D.E. Harrison, 1996. Scales of variability in the equatorial Pacific inferred from the Tropical Atmosphere-Ocean
(TAO) buoy array, .1. Climate, in press.
Kleeman, R., A.M. Moore, and N.R. Smith, 1995. Assimilation of subsurface thermal data into an intermediate tropical coupled ocean-atmosphere model, Mon. Wea. Rev, 123, 3103-3113.
Kleeman, R., R. Colman, N.R. Smith and S.B. Power, 1996. A recent change in the mean state of the Pacific Ocean. J. Geophys. Res., in print.
