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Natural Climate Variability on Decade-to-Century Time Scales
Seasonal-to-Interannual Fluctuations in Surface Temperature over the Pacific: Effects of Monthly Winds and Heat Fluxes
DANIEL R. CAYAN1, ARTHUR J. MILLER1, TIM P. BARNETT1, NICHOLAS E. GRAHAM1, JACK N. RITCHIE1, AND JOSEF M. OBERHUBER2
Monthly heat fluxes and wind stresses are used to force the Oberhuber isopycnic ocean general-circulation (OPYC) model of the Pacific basin over a two-decade period from 1970 to 1988. The surface forcings are constructed from COADS marine observations via bulk formulae. Monthly anomalies of the fluxes and stresses are superimposed upon model climatological means of these variables, which were saved from a long spin-up. Two aspects of this work are highlighted, both aimed at a better understanding of the atmosphere-ocean variability and exchanges and at diagnosing the performance of the OPYC model in simulating monthly to decadal-scale variability. The first is the evaluation of the data used to force the model ocean, along with its relationship to other observed data. The second is the diagnosis of the processes revealed in the model that are associated with sea surface temperature (SST) variability, including their seasonal and geographic structure.
Although both random and systematic errors arise from the marine data and the bulk formulations, large signals in the air-sea fluxes are nonetheless consistent with the large-scale atmospheric circulation anomalies over the Pacific. This signal is large in a composite prepared from months with similar circulation modes. Also, latent and sensible heat-flux anomaly patterns correspond well to those of SST anomaly tendencies. Considering short-period variations, SST anomaly tendencies have typical magnitudes of 0.3°C mo-1. These are associated with monthly mean flux anomalies having typical magnitudes of 50 W m-2 and are consistent with observed mixed-layer depths. Decadal anomalies have much smaller magnitudes, perhaps reduced by two orders of magnitude, and it is here that the signal-to-noise problem is more severe. The forcing terms are generally products of variables, so realistic means and fluctuations of these variables are crucial for a successful simulation.
The 19-year simulation of the Pacific basin by the monthly marine data-forced OPYC model displays good skill in reproducing SST variability. These results represent the first
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
Max-Planck-Institut für Meteorologie, Hamburg, Germany