Cayan, D. R., A. J. Miller, T. P. Barnett, N. E. Graham, J. N. Ritchie and J. M. Oberhuber,
fluctuations in surface temperature over the Pacific:
Effects of monthly winds and heat fluxes
In: Natural Climate Variability
on Decadal-to-Century Time Scales, National Academy Press, 133-150.
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 decadalscale
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-tonoise
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
hindcast of which we are aware that uses both observed total heat-flux and wind-stress anomalies as forcing for such a long time
interval. There is close agreement between the model SSTs and those observed in many regions of the Pacific, including the tropics and
the northern extratropics. Besides performing credibly on the monthly time scale, the model captures the essence of low-frequency
variability over the North Pacific, including aspects of a marked basin-wide change that occurred in 1976-1977. In the model's detailed
heat budget, the anomalous air-sea heat fluxes, entrainment, and to a lesser extent horizontal advection, force thermal-anomaly changes
in the mixed layer. Each of these components was apparently involved in the 1976-1977 decadal SST shift.