Miller, A. J., D. R. Cayan, T. P. Barnett, N. E. Graham and J. M. Oberhuber, 1994:
Interdecadal variability of the Pacific
Model response to observed heat flux and wind stress anomalies
Variability of the Pacific Ocean is examined in numerical simulations with an ocean general circulation model forced
by observed anomalies of surface heat flux, wind stress and turbulent kinetic energy (TKE) over the period 1970-88.
The model captures the 1976-77 winter time climate shift in sea surface temperature, as well as its monthly, seasonal
and longer term variability as evidenced in regional time series and empirical orthogonal function analyses.
Examination of the surface mixed-layer heat budget reveals that the 1976-77 shift was caused by a unique concurrance
of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi-month period
preceding the shift in both the central Pacific region (where cooling occurred) and the California coastal region (where
warming occurred). In the central Pacific, the warm conditions preceding and the cold conditions following the shift
tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux (TKE input)
into the mixed layer (which deepens in the model after the shift) from the early 1970s to the late 1970s and 1980s.
Since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of
the 1976-77 shift, it appears that the midlatitude part of the shift was driven by the atmosphere, although effects of
midlatitude ocean-atmosphere feedback are still possible. The surface mixed-layer heat budget also reveals that, in the
central Pacific, the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while
horizontal advection anomalies are roughly half that size. In the California coastal region, in contrast, where wind
variability is much weaker than in the central Pacific, horizontal advection and vertical mixing effects on the
mixed-layer heat budget are only one-quarter the size of typical heat flux input anomalies.