North Pacific basin regions during winter. Model advection is generally not an important influence on SST anomalies in the Coastal region. There is an interesting seasonal cycle in the link between MLD and , which has its strongest expression for the North Pacific basin average. In fall-early winter (October-January), the two are out of phase: The SST anomaly tends to decrease ( is negative) when the mixed layer is anomalously deep, and vice versa. This implies that when large heat losses and strong wind mixing produce decreasing winter SST anomalies, they also produce a deeper MLD. In summer, the sign of the relationship reverses: The SST anomaly tends to increase when the mixed layer is anomalously deep. This suggests that in summer, when MLD is shallow, the mechanisms that extract heat from the mixed layers produce decreasing SST anomalies when the layer is thinner and the gradient below the mixed layer is stronger (and vice versa).

Analogous correlations (not shown) were also computed for the Niño 4, Niño 3, and Niño 2 regions along the equator from the international date line to South America. The largest contributions to model SST-anomaly variability in these equatorial regions are entrainment, advection, and diffusion. Strong correlations linking equatorial Pacific SST anomalies to advection and entrainment are indicated by the maps in Figures 5b and 5c. In the tropics, SST anomaly tendencies are generally in phase with MLD anomalies (warming coincides with deeper mixed layers), but not strongly so.

The 1976-1977 SST Shift

To evaluate the model's performance in simulating the spatial pattern of the 1976-1977 shift, the wintertime SST difference field for the six years after the shift (December 1976-February 1977 through December 1981-February 1982) minus the six years before the shift (December 1970-February 1971 through December 1975-February 1976) was calculated, following Graham (1991). Figure 9 (see color well) shows these fields, with the observed SST differences for comparison. The model captures the principal extratropical observations, namely, a warming in the Coastal region and a cooling in the Mid-Pacific region. In the tropics and subtropics, observations contain a swath of warm water throughout the equatorial region and across the southeastern North Pacific. The model captures only a vestige of this warming, which appears mostly in the western half of the tropics. This discrepancy may relate to the use of Newtonian damping rather than observed fluxes in the ± 20° zone. The warming in the equatorial Pacific points to possible dynamic effects due to wind-stress anomalies alone. Mechanisms for the SST shift are suggested by the comparable-difference maps of the winter fields of SLP, net heat flux, and pseudo-rate of kinetic energy transfer by the wind shown in Figures 10a, 10b, and 10c. These maps clearly show a deepened winter Aleutian Low during the post-1976 period (see also Venrick et al., 1987; Trenberth, 1990; Trenberth and Hurrell, 1995, in this volume). The deepened low sustained stronger winds and greater storm activity across the central North Pacific, and generally warmer, moister air masses along the West Coast. This pattern represents a tendency for strong positive PNA patterns in the post-1976 period. These conditions resulted in a large region of increased heat loss in the western and central North Pacific and decreased heat loss in the eastern North Pacific. Increased wind mixing apparently occurred in the central North Pacific, particularly between 30° and 40°N along the southern fringe of the anomalous low-pressure center. Both the heat-flux and wind-mixing patterns are consistent with the SST difference field, shown in Figure 9. Also, the heat-flux difference map is quite similar to the latent-and-sensible heat-flux anomaly maps associated with months having large PNA EOF amplitudes, which appear in Figures 1 and 3.

Inspection of the seasonal time series of the heat-budget components at the individual regions helps to understand

FIGURE 10

Difference fields of observed winter SLP (mb; top), heat flux (W m-2; middle), and {w3}, the pseudo-total kinetic energy (m3 s-3; bottom), for the winters of 1976-77 through 1981-82 minus those of 1970-71 through 1975-76.



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