A full review of Pacific circulation variability cannot be included here. The vast amount of work on the El Niño/Southern Oscillation phenomenon is reviewed elsewhere [ e.g., Philander, 1990; National Research Council, 1990]. Several observations regarding variation in gyre transports as related to Sverdrup transport are reported above. An important mode of North Pacific variability which has received attention is a change in the climatic regime of the upper ocean in 1976-77, and which might have ended in 1988 [reviewed in UNESCO, 1992; Trenberth and Hurrell, 1994]. Miller et al. [1994] review the observations (Fig. 6) and model the shift, which occurred in virtually every observable quantity [ Ebbesmeyer et al., 1991]. A dominant pattern of sea surface temperature variability in the northern North Pacific, as evident in the empirical orthogonal eigenfunctions (EOFs) is associated with the Aleutian Low, as demonstrated earlier [e.g., Davis, 1976; Chelton and Davis, 1982]. Tanimoto et al. [1993] demonstrate the regime shift through the variation of the sea surface temperature EOF which is associated with the Aleutian Low. The regime shift takes the form of a change in the strength of the Aleutian Low, sea surface temperature, winds, etc., and affected the central mid-latitude Pacific and the California Current region. It appears not have been associated directly with El Niño, whose strong mid-latitude effect is in the western North Pacific. A mechanism for creating the regime shift is modeled by Miller et al. [1994].
Barotropic variability for short time
scales has been well-documented in the
northern North Pacific, in response to
changing winds [e.g. Niiler and
Koblinsky, 1985; Luther et al., 1990;
Chave et al., 1991]. They show that the
ocean's barotropic mode near 40
N
in the central Pacific is coherent with
several atmospheric variables; the
response is non-local suggesting
propagation of Rossby waves over the
rough topography. Kelly et al. [1993]
used the short existing Geosat altimetry
record to show the two modes of the
northeastern Pacific, which correspond
to a strong or weak California Current
[ Chelton and Davis, 1982], and that the
variations in surface height are related
to local Ekman pumping, rather than to
Rossby wave propagation.
A review of variability in the eastern boundary region of the Pacific this region would be a complete paper in itself, and so only brief mention of some features of the eastern North Pacific (California Current and Alaska Stream) are mentioned here. Strong seasonal variation in upwelling-favorable winds creates seasonally dependent currents along the west coast of the U.S. Lynn and Simpson [1987] show a maximum in variability about 200-400 km west of the California Coast, and the suggestion is that the California Current is composed of large eddies around which the weak current meanders. The effect of the large 1982-83 El Niño on the west coast of the U.S. was pronounced, with its effects being a combination of distant effects and changes in local forcing [ Huyer and Smith, 1985]. Circulation in the Gulf of Alaska and bifurcation of the North Pacific Current into the California Current and the Alaskan Stream depend on the strength of the Aleutian Low (atmospheric low pressure zone) [ Davis, 1976; Chelton and Davis, 1982], and there is a weak link between its strength and the El Niño cycles [ Van Scoy and Druffel, 1993; Kelly et al., 1993]. In the Gulf of Alaska, the Alaskan Stream has very little annual variation in transport despite large variations in wind-stress curl, except very close to the coast where direct effects of seasonal variation in runoff are important [ Musgrave et al., 1992]. Instead interannual variation dominates.