Figure 2 Power spectra of the temporal coefficients of the principal EOFs (EOF1: thick solid lines, EOF2: dashed lines, EOF3: thin solid lines) for (a) observation, (b) AOGCM and (c) SGCM.
It is found that the AOGCM reproduces overall spatio-temporal structure of the observed SST variability. On the other hand, the SGCM shows somewhat similar spatial pattern witt AOGCIVI but does not simulate observed temporal structure. These suggest the temporal structure of the variability in the Pacific is associated with dynamical processes in the ocean.
FIRST MODE
In order to investigate variations with temporal evolution which is associated with the time scale, a complex empirical orthogonal function (CEOF) analysis are performed for vertically averaged temperature (VAT) of the upper ocean (0-600m) in the Pacif region (120｡?-70｡?, 30｡?-60｡?). We applied band-pass filter with 8-50 year truncation perids for the VAT data before the calculation of CEOF.
Fig. 3 shows the reconstructed spatial pattern of the first mode CEOF (CEOF1) at two 90° different phases. This mode accounts for 42.5% of the filtered total variance. The temporal consistent of this made has a spectral peak around 21 year period, which is consisitent with that for the first EOF of SST (Fig. 2b) A wedge shaped pattern in the eastern Pacific is quasi-siymmetric etric about the equator (φ＝120°). It propagates westward in the northern subtropics and is reinforced again at the central basin during the propagation (φ= 210°).
Associated spatial pattern of SST and the surface wind stress are obtained from regressions on CEOF1 of VAT. At the time of φ=120°, the positive VAT anomaly appears in the eastern Pacific (Fig. 3a), there is a westerly wind anomaly in the western-central tropical Pacific (Fig. 4a) and a positive SST anomaly in the central-eastern tropical Paciflc (Fig. 4b). The SST pattern is almost idenitical to EOF1 of SSI (Fig. 1b). Along the equator, vertical structur of temperature anomaly (Fig. 5) shows opposite sign between the western subsurface and the eastern surface, which implies an east-west seesaw of the modes' thermocline at the equator. These features are similar to the model ENSO, except that the latitudinal extent of the interdecadal signal is broader.
Acompanied by east-west seesaw of the equatorial subsurface temperature (Fig. 6b), the SST and zonal wind stress along the equator oscillatis almost stationary in the central Pacific (Fig. 6c, d). The tropical westerly wind stress anomaly suppresses cold advecion and equatorial upwelling, that results in maintaining or enhancing positive SST anomaly. This implies that the variations of SST, wind stress and thermocline are in a positive feedback for this mode.
The wind stress variation accompanies a notable cyclonic pattern in the subtropical North Pacific with its center around Hawaii (Fig. 4b). This cyclornic wind anomaly leads to increase of the SST due to decreased evaporative cooling in the trade wind zone, and increased warm advection and downwelling along the American coast.
