In our model, variation of northward heat transport by the subtropical gyre is in-phase with the midlatitude SST anomaly, implying that meridional heat transport is not a leading factor for changing the SST in midlatitudes but has a positive feedback effect to amplify the SST anomaly. Therefore the spatial appearance of this second mode is similar to the mode by Latif and Bamett (1994), but the mechanism seems different from that proposed by them where variation of heat transport by the western boundary current acts as a delayed negative feedback to cause coupled instability. It is suggested that the midlatitude SST variation for our second mode is excited by the atmospheric stochastic forcing associated with the atmospheric internal mode, and its timescale is selected by the oceanic response associated with the clockwise rotation of the temperature anomaly around the North Pacific subtropical gyre.
Figure 8 Anomalies of (a) SST and (b) surface wind stress corresponding to the second mode at the phase lag 80゜, obtained from linear regressions on the temporal CEOF for the VAT with the phase lag.
Figure 9 Ocean temperature anomaly (thin contours) for the second mode along the meridional vertical section at 180°E, at phase lags (a) 60° and (b) 120°, obtained from linear regressions on the temporal CEOF for the VAT with each phaselags. Contour interval is 0.02K, and values larger than 0.06K are shaded Dashed thick contours show isopycnal surfaces in the model climatology (unit is σ).
