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Figure 5. Lagged correlation coefficient between Jul-Aug mean Nino 3,4 SST and SST at grid points of (a) Jul-Aug, (b) Jan-Feb, or (c) Jul-Aug of next year. The left-hand pane is for the solar max, and the right-hand pane is for the solar min. Correlation is calculated for 1958 to 1998. The contour interval is 0.1, and absolute values less than 0.4 are suppressed. Dashed lines indicate negative values. From Kodera (2005).

than during the solar max. Recently it was suggested (Kodera, 2005) that this is also produced through a stratospheric process.

The ENSO phenomenon, in general, develops from summer to autumn, and the influence of the ENSO extends from the Pacific to the Indian Ocean. However, as discussed above, the convective activity over the Indian Ocean is suppressed in summer during the solar max. Therefore, the influence of ENSO on the Indian Ocean is likely to differ between solar max and min. Figure 5 depicts the lagged correlation coefficient between Jul-Aug mean SST of the Nino 3, 4 region (170°E-120°W, 5°S-5°N) and SST at each grid point of (a) Jul-Aug, (b) Jan-Feb, and (c) Jul-Aug of the following year calculated separately for solar max (left) and min (right) (Kodera, 2005). In both cases, ENSO develops from summer to winter, but in the solar min case, its influence is stronger over the Indian Ocean. In the following summer, the positive correlation persists over the equatorial eastern Pacific during the solar max, but the correlation changes sign to negative for the solar min case. This means that the TBO tends to occur more frequently during the solar min, conforming to Barnett's result.

The difference of the evolution of the SSTs may be understood by the difference of the Walker circulation. Figure 6 exhibits the same correlation coefficient

Figure 6. Same as in Figure 5, but for the correlation coefficient between Jul-Aug mean Nino 3, 4 SST and (a) Jul-Aug or (b) Oct-Nov mean pressure coordinate vertical velocity over equator at each longitude-height grid point. From Kodera (2005). (c) Schematic presentation of (left) single and (right) double-cell regimes of anomalous Walker circulation (arrows). Shading indicates higher SST regions.

Figure 6. Same as in Figure 5, but for the correlation coefficient between Jul-Aug mean Nino 3, 4 SST and (a) Jul-Aug or (b) Oct-Nov mean pressure coordinate vertical velocity over equator at each longitude-height grid point. From Kodera (2005). (c) Schematic presentation of (left) single and (right) double-cell regimes of anomalous Walker circulation (arrows). Shading indicates higher SST regions.

between the Jul-Aug mean SST of Nino 3,4, but with (a) Jul-Aug or (b) Oct-Nov mean pressure-coordinate vertical velocity over the equator (5°S-5°N) at each longitude and height grid point. In Jul-Aug, anomalous upwelling over the warm SST of the East Pacific and downwelling over the Maritime continent region are observed in both cases. The anomalous upwelling over the Eastern Pacific extends up to the stratosphere for solar min, but it stops at 150 hPa for max. Accordingly, anomalous downwelling is stronger over the Indian Ocean sector for solar max, whereas no anomalous downwelling, or even upwelling, is found over the Indian Ocean for solar min.

In autumn, anomalous upwelling is formed over the western Indian Ocean in the minimum case. This means that anomalous Walker circulation changes from the single-cell type to the double-cell type for the solar minimum, but the single-cell regime persists for solar max, as schematically depicted in Figure 6c. The development of the double-cell regime is an important condition for the transition of the ENSO cycle in the following year (for more details see Kawamura et al., 2003). We suggest that the ENSO tends to change phase every other year, i.e., biennial oscillation tends to occur during the solar min.

The reason why the double-cell regime does not develop during the solar max may be that the convective activity over the Indian Ocean is suppressed during solar max as can be seen in Figure 4.

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