However, the WMI-ENSO relationship seems
not consistently highly correlated. The correlations
among WMI, ENSO and PDO indices in
21-yr moving window are shown in Fig. 3, two
full oscillations of the interdecadal WMI-ENSO
mode between 1905–1992 indicate a period of
about 44 yr. There is an increasing positive correlation
coefficient between WMI and ENSO
during two period (1910s–1920s; 1950s), and
this in-phase interdecadal relationship seems
break down during other two periods (1930s–
1940s; the mid-1960s–1980s). Thus the correlation
between the WMI and ENSO clearly
undergoes low-frequency oscillation. Note that
the WMI and PDO index is not related very well
during the mid-1920s to the mid-1970s, a significant
anti-phase relationship occurs before the
mid-1920s, while a significant in-phase relationship
appears in 1980s. Moreover, it is clear that
ENSO and PDO are significantly positive correlated
during 1930s–1940s, but experience a remarkable
change from positive correlation to
negative correlation and significant at 95% con-
fidence level during the late 1960s to the early
1970s. It is also interesting to find that WMI has
poor relationship with PDO or ENSO index almost
when both ENSO and PDO are strongly inphase.
The temporal evolution of correlation
clearly indicates the interdecadal variations of
the relationship between the WMI and ENSO.