Interdecadal variations of phase delays between two Niño indices at different time scales

Phase delays between two Nino indices—sea surface temperatures in Nino regions 1+2 and 3.4 (1950–2001)—at different time scales are detected by wavelet analysis. Analysis results show that there are two types of period bifurcations in the Nino indices and that period bifurcation points exist only in the region where the wavelet power is small. Interdecadal variation features of phase delays between the two indices vary with different time scales. In the periods of 40–72 months, the phase delay changes its sign in 1977: Nino 1+2 indices are 2–4 months earlier than Nino 3.4 indices before 1977, but 3–6 months later afterwards. In the periods of 20–40 months, however, the phase delay changes its sign in another way: Nino 1+2 indices are 1–4 months earlier before 1980 and during 1986–90, but 1–4 months later during 1980–83 and 1993–2001.