Geoscience Reference
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Fig. 1. Time series of the tropical Pacific-Indian Ocean temperature anomaly mode
index (solid line), and SSTA in Nino
3
.
4
(dash line).
changes the phase of ENSO index. This indicates that studying composite
mode and its index
I
com
are very important.
In order to further explain the difference between the composite mode
and the pure Pacific ENSO mode, the two types of positive phase and quasi-
normal phase are compared. Based on the tropical Pacific-Indian Ocean
temperature anomaly mode index, we select the positive phase (1951, 1965,
1972, 1982, 1983, 1987, and 1997) with the composite index more than or
equal to 3.8 and the quasi-normal phase (1952, 1956, 1960, 1967, 1968,
1979, 1980, 1981, and 1990) with the index near zero. For comparing with
the pure ENSO mode, the pure El Nino year is chosen when the west-east
difference of SSTA in the Indian Ocean (the Indian Ocean dipole index) is
small: 1951, 1953, 1957, 1963, 1965, 1969, 1976, 1986, and 1991, and the
quasi-normal year of the Pacific is also picked when the Nino
3
.
4
SSTA is
about zero: 1959, 1960, 1962, 1980, 1981, and 1990.
The composite summer SSTA patterns of the above-mentioned types
are shown in Fig. 2 respectively. The quasi-normal year of the composite
index reflects the quasi-normal feature of SST better than that of the
ENSO index. The SSTA in equatorial sea areas is very small for the quasi-
normal year of the composite index. Moreover, there are a lot of differences
between the positive phase year of the composite index and that of the
ENSO index. The composite index displays the west-east difference of the
tropical ocean SSTA, not only in the Pacific, but also in the Indian Ocean.
These comparing results once again indicate that the tropic Pacific-Indian
Ocean temperature anomaly composite mode considerably differs from the
