Tropical Pacific Indian Ocean Temperature Anomaly Mode

El Niño and La Niña are defined by the SSTA in the equatorial eastern Pacific. Actually, when there is the positive (negative) SSTA in the eastern equatorial Pacific, the negative (positive) SSTA in the western equatorial Pacific occurs. On the other hand, the so-called Indian Ocean dipole is defined by the difference of the SSTA in the western equatorial Indian Ocean from that in the eastern equatorial Indian Ocean, indicating zonal heat contrast of the Indian Ocean SSTA. Although the name dipole is used, it actually does not define the mathematic meaning (SSTA distribution with positive (negative) in the west and opposite in the east).15 Considering the close relationship between the Pacific ENSO mode and the Indian Ocean dipole, the index of the PIM can be defined as the respectively normalized east-west differences of the equatorial areas in the two oceans. As to the SSTA, the SSTA of ENSO is stronger than that in the equatorial Indian Ocean because of the bigger Pacific basin. However, as to the influence of the SSTA on East Asia, a series of numerical experiments clearly indicate that the effect of SSTA forcing of the Indian Ocean is stronger than that of the eastern equatorial Pacific.21-23 So we will define the composite index based on the normalized dipoles respectively in the Pacific and the Indian Ocean. In this way, the normalized dipoles are comparable and the composite index has mathematical and physical base, so does its change. The composite index Icom is defined as

The symbols T1,T2,T3, and T4 are denoted, respectively, by the averaged SSTA in the region (50°E-65°E, 5°S-10°N), (85°E-100°E, 10°S-5°N), (130°W-80°W, 5°S-5°N) and (140°E-160°E, 5°S-10°N). The symbols VTi and VTp are normalized.

Figure 1(c) gives the time series of Icom (tropical Pacific-Indian Ocean temperature anomaly mode index) and SSTA in Niño3 4 (also called ENSO index). It can be found that although Icom is closely related to ENSO index, the difference in them is large. The importing Indian Ocean SSTA mostly makes the Icom stronger than the (normalized) ENSO mode, and partly

Indian Ocean Index
Fig. 1. Time series of the tropical Pacific—Indian Ocean temperature anomaly mode index (solid line), and SSTA in Nino3,4 (dash line).

changes the phase of ENSO index. This indicates that studying composite mode and its index Icom 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 quasinormal 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 Niño 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 Niño3.4 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 quasinormal 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

120E 150E 180 150W 120W 90W

Fig. 2. Composite of SSTA in summer (JJA) for the quasi-normal phase of ENSO mode (a), the quasi-normal phase of the PIM (b), the positive phase (El Nino) of the ENSO mode (c), the positive phase of the PIM (d). Unit: °C. The anomalies greater than 0.2 are in dark shading and those less than —0.2 are in light shading.

60E 90E 120E 150E 180 150W 120W 90W

120E 150E 180 150W 120W 90W

Fig. 2. Composite of SSTA in summer (JJA) for the quasi-normal phase of ENSO mode (a), the quasi-normal phase of the PIM (b), the positive phase (El Nino) of the ENSO mode (c), the positive phase of the PIM (d). Unit: °C. The anomalies greater than 0.2 are in dark shading and those less than —0.2 are in light shading.

Pacific ENSO mode, and proposing and studying the composite index is very significant.

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