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Abstract: Negative Indian Ocean Dipole (nIOD) can exert great impacts on global climate and can also strongly influence the climate in China. Early nIOD is a major type of nIOD, which can induce more pronounced climate anomalies in summer than La Niña-related nIOD. However, the characteristics and triggering mechanisms of early nIOD are unclear. Our results based on reanalysis datasets indicate that the early nIOD and La Niña-related nIOD are the two major types of nIOD, and the former accounts for over one third of all the nIOD events in the past six decades. These two types of nIODs are similar in their intensities, but are different in their spatial patterns and seasonal cycles. The early nIOD, which develops in spring and peaks in summer, is one season earlier than the La Niña-related nIOD. The spatial pattern of the wind anomaly associated with early nIOD exhibits a winter monsoon-like pattern, with strong westerly anomalies in the equatorial Indian Ocean and eastly anomalies in the northern Indian Ocean. Opposite to the triggering mechanism of early positve IOD, the early nIOD is induced by delayed Indian summer monsoon onset. The results of this study are helpful for improving the prediction skill of IOD and its climate impacts.
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Key words:
- Indian Ocean Dipole (IOD) /
- triggering mechanism /
- Indian summer monsoon /
- seasonal cycle /
- negative IOD
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Figure 1. The normalized time series of DMI (averaged during July–October, gray shading). The blue vertical bars indicate La Niña-related nIOD events. The pink bars indicate early nIOD events. The purple bars indicate both types of nIOD appear in the same year. All nIOD events are indicated by blue circles.
Figure 2. Composite seasonal evolutions of DMI (℃) during early nIOD (a) and La Niña-related nIOD (b), and composite seasonal evolutions of thermocline DMI (m) during early nIOD (c) and La Niña-related nIOD (d). The black curves are the mean values for the corresponding year.
Figure 3. Composite seasonal evolutions of equatorial zonal wind (EZW, averaged between 5°S and 5°N, m/s) anomalies during early nIOD (a) and La Niña-related nIOD (b). Values that exceed 90% confidence level are indicated by gray dots.
Figure 4. Composites of SSTA (°C, color shading) and 850 hPa wind anomalies (m/s, vectors) in May of early nIOD (a) and La Niña-related nIOD (b), in the peak month (July) of early nIOD (c) and in the peak month (October) of La Niña-related nIOD (d). Wind vectors and SSTA values that exceed the 90% confidence level are shown in black arrows and gray dots, respectively.
Figure 5. Composite outgoing longwave radiation (OLR) anomaly in spring (from the 6th pentad of April to the 3rd pentad of May) during early nIOD (a–d), and composite zonal wind anomalies at 850 hPa in spring (from the 6th pentad of April to the 3rd pentad of May) during early nIOD (e–h).
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