Abstract
It is well known that the Indian Ocean dipole (IOD) is closely related to El Niño-Southern Oscillation (ENSO). In this study, it is found that spring–summer north tropical Atlantic (NTA) sea surface temperature (SST) anomalies can contribute to the development of the IOD since the mid-1980s as well as ENSO. After the mid-1980s, an anticyclonic circulation over the subtropical northeastern Pacific could be excited by cool NTA SST anomalies in spring and summer due to a Gill-type Rossby-wave response. A low-level cyclonic circulation appears in the west Pacific in turn. The anomalous lower-level southwesterlies on the southeastern flank of the cyclonic circulation reduce the climatological wind speed, and thus, warm SST anomalies appear and extend to the central tropical Pacific via the wind-evaporation-SST (WES) feedback. As a result, Walker circulation over the Indo-Pacific region is changed, with anomalous descending in the eastern tropical Indian Ocean and ascending in the central tropical Pacific. The descending branch of anomalous Walker circulation generates surface southeasterly wind anomalies along the coast off Sumatra and lifts the thermocline in the eastern tropical Indian Ocean, which produces a positive IOD event in autumn. Such a cross-basin mechanism is supported by a coupled model experiment with warm SST perturbations over the NTA. In contrast, although cold NTA SST anomalies can induce warm SST anomalies in the eastern equatorial Pacific through interhemispheric meridional circulation before the mid-1980s, the responses in the tropical Indian Ocean are rather weak. In addition, the enhancement in the NTA-IOD relationship after the mid-1980s is suggested to result from the changes in the SST mean state under the context of global warming, which is confirmed by two coupled model experiments with different mean SST backgrounds.
Highlights
The Indian Ocean dipole (IOD) is an interannual fluctuation in sea surface temperature (SST) anomalies between the western and eastern tropical Indian Ocean accompanied by equatorial surface wind anomalies (Saji et al 1999; Webster et al 1999)
Using observational datasets and model simulations, the present study investigates the linkage between the spring–summer SST anomalies in the north tropical Atlantic (NTA) and the subsequent IOD events and explores the possible mechanism of how the SST anomalies in the NTA influence the IOD
Before the mid-1980s, the relationship between them was rather weak, while a significantly negative relationship appeared after the mid-1980s, indicating that the cool SST anomalies in the NTA in spring could lead to a positive IOD in the following autumn
Summary
The Indian Ocean dipole (IOD) is an interannual fluctuation in sea surface temperature (SST) anomalies between the western and eastern tropical Indian Ocean accompanied by equatorial surface wind anomalies (Saji et al 1999; Webster et al 1999). The IOD could be trigger by external forcing out of the Indian Ocean and internal forcing within the Indian Ocean (Yang et al 2015). For external forcing, more attentions are paid on the roles of El Niño-Southern Oscillation (ENSO), which can influence the IOD through Walker circulation Kucharski et al (2008, 2009) and Wang et al (2009) indicated that the warm SST in the Atlantic Niño region can induce an atmospheric Kelvin wave, which triggers surface easterly wind anomalies in the tropical Indian Ocean and promotes generation of the IOD. Wang et al (2019) pointed out that the processes for an Atlantic Niño inducing a positive IOD need to be further confirmed
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