Solar/QBO Influence on the Indian Ocean Dipole in Boreal Autumn

Abstract:

The boreal autumn in 2019 experienced the strongest positive Indian Ocean Dipole (IOD) over the past six decades. The 2019 IOD event has led drought in Australia and Indonesia and heavy rainfall in East Africa, which might indirectly result in the Australian continuous bush fires and the African locust outbreaks. Thus, some latest studies have addressed the possible causes of this extreme 2019 IOD event. Meanwhile, it is worth noting that the 2019 autumn is very close to the solar minimum (from late 2019 to first half of 2020). Therefore, an interesting question has been raised: is there any association between this extreme positive 2019 IOD event and the solar minimum.

Preliminary analysis reveals that, with the strengthening of solar activity (represented by sunspot number) during 1979-2019, the sea surface temperature (SST) markedly decreases over the western tropical Indian Ocean and increases off the Sumatran south west coast in boreal autumn. This solar-related SST anomalies pattern is quite similar to the classic negative IOD. And the significant relationship between sunspot number and autumn IOD cannot be interrupted by the ENSO. Further exploration indicates that the equatorial stratospheric quasi-biennial oscillation (QBO) westerly phase could avail the downward development of solar signal. In particular, during the westly phase of QBO, the weakened solar activity could first cool the air in the tropical lower stratosphere and strengthen the upward vertical movement over the western tropical Indian Ocean in boreal summer, then in autumn, the convection over eastern equatorial Indian Ocean is suppressed by anomalous subdued Indian Ocean Walker circulation, and finally a positive IOD-like appears in autumn through air-sea interaction. The results suggest both the QBO westly phase and solar minimum that occurred in 2019 might have some contribution to the extreme positive IOD in 2019 fall. The factors that affect IOD are actually complex, and we need to extract the solar influence quantitatively in further.