Impact of two types of La Niña on boreal autumn rainfall around Southeast Asia and Australia

Abstract

The distinct influences of eastern Pacific (EP) and central Pacific (CP) La Niña on rainfall anomalies over Southeast Asia and Australia in boreal autumn (September to November) are explored in this study. Composite results reveal that CP La Niña gives rise to significant and severe flooding over Southeast Asia and Australia, whereas EP La Niña fails to exert any evident impacts on rainfall over this region. This difference can be attributed to the distinct features of cooling sea surface temperature anomalies (SSTAs) between EP and CP La Niña. With a more westward location and stronger intensity of the negative SSTAs during CP La Niña autumn, the highest and lowest SLP anomalies are substantially enhanced and shift westwards too, further causing intense easterly winds over the western Pacific and westerly anomalies over the Indian Ocean driven by this SLP gradient. Subsequently, robust low-level convergence and high-level divergence is observed over the Maritime Continent and Australia, resulting in significant above-normal rainfall anomalies in those regions. In contrast, weak and eastern Pacific–confined cooling SSTAs during EP La Niña produce correspondingly weak low-level convergence over the Maritime Continent conditions that make it hard for significant rainfall anomalies to arise.