Effects of the Australian Winter Monsoon on the Persistence of La Niña Events

Abstract

 In general, La Niña (LN) events are longer than El Niño (EN) events. Using objective analysis data, we herein investigated the effects of the Australian winter monsoon (AWM) on prolonging LN events. Conventionally, EN events are terminated through the eastward shift of the anomalous Walker circulation in the equatorial Pacific during March–August. In contrast, the stronger-than-usual AWM induced by the LN anchors the upflow branch of anomalous Walker circulation in the Indonesian maritime continent (IMC). The strength of the AWM is controlled by the surface temperature difference between the IMC and the northern Australian continent (NAC). The LN has a large impact on the decrease in surface temperature on the NAC through decrease of the downward surface short-wave radiation flux and increase in surface soil moisture on the NAC. In LN events, the strength of the AWM and the anomalous Walker circulation reinforce each other through the common convective ascending in and around the IMC, which may be termed LN–AWM feedback, prolonging the duration of LN events. During EN events, such feedback is weak so that EN events generally end in the period of March–August.