Summary
Influence of the Madden-Julian
Oscillation on Multiweek Prediction of Australian Rainfall Extremes using the
ACCESS-S1 Prediction System
The MJO is a large-scale
eastward moving 'pulse' of cloud and
rainfall near the equator typically recurring every 30 to 60 days. Its position
in relation to Australia, both in terms of the latitude of the eastward moving winds
and the location of the cloud and rainfall pulse, influences Australia’s
climate. The MJO particularly affects the tropical regions of Australia during
summer where it can impact the timing and intensity of active monsoon periods. Researchers
from the Bureau of Metrology have investigated the predictability of the MJO
and associated Australian weather using the seasonal climate model, ACCESS-S1.
Data from 1990 to
2012 demonstrate that the MJO is a driver of rainfall variation across
Australia. For instance, the chances of extreme rainfall in northern Australia
increased when the active phases of the MJO are present in the area. The MJO
also impacted extreme rainfall in more southern locations due to the impact the
MJO has on circulation patterns in the atmosphere.
The analysis
demonstrated that ACCESS-S1 can predict the MJO up 28 days in advance, and the
model did a reasonable job of simulating the changes in extremes of weekly
rainfall for different phases of the MJO across seasons. This allows for
predictability 2 to 3 weeks in advance during spring and summer in eastern and
southern Australia when the MJO is strong. However, this is not the case for forecasting
for central and far north Australia in summer. During autumn and winter, the
strength of the MJO has negligible impacts on forecast skill. Thus, the skill
of ACCESS-S1 is greatest in regions where the MJO has little influence on
rainfall variation.
There are ongoing
efforts to improve the depiction of the MJO and its connections to Australian
weather patterns in ACCESS-S1. Improvements to ACCESS-S1 based on this study
would be particularly helpful to regions that with large variations in rainfall
due to the phase of the MJO.