Melbourne 30-Day Climate Outlook
Today's scientists talk in terms of the continent's large climate variability from season to season, and from year to year. What causes these fluctuations? They are, in part, connected with the climate phenomenon called the Southern Oscillation, a major air pressure shift between the Asian and east Pacific regions whose best-known extremes are El Niño events and La Niña events. The Pacific Ocean is a huge mass of water which controls many climate features in its region. Its equatorial expanse, far larger than the Indian or Atlantic Oceans, is critical to the development of the Southern Oscillation and the El Niño and the La Niña. The Multivariate ENSO (El Niño Southern Oscillation) Index, which combines the Sea Surface Temperature (SST) distribution across the Pacific with various features of the atmospheric circulation, is used to monitor the evolution of the El Niño and Southern Oscillation phenomena. The MEI is a bimonthly index which is calculated at the end of each month. The most recent MEI is 0.2 standard deviations. This reflects a mild El Niño event. This most recent value may be adjusted by taking into account the values of the Southern Oscillation Index averaged over the past 30 and 90 days to yield a more representative expected current value of the MEI. The expected MEI is 0.53 standard deviations. This would reflect a mild El Niño event.
Another region of Sea Surface Temperature variability that impacts on Australian climate is that of the Indian Ocean. One mode of variability that appears to affect Australian rainfall, particularly the south east of the country, is the Indian Ocean Dipole (IOD). The IOD referred to here is defined by an index that is the difference between SST in the western (50°-70°E, 0°S-10°N) and eastern (90°-110°E, 10°-0°S) tropical Indian Oceans. A positive Indian Ocean Dipole Mode Index (DMI) occurs when the western basin is warmer than average and the eastern basin is cool and hence the DMI is positive. These regions were proposed in a paper by Saji et al. (1999) on the Indian Ocean Dipole that showed a modulation in Australian seasonal rainfall with DMI positive and negative years. The current Indian Ocean Dipole Mode Index (DMI) is 1.87 standard deviations. This reflects a very strongly positive Indian Ocean Dipole.
Also impacting upon Australian climate variability is a phenomenon known as the Intra Seasonal Oscillation (ISO) (also known as the Madden-Julian Oscillation (MJO), after Madden and Julian, who first identified it in the early 1970s). They discovered that, at many locations in the tropics, surface pressure and upper atmospheric winds tend to go through a coherent cycle over periods of about 40 to 50 days. It has been found that a broad area of active cloud and rainfall propagates eastwards around the equator at intervals of between about 40 and 50 days. These are not strict time limits - research over the years has pushed the limits of the oscillation's period to between about 30 and 60 days. The Madden-Julian Oscillation (MJO) is presently operating in Phase 5. This is reflected in the near-equatorial enhanced convection being found over the over the eastern part of what is referred to as the Maritime Continent (Indonesia). Following Phase 5, the region of enhanced convection often moves from the eastern part of Indonesia to the western Pacific.
In Melbourne, at this time of the year, a combination of the MEI, the DMI, and the MJO Phase, such as what we have operating now, suggests, over the following 30 days:
RAINFALL: There is a 21% chance of it being wet, a 31% chance of normal rainfall, and a 48% chance of it being dry.
OVERNIGHT TEMPERATURES: There is a 19% chance of warm nights, a 31% chance of normal overnight temperatures, and a 50% chance of cool nights.
DAYTIME TEMPERATURES: There is a 35% chance of warm days, a 33% chance of normal daytime temperatures, and a 32% chance of cool days.