Ground Cover North : Ground Cover 058 October-November 2005 - North
30 Weather: northern zone UNDERSTANDING THE MJO By LEXIE DONALD, Research Scientist, QDPI&F n User feedback indicates that many people are aware of the Madden-Julian Oscillation (MJO, also known as the 40-Day Wave), its impact, and where they can source information. However, some of the more technical aspects of the MJO analysis remain confusing, so following are some frequently asked questions. For those unfamiliar with the MJO, see www.apsru.gov.au/mjo/. WHAT IS THE MJO? The MJO is a wavelike atmospheric disturbance that can be observed in the eastern hemisphere tropics using various meteorological measurements, such as mean sea level pressure. An area of convection (the active part of the MJO) forms off the east coast of Africa and travels eastward at a distinctive speed of five to 10 metres a second (m/s). Other features of the MJO, like the westerly wind bursts that usually follow convection, are also used to define an MJO event. Although the MJO is most prominent in the eastern hemisphere, it can still be detected west of the dateline. However, it will have less moisture associated with it, increased speed (up to 15m/s) and often will have little local impact. Our aim is to use the MJO to improve long-term weather forecasting (10 to 60 days). Areas of further research -- given appropriate funding -- will include MJO- ENSO (El Nino-Southern Oscillation) interactions and MJO-temperature forecasts. HOW DO I READ THAT SPIDERWEB DIAGRAM? The phase space diagram (found at www.bom. gov.au/bmrc/clfor/cfstaff/matw/maproom/ RMM/phase.Last40days.gif, or follow the link from www.apsru.gov.au/mjo/) is a method to observe the amplitude of the signal (weak if inside the centre circle); the octants reveal where in the tropics (what phase) the active centre of the MJO is located. HOW DO I ESTIMATE THE TIMING OF THE MJO? The MJO is in each of the eight phases for approximately six to 10 days. If you are in Phase 7 and hoping for rain in Phase 4, then you are planning 20 to 40 days ahead. However, the timing might be influenced by seasonal conditions and is subject to ongoing research. THE MJO APPEARS TO HAVE GONE BACKWARDS? The MJO only travels east, and cannot retreat to the west. However it can stall, and/or the signal can be so obscured as to imply westward movement on the 'spiderweb'. MJO SIGNAL STRENGTH -- WHAT DOES IT MEAN? The signal strength, or amplitude, indicates how well the Bureau of Meteorology's Real- Time Multivariate MJO (RMM) index is able to distinguish between the MJO and other atmospheric phenomena based on observed data (outgoing longwave radiation and upper level wind anomalies). When the level of amplitude falls below one, other climatic features such as monsoonal activity may be obscuring the signal. The MJO signal is then beyond the focus of the index. WHEN DOES THE MJO HELP TRIGGER RAIN, AND HOW MUCH WILL I GET? Some MJO phases tend to suppress the probability of rainfall while others enhance it. Generally, Phases 4 and 5 are associated with increased chances of rain. However, the impact is seasonally modified (Figure 1, A-D). Sometimes the passage of the MJO results in only increased cloud cover, but little or no rain. Our seasonal analyses have identified clear regional differences, which are shown on our website. However, to answer this question fully we need to investigate MJO and ENSO interactions more closely. GRDC Research Code DAQ469 For more information: Lexie Donald, Alexis.Donald@dpi.qld.gov.au Figure 1: Australian rainfall-stations' significant enhanced (blue) or suppressed (red) rainfall by phase and season. 1A May-Oct Phase 4 1B May-Oct Phase 5 1C Nov-Mar Phase 4 1D Nov-Mar Phase 5 Note: these are experimental results, and have yet to be peer-reviewed. HARVEST RADIO www.grdc.com.au/radio/main.htm GROUND COVER OCTOBER/NOVEMBER 2005 possibilities for seasonal climate forecasts, it has also provided a lot of learning opportunities for farmers, agronomists and people like myself to learn more about our climate by better understanding the weather. GRDC Research Code LWR25 For more information: Dr Peter Hayman, 08 8303 9729 Weather or climate -- a matter of time By PETER HAYMAN, Principal Scientist, Climate Applications, SARDI n A useful distinction is often made between a weather forecast for the next few days, the climate outlook over the next season, and climate change projections which refer to changes over decades. The distinction between weather and climate is important because weather forecasts use numerical models to predict the timing and amount of rainfall for up to 10 days ahead. By contrast, seasonal climate forecasts typically use atmospheric (for example, Southern Oscillation Index, SOI/mid-latitude SOI) or ocean (Sea Surface Temperature, SST) indicators to give the chance of the next three to six months being wetter or drier, or hotter or cooler, than the long-term average. Global circulation models are used to project what the climate for a broad region will be like in say 2030 or 2070. The term projection is preferred to prediction because about half the uncertainty in what the climate will be like in 2030 or 2070 is due to the amount of greenhouse gases we as a global community emit in the meantime. Back in 1998, in a GRDC-funded project with the University of Western Sydney, we sent a group of farmers each week a fax of weather and climate forecasts and asked them to tell us if it was a help or hindrance to their management. We learnt that while the distinction between weather and climate was useful for us, it was harder to make the distinction on the farm. Farmers waiting to sow might get a climate signal that the risk of frost is reduced and the odds for a good season were increased, but if it did not rain, they could not sow. So the little wheel of weather dictated how they could respond to the big wheel of climate. Then on ANZAC day, 1988, it did rain in NSW, and rained so much that it was too wet to sow. We not only learnt that farming is easier by computer, we also saw how short- term weather influenced how farmers could respond to the climate. While climate science might look at El Niño as a shift in the probability distribution of rainfall, farmers experience El Niño as day after day of clear weather, or what Dorothea MacKeller referred to as "the pitiless blue sky". The climate of El Niño or La Niña is delivered through a series of weather events. An important aspect of the diagram is that there is a connection between global climate change and what happens in the back paddock, and this will be through changes in seasons and individual weather events. The very hot and dry autumn of 2005, or the hot day on 12 October last year, lead to the obvious question -- is this climate change or just an unusual bit of weather? Obviously it is hard to say. There are many clever mathematicians working on questions of detection (what are the trends in climate) and attribution (what is the cause of the trends). We can say that climate change will be delivered through changes in weather events. Although the trend in the average seasonal minimum and maximum temperatures is the most convincing aspect of trends in climate, changes in weather events, especially extreme weather events, are among the most worrying aspects of climate change. The Managing Climate Variability Program (MCVP), in which the GRDC is a significant partner, focuses primarily on the time period of the climate of a growing season, but in doing so recognises the importance of longer- term trends in climate and the fact that climate is delivered through weather events. One of the interesting aspects of the MCVP project on Oceans to Grains, led by Dr Peter McIntosh from CSIRO, is the explicit link between climate and weather. This project is studying the synoptic systems responsible for rainfall and relating the frequency and intensity of these systems to longer-term patterns in the atmosphere and oceans. Not only does this project offer new A B C D 1010 109 108 107 106 105 104 103 102 10 1 11 01 0 2 103 104 105 106 107 108 109 Characteristic size (metres) Dust Devil Cumulus cloud Severe storm Cold front Highs and lows Monsoon Walker circulation Global warming 1 km 10 km 100 km 1000 km 10,000 km100,000 km 1 hour 1 day 1 month 1 year 1 decade 1 century 2 hours 12 hours 3 days 1 month 2 years Climate prediction Extended-range weather forecasting Medium-range weather forecasting Short-range weather forecasting Very short-range weather forecasting Nowcasting Long-range forecasting (seasonal to interannual climate prediction) Characteristic lifetime (seconds) L H The connection between the weather in the back paddock today and global climate change over centuries: the duration (time scale) and extent (space scale) of atmospheric phenomena.
Ground Cover 059 December-January 2006 - North
Ground Cover 057 August-September 2005 - North