The big question on everyone’s mind right now is whether the Madden-Julian Oscillation (MJO) that looked like it was going to occur in the Indian Ocean this week is actually going to happen!
This Wheeler-Hendon diagram shows the state of the MJO between September 13 and October 22, 2011. The MJO signal propagates eastward or counterclockwise around the diagram. Strong MJO signals are indicated by points outside the middle circle.This plot shows a method of monitoring the MJO, known as a Wheeler-Hendon diagram, named after the two scientists, Matthew Wheeler and Harry Hendon of the Australian Bureau of Meteorology, who developed it. In this method, the anomalies (difference from the long-term average) in the precipitation, the upper atmosphere winds, and the low-level winds along the equator are examined. Wheeler and Hendon found that, over time, two dominant patterns (here simply called RMM1 and RMM2) in these anomalies explained most of the precipitation and wind anomalies associated with the MJO. By plotting the strength of the current anomalies and how well they correlate with the RMM1 and RMM2 patterns, you can understand the current state of the MJO. Since the RMM1 and RMM2 patterns include spatial structure along the equator, their values are also associated with the geographic location of the peak precipitation along the equator, as indicated by the labels in the diagram. For instance, negative values of RMM2 and small values of RMM1 are associated with the Indian Ocean.
In an MJO, the wind and precipitation anomalies will be strong and also strongly correlated (either positively or negatively) with RMM1 or RMM2 or their combination. Thus, the distance from the origin is an indicator of the strength of the MJO signal. The diagram shows that in mid-October the MJO signal was quite strong over Africa; in fact, it was at near-record levels. However, as the signal has moved to the Indian Ocean, it has dramatically weakened in the last few days.
Will this MJO signal die completely? If so, why? Will an MJO re-initiate in the Indian Ocean, but at a lower amplitude? If this MJO signal fades, what will trigger the next MJO? About half of all MJO events that initiate in the Indian Ocean appear to be triggered by previous MJOs, while the other half are known as “primary” MJOs, with no clear precursors. So we’ll stay tuned to see what happens over the next few days.
— Sally McFarlane
