Saturday, October 22, 2011

Why MJO Initiation is Being Eagerly Anticipated

Much chatter has occurred over the last several days about the imminent blossoming of MJO precipitation in the Indian Ocean. One piece of supporting evidence for this sentiment is that if you look at the average evolution of MJO events over the historical record, they tend to follow a well-defined evolution of eastward-moving precipitation and wind patterns, and hence if we know the state of the MJO now, we might anticipate its evolution in the coming days based on past events. Statistical prediction models for MJO evolution have been developed using data from many past events.

The most widely-used measure to document the current state of the MJO is the monitoring method developed by Matthew Wheeler and Harry Hendon at the Australian Bureau of Meteorology. The diagnostics I will describe here can be found on Matt Wheeler’s website: How this method works is the following. If one takes the state of the atmosphere at the current time along the equator, at all longitudes from Africa eastward through the Pacific and South America and then onward to Africa, the pattern of upper and lower level east-west wind and precipitation anomalies associated with the MJO can be largely be explained by two dominant structures, as shown below (multiply the quantity OLR by negative one to get precpitation). We can think of “anomalies” as being the departure from some sort of long term average, much like a very cold night in the winter is often referred to by the weatherman as being “colder than average”. In this case, this would correspond to a negative anomaly.

With each of these structures (called empirical orthogonal functions, or EOFs), east-west wind anomalies in the upper troposphere (about jet aircraft cruising level, U200) are of opposite sign to those near the surface (U850), and enhanced precipitation occurs where low-level winds come together and upper level winds move apart. However, there are two such patterns displayed here, with the second pattern shifted eastward relative to the first one. This is because the MJO moves eastward with time, and therefore one pattern cannot always be the best match to what the MJO looks like at the current time. In fact, sometimes it is the combination of the two patterns that best matches the MJO at the current time, when the state of the MJO is transitioning between these two patterns.

How the Wheeler and Hendon method documents the state of the MJO is to determine how well the current state of the tropical atmosphere looks like these two patterns associated with the MJO. If the atmospheric wind and precipitation anomalies are strong and greatly resemble one of these patterns (or the combination of them) at the current time, then the MJO can be said to be strong at the current time. A nice way of documenting the strength of the MJO is to plot a diagram showing how much the current state of the atmosphere resembles the first pattern (called RMM1 in the plot to the left) on the horizontal axis, and how much the atmosphere resembles the second pattern on the vertical axis (RMM2). If the strength of the signal is strong and the resemblance to the pattern is strong, then a high value of the RMM is reached. Since the MJO moves eastward, a typical MJO event will move counterclockwise around this diagram, progressing from positive RMM1 to positive RMM2 to negative RMM1 to negative RMM2. Why negative? It is just as reasonable for the atmospheric anomalies to resemble the opposite of the patterns shown above, and hence the RMMs can be negative.

This “phase diagram” shown here is nearly (but not quite) updated to the current day. The last day is October 20. Significant MJO events are typically defined as having a combined strength of RMM1 and RMM2 occurring outside the middle circle. In fact, the strength of the MJO has been well outside the middle circle for the past two weeks, and is near record strength at the current time. The phase diagram also shows geographical pointers indicating where the area of enhanced precipitation associated with the MJO was occurring at that time. The plot currently indicates enhanced MJO precipitation moving out of Africa and into the Indian Ocean. This has been the reason for all of the excitement over the last week, in that everyone is anticipating a blow-up of precipitation associated with the MJO in the DYNAMO observation area. Several statistical models also predict that MJO precipitation will blossom in the Indian Ocean.

However, not all measures are optimistic that this will occur. Numerous weather prediction models are run around the world that predict the state of the atmosphere out to several weeks from now. These models, while admittedly imperfect, include a relatively sophisticated representation of the physics of the atmosphere, albeit constrained by non-infinite computational resources, an imperfect knowledge of atmospheric physics, inherent limits of predictability of some aspects of the atmosphere, and other factors. Just as in the real atmosphere, we can use the structures shown above to document the strength and evolution of the MJO in the predicted atmosphere many days from now. Shown in the plot to the right is one prediction of the MJO for the next two weeks (relative to October 20) from the weather forecast model maintained by the NOAA National Centers for Environmental Prediction. Most models around the world demonstrate a similar prediction to what is shown here.

Basically, most models predict a plunge in MJO strength toward zero in the next week. Enhanced precipitation is anticipated to be relatively weak in these models in Indian Ocean, and with the lack of strong precipitation anomalies, the wind structure associated with the MJO cannot be maintained. Models do have a hard time predicting the initiation of the MJO in the Indian Ocean, and so these predictions should be taken with a grain of salt. However, over-exuberance for the imminent initiation of the MJO in the Indian Ocean is probably not warranted. Things could just as plausibly peter out.

On an unrelated note, here is a nice image of a rainbow taken last night on my bike ride back to the hotel. It was a beautiful night, with many people out and about on the street, because Friday is the local population’s holiday.

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