Wednesday, January 4, 2012

Resurrected or reborn?

It's now getting suppressed again over the Indian ocean, as the active phase we had a couple of weeks ago has moved to the Pacific.  The previous event never really got out of the western Pacific.  When it got there, it died, and not much more than a week later, it was the active phase again in the eastern Indian, where it had just been about three weeks ago.   

Here is the RMM phase diagram from the last two months.  Note the loop on the right side, indicating amplitude decrease (line moving in towards the center of the circle) in phases 4-5, followed by an increase again and then further eastward propagation (counterclockwise motion, ending where we were as of a couple days ago, near the vertical line between phases 6 and 7).

Was this a new MJO event, or a resurrection of the previous one?  This was an issue that was discussed in today's episode of the weekly teleconference that the DYNAMO scientists all over the world have been having since the start of the experiment. 

The three-week period separating the two active phases in the Indian ocean was shorter than we usually associate with the MJO.  We typically say the period of the MJO is 30-60 days, but this was only 20.  So does that mean it couldn't be a new event?  Not really.  These numbers are only rough guides.  The spectral peak associated with the MJO (see here for some background) is broad, meaning the period is variable.  Just eyeballing the OLR Hovmoeller plot, the latest event sure looks like the latest one in a series of similar ones (the three blue raggedy stripes going down and to the right in the lower left of the plot);  ok, it's a little cut off on the left (indicating that it started in the eastern IO rather than western) but  it doesn't appear to have grown directly out of the previous event.

While it looks like a new event to me, the honest answer, I think, is that there is no honest answer.  The MJO is a rough, large-scale envelope of weather, rather than a tightly defined weather system whose birth and death we can define precisely.  I don't think we'll ever give MJO events names, like we do with tropical cyclones.  And if we did want to define the start and end of an event with precision, ultimately the way we do that should depend on an understanding of MJO dynamics, not just on statistical or phenomenological features that have been defined purely by subjective convention.  We don't have that dynamical understanding yet.

Another reasonable question to ask might be:  does it matter whether it's a new event or the same old one?  It's pretty clear that it's the MJO one way or another, and the more interesting question is what has made it evolve in this unusual way just now.  

1 comment:

  1. Please forgive my ignorance and/or presumptive attitude... but has the lay of the geomagnetic field in the Indian Ocean at the presumed start of the MJO been taken into account? Also, any increase in the cosmic ray counts (mainly muons and/or neutinos) or density of the solar wind stream at that time?
    It seems to me that dealing with such an airy weather pattern at such a high altitude, something as simple as static charges giving small, but cascading lift to different layers at differing temps would be simple to connect to electromagnetic currents and charged particles. A simple warming of an extremely cold layer... the butterfly flaps its wings.