(Note: A comment by reader Lauren H brought my attention to an analysis I’d overlooked by reader MPat. As Lauren H points out, it’s as timely now as it was when MPat first aired it back in March. — JW)
The potential arrival of more debris in the East African region is triggering interest once more in the currents and drift patterns in the SIO. To sense check the concept that debris could drift from the current search area to these regions I did a little research of my own, the premise being that the observed behaviour of real floating objects (and I am considering of course the buoys of the Global Drifter Program) should be a useful indicator of possible drift pathways, as a counterpoint to cell-based drift simulation models (which may be calibrated to high level drifter behaviour but typically lack the resolution to reproduce drifter movement in detail).
The full drifter database contains meta-data and trajectories for almost 19800 buoys worldwide (some 1400 are currently active). The meta-data includes timing of drogue loss, and a ‘death’ code to categorise the end of life status of buoys that cease transmitting. It is clear from this that drogues are typically lost in a surprisingly short timeframe. It is also notable that only 20% of all the buoys have ended their lives by running aground, with 66% simply ceasing transmission for undocumented reasons.
I have filtered out buoys that have at any time in their lives passed through the locality of the current search zone, based on a rectangle bounded by longitudes 88 to 96 degrees and latitudes -32 to -39 degrees. None were present in this area at the time of the crash, but I consider in any case all buoys that have ever been in this location (dates range from 1995 to 2014). There are 177 in this category. Of these, 39 are listed as having subsequently run aground. The locations at which they washed up are shown in the plot above.
Of the 39, 31 beached on East African coastlines, only 7 in Western Australia, and 1 in Sumatra. An example of 3 randomly chosen trajectories from the 31 that drifted west are shown below together with the box defining search locality :
The average time for buoys to reach their western beaching point after leaving the search box is 534 days (~ 18 months) with minimum 234 days (~ 8 months) and maximum 1263 days (~ 42 months). All but 3 were un-drogued during this journey, and those 3 lost their drogues en-route. For those arriving in Western Australia, the average time to beach was 362 days, with minimum 79 days and maximum 513 days.
If we relax the criterion that the buoys must end by running aground, and simply look at the locations where they eventually stopped transmitting after leaving the search area, we see the following three plots which display the 54 buoys that ended up west of longitude 55 deg (the longitude of Reunion Island),
the 12 that ended east of longitude 109 deg (coast of Western Australia),
and the 111 that remained in between:
Clearly the transport qualities of the ocean currents and weather systems will vary from month to month and year to year. It is also not clear how representative the buoys would be of the drift characteristics of floating debris resulting from a crashed aircraft. Neverthless I believe it is reasonable to propose from the buoy behaviour noted above across a 20 year drifting history that :
i) there is a strong tendency for objects that have been present in the current search area to remain trapped in the mid ocean gyre over extended periods
ii) a proportion, perhaps as high as 10% of robustly floating debris, might be expected to make landfall within 18 months of the crash
iii) the vast majority of the debris making landfall is likely to do so across the coastlines and islands of eastern Africa, with relatively little beaching in Australia.
For what it is worth, I have more background and analysis in a write-up that I hope to post soon.
Please also note that a vastly more expert analysis of drifter behaviour has been performed in October last year by David Griffin of CSIRO, in which he uses composite drifter trajectories to infer a likelihood function for where the MH370 flaperon may have originated. This is well worth a read.
UPDATE 79/2016: Reader Richard Cole has posted a link to a .kml file that shows the trajectories of the drifters that reached Australia. Here’s a screenshot of what it looks like if you drop the file into Google Earth. Interesting to note that the greater part of the debris winds up on the southern coast and Tasmania rather than the western coast.