MH370 - drift analysis David Griffin



26 October 2018

Why was the Pemba flap the only debris item reported north of Madagascar?

If you step through the simulation, you see that many of the magenta-coded items (originating from 23°S to 19°S along the 7th arc) pass north of Madagascar, as early as 15 Sep 2014. Their trip west starts earlier than for neighbours originating north or south along the 7th arc for several reasons, the main one probably being that the wind was persistently from the SE in that range of latitudes.

Analysing the spatial distribution of the modelled items beaching on African shores as a function of their starting latitude, we see (Figure 5a at right, or Figure 5c) that for crash latitudes north of 23°S, most of the modelled beaching occured in Tanzania, Kenya and Somalia rather than in Madagascar, Mozambique and South Africa. This is in stark contrast with the distribution of reported beachings, which were all (but one) in Madagascar or southwards.

Possible explanations of the near-absence of reported beachings north of Madagascar include

MH370 did not crash north of 23°S many items beached in Tanzania, Kenya and Somalia but were either not noticed or not reported, or MH370 crashed north of 23°S but the debris was not taken north (under the influence of the SE trade winds) as much as happens in our model.

Figure 5a also shows that for crash latitudes south of 23°S, the probability of beaching at various locations is much the same, i.e. highest in Madagascar, least elsewhere. This says that the observed distribution of beaching location does not, by itself, point towards a most-likely crash latitude south of 23°S. (For confirmation that Madagascar rather than mainland Africa at the same latitudes is the predicted beaching 'hot spot', see Figure 5b, for which beachings in Madagascar are excluded).

Madagascar was where a large fraction of the 29 reported debris items were found. Many of these discoveries were thanks to the efforts of Blaine Gibson and the MH370 families who targetted Madagascar because model predictions indicated that this was where he was most likely to find debris. The Madagascar findings, both their location and timing, therefore have the potential to exaggerate or misrepresent the true pattern of debris beachings.

We noted above that many of the magenta-coded model items crossed the Indian ocean very quickly, with many beaching north of Madagascar. Our earlier analysis of the reported and modelled timing of beaching was therefore potentially skewed by failing to separate the north-of-Madagascar (modelled) beachings from the others. To correct for this, and the 'Gibson and MH370 families effect' on reported beachings, we have re-analysed the modelled and reported beaching timings, separately for the three regions delineated in Figure 4 above.

First consider beachings in the north. If the crash latitude was north of 24°S, the median modelled beaching date - February to June 2015 - is more than a year before the Pemba flap was reported. Even the 90th percentile modelled beaching date is well before (more than 6 months) the reported date. Furthermore, this modelling does not factor in the possibility that the flap drifted faster than low-windage debris, in which case it would have crossed the ocean even faster. The flap was reported not to be freshly washed up, but it is unknown if it could have beached as much as a year (or more if it had higher windage) before it was reported. If the likely delay between beaching and reporting was just a few months, then this piece becomes consistent with the crash being at any latitude south of 24°S. If the crash was at 35°S, the flap would have had to take a lower than median (but still highly probable) time in order to arrive a month or two before being reported.

Now consider beaching in the east (Madagascar, Mauritius and Rodrigues) region. For crash latitudes near 25°S or north, the 10th percentile modelled beaching date is earlier than June 2015. This pre-dates the earliest reported finding (an unidentified piece of honeycomb composite at St Luce in December 2015) by 6 months. It also pre-dates Blaine's August 2015 visit to Mauritius and Rodrigues where he found nothing, and all the media attention on the flaperon finding in Reunion which presumably would have stimulated reports of other debris had any washed up on those islands. Blaine reports that the Madagascan locals were certainly not on the lookout for parts of MH370 before his first visit in June 2016. It was only after he and the MH370 families raised awareness that the reports of earlier sightings were made. The median reported date (June 2016), is therefore certainly later than the median beaching date. The question is 'by how much'? To be consistent with the crash site being north of 25°S, the delay needs to be 8 months (like the delay between the modelled and reported 10th percentiles). For the crash latitudes to be south of 25°S, this delay need only be 5 months north of 30°S tapering to 0 south of 34°S. If the 3 items found at St Luce turn out not to be parts of MH370, of course, the picture changes but only slightly.

Lastly, the south (South Africa and Mozambique) beachings. Here, the distribution of modelled beaching dates is similar to the dates for the east region but later by a month or two - except for a narrow band of crash latitudes near 22°S where they are the same. The median reported beaching date, in contrast, is slightly earlier than for the east. The most likely explanation of this is that it is not skewed by the large number of reported Riake beach findings in June 2016. These opposing shifts both reduce the delay between modelled and reported beaching but for crash latitudes north of 24°S the delay is still 5 months or more for the medians. The lower number of modelled beachings in the south compared to the other regions is presumably due to the combined effect of the reduced prevalence of onshore wind and faster alongshore drift velocity in the Agulhas current. In this regard, the model agrees with the southern reduction of where drifting buoys come ashore). The relatively large number of debris items found in the south is not, therefore, due to this being an area of greater likelihood for items to wash ashore. This leaves the most likely explanations being 1) greater reporting probability, and/or 2) southern Indian Ocean origin.

Conclusion

Splitting the reported and modelled beachings of debris has certainly been of value, highlighting differences between results for the north, east and southern components. Our conclusion, however, is much the same: the possibility that MH370 could have crashed in the unsearched segment of the 7th arc north of 25°S hinges on the question of whether large fractions of the debris could have beached on African shores in places 1) where very little (Tanzania) or none (Kenya) has been reported, and 2) where much was reported, but several (8 or 5) months before reporting began.

Acknowledgement: The analysis and discussion above arose from suggestions, questions and information gratefully received from Blaine Gibson and several members (including David, Richard Godfrey, DrB and Niels) of the MH370 forum hosted by Victor Iannello.

17 September 2018

The extraordinary tale of drifter 101703

It has been known since 2014 that GDP drifter 101703 crossed the 7th arc (at 23.6°S), by extraordinary coincidence, on 8 March 2014. It has also been noted that this drifter did not start crossing the Indian Ocean until December 2014, getting much of the way (to 60°E) by 22 July 2015 (when it ceased transmission). This is near Reunion, where the flaperon was discovered just a week later, and 5 months before Roy was first photographed on a South African beach. Does the near-agreement of drifter 101703's trajectory with that of the flaperon prove that 23.6°S (a region yet to be searched) is where MH370 crashed into the ocean? Unfortunately not. All that drifter 101703 proves is that if MH370 had crashed at 23.6°S then at least one of the thousands of pieces of the aircraft might have taken the same path and time to cross the Indian Ocean. Our model agrees with this, and helps to explain why so little can safely be concluded. The track of drifter 101703 is well within the envelope of 120 model counterparts (all initially within 5nm of the drifter and thus equally representative) - but that envelope is very wide.

The animation at right explains why travel times across the ocean differ so widely, for both real and modelled drifters. In this part of the ocean, the mean flow velocity is very small compared to the swirling motion associated with eddies. The erratic trajectories of drifting items is the net result of the combined effect of the wind and random encounters with these eddies. Two items travelling along side by side sometimes suddenly go in very different directions. This is true in the real world and in the model, but the model only represents this approximately since it cannot represent all the small-scale shears that exist in the ocean. The model knows where the eddies are, and their approximate shape and intensity, thanks to satellite altimeter observations of sea level, which is shown in colour. The flow is clockwise around the lows, and anticlockwise around the highs, as observed and modelled (independently of the tracks of the drifters). Three levels of windage are used - zero to mimic the drifters when their drogues are still attached (model coded black, drifters coded blue), 1% to mimic undrogued drifters (model red, drifters magenta) and 3% to mimic 'high windage' items of aircraft debris. The positions of the items on 13 March 2014 shows how trajectories are sensitive to windage. (The red model drifters match the position of 101703 best, in agreement with the GDP's assessment that the drifter had lost its drogue by this time.) This sensitivity to windage further amplifies the dispersal of items due to the chaotic eddies, by being a cause of initial separation.

We said last month that "We can not completely rule out the possibility that the aircraft is located either north or south" because the model does indeed say that either is possible. But it also says they are unlikely (unless many debris items beached before any were reported, which is not for us to assume). Figure 2 of last month's post suggests that a small region near 23.6°S is the location north of 25°S that is closest to being compatible with the observed dates of miscellaneous debris. But with the ocean's randomness in mind, we choose to refrain from saying that analysis of the limited number of debris item trajectories that crossed the whole Indian Ocean actually supports searching north of 25°S.

10 August 2018 (and 19 Sept, 18 Oct)

Now that Ocean Infinity has searched as far north as 25°S along the 7th arc without success, the mystery of where MH370 lies is now deeper than ever. The crash site must be either:

north of 25°S, south of 39.6°S, between those latitudes but farther off-arc than has been searched, within the area searched but overlooked, or somewhere else we can't imagine.

Here, we expand on what we have said earlier about the possibility of the crash site being north of 25°S or south of 39.6°S - the only two possibilities where ocean currents are relevant. We have not used any new information to do this - we have just used the same ocean model discussed in our earlier reports to ATSB, applying it to potential crash sites farther north.

The flaperon

Figure 3.2 of our 2nd report to ATSB showed the probability, as a function of crash site latitude (along the 7th arc, from 42°S to 26°S), and subsequent time, of the flaperon being in the vicinity of Reunion (centre panel). Informed by drift tests of a genuine 777 flaperon, and an ocean 'reanalysis' model fed with satellite data and carefully compared with trajectories of buoys drifting freely on the surface of the ocean, this showed that crash sites from the southern limit of the search area to about 30.5°S were all consistent with the observed arrival date of 29 July 2015. Latitudes from 30.5°S to 26°S were less consistent but not inconsistent. Figure 1 extends the latitude axis all the way to Java (8°S) and shows that in order to explain the flaperon evidence using our model, the northern limit of plausible crash sites was reached by Ocean Infinity's search. Crash sites farther north of the searched area, to 23°S, have high probability of the flaperon reaching Reunion - but only on dates many months earlier than observed. For crash sites north of 23°S the modelled trajectories of the flaperon do not enter the vicinity of Reunion (defined fairly broadly as spanning 10° of latitude). Stepping through the simulation we see this is because the trajectories pass far north of Reunion. According to our model, the flaperon's arrival at Reunion is not consistent with crash sites north of 25°S. 19 Sept update: If we increase the assumed angle of the flaperon drift with respect to the wind from 20° left to 30° then the modelled flaperon trajectories shift south, so more come near Reunion. Crash sites as far north as 23°S become not inconsistent with the July 2015 arrival date (the most likely arrival date remains in late 2014). But could the drift angle have been 30°? The DGA, using computational fluid dynamics, found the angle to be 18° if the trailing edge was to the wind and 30° if the leading edge was to the wind. Our field testing showed that the flaperon spent very little time with the leading edge to the wind. The drift angle varied from deployment to deployment, and between the early and late half of each. Values from 8° right to 33° left were estimated (see Fig 2.3.1 of our 2nd report), with several near the DGA's 18° estimate. The combination of 30° being less likely, and associated trajectories not very consistent with the arrival at Reunion, implies that (if our modelling is correct), the flaperon evidence suggests that crash sites as far north as 24°S or 23°S are possible, but very unlikely.

Other debris

The lower panel of Figure 3.2.1 in our first report to ATSB showed that low-windage debris such as the items found on African shores had high probability of arriving from December 2015 onwards, as observed, if the crash site was between 37°S and 33°S. Crash sites north of 33°S are only consistent with the debris evidence if one assumes that arrivals occurred before December 2015 but remain undocumented, which is possible, but increasingly unlikely for longer intervals of non-detection and non-reporting. Extending the latitude axis farther north as above, Figure 2 shows that the required non-reporting interval is greatest (more than a year) for crash sites between 23°S and 21°S. As described in our reports, Figure 2 shows the probability of being within a defined region.

Updated 18 Oct 2018, after extending the simulation to Dec 2016 and fixing a bug ('beachings' wrongly counted at the end and western boundary of the simulation shifted the median and 90th percentiles right in the original Figure 3: Another way to present the model results is to look at the statistics (Figure 3) of the dates on which modelled debris items are blown onto the coast by wind. This shows that for all potential crash sites progressively south of 25°S, the median and 10th percentile modelled beaching dates are progressively less than 8 months earlier than the corresponding percentiles of the dates on which debris was reported. For crash sites near 36°S, the delay is down to about 1 month. For latitudes north of 25°S, in contrast, the delay is greater - much greater in the case of crash sites between 23°S and 20°S where the delay is more than a year.

Assuming our simulation is correct, the question of whether Ocean Infinity has exhausted the range of northern latitudes that are worth searching therefore hinges on the question of whether more than 50% of the debris from MH370 could have washed up on African shores, unnoticed, throughout much of 2015. We are not prepared to assume this is true. Consequently, we think the available evidence from African shores is consistent with the flaperon evidence, and that MH370 is much more likely to have crashed south of 25°S.

Far south

As we said in our first report to ATSB, the two arguments against the crash site being south of 39.6°S are that 1) debris would not have arrived in Africa as early as was observed (see above), and 2) an observable amount of debris would have arrived on Australian shores - where targetted searches found none. Hard evidence (that was known to us in 2016 but not included in our report) that debris floating in the SE Indian Ocean finds its way to Australian shores despite the frequently-adverse winds is provided by the trajectories of (undrogued) drifting buoys, just as many of which arrive on West Australian shores as on Southern African shores. This also had a bearing on how we identified 35°S as the most likely region of the crash.

Limitations

The two most powerful tools available to the drift modeller are 1) data-assimilating hydrodynamic models and 2) the archived trajectories of satellite-tracked drifting buoys. But as discussed in our first report to ATSB, neither of these tools is actually fit for the purpose of modelling the drift of debris. This is because both were designed for simulating or observing the movement of water in the surface mixed-layer, which is tens of meters thick. The movement of items floating right at the surface is significantly different, due to the combined influence of Stokes drift, direct wave forcing, and direct wind forcing. This difference can be seen by comparing the average velocity of drifters once they have lost their drogues and when the drogues are still attached with a reference velocity, in this case the BRAN2015 ocean model evaluated along the path of each drifter. The drifter database is approximately 50:50 drogued and undrogued data points. Failure to isolate the two halves will inevitably lead to misleading results, either directly or via a mis-calibrated ocean model. If there were many more (undrogued) drifters in the ocean in 2014-15 than there were, a data product derived solely from them might be worth using but unfortunately this is not the case. Using many years increases the data count at the expense of direct applicability to 2014-15. Another problem is that since the drifters lose their drogues about halfway through their lifetime, trajectories right across the ocean in one state or the other are relatively few. To avoid misleading results, it is therefore necessary to use the drifter information very carefully, and combine it with information from many other sources in order to accurately simulate the drift of aircraft debris, especially items with complex drift characteristics, such as the flaperon. We did this in 2016 to the best of our ability at that time. As modelling techniques improve, retrospective drift analyses will undoubtedly become more accurate, possibly leading to different conclusions to ours and shedding more light on the location of MH370. In particular, models with finer spatial resolution will undoubtedly result in a wider range of simulated travel times across the ocean. Coupled with better models of the Stokes Drift, and items' response to wave forcing, a different picture may emerge. It is also possible that refined analysis of the flaperon barnacle samples will produce age and temperature estimates accurate enough to be used in conjunction with drift modelling. This has been attempted already but our understanding is that the uncertainties of the barnacle analyses have lead to doubtful conclusions.

Potential modelling improvements cannot, unfortunately, offset a key problem limiting the utility with drift modelling, which is that the number of debris items located on African shores is small, their dates of first arrival are uncertain, and it is unclear (due to the absence of directed searching) if items washed up during 2015 unnoticed.

Conclusion

We do not think that our drift modelling supports extending the search either northwards or southwards. We can not completely rule out the possibility that the aircraft is located either north or south, but the available information suggests to us that the intermediate latitudes are much more likely. We leave it to relevant domain experts to assess whether the search should extend either farther from the 7th arc, possibly at just a selection of latitudes, or focus on the so-called 'holidays' (gaps in the sonar coverage where terrain is difficult). We hope to revisit our conclusions at some future point but in the meantime express our sincere regrets that the committed efforts of many dedicated people have not yet solved this enduring mystery.

3 October 2017

Our 4th report to the Australian Transport Safety Bureau was released today, in support of the ATSB's Final Report on the Search for MH370. Ours is a short report providing a little more detail on our 2016 analysis of the 2014 surface search for debris, i.e., whether the surface search helps to rule the 32°S to 26°S segment of the 7th arc as being the location of the impact. In this report, we look specifically at whether debris from an impact at 30.5°S, 97.86°E would have been detected. The panel at right shows that according to our model, the resulting (hypothetical) debris cloud would have been overflown by many of the search tracks completed on 31 March. So even if the probability of detecting debris on any single overflight may have been low (and probably very low for any particular item), the cumulative probability of detecting at least one of the many items afloat, during several days of searching, was quite high. Our earlier conclusions (which rest only partly on this question anyway), therefore, remain unchanged.

16 August 2017

Our 3rd report to the Australian Transport Safety Bureau was released today, along with a report by Geoscience Australia. The Geoscience Australia report presents their analysis of 4 Pleiades 1A images taken on 23 March 2014 to the west of the 7th arc. The authors identified 9 objects in one of the images (denoted PHR4) that were "probably man-made", in addition to other objects that were "possibly man-made".

The winds and currents are predominately towards the east, so west of the 7th arc would seem to be an unlikely place to find debris associated with MH370. Our task, therefore, was to assess whether the location of these objects on 23 March was consistent with the aircaft impacting the water on 8 March somewhere east or south-east of the image locations, within the area that other evidence had led us to. The answer, as shown at right, was yes.

The figure at right (click to expand, or see the animation) shows a line of potential debris items on 8 March in white, and on 23 March in red, blue and black (denoting deep-drafted, shallow-drafted and highly buoyant items, respectively). The distortion of the line of potential debris items as time goes by is due to the stronger surface current near 35.5°S (which we have argued earlier as providing an explanation for the absence of debris finds on Australian shores). It is only those items drifting in the core of the current that reach the location of image PHR4 by 23 March, thus suggesting that if the objects seen in that image were pieces of the missing aircraft then 35.6°S 92.8°E is likely to be the location of the aircraft. We performed similar analyses for potential debris items from other portions of the proposed search area, finding that two locations west of the 7th arc (on the flanks of the current) are also possibly consistent with the imagery, but the northern half of the search zone is not.

For further details please see the [ATSB announcement] and our ECOS article. For a brief summary of the 8 clues that point to the location of MH370, see our poster.

FAQ: why not track the locations of the images backwards (rather than tracking a line of points forwards)? Wouldn't that reveal the location of the impact?

Answer: We can do that. But have a look at this back-track of three rings of particles. Why three rings? Because 9 'probably man-made objects' were found in image PHR4 and only 2, 1 and 0 in PHR3, 1 and 2, respectively. Needless to say, we don't have any other images. The three rings is a crude representation of the bounds and density of the real debris field. The three rings do not, of course, collapse to a dot when tracked backwards in time. This is because we cannot 'undo' diffusion due to the (unmodelled) turbulence that is important over this time-frame. So what we presented in the report to ATSB was 1) use a no-diffusion forward model, with many start points, to find the one that connects best with the location of PHR4, then 2) verify that adding a plausible amount of diffusion (via random walk) created a debris field large enough on 23 March to span (or nearly span) the distance to the other images, thereby accounting for the few items identified there (e.g. Fig. 3.4.3).

21 April 2017

Our 2nd report to the Australian Transport Safety Bureau was released today. This report focusses on field testing of a genuine Boeing 777 flaperon. This testing confirmed predictions by Pengam (2016) that the flaperon's motion with respect to the water is about 20 degrees left of the wind. Arrival of MH370's flaperon on Reunion in July 2015 now makes perfect sense, rather than just being plausible according to our earlier simulations. Our estimate of the location of the crash, however, is unchanged by this work, because the drift of the flaperon was just one of several considerations. The report is published by CSIRO DOI: 10.4225/08/58fba83e73f2b and is available from the ATSB. See also today's [ECOS article] and yesterday's [seminar].

21 January 2017

Interview with Bob McDonald on the CBC's Quirks and Quarks program explaining why we think that within the proposed new 25,000 km2 search area there is a much smaller region where the plane is most likely to be:- either NW or SE of the 7th arc at 35°S.

20 December 2016

One of the critical elements of our work was to address the question of how quickly aircraft parts drift downwind compared to oceanographic drifters, whose rates of drift across the ocean are quite well known. We did this by deploying life-size, GPS-equipped replicas of the flaperon and two other found parts of the aircraft alongside oceanographic drifters. With the 'effective windage coefficient' determined, we then used daily estimates of the wind (from ECMWF) and surface currents from the latest version of our global ocean model, to compute trajectories of hypothetical items of debris. The ocean model is informed by continuous satellite measurements of sea level (dotted in the Figure at right), enabling it to be quite accurate on a daily basis.

The first figure at right (click it to expand or [here to animate]) shows the outline in red of the area of sea floor that has been searched to date. Computed positions on 20 March 2014 (12 days after the aircraft went missing) of many hypothetical debris items are shown in black. These correspond to hypothetical accident sites either side of the 7th arc, from latitude 40°S to 31°S. The report describes why the northwestward displacement of items originating near 36°-35°S would explain the absence of detections during the surface search in March-April 2014 as well as the absence of debris findings on the WA coastline. This suggests the crash occurred near 36°S-35°S. The report also explains why we think that the accident is very unlikely to have occurred north of 32°S or south of 39°S.

Now that the 39°S-36°S segment of the arc has been searched, the 36°-32°S segment is all that remains. A zone close to the arc has already been searched, so the only possible locations remaining are outside that zone but within about 25NM of the 7th arc (see 2nd Figure at right from the ATSB report, which can also be clicked to expand).

For more details of how we have come to this conclusion, please see our and the ATSB's reports.

28 February 2016 update: 'No step' item found on a sandbank off Mozambique An item which appears to be a wing fragment, with NO STEP printed on it, has been found by Blaine Gibson on a sandbank in the Mozambique Channel. The item has been handed in to authorities for investigation. If it can be identified as being from MH370, this brings to two the number of items potentially usable for oceanographic back-tracking.

Unlike the flaperon found on La Reunion, however, this item is not heavily encrusted with sea life, so it has probably spent a significant length of time either weathering in the sun and/or washing back and forth in the sand at this or some other location. The time at sea is therefore possibly much less than the 716 days that have elapsed since 14 March 2014, and the path taken may have been two or more distinct segments.

The tracks of Global Drifter Program drifters arriving in the Mozambique Channel (during 1985-2015) were overwhelmingly from the east, as shown at right. Two were close to the ATSB search area 700d prior to being in the Mozambique Channel, suggesting that the 'no step' item could very well have followed a similar path. But other drifters arrived near Mozambique from very different points along the 7th arc, showing that those, too, could also be regarded as possible origins of the item.

As with the flaperon, therefore, we conclude that while the location of this finding does not cast doubt on the ATSB's choice of search area (based on the Inmarsat handshakes), it can not provide particularly strong support for it either, because the trajectories of drifting items are so chaotic.

Trajectories of Global Drifter Program drifters

2 October 2015 update: Composite trajectories

Introduction

Few drifters complete journeys long enough to be directly relevant to the task at hand, especially since we are only interested in the subset of the drifters' trajectories that are before or after specific places, and with or without the drogue attached. We have therefore elected to create composite trajectories by adding shorter trajectories together head-to-tail. There are gaps in time and space involved, but we think that the benefits of the vastly greater number of composite trajectories more than compensates for the damage done by these gaps.

Method

Our method is simple: at the end of a partial trajectory, we do a search for all trajectories that meet distance-away, time-of-year and drogue on/off criteria. At the ends of all those trajectories, another set of searches is done. Drifter data density is quite uneven, potentially biasing the results, so we under- or over-sample the available trajectories appropriately (by time-shifting in the latter case). We have limited the number of partial trajectories to three or four for the results shown here, so they are either 538/3 or 538/4 days long. This results in a large number of trajectories. Using the trajectories back from La Reunion, we can estimate the possible location of the flaperon 508 days earlier (the time between 8 March 2014 and 29 July 2015) by mapping the density of points 508days (+/- 30days, center-weighted) back along all the trajectories.

One way to rationalise this approach is to consider that the best way to estimate where a drifter might go, having come to a certain point, is to look at the paths of other drifters that have passed near that point. This is, after all, what we were doing by looking at single-drifter trajectories, anyway. So now, as well as single-drifter trajectories, we have very many more three- or four-drifter trajectories. We have done this for a selection of La Reunion-backward and drogued and un-drogued trajectories [list].

Results

The image above shows our estimate of the likelihood function of where the flaperon was on 8 March 2014. The 7th arc is added for reference, with the 'high priority' search zone shown in bold. The sea-floor search zone is clearly within the zone of likely origins. The southern end of the search zone is close to the southern flank of the likelihood function, while the northern end of the search zone is towards the eastern flank, but at slightly higher likelihood than the southern half. But why is there a broad maximum NW of the arc, e.g. near 35S, 75E - 30S, 90E? Because this is where drifting buoys are evidently concentrated, moving at lower average velocity and therefore spending more time. We have verified that this is not an artefact of where drifters are deployed or lose their drogues. There is a (leaky) mechanism that concentrates them here in the centre of the gyre. Our back-tracking inevitably concludes that the most likely prior location of the flaperon was where it probably spent a lot of time, regardless of exactly where it entered the water nearby. This assumes that the flaperon drifted exactly like an undrogued drifter, which is probably not the case, being a very different shape. But it probably drifted much more like an undrogued drifter than a drogued one (in which case we would conclude that it entered the water much farther north).

The end points of forward-tracks originating near the northern half of the high-priority seafloor search zone are distributed fairly evenly around La Reunion. A similar result is obtained for the southern half, reinforcing our earlier conclusion that if the plane had crashed where it is thought to have, that Reunion would be one of the most likely places where debris would be found.

Summary

Analysis of the trajectories of satellite-tracked drifting buoys deployed in the Indian Ocean over the last 30 years confirms our earlier conclusion based on computer modelling that the MH370 flaperon found on La Reunion in July 2015 is consistent with MH370 having crashed near the 39°S-32°S segment of the 7th arc on 8 March 2014. With just one piece of MH370 found, however, the buoy data, like the computer modelling, can not significantly refine the ATSB's sea-floor search area - it just increases our confidence that the flight path analysis underpinning the choice of sea-floor search area is not wrong.

Introduction

Our 5 August news item described how the 29 July 2015 discovery of a flaperon (now confirmed as being from MH370) on La Reunion island did not cast doubt on the MH370 sea-floor search area chosen by the Australian Transport Safety Bureau based on analysis of the satellite ping data (see the ATSB Fact Sheet). This conclusion was based on ocean modelling. Here, we discuss additional information that lends further support to our 5 August conclusion.

The Global Drifter Program is a highly-valued legacy of the World Ocean Circulation Experiment. The satellite-tracked drifting buoys ('drifters') measure ocean surface temperature for calibration of satellites, and atmospheric pressure for improving the accuracy of weather forecasts. The trajectories of the drifters tells us about ocean surface currents.

The term 'surface current' however, needs discussion before we connect it with the drift of the flaperon.

The flaperon is buoyant, flat, and only a few meters long. Depending on exactly how it floated, it would have drifted at the velocity of the water averaged over the top 0.5-2m. This is certainly close enough to the ocean surface for the effect of waves to be important, so the Stokes Drift would have contributed to its drift. If the flaperon had any non-negligible freeboard (projection into the air) then it would also catch the wind and 'sail' slowly through the water at some small fraction of the wind speed. This down-wind velocity is often referred to as 'leeway' and expressed as a percentage of the wind speed. We do not have an estimate of the leeway factor of the flaperon, so our modelling work was done for a range of plausible leeway factors. As an aside, we note that winds and waves are usually strongly correlated so empirical leeway factors will include the Stokes Drift if the influence of waves is not deliberately isolated.

Global Drifter Program drifters are fitted with 10m-long sea-anchors (or 'drogues') centred at about 15m depth so that, by design, they do not drift downwind like buoyant items such as the flaperon. There are several good reasons for this design, both scientific and practical. Fortunately for our present purpose, however, the drifters lose their sea-anchors after some time (Lumpkin et al. 2013), making them much more relevant to the question we now face with the flaperon than they were designed to be. Their leeway is possibly slightly higher than the flaperon's, but we think that the error associated with this is smaller than the errors of any other source of information available. We refer below to these as undrogued drifters. Trajectories of the drogued drifters are also shown to highlight the importance of the difference, and for comparison with models that have not included the effects of winds and waves, such as the recent GEOMAR study.

Results

We have extracted from the 1985-2015 database the trajectories of all undrogued drifters relevant to MH370, i.e. drifters that were near the present MH370 sea floor search area (the high-probability segment of the Inmarsat 7th arc between 39°S and 32°S) in February-April, or near La Reunion in April-July, in any of the last 30 years.

Forward drift from 39°S-32°S near 7th arc

Seventy four undrogued drifters (right, click to expand) passed through an area surrounding the present sea-floor search area for MH370 and reported data for at least 200 days after that. A substantial fraction of them went near La Reunion within 500 days. These are drifters that transited the blue rectangle within 6 weeks of 8 March. Relaxing that criterion, we see that trajectories at other times of the year were not very different. One drifter actually beached on La Reunion.

Water temperature and barnacles

The drifting buoys record the surface temperate of the ocean. These data, along with the general northward then westward drift post-crash, show that the flaperon probably entered water warmer than 18°C within a month or two of the crash, so barnacle nauplii may have started settling and growing on the flaperon for most of the voyage.

Forward drift from other segments of the 7th arc

We have also looked at the trajectories of drifters passing though segments of the 7th arc either south of 39°S or north of 32°S. Drifters passing through the southern region have an increased tendency to go east and a lower chance of going to La Reunion. Conversely, drifters passing through the northern region mostly went west across the Indian Ocean but passed north of La Reunion, several beaching on Madagascar or Africa in less than 300d. The flaperon finding is therefore too late as well as too far south to be consistent with a crash site north of the present sea-floor search region.

Comparison with model results

The undrogued drifter trajectories differ from our model trajectories in that the real drifters passed by La Reunion principally north of La Reunion, while our model drifters were principally south of La Reunion. This is precisely the sort of model error we had in mind when we said taking model errors into account in our 5 August news item.

Trajectories leading to La Reunion

Turning the question around, the origins of undrogued drifters that passed close to La Reunion any time in 1985-2015 were principally in the southern Indian Ocean, including regions near the supposed MH370 crash site, as shown at right. Selecting only those that arrived near La Reunion in April-July does not greatly alter the picture. What does change the picture, as mentioned above, is selecting for tracks of drogued drifters that (ultimately, possibly with the drogue off, like this one) go near La Reunion. These drifters are more likely to have originated in the tropical Indian Ocean. As discussed above, however, the flaperon floated close to the ocean surface, so the tracks of drogued drifters are much less directly relevant than those of the undrogued drifters.

Conclusion

Taking the modelling and drifter observations together, we stand by our earlier conclusions: the finding of the flaperon is not a reason to doubt the present choice of sea-floor search area. And with only one piece of MH370 found, the presence of ocean eddies makes it essentially impossible to refine the sea-floor search area with any confidence. The flaperon finding does, however, support the flight-path analysis conclusion that the 39°S-32°S segment of the 7th arc is indeed the highest-priority search region for MH370.

Graphics Archive

Model simulations using splashpoints on the 39°S to 32°S segment of the '7th arc'