A good summary of what has been used to choose the southern arc can be found here.
http://www.rin.org.uk/Uploadedpdfs/s...at%20Paper.pdf
Note please, that the following initial basics did not change throughout the whole process. Every refinement and the final conclusion are based on the asumptions of this initial reconstruction attempt. (bolding by me)
Initial flight path reconstruction attempts were based on the aircraft flying at a steady speed on a relatively constant track consistent with an aircraft operating without human control. It was initially thought that for the fuel to have lasted until 00:19 UTC the aircraft would have needed to be flying at high altitude, where the air is thinner and drag is reduced, which would have resulted in its flying at close to its maximum speed of just over 500 knots (926 kph). This gave two solutions, one in a northerly and the other in a southerly direction, as illustrated in Figure 6, where thered lines indicate the flight paths prior to 00:19 UTC and the green lines indicate the potential additional flight paths between the last signal at 00:19 UTC and the failure to respond to the LOI message sent by the GES at 01:15 UTC.
It is important to remember that these initial flight paths, while consistent with the BTO timing data and the aircraft performance, were based on a number of assumptions: that the aircraft travelled at a steady and high speed and did not make any manoeuvres beyond a turn to the north or south shortly after its last radar detection.
The changing BFO data caused by the wobbling of the Sattelite and the above assumptions led to the southern path. The sattelite moves north south with a maximum of 2,412 km per day which results in a comparable low BFO change opposed to a climb, descent or a speed change.
Read also on the Blog of Duncan Steel.
The Bottom Line: A northerly route for MH370 deep into central Asia cannot be excluded on the basis of the publicly-available Inmarsat-3F1 satellite data.
The Inmarsat-3F1 Doppler Data Do Not Exclude a Northerly Flight Path for MH370 | Duncan Steel