Edim,
There is absolutely no evidence for a decreasing airborne fraction. As I’ve noted there is some hint of an increase but nothing conclusive.
Manacker’s graph fails to take into account anthropogenic emissions due to land usage change, around 25% of the historical total. There is little trend in this source over the past few decades so of course failing to include it will produce a result which suggests decreasing airborne fraction.
By the way, what do you think causes the seasonal CO2 cycle, SST cycle or NH greening?
In the absense of any great understanding on my part, I’ll refer to the annual carbon cycle diagram from AR4: http://ipcc.ch/publications_and_data/ar4/wg1/en/figure-7-3.html.
Over land there is about 120 GtC/yr going up and down related to vegetation etc. I would presume the timing of these fluxes would follow seasons, particularly relating to the NH growing season.
Over sea there is about 70 GtC/yr up and down. Again, I would presume there would be seasonal differences, relating to SSTs.
The shape of the overall annual cycle in the Mauna Loa, or global, data will be largely determined by a combination of these.