As I wrote here
I am sort of still checking back wether the ugamp page shows a longitude independent or averaged image, by looking at your oscillations of Singpore (approx. at the equator) at 70hPa it looks as if easterlies and westerlies last approx. the same time at 70hPa. That's different from fig 1 of the ugamp page.
So it seems one has to find out what a "UKMO assimilated dataset" is. argh.