Julia Sligo (and colleagues) have modelled Chinese rainfall for the last thousand years - so they've more or less got the monsoon variation right.
John Larkin wouldn't know about that - he probably doesn't even know what a monsoon is.
Well enough understood that a lot of the variation can be modelled. As the Argo buoy data comes in on the deep ocean currents the last major uncertainty will be reduced.
That's how John Larkin works. He doesn't realise that there's a better - and more informative - way to do it.