It has been noted that East Asians are positive outliers for the latitude ~ IQ pattern, they’re too far south for their high IQs.

Some possible reasons for this are:

The peoples evolved further north and migrated south in recent times, and their intelligence level is related to their recent origin, not current location, just as it is for e.g. Europeans in Australia, South Africa and Brazil. This would be particularly applicable to the southern Chinese. The East Asian IQ has been over-estimated and is not actually that high, perhaps only around 100. This would leave a smaller residual, but a residual nonetheless. According to David Becker’s latest (v1.2) recalculations, the IQs are: China 105.1, South Korea: 97.2, Japan 107.6. So there doesn’t seem to be much support for this idea, except for the odd value for Korea, probably a fluke. Latitude does not proxy the relevant climate causes well in this case. A version of this is that Chinese area used to be colder in the past. The high intelligence of East Asians is primarily due to some other factors that doesn’t have to do with climate, e.g. social organization.

Regarding the latter, I was reviewing evolution of human skin pigmentation (this review) and found some UV radiation maps that are in line with (3). Notice especially China.