Pressure data from Indonesia and Tahiti for years before 1866 are used to extend the Southern Oscillation Index (SOI) back to 1841, with a gap between 1861 and 1865. Extension further is possible using an index of Jakarta rainday counts back to 1829. Rainday counts correlate (r=0.60) with average Jakarta pressure for the June-November dry season over the 1876-1944 period. Although low, this correlation is still better than the correlation of tree rings with pressure or SOI. After 1950 the rainday count/pressure relationship alters, and by the 1990s 18% more raindays (an increase of seven per dry season) occur than the pressure would indicate. The dramatic increase in the size and population of Jakarta since 1950 is considered the most likely reason.
GP Können, PD Jones, MH Kalthofen, RJ Allan. extensions of the Southern Oscillation Index using early Indonesian and Tahitian meteorological readings
published, J. Climate, 1998, 11