Calculation of SOI
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Note the anomalies are departures from the 1951-1980 base period. 

SOI = (Standardized Tahiti - Standardized Darwin) / MSD , where

Standardized Tahiti = (Actual Tahiti (SLP) - Mean Tahiti (SLP))  / Standard Deviation Tahiti , and

Standard Deviation Tahiti = SQRT( SUMMATION(1) / N ) , where

SUMMATION(1) - is the sum of all ((TA) ** 2) 
TA - Tahiti anomaly = (actual(SLP) - mean(SLP)) 
N - number of months 


Standardized Darwin = (Actual Darwin (SLP) - Mean Darwin (SLP)) / Standard Deviation Darwin , and

Standard Deviation Darwin = SQRT( SUMMATION(1) / N ) , where

SUMMATION(1) - is the sum of all ((DA) ** 2) 
DA - Darwin anomaly = (actual(SLP) - mean(SLP)) 
N - number of months 

To calculate the monthly standard deviation: 

Monthly Standard Deviation (MSD) = SQRT( SUMMATION(3) / N)
SUMMATION(3) - is the sum of ((Standardized Tahiti - Standardized Darwin) ** 2)
N - total number of summed months