Butterfly Effect
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   Analyses are subject to a "butterfly effect".  Because of the chaotic
nature of the atmosphere, a butterfly in Brazil can cause changes in 
storms hitting England.  The butterfly has much less effect on the 
but it can be severe over data poor regions such as the Southern ocean.  
We have found (1) compiler changes, (2) computer changes, (3) 
initial analyses, and (4) perturbed observations data sets can cause major 
changes in the Southern ocean.

   The "butterfly effect" occurs because the analyses use a first guess
that is generated by a 6 hour model forecast.  In regions with no data, 
the first guess becomes the new analysis.  For the next analysis, 

In regions of no data, the analyses/forecasts will diverge.  I've seen 
cases where one set of analyses had a Southern ocean storm and an other  
didn't.  (BTW in data-rich regions, the analyses remain close to each other 
but don't converge.)