Coming from the aviation site: What is the westerly index?

The westerly index is:

... the Trenberth Z1 index (Trenberth 1976), which is the departure of the monthly mean pressure difference between Auckland and Christchurch (near 44°S) from its mean over the 1961–90 period.


Those two places are ~750 km apart (not a small distance), and judging by the METAR reports, the QNH difference in the past 72 hours varies from 4 to 7 millibars or so. Is that unusual? What's the significance of that index?


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