Arctic Oscillation Index
The Arctic Oscillation is observed as a fluctuation in 700 mb (~3300 m) and, presumedly, in near sealevel pressure anomalies poleward of 20 degrees N. Essentially, the index is positive if pressures (or heights) are lower in the polar latitudes compared to those in the midlatitudes. The index itself is defined on the basis of zonally averaged 700 mb height anomalies. (Actually, the index itself is based upon a harmonic analysis of these pressure/height fluctuations, so I am greatly oversimplifying here)
The Arctic Oscillation itself is a semi-periodic flluctuation in this index. Clearly, the idea is that the index (AOI) itself should be linked to storminess and precipitation in the middle latitudes and, presumedly, along the West Coast, because when strong cyclones are present in the middle latitudes and, presumedly, along the West Coast the AOI should be negative.
In the view of many atmospheric scientists, this and other indices are misinterpreted and overused as explanations for climate or weather variability. Here are a few reasons why:
- the AOI essentially (but not exactly) estimates differences in pressures or heights between the subtropics and polar regions and not the pressures/heights themselves;
- even if a negative AOI implies lower pressure in the middle latitudes and, presumedly, along the West Coast, it says nothing about the location of the low pressure areas, strength of the low, for example, and other important characteristics that determine precipitation amounts, such as strength of vertical motion fields, amounts of water vapor, and the degree to which orographic effects are present, to name a few;
- even if the AOI allows some meaningful insight into weather and climate variability, mere consideration of that alone creates "brain deadedness" in forecasters who should be considering the ingredients* of patterns, and the details of the patterns themselves, rather than a blind correlation of an index to precipitation, for example.
*some ingredients are italicized in the second bullet point above
Because of these concerns, meteorologists should use composite patterns developed for the AOI with great caution. These composites are useful, but still can lead to overbroad generalizations.
I also want to underscore that an analysis of PAST cycles in the AOI may indeed be useful in foreshadowing something about the pressure or height field in the future. But the low correlation of the AOI with precipitation in California is a sure indication that the Arctic Oscillation itself should be used with GREAT caution when discussing droughts or ends to droughts etc. On the other hand, the correlation of California precipitation with several of the indices used to quanitfy ENSO is very high and linked to a physical mechanism (the impact of SSTs on height patterns) to shift position of jet streams around, to oversimplify. For example, weak La Niña seasons are often associated with drier than average conditions in our part of California.