The concept of the North Pacific Oscillation refers to a theory that there are two states of meteorological conditions and circulation regimes for the North Pacific. According to the theory the North Pacific stays in one of those states for a period of time and then shifts to the other. The National Oceanic and Atmospheric Administration (NOAA) has compiled two relevant indices: the WP (Western Pacific) Index and the EP/NP (Easter Pacific and Northern Pacific) Index. If the ocean behaved strictly as the theory suggests the tabulation of the the indices would look something like this:

The actual histograms (tabulations of frequencies) look like this.

The data for these histograms are for the period January 1950 to October 2009. Some data for the EP/NP index for a few months were not given. These months for the EP/NP Index were left out of the tabulation.

There is not a hint of bimodality in those histograms. So the two states referred to in the theory have be the extreme points of a continuum of states. In this form the theory could still be valid and useful.

The real test of the theory would be in terms of correlations between the meteorological state of the North Pacific and the weather in North America. The data for this comparison have not yet been obtained, but in lieu of that comparison consider the matter of whether there is any correlation between conditions in the western Pacific and conditions in the East-North Pacific. The scatter diagram for this comparison is shown below.

The search for spatial correlations does not look very promising from the looks of the above display.

(To be continued.)