What You Need to Know About the Pacific Decadal Oscillation

Editor’s note: This is a guest post by John Feldt. John is an AMS Certified Consulting Meteorologist (CCM) and has over three decades of professional hydrometeorological experience forecasting, communicating, and warning of weather hazards – from tornadoes and blizzards, to droughts and floods. His subscription service is one of the best. Learn more about Blue Water Outlook here.

The (1)

What is a Teleconnection?

Let’s discuss an important teleconnection called the Pacific Decadal Oscillation (PDO).

First, what is a teleconnection? A teleconnection is a relationship between an observed weather variable in one location with weather impacts at another. Typically, the observed weather variable is a measurement of atmospheric pressure at a specific location at a high latitude with another location at a lower latitude. In essence, a typical teleconnection is a measurement of the differences in atmospheric pressure between high and low latitudes.

Most teleconnections alternate between positive and negative phases. Many are considered short-term atmospheric teleconnections such as the Arctic Oscillation. These short-term oscillations typically cycle within days to weeks. El Niño/Southern Oscillation (ENSO) regimes (El Niño and El Niño) are based on equatorial pacific sea surface temperature anomalies which cycle between 6 and 18 months. However, there are a number of much longer, often decadal, oscillations which can interact with or influence shorter-term oscillations.

Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is a long-lived pattern of climate variability. A warm (positive) PDO is based upon anomalously warm sea surface temperatures near the equator and along the coast of North America, and anomalously cool sea surface temperatures in the central North Pacific. The cool phases for PDO have the opposite patterns of SST anomalies: cool along the equator and the coast of North America and warm in the central north Pacific.

Research indicates that just two full PDO cycles have occurred in the past century: Cool PDO regimes prevailed from 1890-1924 and again from 1947-1976, while warm PDO regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990’s.

Note the alternating phases of the PDO – negative/cold (blue) or positive/warm (yellow).   Blue box – past negative phase Red Circle – current readings
Note the alternating phases of the PDO – negative/cold (blue) or positive/warm (yellow).
Blue box – past negative phase
Red Circle – current readings

 

What is quite interesting here is that the Pacific Decadal Oscillation recently switched from a prolonged negative phase to a possible positive phase. The PDO index turned positive in early 2014 and has remained positive through the current time. In fact, December 2014 recorded an unusually-high positive PDO reading. The image below plots the monthly PDO index reading for 2014 and 2014. Note how the PDO turned positive in 2014 and the unusually- high level this past December (far right). This seems to support a true shift to a positive phase.

Figure2

So, what does this mean for our weather?

Assuming the Pacific Decadal Oscillation has shifted from a negative to a positive phase – significant changes to our weather patterns could be on the way. These changes might not always be obvious, and in fact, could be subtle. However, areas that have experienced persistent drought could find improvement while other areas might become drier.

Here is an interesting study developed by BWO Agriculture Analyst Lydia Rill. It shows that precipitation tends to increase over the U.S. Corn Belt during positive phases of the PDO. Nearly 70% of the years during the past PDO reported above-average precipitation.

Figure3

Again, keep in mind that other shorter-term teleconnections will have an impact, as will a wide variety of other weather factors, such as temperature or precipitation feedback loops. However, if the Pacific Decadal Oscillation remains positive, and it is looking likely that it will, it could foretell an extended period of shifting weather patterns.