Arctic Oscillation: 1 – Trends (Update 1)

arctic oscillation (AO): 1 –  trends Since 1950

In this post, I begin a series on the Arctic Oscillation (AO) .  This post  presents a chart of monthly AO Index from 1950 to the present and introductory information on AO .  I will be updating this chart each month as NOAA updates the data series.  A link to the RClimate script that downloads the source data from NOAA is provided.

Update 1: Reader skrafner noticed that my plot legend indicated a 60-day moving average while the script actually calculated a 60-month moving avg. I’ve updated  the script and plot.


I’ve also created a daily plot of AO for the past 120 days, using NOAA’s daily data file. I have placed this plot in the side bar where I will update it several times a week.


AO is one of the ” major [climate] patterns that wax and wane, stretching for thousands of miles across the atmosphere and shaping Earth’s weather and climate” ( UCAR).

NASA’s Earth Observatory, in discussing the role of Arctic Oscillation (AO) in winter weather (link) defines AO ….

“The Arctic Oscillation is a climate pattern that influences winter weather in the Northern Hemisphere. It is defined by the pressure difference between air at mid-latitudes (around 45 degrees North, about the latitude of Montreal, Canada or Bordeaux, France) and air over the Arctic. A low-pressure air mass usually dominates the Arctic, while higher pressure air sits over the mid-latitudes. This pressure difference generates winds that confine extremely cold air to the Arctic. Sometimes, the pressure systems weaken, decreasing the pressure difference between the Arctic and mid-latitudes and allowing chilly Arctic air to slide south while warmer air creeps north. A weaker-than-normal Arctic Oscillation is said to be negative. When the pressure systems are strong, the Arctic Oscillation is positive.” Source: NASA Earth Observatory

Here are several key points about AO that I have found so far:

  • The Arctic Oscillation (AO) is a climate pattern
  • Impacts  winter weather in the Northern Hemisphere
  • Arctic is dominated by a low-pressure air mass
  • Mid-latitudes, around 45 degrees N (Montreal) governed by high pressure systems
  • Strengths of these  high and low-pressure systems oscillate
  • Strong mid latitude high and Arctic low pressure systems characterize positive AO
    • Positive Phase: Mid Latitudes HP – Arctic LP = +
    • Keeps chilly Arctic air in Arctic region
  • Weaker than normal mid latitude high and Arctic low systems cause the pressure difference between the two to decrease, allowing Arctic air to escape south and warmer air north
    • Negative Phase: Mid Latitudes LP – Arctic HP =
    • Lets chilly Arctic air slide south and warmer air move north

why am i interested in AO?

I have 2 reasons for learning about Arctic oscillation:

  1. I want to understand the impact of AO on my winter weather conditions in Philadelphia, Pennsylvania
  2. I want to understand the impact of AO on Arctic Sea Ice

During the winter of 2009-10, I first heard about the role that the Arctic Oscillation had on snowfall in Pennsylvania and nearby East Coast US states.  James Hansen et al (link) explains the December, 2009 winter conditions as follows:

“It is obvious that in December 2009 there was an unusual exchange of polar and mid-latitude air in the Northern Hemisphere. Arctic air rushed into both North America and Eurasia, and, of course, it was replaced in the polar region by air from middle latitudes. The degree to which Arctic air penetrates into middle latitudes is related to the Arctic Oscillation (AO) index, which is defined by surface atmospheric pressure patterns …

When the AO index is positive surface pressure is low in the polar region. This helps the middle latitude jet stream to blow strongly and  consistently from west to east, thus keeping cold Arctic air locked in the polar region. When the AO index is negative there tends to be high pressure in the polar region, weaker zonal winds, and greater movement of frigid polar air into middle latitudes.

… December 2009 was the most extreme negative Arctic Oscillation since the 1970s. Although there were ten cases between the early 1960s and mid 1980s with an AO index more extreme than ‐2.5, there were no such extreme cases since then until December 2009. It is no wonder that the public became accustomed to the absence of extreme blasts of cold air.”

As I tracked the Arctic Sea Ice Extent (SIE) decline over the summer of 2010 (see my Arctic Update Page), I heard more and more (link)  about the role of Arctic Oscillation on sea ice melt conditions.  This lead me to Dr Jeff Masters blog and his post on 2009 Arctic Sea Ice Melt Season.

I want to learn as much as I can about the Arctic Oscillation and its role on both Philadelphia winter weather and Arctic Sea Ice conditions and the role that changing Arctic Ice may have on the jet stream and future Philadelphia and Northern Hemisphere climate.

5 responses to “Arctic Oscillation: 1 – Trends (Update 1)

  1. Great plots, and thanks a million for sharing the scripts openly. A quick comment on your monthly AO index script; I think you are actually showing the 5 yr moving average rather than the 60 day average. your script calculates the moving average using filter(x, rep(1/60, 60),sides =1) but since the data is monthly, this will be the 60 months prior. You can see this as the moving average line doesn’t begin until around 1955.
    Thanks again, keep up the great work!

    • skrafner

      Thanks for the feedback. You are right! I’ve updated the legend note. If you check the R script you’ll see the update.

      Thanks again.

  2. Have you spent any time looking at an Australian site by Earl Happ called “Climate Change”? What is interesting on this site is that he looks at the polar oscillations and how these are affected by the solar cycle. Just looking at the local weather (mid-Atlantic) and some of the things going on with blocking patterns 2 years after solar minimums, he has some ideas that are interesting to consider.

  3. AO goes from very negative to very positive and back with frequency.

    The past two weeks the swings were quite radical indeed.

    I look forward to read more about what you learn about the AO, Kelly. Maybe the Dipole Anomaly would be interesting after that as a next step?

  4. The interesting aspect of the AO among the atmospheric circulation indices is the frequent and significant changes. Compared to PDO, ENSO the PNW ete. AO goes from very negative to very positive and back with frequency. The lack of trends at the 60 day interval in your graph suggests that is not the best time frame to look at, too long. For a glacier this means that it is not the AO average that is important to examine, it is specific significant deviations.

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