Now that winter has arrived, you will likely be hearing these terms used more frequently by meteorologists. Here, I will discuss each of the following terms and explain the affects they have on winter weather patterns. The terms are: AO (Arctic Oscillation) index, NAO (North Atlantic Oscillation) index, and PNA (Pacific-North America) index.

The first term I want to talk about is the AO index. This is defined by surface atmospheric pressure patterns. When the AO index is positive, surface pressure is lower than normal in the polar region and higher than normal in the mid-latitudes. This causes westerlies to increase over Canada and keeps the cold air farther north.

The warm (positive) phase of the Arctic Oscillation (AO). Courtesy NWS Little Rock.

When the AO is negative, surface pressure is higher than normal in the polar region and lower than normal in the mid-latitudes. This means the westerlies are not as strong since the pressure gradient force is weaker, and the cold air is able to move southward into the mid-latitudes.

Currently the AO is extremely positive, thus the frigid air is being bottled up in the polar regions. The AO is forecast to become less positive over the next two weeks, and this should allow for periods of cold air to sink southward into eastern parts of the country, especially as it approaches zero during the first week of January.

Observed AO (black line) and the forecast AO (red lines).

The NAO is closely related to the AO. Essentially, the NAO index is based off the difference of the atmospheric pressure at sea level between the Icelandic low and the Azores high. The strengths and location of the the Icelandic low and the Azores high vary from year to year and this variation is known as the NAO.

Comparison of a positive and negative NAO.

Much like the AO, if the pressure gradient between the Icelandic low and Azores high is strong, then the corresponding westerlies are strong. This is known as a positive NAO. Since the Azores high is stronger (and typically farther west), more of southwesterly flow is present across the northeastern United States, meaning warmer than normal temperatures. When the high is weaker (and farther east), the Icelandic low tends to influence the northeastern US more than the high. This means more of a northerly component to the wind, thus allowing the colder air to intrude farther south.

Currently the NAO is positive (as shown below), but is forecast to become less positive over the next two weeks. Again, this will allow for periods of colder air to makes its way into eastern sections of the country, especially towards the latter part of the first week of January.

Observed NAO (black line) and forecast NAO (red lines).

Lastly there is the PNA. When the PNA is positive the surface pressures are higher than normal over Hawaii and the inner-mountain west of the United States. The PNA pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. When the PNA is positive, this jet stream is enhanced and shifted eastward to where the exit region of the jet is near the western United States.

When the PNA is negative, this jet stream is weaker and shifts farther to the west toward eastern Asia. This effectively blocks activity over the high latitudes of the North Pacific and causes a strong split-flow to develop over the central North Pacific.

The positive phase of the PNA is associated with above average temperatures across the far western US and western Canada, and below average temperatures across the south-central and southeastern U.S. This also tends to lead to above average precipitation across the Gulf of Alaska and the Pacific Northwest and below average precipitation across the Upper Midwest. The negative phase of the PNA is associated with exactly the opposite.

Temperature anomalies associated with a positive PNA. Courtesy Climate Prediction Center.

Precipitation anomalies associated with a positive PNA. Courtesy Climate Prediction Center.

Currently, the PNA is slightly positive and is not forecast to change to much over the next week. In fact, it will likely rise a little if anything. Perhaps as we head into the first week of January it will try to become neutral or negative, but we will have to wait and see.

Observed PNA (black lines) and the forecast PNA (red lines).

Much of the Pacific Northwest has been unusually dry as of late, but with the PNA forecast to rise a little over the next week, we can expect to see this dry spell come to an end. Much of the Pacific Northwest will see a good amount of rain over the next week, and the PNA supports this.

Hopefully you now better understand how the AO, NAO, and PNA affect the weather patterns across the US. If you have any questions feel free to leave a comment or send me a message.