Arctic Warmth is NOT Due to the AO+

[donsutherland1]

This morning, I received an e-mail about an argument tying warm anomalies in the Arctic to the AO+. I haven't been able to verify the source of the argument, as I found no news stories in a Lexis-Nexis search nor the timeframe in question (possibly the most recent winter given that we're currently in early March?), but as I have looked at that issue quite a bit in devising my own medium-range and monthly forecasts this winter, I will comment. In my view, the argument that the AO+ is responsible for warm Arctic anomalies is incorrect.

In general, predominant AO+ regimes are colder not warmer in the Arctic region, because the cold is bottled up and largely confined there on account of a lack of blocking. In contrast, AO- regimes are warmer in the Arctic, because the blocking distorts the jet stream, which drives the cold farther south into Eurasia and North America, and sometimes both. Winters 2009-10 and 2010-11 were recent examples.

This winter saw the AO average above +0.500 (December-February period). Typically, that strong of a seasonal positive anomaly should produce a cold outcome in the Arctic region. Instead, cold anomalies were limited and there was an expansive area of exceptionally warm anomalies. This winter's Arctic warmth is atypical for strong AO+ patterns. However, such outcomes have become increasingly frequent beginning in the 2000s during which the Arctic region has experienced exceptional warmth, in general. The sample size for that timeframe is still small, so there is a degree of greater uncertainty.

The re-analysis data tells the story. Below are the temperature anomalies for winters when the AO averaged +0.500 or above, prior to winter 1999-00 and then for the winters beginning with 1999-00:

Notice:

1. The prior set of winters had expansive cold anomalies in the Arctic region (the opposite of the AO+ argument).

2. The latter set of winters had notably less cold in the Arctic.

If one examines days during which the AO was strongly positive (+3 or above) or strongly negative (-3 or below)--a chart I posted earlier in this thread--one finds a similar outcome for strong AO+ regimes. The formerly cold (in the Arctic region) AO+ situation has given way to outcomes that are much warmer in the Arctic than had been the case in the past.

In sum, the re-analysis data shows that the argument that the AO+ is the cause of the current or recent Arctic warmth is incorrect.

[dabize]

Where does the Arctic Dipole fit into this?

I've always thought of it as being an extreme negative AO, since it favors the transport of heat to the Arctic - does it show up as a correlation on the AO index?

[donsutherland1]

Where does the Arctic Dipole fit into this?

I've always thought of it as being an extreme negative AO, since it favors the transport of heat to the Arctic - does it show up as a correlation on the AO index?

The Arctic Dipole pattern may be a fairly recent phenomenon, so there's a lot of uncertainty. It could be playing a role in leading to what have been warmer outcomes in the Arctic region regardless of AO and it may be linked to decreased summer ice.

For those who aren't familiar with the pattern, here's a brief piece that describes it: http://nsidc.org/asina/2010/070610.html

[Vergent]

High arctic temperatures in winter with a strong AO+(the air is trapped) are a function of:

1. The black body temperature of the universe.
   
2. The black body temperature of the surface.
   
3. The GHG concentrations in between.
   
4. The heat flux from under the ice(thermal current). This is what maintains the surface temperature.
   
5. Misc. minor heat sources(imported wind energy etc.).
   

#1 is constant, #2is confined to a narrow range around the melt/freeze point of sea water; -1.5C., #3 has changed by a small percentage over the recent years. #5 is minor.

I conclude that #4 has increased significantly, keeping #2 near the top of its range. This is probably due to thin ice( less insulation), and increased underlying water temps due to increased import of heat from the North Atlantic and Japanese currents.

[donsutherland1]

FWIW, I just learned that the argument about the AO+ and warm Arctic anomalies was not from any news story (hence the failure of Lexis Nexis to find anything related). It came from a tweet from Joe Bastardi. JB stated:

Remember arctic "warmth" is because of well mixed air ( stronger than normal winds) due to positive AO.

I have no arguments about the idea that mixing leads to smaller temperature anomalies.

My point about the AO+ still stands and it's supported by the re-analysis data. One cannot generalize that an AO+ leads to warm anomalies in the Arctic. In fact, until recently, the opposite has typically been the case. Recent winters have marked a departure from that earlier norm. In fact, regardless of the pattern (AO+ or AO-), the outcome is warmer than it had been in the past for similar synoptic patterns.

Therefore, something beyond the synoptic pattern in driving the warmer outcomes. Longer cycles are contributing, but the warmth in the Arctic has been far greater than during the 20th century warm cycle. I believe ongoing climate change is another important factor, and Arctic warming is forecast by the climate models.