By Andy May
The Arctic Oscillation (AO) is closely related to the NAO (the North Atlantic Oscillation discussed below) but they are not the same. The NAO is usually measured using the SLP (sea level air pressure) difference between the Azores or the Iberian Peninsula and Iceland and is a North Atlantic regional phenomenon, whereas the Arctic Oscillation is the SLP difference between the northern mid-latitudes and the Arctic, and is evident in all longitudes (Thompson & Wallace, 2001). The AO accounts for more of the variance in Northern Hemisphere surface air temperature than the NAO and is tightly connected to the stratospheric polar vortex (Higgins, et al., 2000) and (Thompson & Wallace, 1998). We will discuss these oscillations together in this post.
The Arctic Oscillation
The Arctic Oscillation (AO) is also called the Northern Annular Mode or NAM. It is analogous to the Southern Annular Mode or SAM discussed in Climate Oscillations 5. However, there is a large difference, whereas SAM is an oscillation over an ocean that surrounds land, NAM is an oscillation over land that surrounds a polar ocean. Thus, they act differently.
When NAM or the AO Index is positive (lower than normal pressure in the Arctic, and/or higher pressure in the mid-latitudes), the high latitude westerly polar jet-stream winds move closer to the pole and storms (which transport heat) move northward. When it is negative (higher pressure in the Arctic), the jet stream weakens, becomes more loopy or wavier, and moves south allowing Arctic air to spill into the middle latitudes causing colder mid-latitude winters. The AO Index is only computed using data from December through February because it only has a significant impact in the winter months (Baldwin & Dunkerton, 1999).
The tropopause is quite low in the Arctic, only about 8 km above the surface, so it is not surprising that the AO is strongly connected to, and influenced by, the stratosphere (Baldwin, et al., 2019), especially in the winter months when the tropospheric and lower stratospheric circulations are coupled in the polar regions (Thompson & Wallace, 2001). A large positive AO Index represents a strong well-organized polar vortex in the stratosphere above the North Pole (Baldwin & Dunkerton, 2001) and (Thompson & Wallace, 1998), just as a positive SAM indicates a strong polar vortex above the South Pole.
The changes in the Sun over the course of the 11-year Schwabe solar cycle affect the stratosphere more than the surface because the shorter wavelength UV (solar ultraviolet radiation) content of sunlight changes more than the longer wavelength visible light that makes it to the surface. The amount of UV absorbed in the stratosphere can increase by 10% or more at the peak of the 11-year Schwabe Cycle. The UV absorbed in the stratosphere both warms it and contributes to stratospheric ozone which also absorbs UV and contributes to further warming. The UV warming affects stratospheric circulation and the strength of the polar vortex which transmits some of the stratospheric changes to the troposphere affecting global weather patterns (Haigh, 2011). We will discuss this later in the series, but in essence most of the ozone is produced in the tropics, which receives the most solar radiation. There is an upward transport of tropospheric air to the stratosphere in the tropics that sets up a transport of stratospheric air toward the poles (the Brewer-Dobson Circulation), where air is taken down from the stratosphere to the troposphere via the polar vortex (Baldwin, et al., 2019). The El Nino/Southern Oscillation (ENSO) process is involved in modulating the tropical transport of tropospheric air into the stratosphere. The AO has been called the “dominant mode of variability in the [northern] extratropics” (Higgins, et al., 2000).
Trends in the AO
As shown in figure 1, the AO is steadily increasing during the 20th century, but not as strongly as the SAM is ( see figure 3, here). This tells us, that on average, the northern polar vortex is strengthening, which leads to warming in the middle northern latitudes.
Figure 1 shows a slight increase in the full-year average AO and suggests cooling (more negative) from the late 1940s to the 1970s. It also shows warming from the 1970s to the early 1990s. The trend toward a more positive AO has reduced the severity of winter weather in the middle- and high-latitude Northern Hemisphere continental regions (Thompson & Wallace, 2001). The polar vortex is much stronger in the winter storm season in the Arctic, so we show the winter average over the same period in figure 2.
As can be seen in figure 2, the severe winter weather observed in the northern mid-latitudes from the late 1950s to 1970 and from 1976-1985 appears in the AO record. Unusual winter weather in these periods is documented here, here, here, here, here, and here. Mild winter weather was observed in the early 1970s and late 1980s to the early 1990s as shown here. The 1960s were also very cold in Asia, but there has been a warming trend since then (Kim & Choi, 2021).
The North Atlantic Oscillation
The NAO or the North Atlantic Oscillation is a very important oscillation in both climate prediction and weather prediction. However, when researchers compute the NAO indices with CMIP5 and CMIP6 climate model results they look like white noise with almost no serial correlation (Eade, et al., 2022).
Long-term weather observations from across the globe reveal patterns and links between seemingly random events and disconnected places. These long-distance relationships reveal changes in the meridional transport of energy from the tropics to the poles. For example, when you stitch together daily observations of air pressure across the Northern Hemisphere, you see large areas of high and low sea level air pressure (SLP) that flow and shift from place to place. These shifts in surface air pressure represent shifts in atmospheric mass from place to place. There’s a pattern to the shifting that is sort of like water sloshing back and forth in a bowl. The atmosphere sloshes northward; air pressure strengthens over the Arctic and weakens over the midlatitudes (either the Atlantic or Pacific Oceans, or both). Then the atmosphere sloshes southward; air pressure strengthens over the midlatitudes and weakens over the Arctic.
North of the equator, the most significant “long-distance relationship” in the Atlantic is between an area of persistently low pressure in the vicinity of Iceland and an area of persistently high pressure over the Azores Islands or the Gibraltar area. When the pressure is lower than average over Iceland and higher than average over the Azores Islands and Gibraltar, the North Atlantic Oscillation is said to be in its positive mode. When the opposite occurs, the North Atlantic Oscillation is said to be negative.
The Arctic and North Atlantic Oscillations are related to one another, and to the AMO (see here and here). Meteorologists often call these relationships and long-distance oscillations “teleconnections.” Teleconnection is as good a name as any, they are actually components of, and indicators of, changes in meridional transport.
Both the Arctic Oscillation and the North Atlantic Oscillation are defined with sea level air pressure or SLP, and the patterns are well illustrated by Rebecca Lindsey here (Lindsey, 2011). These patterns and the resulting NAO surface temperatures are shown in figure 3.
Past prolonged NAO (see figure 4) trends that last several decades cannot be explained by current climate models. The models clearly do not reflect multidecadal meridional transport regimes. Climate model simulations of NAO indices resemble white noise without serial correlation according to Rosie Eade and her colleagues at the MET office in the UK (Eade et al. 2021). There is a very small chance (1 in 20) that climate models emulate the observed NAO since 1860. Yet, figure 3 suggests that NAO trends are a key indicator of meridional transport (MT) strength. During the negative phase a lot of heat is transported poleward warming the polar region and during the positive phase of the NAO, little heat is transported to the polar region and it stays cold.
If the models cannot simulate meridional transport or the NAO, they cannot explain climate change. As discussed above, the polar vortex is strongest in the winter months and when the AO is positive. A strong winter polar vortex keeps the cold air in the Arctic and keeps warm air from being transported to the pole, thus delaying its expulsion to space and warming the middle latitudes, including the United States and Europe. In figure 4 we plot the AO in winter as a proxy for polar vortex strength and we see that the NAO is generally positive when the AO is positive (Wallace, 2006), we also see that cooler periods in the Northern Hemisphere and globally (1950s, 1960s, and early 1970s) show a declining NAO trend and a negative winter AO.
Figure 5 shows the same thing as figure 4, but only the winter NAO values are averaged. As David Parker and colleagues (Parker, et al., 2007) have noted the increase in the winter NAO from 1965 to 1995 is dramatic. It can be seen in the whole-year average shown in figure 4, but it is much more obvious in figure 5. It also shows a strong correlation to the winter AO and thus the strength of the winter polar vortex.
The NAM and the AO are two names for the same oscillation. The true measure of the strength of the polar vortex is the “PCH” or the composite geopotential height anomaly (“polar cap height”) averaged from 65°N to the pole and normalized by its standard deviation (Kim & Choi, 2021). Except for the PCH, the AO is the strongest proxy for the winter polar vortex strength, but the winter NAO can also be used as illustrated in figure 5. Data quality prior to 1950 is poor, but since then there is a good correspondence between the AO and the NAO in winter.
James Hurrell (Hurrell, 1995) points out the rapid rise in the winter NAO since 1965, and especially from the 1980s to the early 1990s. He adds that past decade-long changes in the NAO, and associated changes in atmospheric circulation, have contributed substantially to regional warming which complicates the interpretation of the effect of greenhouse gases on climate. He adds that the relationship of the NAO to greenhouse gas forcing should be examined. He asks that we investigate how well the climate models simulate the NAO, since it has a large effect on the climate over much of the world. Later Rosie Eade (Eade, Stephenson, & Scaife, 2022) did such a study and could not find the critical NAO in the models at all.
Conclusions
The Arctic and North Atlantic Oscillations are the dominant modes of variability in Northern Hemisphere climate. The observed positive trend in the AO/NAO in recent decades (see figures 2 & 5) is not reproduced in the CMIP5 or CMIP6 climate models, in fact the multi-model multi-member ensemble mean of the trend is zero (IPCC, 2021, p. 490). AR6 adds, on the same page, that the observed NAO trend lies outside the 5th-95th percentile range of the CMIP6 climate model distribution and the AO trend lies above the 90th percentile. It seems very unlikely that the models are useful with results like this.
Download the bibliography here.
Previous posts in this series:
Climate Oscillations 1: The Regression
Climate Oscillations 2: The Western Hemisphere Warm Pool (WHWP)
Climate Oscillations 3: Northern Hemisphere Sea Ice Area
Climate Oscillations 4: The Length of Day (LOD)
Climate Oscillations 6: Atlantic Meridional Model
Climate Oscillations 7: The Pacific mean SST
Climate Oscillations 8: The NPI and PDO
Concerning your concluding sentence, this is just one more example of lack of climate model utility.
There is a fundamental climate model problem. Essential processes (Tstorm convection cells is the on I have illustrated in writings and posts) occur on fine scales that are computationally intractable because of the CFL constraint on numerically solved PDEs. So the models must be parameterized, with (by CMIP rules) parameters tune to best hindcast 30 years. Tuning necessarily drags in the attribution problem of natural variation, which models by and large ignore.
Your post discusses NH natural ocean/air variations that climate models ‘reproduce’ as white noise when you show they obviously aren’t.
Weather is chaotic, that is, a case on nonlinear dynamics.
chaotic systems exhibit what is called strange statistics meaning the averages oscillate aperiodically. Your figures are clearly the footprint of chaos so it is no wonder the averages called climate oscillate unpredictably. The models ignore this of course.
Just to wax a bit the figures all show rapid large scale oscillations within a clearly defined range which is the strange attractor. This is a powerful form of stability that you get when you cram an infinite number of system trajectories into a finite state space. Poincaré discovered this math in 1910 or so. It is not about complexity as the simplest nonlinear equations can exhibit chaos.
The price of the stability is intrinsic unpredictability because an infinitesimal difference in system state quickly yields a big difference in behavior and you cannot know the system state to that accuracy. The unpredictability is epistemically not deterministic.
What cannot be predicted also cannot be explained when explanation is prediction after the fact as it often is.
Hear! Hear!
The indirect solar forcing of weekly NAO/AO anomalies can be predicted years ahead by using heliocentric planetary analogues. Kepler predicted the extreme cold winter of 1595 by such methods, but 20th century science lost the plot by insisting that weather is chaotic.
David,
I do not believe that any of the oscillations I discuss in this series are the result of chaos. All the oscillations are significant statistically and many of them back as far as 1600 or earlier through proxies. You may believe that they are simply random time series that happen to organize themselves into these chaotic oscillations and the IPCC clearly believes this as they renamed the AMO to the AMV and PDO to the PDV, but they are not.
Michael Mann showed in 1999 that the AMO and global SSTs have a statistically significant ~70-year oscillation at the 99% level and the ENSO is tied to it at a higher frequency.
https://www.sciencedirect.com/science/article/abs/pii/S0065268708600266
ENSO and the PDO have a less regular pattern. The statistical strength of the oscillations varies, but the AMO and the global cycle are strongly significant.
David,
Another good source for the validity of the AMO and the global SST cycle is Scafetta, 2010:
https://www.sciencedirect.com/science/article/abs/pii/S1364682610001495
He also finds the global cycle is well above the 99% level using multiple methods. He also finds a correspondence with celestial forcing. The cycles in ocean temperatures are similar to changes in the Sun (a 22-year Hale cycle). The origin of the 60-70-year cycle may be in the Jupiter and Saturn orbits.
Another really good source for astronomical forcing is Stefani, 2023:
https://link.springer.com/article/10.1007/s11207-024-02295-x
AMO cycles follow centennial solar minima not Jupiter-Saturn trigons. Every other warm AMO phase occurs during each centennial solar minimum, so the AMO cycles have to vary in length according the varying intervals between the centennial minima.
The statement attributed to James Hurrell demonstrates he has limited understanding of the processes that regulate Earth’s uptake and release of energy.The reason should be evident to anyone who has taken the time to think about it.
The average radiating temperature of Earth is well known to be 255K. The dominant emitter of long wave heat to space is H2O. At 255K H2O will be solidifying to ice and just 1mm of ice in the atmosphere has an emissivity of 90%.
If you look across global oceans on any day you will not find anywhere with an effective radiating temperature higher than 273K,
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That means ice dominates both the release of energy to space and the amount of solar EMR that is thermalised.
Anyone wanting to understand Earth’s climate must first understand a good deal about ice and its formation/loss on land, on oceans and in the atmosphere. “Greenhouse gasses” is an invention of scammers wanting to promote their scam.
If the oscillations don’t fit, they must acquit.
The cyclical nature of AO and the NAO (and the AMO) mimic the cyclical nature of the written, historic, regional temperature records.
I think you are on to something, Andy! 🙂
Good article.
Thanks!
The reality is in the oscillations. If the models don’t reproduce the oscillations, the models are wrong, not the oscillations.
Agreed. Wrong, and useless. Apart from that, they’re great!
The models predict increasingly positive NAO/AO with rising CO2 forcing:
https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/ch10s10-3-5-6.html
The models cannot predict oscillations without the indirect solar forcing of the NAO/AO:
https://www.sciencedirect.com/science/article/abs/pii/S0273117713005802
Ulric,
The article is typical IPCC gobbledy gook, but your summary of it is incorrect. All IPCC models use CO2 forcing, unless it is deliberately excluded, in which case there is no trend at all. “Some” of the models show a rising NAO and AO trend since the 1960s as is seen in nature, but the observations fall outside the 95% confidence range of the models which makes the models statistically invalid as noted in the IPCC main text, which is cited in the article.
The consensus of global circulation models predict increasingly positive NAO/AO conditions with rising CO2 forcing, that’s an accurate summary. It’s why the UK Met Office predict that UK-Euro heatwaves like in 2003 and 2018 will be happening every other year by 2050. Which is actually crazy as they were discretely solar driven heat events.
From AR6, page 490:
Bold added. This is a more accurate summary than yours.
That is a summary of attempts to model past observations, refer to the text in my link for the model projections.
I’m not sure what you mean, when you write there is no “solar forcing of the NAO/AO.”
But, with regard to the article cited:
Yes, the NAO/AO are strongly influenced by the solar wind. There are many good correlations between climate and weather and solar activity as detailed in Zhou, et al. and in the references given in that article. There may be some influences from increasing greenhouse gases, but this is not as well documented. Note that Zhou suggests that there is evidence of solar influence throughout the winter season.
“I’m not sure what you mean, when you write there is no “solar forcing of the NAO/AO.””
I did not write “no”, you did. CO2 based models can only predict an increase, but not an oscillation, as they do not expect the CO2 forcing to oscillate.
OK, that is much more clear. Thanks for clarifying, I did not understand what you meant.
I think that the importance of the AO and NAO are underlooked by most. Together with ENSO, the impacts in the Northern Hemisphere, and North America especially, is huge. I’ve lived in Colorado for about 30 years now, and have seen the changes and effects of ENSO and the AO trends here. In layman’s terms, ENSO stacks the deck, the Arctic Oscillation deals the hand. Natural patterns are driving the weather year-to-year, but naturally everyone wants to blame CO2 for every fire, drought, flood, hurricane, or unusual blizzard (James Hansen blamed anthropogenic global warming for the 1988 drought in the Midwest, the 1993 flooding in the Midwest, and the 1996 blizzards on the East Coast).
This sounds nonsensical, but maybe someone could explain what force propels air (or anything else) at the tropics to venture polewards. Just saying “it’s the Sun, stupid” is not an explanation, as the action of nighttime undoes the work of the day.
If someone says “Oh, it’s too complicated to explain,”, they are just demonstrating their ignorance of the subject. The same goes for the silly and irrelevant analogies employed (even at Harvard) to avoid admitting the ignorance of the analogy provider.
Can anyone at all provide a sensible explanation of the Brewer-Dobson circulation?
A scientific explanation would be nice.
Michael,
In a word the air and water vapor are carried to the stratosphere via thunderstorms and deep convection, mainly in the ITCZ. The ITCZ is the center of this deep convection and it is constantly moving with the sun, so it is hard to track, but it has been done:
https://link.springer.com/article/10.1007/s00382-021-05772-2
Also the ENSO cycle modulates the magnitude of the deep convection to the stratosphere.
Andy, your link states –
On other words, the Brewer-Dobson circulation is just assumed to exist, which might indicate that the 15 authors are just as ignorant and gullible as anyone else who believes that air or water are magically attracted to the poles, the equator, or some other location on the planet.
Sorry, but you didn’t seem to understand my request. Thanks for the response, anyway.
Here is another article on tropospheric/stratospheric exchanges in the tropics and subtropics. It emphasizes the importance of atmospheric waves in the process. They also play a role when the air returns to the troposphere in the polar regions.
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005JD006947
There are many other sources, but these should get you started.
Andy, from your link –
The “climatology of tropospheric ozone”? More meaningless nonsense, showing that “pay to publish” is thriving.
If you can find any explanation of the alleged Brewster-Dobson circulation, backed up by solid science (including reproducible experimental support), I’d be glad to see it. Quoting seemingly random papers based on speculation at best, is not terribly convincing. Thanks anyway, but I wonder if you subject any of your links to critical appraisal before you accept them as fact, rather than wishful thinking.
No offense intended.
Michael,
The Brewer-Dobson circulation has been known since the 1950s, and there is a lot of evidence for it. I do not plan to rehash that old ground in this series. You can easily research that yourself without my help, at least I hope so. I will mention it again in my post on the ONI, which is in a week or so. The air and water vapor are injected into the stratosphere in the tropics and make it back to the troposphere in the polar regions through the polar vortex. Atmospheric waves make a big difference in this process and supply a lot of the power.
This circulation is not the subject of this series.
I didn’t say it was, but you did write –
This statement is just pure nonsense, unless you can support it with more than speculation.
Just saying “it has been known since . . . ” is meaningless.
Sorry Andy, but you seem to be blindly accepting assertions without foundation, in lieu of testable science. I prefer to accept the untestable assertion that the atmosphere is chaotic, and hence it is impossible to usefully predict future atmospheric states.
I believe my assertion has more practical use than all of yours put together. Can you demonstrate otherwise? If not, I win.
Parts of America are experiencing record cold now, records held since 1875.
“The Arctic and North Atlantic Oscillations are the dominant modes of variability in Northern Hemisphere climate”
If you look at correlations as proof of cause then the variation in Northern Hemisphere Temperatures is directly related to the position of the North Magnetic Pole. See Fig. 2 on this site.
https://adriankerton.wordpress.com/climate-change-and-the-earths-magnetic-poles-a-possible-connection/
“Story tip”