Climate Change Leading to More NAO and PNA Extremes

[bluewave]

I saw the near record +PNA rise being discussed in Don's great thread. So I did some research into how it’s becoming more extreme with climate change. The PNA index isn’t the only one becoming more amplified. A study from 2009 documented how the NAO has also exhibited much bigger swings from high to low as the climate has warmed. In fact, the summer of 2009 to the present has exhibited record breaking highs and lows. The AO is showing these same extremes from record lows in the summer of 2009 through March 2013. Then we experienced a rapid reversal with record positive levels in the winter of 14-15 while the PNA was also in a very positive phase. This past winter featured an historic February reversal from record daily negative readings to very positive.

https://phys.org/news/2009-01-links-north-atlantic-oscillation-variability.html

Study links swings in North Atlantic oscillation variability to climate warming

 

Using a 218-year-long temperature record from a Bermuda brain coral, researchers at the Woods Hole Oceanographic Institution (WHOI) have created the first marine-based reconstruction showing the long-term behavior of one of the most important drivers of climate fluctuations in the North Atlantic.

The North Atlantic Oscillation (NAO) is a wide-ranging pressure seesaw that drives winter climate over much of North America, Europe and North Africa. Past reconstructions of the NAO have relied mainly on terrestrial, or land-based records, such as tree ring chronologies combined with ice cores and historical climate data. Those records do not fully capture oceanic processes linked to NAO variability, and short instrumental records from relatively few locations limit the understanding of ocean-atmosphere dynamics with regard to NAO behavior.

By analyzing the coral, we were able to look at changes in the ocean relative to changes on land," said Nathalie Goodkin, lead author of the study published in the December issue of the journal Nature Geoscience. "Because they are slow growing and have long life-spans, corals can provide high resolution records that are well dated and centuries long." 

As they grow, corals accrete seasonal and annual growth layers, similar to tree rings. The proportions of trace elements versus the major element (calcium) found in the layers of the skeleton largely depend on the temperature of the seawater in which it was formed. By analyzing the strontium to calcium ratio in the Bermuda brain coral, Goodkin and colleagues — WHOI scientists Konrad Hughen, Scott Doney and William Curry — were able to reconstruct monthly changes in ocean temperatures and evaluate variability of the NAO during both cold and warm periods from the Little Ice Age (1800�) to modern day. 

The research team found the variability of the NAO decade-to-decade (multi-decadal scale) has been larger, swinging more wildly, during the late twentieth century than in the early 1800s, suggesting that variability is linked to the mean temperature of the Northern Hemisphere. This confirms variability previously reported in past terrestrial reconstructions. 

"When the Industrial Revolution begins and atmospheric temperature becomes warmer, the NAO takes on a much stronger pattern in longer-term behavior," said Goodkin. "That was suspected before in the instrumental records, but this is the first time it has been documented in records from both the ocean and the atmosphere." 

The North Atlantic Oscillation is described by the NAO index, calculated as a weighted difference between the polar low and the subtropical high during the winter season. In a positive phase, both the low-pressure zone over Iceland and high pressure over the Azores are intensified, resulting in changes in the strength, incidence, and pathway of winter storms crossing the Atlantic. In a negative phase, a weak subtropical high and a weak Icelandic low results in fewer and weaker winter storms crossing on a more west-east pathway. 

The NAO index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting more than a decade. An unusually long period of positive phase between 1970-2000 led to the suggestion that global warming was affecting the behavior of the NAO. 

"Anthropogenic (human-related) warming does not appear to be altering whether the NAO is in a positive or negative phase at multi-decadal time scales," said WHOI paleoclimatologist Konrad Hughen. "It does seem to be increasing variability. Clearly, this has implications for the future." 

"As temperatures get warmer, there's potential for more violent swings of the NAO — the phases becoming even more positive and even more negative," Hughen added. "If the NAO locks more into these patterns, intense storms will become more intense and droughts will become more severe." 

The climatic influence of the NAO extends from the eastern United States to Western Europe, impacting human activities such as shipping, oil drilling, fisheries, hydroelectric power generation and coastal management. Improving the ability to predict shifts in the phase and intensity of the NAO is a prerequisite to mitigating the economic impacts of future climate change. 

While additional modeling and palaeoclimatic studies are needed, a broad distribution of marine records could advance our knowledge of NAO variability and serve to improve future projections, said Goodkin, now an assistant professor in the Department of Earth Sciences at the University of Hong Kong. 

Nature GeoScience paper: Increased multidecadal variability of the North Atlantic Oscillation since 1781 www.nature.com/ngeo/journal/va … nt/full/ngeo352.html

Source: Woods Hole Oceanographic Institution

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL086309

Robust Increases in Extreme Pacific North American Events Under Greenhouse Warming

Zhongfang Liu, Xiaogang He, Wentao Ma, Yue Wang

First published: 03 January 2020

 

https://doi.org/10.1029/2019GL086309

Citations: 1

Read the full text

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Abstract

The Pacific North American (PNA) pattern is an internal mode of the Northern Hemisphere atmosphere, which strongly affects the hydroclimate and ecosystems of the Pacific-North American sector. Recent studies have suggested a more positive PNA pattern in response to greenhouse warming, but how extreme PNA events will change remains unclear. Based on results from the Coupled Model Intercomparison Project Phase 5 multimodel ensemble, we find a 50% increase in the frequency of extreme positive PNA events and a 32% increase in extreme negative PNA events in the 21st century relative to the twentieth century. The increased frequency arises from a shift toward a more positive PNA pattern and is enhanced by more frequent ENSO events under greenhouse warming. Our study suggests that a continued increase in greenhouse-gas emissions is likely to cause increased occurrence of extreme PNA events, thus increasing the risk of extreme weather and climate in North America.

https://www.nature.com/articles/s41467-021-21830-z

The PNA, both in winter and summer, has exhibited a significant positive trend since the satellite era^45,46, with a magnitude that seems unprecedented over the past millennium⁴⁷. This trend is particularly strong for the summertime PNA during the period 1979–2016 (0.27 per decade, p < 0.01), with the pattern shifting from a negative phase in the 1980s through the mid-1990s to a strong positive phase after the mid-1990s (Fig. 1c).

 

[LibertyBell]

This explains a lot about the radical climate shift we saw beginning in the mid 90s, with tropical weather, with snowfall seasons and with extreme temps.....

 

[donsutherland1]

1 hour ago, bluewave said:

I saw the near record +PNA rise being discussed in Don's great thread. So I did some research into how it’s becoming more extreme with climate change. The PNA index isn’t the only one becoming more amplified. A study from 2009 documented how the NAO has also exhibited much bigger swings from high to low as the climate has warmed. In fact, the summer of 2009 to the present has exhibited record breaking highs and lows. The AO is showing these same extremes from record lows in the summer of 2009 through March 2013. Then we experienced a rapid reversal with record positive levels in the winter of 14-15 while the PNA was also in a very positive phase. This past winter featured an historic February reversal from record daily negative readings to very positive.

https://phys.org/news/2009-01-links-north-atlantic-oscillation-variability.html

Study links swings in North Atlantic oscillation variability to climate warming

 

Using a 218-year-long temperature record from a Bermuda brain coral, researchers at the Woods Hole Oceanographic Institution (WHOI) have created the first marine-based reconstruction showing the long-term behavior of one of the most important drivers of climate fluctuations in the North Atlantic.

The North Atlantic Oscillation (NAO) is a wide-ranging pressure seesaw that drives winter climate over much of North America, Europe and North Africa. Past reconstructions of the NAO have relied mainly on terrestrial, or land-based records, such as tree ring chronologies combined with ice cores and historical climate data. Those records do not fully capture oceanic processes linked to NAO variability, and short instrumental records from relatively few locations limit the understanding of ocean-atmosphere dynamics with regard to NAO behavior.

By analyzing the coral, we were able to look at changes in the ocean relative to changes on land," said Nathalie Goodkin, lead author of the study published in the December issue of the journal Nature Geoscience. "Because they are slow growing and have long life-spans, corals can provide high resolution records that are well dated and centuries long." 

As they grow, corals accrete seasonal and annual growth layers, similar to tree rings. The proportions of trace elements versus the major element (calcium) found in the layers of the skeleton largely depend on the temperature of the seawater in which it was formed. By analyzing the strontium to calcium ratio in the Bermuda brain coral, Goodkin and colleagues — WHOI scientists Konrad Hughen, Scott Doney and William Curry — were able to reconstruct monthly changes in ocean temperatures and evaluate variability of the NAO during both cold and warm periods from the Little Ice Age (1800�) to modern day. 

The research team found the variability of the NAO decade-to-decade (multi-decadal scale) has been larger, swinging more wildly, during the late twentieth century than in the early 1800s, suggesting that variability is linked to the mean temperature of the Northern Hemisphere. This confirms variability previously reported in past terrestrial reconstructions. 

"When the Industrial Revolution begins and atmospheric temperature becomes warmer, the NAO takes on a much stronger pattern in longer-term behavior," said Goodkin. "That was suspected before in the instrumental records, but this is the first time it has been documented in records from both the ocean and the atmosphere." 

The North Atlantic Oscillation is described by the NAO index, calculated as a weighted difference between the polar low and the subtropical high during the winter season. In a positive phase, both the low-pressure zone over Iceland and high pressure over the Azores are intensified, resulting in changes in the strength, incidence, and pathway of winter storms crossing the Atlantic. In a negative phase, a weak subtropical high and a weak Icelandic low results in fewer and weaker winter storms crossing on a more west-east pathway. 

The NAO index varies from year to year, but also exhibits a tendency to remain in one phase for intervals lasting more than a decade. An unusually long period of positive phase between 1970-2000 led to the suggestion that global warming was affecting the behavior of the NAO. 

"Anthropogenic (human-related) warming does not appear to be altering whether the NAO is in a positive or negative phase at multi-decadal time scales," said WHOI paleoclimatologist Konrad Hughen. "It does seem to be increasing variability. Clearly, this has implications for the future." 

"As temperatures get warmer, there's potential for more violent swings of the NAO — the phases becoming even more positive and even more negative," Hughen added. "If the NAO locks more into these patterns, intense storms will become more intense and droughts will become more severe." 

The climatic influence of the NAO extends from the eastern United States to Western Europe, impacting human activities such as shipping, oil drilling, fisheries, hydroelectric power generation and coastal management. Improving the ability to predict shifts in the phase and intensity of the NAO is a prerequisite to mitigating the economic impacts of future climate change. 

While additional modeling and palaeoclimatic studies are needed, a broad distribution of marine records could advance our knowledge of NAO variability and serve to improve future projections, said Goodkin, now an assistant professor in the Department of Earth Sciences at the University of Hong Kong. 

Nature GeoScience paper: Increased multidecadal variability of the North Atlantic Oscillation since 1781 www.nature.com/ngeo/journal/va … nt/full/ngeo352.html

Source: Woods Hole Oceanographic Institution

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL086309

Robust Increases in Extreme Pacific North American Events Under Greenhouse Warming

Zhongfang Liu, Xiaogang He, Wentao Ma, Yue Wang

First published: 03 January 2020

 

https://doi.org/10.1029/2019GL086309

Citations: 1

Read the full text

PDF

TOOLS 

SHARE

 

Abstract

The Pacific North American (PNA) pattern is an internal mode of the Northern Hemisphere atmosphere, which strongly affects the hydroclimate and ecosystems of the Pacific-North American sector. Recent studies have suggested a more positive PNA pattern in response to greenhouse warming, but how extreme PNA events will change remains unclear. Based on results from the Coupled Model Intercomparison Project Phase 5 multimodel ensemble, we find a 50% increase in the frequency of extreme positive PNA events and a 32% increase in extreme negative PNA events in the 21st century relative to the twentieth century. The increased frequency arises from a shift toward a more positive PNA pattern and is enhanced by more frequent ENSO events under greenhouse warming. Our study suggests that a continued increase in greenhouse-gas emissions is likely to cause increased occurrence of extreme PNA events, thus increasing the risk of extreme weather and climate in North America.

https://www.nature.com/articles/s41467-021-21830-z

The PNA, both in winter and summer, has exhibited a significant positive trend since the satellite era^45,46, with a magnitude that seems unprecedented over the past millennium⁴⁷. This trend is particularly strong for the summertime PNA during the period 1979–2016 (0.27 per decade, p < 0.01), with the pattern shifting from a negative phase in the 1980s through the mid-1990s to a strong positive phase after the mid-1990s (Fig. 1c).

 

These are interesting papers and I look forward to reading them.

[GaWx]

To give me a feel for the PNA trends, I looked at the monthly data since 1950, which is shown here:

 https://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/norm.pna.monthly.b5001.current.ascii.table

 I also looked at this site, which is great because it uses the same data (completed through Feb of 2019) while also showing the yearly and monthly averages as well as using color coding to make it easier to see trends:

 https://www.daculaweather.com/4_pna_index.php

 Looking at the trends:

 

1. Average annual by decade (2010s includes 2020's +0.21):

1950s: -0.07

1960s: -0.03

1970s:  -0.11

1980s: +0.15

1990s: +0.07

2000s: +0.19

2010s: +0.16

 This tells me the trend was fairly flat from the 1950s to the 1970s. Then there was a pretty sharp increase from the 1970s to the 1980s. However, although the sharp 1980s increase has held, it actually has been pretty flat overall since as the 2010s were about the same as the 1980s. 

 Now when looking at the last 7 years, alone, they have all been positive. This is the first time since 1950 to have 7 positive years in a row and they've averaged +0.24. However, way back during 1977-1983, those 7 years averaged about the same at +0.23. Also, the highest 7 year average was the +0.34 of 2004-10. Furthermore, the first half of 2021 is the 3rd most most negative since 2000. Even with August likely ending up strongly positive (at least close to +1 and probably a good bit higher), Jan-Aug will likely still average negative and the odds are that 2021 will remain negative considering no El Nino. So, that 7 year positive streak has a good chance to end in 2021.

 

 2. Next I looked at the trend for each month and found some interesting things:

Jan: After averaging negative in the 1950s-70s, it skyrocketed to +0.78 in the 1980s, came back down but remained solidly positive in the 1990s and 2000s, and then went back up to +0.78 in the 2010s.

Feb: also rose pretty sharply in the 1980s to +0.44, fell to neutral in the 1990s, followed by a rise back to 0.30 in the 2000s, and then followed by a fall back to neutral. So, certainly no positive trend since the 1980s.

Mar: similarly rose to +0.41 in the 1980s but like Feb has fallen back to neutral in the 2010s.

Apr: similarly rose to +0.37 in the 1980s but fell back to -0.25 in the 2010s.

May: neutral in the 1980s, rose to +0.25 in the 1990s, but has since plunged to -0.48 in the 2010s.

Jun: rose to +0.46 in the 1980s, dropped sharply to neutral in the 1990s and then -0.57 in the 2000s, but then rose sharply back to neutral in the 2010s.

Jul: rose to +0.20 in the 1980s and has been solidly positive and rising significantly since with +0.70 in the 2010s.

Aug: was unusual with it falling to -0.57 in the 1980s but then followed by sharp rises to neutral in the 1990s, +0.42 in the 2000s, and +0.67 in the 2010s.

Sep: like Aug it plunged to -0.50 in the 1980s followed by sharp rise to -0.13 in the 1990s, +0.65 in the 2000s, and then a slight drop to a still solid +0.47 in the 2010s.

Oct: neutral in the 1980s, then fell to -0.37 in the 1990s, rose to neutral in the 1990s/2000s,  and +0.25 in the 2010s.

Nov: rose to +0.30 in the 1980s, dropped to -0.17 in the 1990s, rose to +0.18 in the 2000s,  and then plunged to -0.32 in the 2010s.

Dec: rose to +0.35 in the 1980s, dropped to neutral in the 1990s, skyrocketed to +0.68 in the 2000s, and then plunged back to +0.12 in the 2010s.

 

 So, my analysis by month shows that Jul-Oct were the months that suggest a rising trend since the 1980s with Jan and Jul-Sep impressively positive in the 2010s. However, the 2010s for Feb-Jun and Nov have fallen a good bit below their respective 1980s levels. So, there are very mixed messages regarding the trend since the 1980s depending on the season. Mid summer through early to mid autumn has risen substantially since the 1980s while late winter through early summer has done the opposite. Does anyone have a feel for why there are these opposing seasonal trends? I'll note that mid summer through early autumn is when SSTs max out. Would there be any connection between that and the sharp rise in the PNA then? If so, why?

 

From the paper bluewave quoted from: "This trend is particularly strong for the summertime PNA during the period 1979–2016 (0.27 per decade, p < 0.01), with the pattern shifting from a negative phase in the 1980s through the mid-1990s to a strong positive phase after the mid-1990s (Fig. 1c)."

--------------------------------------------------------------

  Regarding the summer, I noted that July and August both showed large increases since the 1980s and that Aug averaged solidly negative in the 1980s followed by a sharp rise. However, June and July both averaged positive during the 1980s making for an overall neutral JJA then (about half the years positive and the other half negative). Also, I have JJA averaging pretty much neutral during the 1990-1996 period (only very slightly negative). So, I don't see why it refers to a negative phase in the 1980s through mid 1990s for the summer. I'd call it neutral. But I do agree with calling it a strong positive phase after the mid 1990s thanks solely to Jul and Aug.

[GaWx]

 The amazing early August +PNA event has just ended. There were amazing new records set (records back to 1950):

 1. The old record high PNA for any date prior to this event was the 3.40 of 9/24/2008. This was obliterated by a 4.00 on 8/6/21 (right at the top of the chart):

 

2. This event had four days in a row of a 3+ PNA. This is amazing because during the prior 71 years, there was only one time that there were as many as two days in a row (Sep of 2008)!

3. This event had 6 days of 2.75+, easily exceeding the previous event record of 3 days set several times.

4.  So, this event is by far the greatest +PNA event in terms of magnitude during the 71 years of recorded history. Could this have been a once in several hundred+ year event? Quite possibly.

5.  Not surprisingly, this event occurred during the July-early Oct period (see my earlier post on this). All six PNA events on record with a peak of 3+ occurred during the July-Sep period. For whatever reason, very strong PNA events like to occur at this time of year. In contrast, met winter has had only three days of 2+ (highest 2.35) and those were in very late Feb of two Febs (1968 and 1983).

 

 Link to thread following this as it was happening: