When will the multi-decade active era that began in 1995 end?

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Re: When will the multi-decade active era that began in 1995 end?

#421 Postby Category5Kaiju » Thu Mar 04, 2021 10:27 pm

Weather Dude wrote:I find it kind of odd that this guy comes up with this idea of a natural cycling Atlantic, which makes sense when looking at the charts of past hurricane seasons, and then all of a sudden when we are on an active streak, he comes out and says that it's because of human caused climate change... My question is, what makes this active phase any different than past ones? I mean it's not like we had anything near the level of technology back then like we do today so of course it's going to seem like the number of storms is much higher now than it was back then. To me the climate is always changing and it will continue to change either way over time. I just can't stand how nowadays something happens (tornado, hurricane, blizzard, etc.) and people just blame it on climate change. I could go on but I don't want start a dispute so I'll stop here. It just seems weird how he would just completely flip like that. But who knows, maybe it's just me.


Yeah I agree; I simply think we need more data and empirical studies/evidence to know for sure. Mother Nature will always be several steps ahead of our understanding as it is a highly mysterious and intricate force, and as a result especially with these highly debated and complex topics (climate change in particular), we should definitely present theories and hypotheses with one another, but I'm not sure if we are to the point where we can be 100% confident with everything that is going on.
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Re: When will the multi-decade active era that began in 1995 end?

#422 Postby aspen » Thu Mar 04, 2021 10:48 pm

Weather Dude wrote:I find it kind of odd that this guy comes up with this idea of a natural cycling Atlantic, which makes sense when looking at the charts of past hurricane seasons, and then all of a sudden when we are on an active streak, he comes out and says that it's because of human caused climate change... My question is, what makes this active phase any different than past ones? I mean it's not like we had anything near the level of technology back then like we do today so of course it's going to seem like the number of storms is much higher now than it was back then. To me the climate is always changing and it will continue to change either way over time. I just can't stand how nowadays something happens (tornado, hurricane, blizzard, etc.) and people just blame it on climate change. I could go on but I don't want start a dispute so I'll stop here. It just seems weird how he would just completely flip like that. But who knows, maybe it's just me.

Yeah it is annoying how frequently weird weather events are automatically linked to climate change. Odds are at least some of them definitely are, and there are other clear signs that the climate is changing; for example, I’ve noticed various species of insects developing additional breeding generations here in New England as warm temperatures are around for longer. However, as you said, the climate is always going to change in some ways, and we currently have technology that we didn’t have decades ago. 1887 and 1933 could’ve been closer to 2020’s insanity than we currently believe, but due to a lack of recon or satellites, storms easily could’ve been missed or under-estimated. Not only that, but even if a weather event is unlikely, it’s still going to happen at one point or another, like the TX/OK/AR cold snap or the May 2018 New England Derecho.

This most recent active streak (2016-present) is unusual in its duration, as well as its continuing streaks of at least one pre-June storm and at least one Cat 5 — after a 9-year gap without a single one. This incredibly active and destructive hurricane season streak, combined with more studies about the dangers and impacts of anthropogenic climate change, is likely why the two are being associated, even if there are natural cycles involved. In my unprofessional opinion, there appear to be two cycles — one short period (as seen with the fluctuations of active and inactive seasons since 1995) and one long period (the overall inactive 1970s-1995 phase and the overall active 1995-present phase).
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Re: When will the multi-decade active era that began in 1995 end?

#423 Postby Steve » Thu Mar 04, 2021 11:20 pm

Weather Dude wrote:I find it kind of odd that this guy comes up with this idea of a natural cycling Atlantic, which makes sense when looking at the charts of past hurricane seasons, and then all of a sudden when we are on an active streak, he comes out and says that it's because of human caused climate change... My question is, what makes this active phase any different than past ones? I mean it's not like we had anything near the level of technology back then like we do today so of course it's going to seem like the number of storms is much higher now than it was back then. To me the climate is always changing and it will continue to change either way over time. I just can't stand how nowadays something happens (tornado, hurricane, blizzard, etc.) and people just blame it on climate change. I could go on but I don't want start a dispute so I'll stop here. It just seems weird how he would just completely flip like that. But who knows, maybe it's just me.


Sure - maybe wolf gets cried sometimes. But screw that if it's used as a catch all or an excuse. It's been hot. And it's been getting hotter. I believed in the AMO too. But maybe warming and cooling of the Atlantic just happens from time to time and those obsolete coupled models that we probably were laughing at back then caught some past patterns. I'm sure the Atlantic will cool again and probably that will lead to somewhat lower continental US temperatures. That's not really the point though. You're talking about the guy who came up with the AMO. It's not like he looked at a swinging pendulum or was trolling a crazy conspiracy site somewhere. It's Mann.
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Re: When will the multi-decade active era that began in 1995 end?

#424 Postby aspen » Fri Mar 05, 2021 4:30 pm

Unprofessional opinion: I’ve been thinking...perhaps we’re looking at AMO the wrong way. Maybe it’s not nearly as simple as a +phase and a -phase. It’s not like it’s the only cycle involved.

Even if you take AMO out of the equation, there are still so many things, both long-term and short-term, that influence Atlantic hurricane season activity:
—ENSO phase: La Niña or El Niño
—Transitions between very warm and very cool ENSO phases
—Atlantic background states
—Anthropogenic ocean temperature warming
—Abnormalities in the Thermohaline Cycle (see 2013)
—Large volcanic dust plumes (Krakatoa ‘83, Pinatubo ‘91)
—Presence/position of rising or sinking air masses that could fuel or surprise MDR activity
—EPac activity that could suppress NAtl activity (can be associated with ENSO or happen by chance)

There are a LOT of competing factors from a variety of sources — short-term events, anthropogenic activity, and moderate to long-term cycles. Since I’m not a professional meteorologist, I’m not sure if some of the things I mentioned are more linked together than I assumed (like rising air masses/PDO and ENSO state), but there are plenty of professionals here to correct me on that.
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Re: When will the multi-decade active era that began in 1995 end?

#425 Postby Category5Kaiju » Fri Mar 05, 2021 7:32 pm

aspen wrote:Unprofessional opinion: I’ve been thinking...perhaps we’re looking at AMO the wrong way. Maybe it’s not nearly as simple as a +phase and a -phase. It’s not like it’s the only cycle involved.

Even if you take AMO out of the equation, there are still so many things, both long-term and short-term, that influence Atlantic hurricane season activity:
—ENSO phase: La Niña or El Niño
—Transitions between very warm and very cool ENSO phases
—Atlantic background states
—Anthropogenic ocean temperature warming
—Abnormalities in the Thermohaline Cycle (see 2013)
—Large volcanic dust plumes (Krakatoa ‘83, Pinatubo ‘91)
—Presence/position of rising or sinking air masses that could fuel or surprise MDR activity
—EPac activity that could suppress NAtl activity (can be associated with ENSO or happen by chance)

There are a LOT of competing factors from a variety of sources — short-term events, anthropogenic activity, and moderate to long-term cycles. Since I’m not a professional meteorologist, I’m not sure if some of the things I mentioned are more linked together than I assumed (like rising air masses/PDO and ENSO state), but there are plenty of professionals here to correct me on that.


Yes absolutely. Not a professional meteorologist either, but I will say that even if we were in a -AMO phase, just remember we can still get punishing Cat 5 storms. Anita, David, Allen, Gilbert, Hugo, and Andrew would like to have a word.
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Re: When will the multi-decade active era that began in 1995 end?

#426 Postby MarioProtVI » Fri Mar 05, 2021 8:37 pm

If the AMO doesn’t exist, then explain 1970 to 1994.
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Re: When will the multi-decade active era that began in 1995 end?

#427 Postby Weather Dude » Fri Mar 05, 2021 9:14 pm

MarioProtVI wrote:If the AMO doesn’t exist, then explain 1970 to 1994.

And all the other inactive periods...
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Re: When will the multi-decade active era that began in 1995 end?

#428 Postby Steve » Sat Mar 06, 2021 1:12 am

Atlantic was colder in that period. No one would argue that. It doesn’t mean AMO is an ongoing cycle just because two models looking back suggested it was. It doesn’t mean the current warm (Atlantic) period is necessarily manmade either. Mann feels like it might be, and you have to give him his due IMHO. It doesn’t mean he’s right, but you have to take his viewpoint into consideration because of the respect he commands on the topic. Correct me if I’m wrong, but I think the Atlantic Ocean is about 145-150,000,000 years old. Cycles could be temporary or in some ways never repeat. I don’t have an agenda as to manmade arguments, but I’m kinda doubting we are helping things.
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Re: When will the multi-decade active era that began in 1995 end?

#429 Postby wxman57 » Sat Mar 06, 2021 9:48 am

Shell Mound wrote:
wxman57 wrote:Klotzbach and I were discussing this and it looks like it may have flipped in 2013 (cool AMO began). Tropics have yet to respond, though.

If the tropics have yet to respond, does that imply the +AMO is still in effect, or are other factors offsetting the onset of the -AMO?


Clearly, the AMO, related to the Atlantic thermohaline circulation, is only one factor that governs activity. Temperature and salinity measurements over the past 7-8 years do indicate that a cool cycle has begun. However, the tropics have yet to respond. Also, I think we put too much weight on the number of named storms when determining activity. With better detection and a changing naming methodology, we should only consider hurricanes and major hurricanes. Weak tropical storms or subtropical storms that last less than 48 hours should be excluded. Many of the storms that are currently named would not have been named in the past.

Looking at the AMO over the past 120 seasons, it appears that it was in a cool phase from around 1900-1925, warm from 1926 to 1969, cool from 1970 to 1994, and warm from 1995-2012. There were some quite bad hurricane impacts during cool cycles. In general, though, the number of hurricanes and major hurricanes decreased during cool cycles.
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Re: When will the multi-decade active era that began in 1995 end?

#430 Postby Shell Mound » Mon Mar 08, 2021 10:13 am

The present study focuses on the 10 most recent Category 5 hurricanes recorded in the Atlantic, from Hurricane Andrew (1992) through Hurricane Felix (2007). These 10 hurricanes are placed into the context of the technology available in the period of 1944–53, the first decade of aircraft reconnaissance. A methodology is created to determine how many of these 10 recent Category 5 hurricanes likely would have been recorded as Category 5 if they had occurred during this period using only the observations that likely would have been available with existing technology and observational networks. Late-1940s and early1950s best-track intensities are determined for the entire lifetime of these 10 recent Category 5 hurricanes. It is found that likely only 2 of these 10—both Category 5 landfalling hurricanes—would have been recorded as Category 5 hurricanes if they had occurred during the late-1940s period. The results suggest that intensity estimates for extreme tropical cyclones prior to the satellite era are unreliable for trend and variability analysis.

Source

Probably many Cat-5 landfalls on the CONUS were missed prior to 1900, along with many more Cat-5 impacts on the Caribbean nations.
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Re: When will the multi-decade active era that began in 1995 end?

#431 Postby InfernoFlameCat » Mon Mar 08, 2021 10:17 am

We might not get much of a cool AMO due to global warming. It might just level out average or slightly below and then in 30 years or so spike real hard when + AMO comes around.
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Re: When will the multi-decade active era that began in 1995 end?

#432 Postby Nuno » Tue Mar 09, 2021 1:46 pm

wxman57 wrote:
Shell Mound wrote:
wxman57 wrote:Klotzbach and I were discussing this and it looks like it may have flipped in 2013 (cool AMO began). Tropics have yet to respond, though.

If the tropics have yet to respond, does that imply the +AMO is still in effect, or are other factors offsetting the onset of the -AMO?


Clearly, the AMO, related to the Atlantic thermohaline circulation, is only one factor that governs activity. Temperature and salinity measurements over the past 7-8 years do indicate that a cool cycle has begun. However, the tropics have yet to respond. Also, I think we put too much weight on the number of named storms when determining activity. With better detection and a changing naming methodology, we should only consider hurricanes and major hurricanes. Weak tropical storms or subtropical storms that last less than 48 hours should be excluded. Many of the storms that are currently named would not have been named in the past.

Looking at the AMO over the past 120 seasons, it appears that it was in a cool phase from around 1900-1925, warm from 1926 to 1969, cool from 1970 to 1994, and warm from 1995-2012. There were some quite bad hurricane impacts during cool cycles. In general, though, the number of hurricanes and major hurricanes decreased during cool cycles.


Why? If other factors indicate IYO that we've entered a cool phase, how much longer do you wait for storms to catch up before one must re-evaluate this time period as an anomalously short cool phase or that it hasn't begun yet?
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Re: When will the multi-decade active era that began in 1995 end?

#433 Postby somethingfunny » Tue Mar 09, 2021 2:35 pm

wxman57 wrote:
Shell Mound wrote:
wxman57 wrote:Klotzbach and I were discussing this and it looks like it may have flipped in 2013 (cool AMO began). Tropics have yet to respond, though.

If the tropics have yet to respond, does that imply the +AMO is still in effect, or are other factors offsetting the onset of the -AMO?


Clearly, the AMO, related to the Atlantic thermohaline circulation, is only one factor that governs activity. Temperature and salinity measurements over the past 7-8 years do indicate that a cool cycle has begun. However, the tropics have yet to respond. Also, I think we put too much weight on the number of named storms when determining activity. With better detection and a changing naming methodology, we should only consider hurricanes and major hurricanes. Weak tropical storms or subtropical storms that last less than 48 hours should be excluded. Many of the storms that are currently named would not have been named in the past.

Looking at the AMO over the past 120 seasons, it appears that it was in a cool phase from around 1900-1925, warm from 1926 to 1969, cool from 1970 to 1994, and warm from 1995-2012. There were some quite bad hurricane impacts during cool cycles. In general, though, the number of hurricanes and major hurricanes decreased during cool cycles.


Going by those timeframes of 1900-1925, 1926-1969, 1970-1994 it seems obvious that, based on observation of 1.5 cycles (not nearly enough data to actually conclude a single thing about the cycle) we have 25 year cool periods and 45 year warm periods. So the current warm period would be expected to end around 2040 based on the single prior warm period we have data for. Not sure why anybody was expecting it to end anytime around 2013-2020.
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Re: When will the multi-decade active era that began in 1995 end?

#434 Postby crownweather » Tue Mar 09, 2021 2:39 pm

somethingfunny wrote:
wxman57 wrote:
Shell Mound wrote:If the tropics have yet to respond, does that imply the +AMO is still in effect, or are other factors offsetting the onset of the -AMO?


Clearly, the AMO, related to the Atlantic thermohaline circulation, is only one factor that governs activity. Temperature and salinity measurements over the past 7-8 years do indicate that a cool cycle has begun. However, the tropics have yet to respond. Also, I think we put too much weight on the number of named storms when determining activity. With better detection and a changing naming methodology, we should only consider hurricanes and major hurricanes. Weak tropical storms or subtropical storms that last less than 48 hours should be excluded. Many of the storms that are currently named would not have been named in the past.

Looking at the AMO over the past 120 seasons, it appears that it was in a cool phase from around 1900-1925, warm from 1926 to 1969, cool from 1970 to 1994, and warm from 1995-2012. There were some quite bad hurricane impacts during cool cycles. In general, though, the number of hurricanes and major hurricanes decreased during cool cycles.


Going by those timeframes of 1900-1925, 1926-1969, 1970-1994 it seems obvious that, based on observation of 1.5 cycles (not nearly enough data to actually conclude a single thing about the cycle) we have 25 year cool periods and 45 year warm periods. So the current warm period would be expected to end around 2040 based on the single prior warm period we have data for. Not sure why anybody was expecting it to end anytime around 2013-2020.


Actually 25 years from 1995 would be 2020.
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Re: When will the multi-decade active era that began in 1995 end?

#435 Postby Nuno » Tue Mar 09, 2021 3:14 pm

Steve wrote:Atlantic was colder in that period. No one would argue that. It doesn’t mean AMO is an ongoing cycle just because two models looking back suggested it was. It doesn’t mean the current warm (Atlantic) period is necessarily manmade either. Mann feels like it might be, and you have to give him his due IMHO. It doesn’t mean he’s right, but you have to take his viewpoint into consideration because of the respect he commands on the topic. Correct me if I’m wrong, but I think the Atlantic Ocean is about 145-150,000,000 years old. Cycles could be temporary or in some ways never repeat. I don’t have an agenda as to manmade arguments, but I’m kinda doubting we are helping things.


This is kind of how I'm open to new perspectives on our current understandings. We have only reliably what, a century plus some change worth of data and information? We need to be careful not to make assumptions as a wx community about "how things should be" when we truly don't even know what a long-term baseline is for the Atlantic ocean. We make a lot of assumptions about the length of cycles based on only the most recent four or five that we know of. We have no idea if anything in the 20th century is anomalous compared to other time periods we simply cannot know.
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Re: When will the multi-decade active era that began in 1995 end?

#436 Postby Steve » Sun Mar 14, 2021 9:07 am

crownweather wrote:
somethingfunny wrote:
wxman57 wrote:
Clearly, the AMO, related to the Atlantic thermohaline circulation, is only one factor that governs activity. Temperature and salinity measurements over the past 7-8 years do indicate that a cool cycle has begun. However, the tropics have yet to respond. Also, I think we put too much weight on the number of named storms when determining activity. With better detection and a changing naming methodology, we should only consider hurricanes and major hurricanes. Weak tropical storms or subtropical storms that last less than 48 hours should be excluded. Many of the storms that are currently named would not have been named in the past.

Looking at the AMO over the past 120 seasons, it appears that it was in a cool phase from around 1900-1925, warm from 1926 to 1969, cool from 1970 to 1994, and warm from 1995-2012. There were some quite bad hurricane impacts during cool cycles. In general, though, the number of hurricanes and major hurricanes decreased during cool cycles.


Going by those timeframes of 1900-1925, 1926-1969, 1970-1994 it seems obvious that, based on observation of 1.5 cycles (not nearly enough data to actually conclude a single thing about the cycle) we have 25 year cool periods and 45 year warm periods. So the current warm period would be expected to end around 2040 based on the single prior warm period we have data for. Not sure why anybody was expecting it to end anytime around 2013-2020.


Actually 25 years from 1995 would be 2020.


Hey Crown. I think he was referencing a 45 year warm cycle not just 25 year alternating ones.
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Re: When will the multi-decade active era that began in 1995 end?

#437 Postby Steve » Sun Mar 14, 2021 9:24 am

Nuno wrote:
Steve wrote:Atlantic was colder in that period. No one would argue that. It doesn’t mean AMO is an ongoing cycle just because two models looking back suggested it was. It doesn’t mean the current warm (Atlantic) period is necessarily manmade either. Mann feels like it might be, and you have to give him his due IMHO. It doesn’t mean he’s right, but you have to take his viewpoint into consideration because of the respect he commands on the topic. Correct me if I’m wrong, but I think the Atlantic Ocean is about 145-150,000,000 years old. Cycles could be temporary or in some ways never repeat. I don’t have an agenda as to manmade arguments, but I’m kinda doubting we are helping things.


This is kind of how I'm open to new perspectives on our current understandings. We have only reliably what, a century plus some change worth of data and information? We need to be careful not to make assumptions as a wx community about "how things should be" when we truly don't even know what a long-term baseline is for the Atlantic ocean. We make a lot of assumptions about the length of cycles based on only the most recent four or five that we know of. We have no idea if anything in the 20th century is anomalous compared to other time periods we simply cannot know.


100%.

We have some ship logs and other things that can help (probably going back to the 1600's). But we really don't know that much about the open oceans except at given points in time. And even 400+ years is 0.0002667% of the ocean's existence. Scientists take core samples of ice and earth and can approximate weather going back through certain periods of time. But that isn't even a torn off corner of a puzzle piece in a 100,000 piece expert puzzle set. Modeling will get better with time and continued observation as well as with artificial intelligence. So we may someday be able to say whether there really is a true AMO cycle. Michael Mann is a geophysicist and climatologist. I think he currently leads the Penn State Earth System Science Center or something close to that. He came up with the term in 2000 (based on research from the mid-1990's). And now he's saying that older modeling that led to the research was probably faulty. I'm trusting his judgment if not necessarily his conclusions. I felt like some of those earlier posts were sort of ways around the TOS to drop in some political points about global warming.
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Re: When will the multi-decade active era that began in 1995 end?

#438 Postby aspen » Sun Mar 14, 2021 10:09 am

Nuno wrote:
Steve wrote:Atlantic was colder in that period. No one would argue that. It doesn’t mean AMO is an ongoing cycle just because two models looking back suggested it was. It doesn’t mean the current warm (Atlantic) period is necessarily manmade either. Mann feels like it might be, and you have to give him his due IMHO. It doesn’t mean he’s right, but you have to take his viewpoint into consideration because of the respect he commands on the topic. Correct me if I’m wrong, but I think the Atlantic Ocean is about 145-150,000,000 years old. Cycles could be temporary or in some ways never repeat. I don’t have an agenda as to manmade arguments, but I’m kinda doubting we are helping things.


This is kind of how I'm open to new perspectives on our current understandings. We have only reliably what, a century plus some change worth of data and information? We need to be careful not to make assumptions as a wx community about "how things should be" when we truly don't even know what a long-term baseline is for the Atlantic ocean. We make a lot of assumptions about the length of cycles based on only the most recent four or five that we know of. We have no idea if anything in the 20th century is anomalous compared to other time periods we simply cannot know.

And in that 100-150 years of data, we only have a few decades of reliable measurements from a combination of specialized recon planes and extensive satellite coverage. How many storms in seasons like 1950, 1933, and 1887 were either missed or underestimated due to a lack of observational data we’ve had access to since the 60s/70s? Even with such limited data, it’s been determined that 1933 had the third-higher number of named storms and highest ACE total of any season dating back to the 1850s; could those numbers have been higher? I’m inclined to say yes, because a storm’s peak intensity could’ve easily been missed by a ship that wasn’t in the right place at the right time, and really short-lived storms like Dolly, Edouard, and Vicky could’ve been missed altogether.

I don’t doubt that there were seasons on the level of 2005, 2017, and 2020 in the decades and centuries prior to recon and satellite, but the technology to get a full grasp of them just didn’t exist. Therefore, our understanding of Atlantic hurricane seasons and long-term trends is going to be biased towards what we’ve observed in the last 40-60 years, which could also be influenced by anthropogenic climate change and ocean warming. We also don’t know what the Atlantic hurricane seasons were like prior to the start of the Industrial Revolution and the widespread use of fossil fuels. Did humans rapidly change tropical cyclone frequency and intensity once we began pumping greenhouse gases into the atmosphere? Or has it been a slow change over the last few centuries?
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Re: When will the multi-decade active era that began in 1995 end?

#439 Postby wxman57 » Sun Mar 14, 2021 10:16 am

Nuno wrote:
wxman57 wrote:
Shell Mound wrote:If the tropics have yet to respond, does that imply the +AMO is still in effect, or are other factors offsetting the onset of the -AMO?


Clearly, the AMO, related to the Atlantic thermohaline circulation, is only one factor that governs activity. Temperature and salinity measurements over the past 7-8 years do indicate that a cool cycle has begun. However, the tropics have yet to respond. Also, I think we put too much weight on the number of named storms when determining activity. With better detection and a changing naming methodology, we should only consider hurricanes and major hurricanes. Weak tropical storms or subtropical storms that last less than 48 hours should be excluded. Many of the storms that are currently named would not have been named in the past.

Looking at the AMO over the past 120 seasons, it appears that it was in a cool phase from around 1900-1925, warm from 1926 to 1969, cool from 1970 to 1994, and warm from 1995-2012. There were some quite bad hurricane impacts during cool cycles. In general, though, the number of hurricanes and major hurricanes decreased during cool cycles.


Why? If other factors indicate IYO that we've entered a cool phase, how much longer do you wait for storms to catch up before one must re-evaluate this time period as an anomalously short cool phase or that it hasn't begun yet?


I was discussing this with Klotzbach the other day. Taking a closer look at the AMO data on his website, it appears that in 3 of the past 4 seasons (not 2018), the AMO was cool all year EXCEPT the peak of hurricane season. Here's a zoomed in plot that I made. The AMO was quite cool outside of peak season in 2017, 2019, and 2020.

Image
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Re: When will the multi-decade active era that began in 1995 end?

#440 Postby Shell Mound » Sun Mar 14, 2021 3:12 pm

wxman57 wrote:
Nuno wrote:
wxman57 wrote:
Clearly, the AMO, related to the Atlantic thermohaline circulation, is only one factor that governs activity. Temperature and salinity measurements over the past 7-8 years do indicate that a cool cycle has begun. However, the tropics have yet to respond. Also, I think we put too much weight on the number of named storms when determining activity. With better detection and a changing naming methodology, we should only consider hurricanes and major hurricanes. Weak tropical storms or subtropical storms that last less than 48 hours should be excluded. Many of the storms that are currently named would not have been named in the past.

Looking at the AMO over the past 120 seasons, it appears that it was in a cool phase from around 1900-1925, warm from 1926 to 1969, cool from 1970 to 1994, and warm from 1995-2012. There were some quite bad hurricane impacts during cool cycles. In general, though, the number of hurricanes and major hurricanes decreased during cool cycles.


Why? If other factors indicate IYO that we've entered a cool phase, how much longer do you wait for storms to catch up before one must re-evaluate this time period as an anomalously short cool phase or that it hasn't begun yet?


I was discussing this with Klotzbach the other day. Taking a closer look at the AMO data on his website, it appears that in 3 of the past 4 seasons (not 2018), the AMO was cool all year EXCEPT the peak of hurricane season. Here's a zoomed in plot that I made. The AMO was quite cool outside of peak season in 2017, 2019, and 2020.

http://wxman57.com/images/AMO.JPG

My personal hypothesis is that interglacial warm periods, including the recent spate of global warming, may actually be correlated with -AMO rather than +AMO historically. If I recall correctly, a number of studies have suggested that the Atlantic basin was hyperactive during the Little Ice Age but less active during the Medieval Warm Period. This makes sense, actually. A warmer globe correlates with a weaker AMOC due to freshwater ice melt in the Arctic. This contributes to decreased salinity and allows North Atlantic Deep Water to percolate southward into the MDR. The same forces that contribute to a weaker AMOC also may result in weaker tornado seasons over parts of North America due to expanding Hadley cells pushing the jet stream farther north. (Incidentally, a warmer climate also tends to amplify the ITCZ vis-à-vis the African monsoon and a wetter Sahel, so storms tend to exit West Africa much farther north and hence fail to develop as they would had they stayed in the MDR.) So I think that AGW may have led to this recent +AMO having ended much earlier than previous +AMOs in the nineteenth and early twentieth centuries, which lasted longer because the global climate or base state was cooler then. I never understood why many studies pointed to storms becoming stronger due to AGW. My view has always been that AGW may actually lead to less extreme Atlantic TC and North-American tornado seasons, owing to changes in the oceanic-atmospheric circulation and feedback (loop).
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