Lack of vertical instability

[TropicalAnalystwx13]

2011 and 2012. Two very busy hurricane seasons in terms of numbers. 18 named storms [and an unnamed system] for the former year, 19 named storms for the latter. A combined total of 38 tropical storms. 7 hurricanes in 2011, 10 hurricanes in 2012; a combined total of 17 hurricanes. 4 major hurricanes in 2011, 2 in 2012; a combined total of 6.

Vertical instability is screwing up the intensity of systems lately. We continue to see a high number of tropical storms and minimal hurricanes (Cat 1/2), but the number of major hurricanes has decreased substantially since 2010 and before. Of the two major hurricanes last year--Sandy and Michael--both only lasted for 6 hours. A major hurricane day of 0.5 for the entire season.

But why has vertical instability been so low lately?

I've been searching for weeks and cannot come up with an answer.

I've heard that perhaps it's above-average temperatures at 700mb. But if we take a look at 700mb air temperature anomalies from July-October 2005...



...and compare it to 700mb air temperature anomalies from July-October 2011 and 2012...



...there is no difference.

I've also heard it's due to a lack of relative humidity at the same level. But again, if we take a look at 700mb relative humidity anomalies from July-October 2005...



...and compare it to the 700mb relative humidity anomalies from July-October 2011 and 2012...



...there is once again no difference.

One of initial theories was that above average MSLPs would cause drier air and a lack of instability due to a tendency for high pressures to develop...



...but they were below average.

I really have no idea. Does anybody else have any theories?

[Ntxw]

First, I have virtually no knowledge of vertical instability so take what I say about it with a grain of salt. We know instability is the relationship between moisture and temperature. In winter the big storms are fueled from the tropics (huge streams of storms/clouds coming up into the higher latitudes). No connection and you'll just get a bunch of weaker systems passing by. Is the lack of this connection causing the same thing during hurricane season? Source would be the ITCZ which generally originates in Africa (drought) and is fueled by the Indian Monsoon (tropical connection). Last year the monsoon began late and when it got going in August we had that frenzy of storms form. 2011 also started out slow for the them as well. It seems the MJO phases that favor Atlantic formation is 2-3 which are the Indian Ocean convection phases which shows the connection to the tropics (from the easterly trades). Just food for thought no real serious research here.

[TropicalAnalystwx13]

First, I have virtually no knowledge of vertical instability so take what I say about it with a grain of salt. We know instability is the relationship between moisture and temperature. In winter the big storms are fueled from the tropics (huge streams of storms/clouds coming up into the higher latitudes). No connection and you'll just get a bunch of weaker systems passing by. Is the lack of this connection causing the same thing during hurricane season? Source would be the ITCZ which generally originates in Africa (drought) and is fueled by the Indian Monsoon (tropical connection). Last year the monsoon began late and when it got going in August we had that frenzy of storms form. 2011 also started out slow for the them as well. It seems the MJO phases that favor Atlantic formation is 2-3 which are the Indian Ocean convection phases which shows the connection to the tropics (from the easterly trades). Just food for thought no real serious research here.

Well, tropical cyclones, as you are aware, develop from warm sea surface temperatures. These have not been an issue the past few years; in fact, 2010 featured record high levels of SSTs across the eastern Atlantic. A friend of mine suggested that it may be because of higher-than-normal SAL coming off the western coast of Africa, but he has no way to prove this because as far as I'm aware there are no SAL records; this would go along with what you said about moisture though.

Then again, as aforementioned, 2010 featured very warm SSTs across the Atlantic basin, as well as a well-defined tripole. This focused the MJO (upward motion, more moisture) into the basin. Yet, vertical instability still turned out below average.

So I have no idea.

[Ntxw]

Well, tropical cyclones, as you are aware, develop from warm sea surface temperatures. These have not been an issue the past few years; in fact, 2010 featured record high levels of SSTs across the eastern Atlantic. A friend of mine suggested that it may be because of higher-than-normal SAL coming off the western coast of Africa, but he has no way to prove this because as far as I'm aware there are no SAL records; this would go along with what you said about moisture though.

Then again, as aforementioned, 2010 featured very warm SSTs across the Atlantic basin, as well as a well-defined tripole. This focused the MJO (upward motion, more moisture) into the basin. Yet, vertical instability still turned out below average.

So I have no idea.

Yeah it's very mind boggling. I guess what I was trying to say was that a weaker Indian monsoon may allow greater SAL (drought) from Africa and it's tougher for the basin to recover moisture wise (an ingredient to the instability factor) as easterly trades don't have much to bring to the basin from Africa through the monsoon. Temperatures are certainly there for strong formation so it has to be moisture. Have you noticed systems having to fight off dry air more than usual the past few years?

2005 is remembered as one of the most active seasons in the Atlantic, at the same time over in India it was one of the wettest/deadliest monsoon seasons that year and began in July earlier than usual.

[cycloneye]

Let's see if our pro mets can help to explain this important question that you are asking.

[Riptide]

This will not contribute much to the excellent discussion here but I believe 2013 will break the trend of weak instability. The ingredients are there, decent MSLP, warm SST's, neutral ENSO. Mabye the low instability has something to do with wind shear as there has been consistent mid-laditude blocking, creating downstream shear in the tropics.

[wxman57]

I spoke with Phil Klotzbach last year about the instability (or lack thereof). Our conclusion was that the decreased instability (increased stability) was due to the presence of sinking air (warm temps aloft) due to a stronger-than-normal Azores-Bermuda High the past 2 years. The stronger high was also responsible for a greater amount of dry air in the deep tropics the past two seasons.

Early indications are that the high will be more normal in 2013, so the instability should increase.

[TropicalAnalystwx13]

I spoke with Phil Klotzbach last year about the instability (or lack thereof). Our conclusion was that the decreased instability (increased stability) was due to the presence of sinking air (warm temps aloft) due to a stronger-than-normal Azores-Bermuda High the past 2 years. The stronger high was also responsible for a greater amount of dry air in the deep tropics the past two seasons.

Early indications are that the high will be more normal in 2013, so the instability should increase.

That would make sense, but 2011 and 2012 featured a mainly negative NAO didn't it? And thus a weaker-than-average Bermuda high?

[Riptide]

I have long suspected that weak thermal gradients have been detrimental for the development of major hurricanes. The polar regions are not as cold as they were in the past, especially the arctic, which had a record low ice extent in September 2012. This changes the dynamic of the ocean-atmosphere relationship and instability needs an atmosphere that is cold-aloft to thrive. Interestingly enough, the global warming theory predicts that Tropical Cyclone intensity will increase and storm frequency will decrease. In recent years, this correlation has been reversed with numerous non-major TC's.

Activity shifting away from the MDR and into the subtropics is possibly key evidence that there is not enough cold air aloft. However, conditions this year are fairly different than 2012 with the split polar vortex displaced on our side of the globe. This should provide the necessary dynamics to get the heat engine moving again in 2013.

[RL3AO]

I think the first thing is you are comparing to 2005. That's not fair. 2005 was a once in a lifetime season where almost every positive for TC development was at near-record high levels. It would be much more reasonable to compare it to an average season.

[cycloneye]

This is a very interesting discussion going on here about this topic. I am going away from the North Atlantic basin to another one more to the east with this question. Is the warming or cooling of the Arabian Sea has a connection with what occurs in the African continent in terms of rainfall and then that translates to the Atlantic basin affecting the instability?

[TropicalAnalystwx13]

I think the first thing is you are comparing to 2005. That's not fair. 2005 was a once in a lifetime season where almost every positive for TC development was at near-record high levels. It would be much more reasonable to compare it to an average season.

Even using 2005, the differences in RH and air temperature are not all that indifferent. Which raises the question...why was vertical instability so low? It shouldn't have been if those two products were the issue.

This is a very interesting discussion going on here about this topic. I am going away from the North Atlantic basin to another one more to the east with this question. Is the warming or cooling of the Arabian Sea has a connection with what occurs in the African continent in terms of rainfall and then that translates to the Atlantic basin affecting the instability?

A cool Arabian Sea would have a positive influence on the strength of tropical waves. But strength of tropical waves last season was not an issue thanks to the cool Gulf of Guinea. Soil moisture in Africa was above average, and the continent saw an above average rainy season.

[CYCLONE MIKE]

Like wxman said about the dry air, I could be wrong but if I remember correctly the gulf has had tons of dry air and shear the last few seasons. Surely that is one of the major reasons storms have had a hard time developing or intensifying out there.

[Cyclenall]

But why has vertical instability been so low lately?

I've been searching for weeks and cannot come up with an answer.

I've been trying to figure this one out for a while too. That makes two of us.

I spoke with Phil Klotzbach last year about the instability (or lack thereof). Our conclusion was that the decreased instability (increased stability) was due to the presence of sinking air (warm temps aloft) due to a stronger-than-normal Azores-Bermuda High the past 2 years. The stronger high was also responsible for a greater amount of dry air in the deep tropics the past two seasons.

Early indications are that the high will be more normal in 2013, so the instability should increase.

As stated, this was already addressed so this explanation doesn't fit. What's your rebuttal?

I have long suspected that weak thermal gradients have been detrimental for the development of major hurricanes. The polar regions are not as cold as they were in the past, especially the arctic, which had a record low ice extent in September 2012. This changes the dynamic of the ocean-atmosphere relationship and instability needs an atmosphere that is cold-aloft to thrive. Interestingly enough, the global warming theory predicts that Tropical Cyclone intensity will increase and storm frequency will decrease. In recent years, this correlation has been reversed with numerous non-major TC's.

Activity shifting away from the MDR and into the subtropics is possibly key evidence that there is not enough cold air aloft. However, conditions this year are fairly different than 2012 with the split polar vortex displaced on our side of the globe. This should provide the necessary dynamics to get the heat engine moving again in 2013.

This sounds plausible but as stated, the correlation is reversed (last 2 or 3 seasons).

I think the first thing is you are comparing to 2005. That's not fair. 2005 was a once in a lifetime season where almost every positive for TC development was at near-record high levels. It would be much more reasonable to compare it to an average season.

I think its fine in this study because he just wants to compare it to a season we all know didn't have problems. Use a different season and we're less sure.

Great discussion going on here.

[SouthDadeFish]

Hello everyone, I would love to join in on this discussion.

I think a big thing that has occurred over the last two years is the lower than normal PWAT (total precipitable water) values over the Main Development Region (9-21.5 degrees North). PWAT is great to look at because it integrates the entire column of atmosphere, not just picking and choosing one level. I used the same time-scale (July-Oct) the original poster did for 2011 and 2012:



Notice the very dry air in the deep tropical Atlantic, much of the Caribbean, and even Gulf of Mexico. It is not a coincidence that the majority of storm tracks for these two seasons were in the subtropical Atlantic where relatively moister air was located in combination with above average SSTs. Just for giggles, here are the storm tracks of the 2011 and 2012 seasons so you can easily compare:

2012:



2011:



These tracks match up nicely with the PWAT anomalies seen above. Also, since the MDR was drier than normal, storms weren't able to take advantage of the usual breeding grounds for major hurricanes, and thus, we saw lower than normal hurricane-to-major-hurricane ratios the past two seasons.

Now compare these anomalies to those years where we saw plenty of hurricanes and major hurricanes:



Notice how much more moist the MDR was. Even without looking at any other factors, we can see a clear correlation to PWAT anomalies in the MDR and active seasons for major hurricanes. Furthermore, I'm not buying into the theory about the stronger Bermuda high yet, since the charts I have seen do not show a stronger than normal Bermuda high. I hope this helps.

[TropicalAnalystwx13]

Hello everyone, I would love to join in on this discussion.

I think a big thing that has occurred over the last two years is the lower than normal PWAT (total precipitable water) values over the Main Development Region (9-21.5 degrees North). PWAT is great to look at because it integrates the entire column of atmosphere, not just picking and choosing one level. I used the same time-scale (July-Oct) the original poster did for 2011 and 2012:

http://imageshack.us/a/img441/5359/9956101181155640.png

Notice the very dry air in the deep tropical Atlantic, much of the Caribbean, and even Gulf of Mexico. It is not a coincidence that the majority of storm tracks for these two seasons were in the subtropical Atlantic where relatively moister air was located in combination with above average SSTs. Just for giggles, here are the storm tracks of the 2011 and 2012 seasons so you can easily compare:

2012:
http://upload.wikimedia.org/wikipedia/c ... ry_map.png

2011:

http://upload.wikimedia.org/wikipedia/c ... ry_map.png

These tracks match up nicely with the PWAT anomalies seen above. Also, since the MDR was drier than normal, storms weren't able to take advantage of the usual breeding grounds for major hurricanes, and thus, we saw lower than normal hurricane-to-major-hurricane ratios the past two seasons.

Now compare these anomalies to those years where we saw plenty of hurricanes and major hurricanes:
http://img9.imageshack.us/img9/1290/995 ... 161241.png

Notice how much more moist the MDR was. Even without looking at any other factors, we can see a clear correlation to PWAT anomalies in the MDR and active seasons for major hurricanes. Furthermore, I'm not buying into the theory about the stronger Bermuda high yet, since the charts I have seen do not show a stronger than normal Bermuda high. I hope this helps.

Well yes. But below-average PWAT anomalies just indicate a drier-than-normal atmosphere. What's causing that?