Global Warming and hurricanes-my thoughts
Global Warming and Hurricanes (for the Atlantic)
This project is going to be on the relationship or lack there of a connection between global warming and the increase in intensity and frequency of tropical systems in the Atlantic basin due to warming global temperatures. Although there has been no severe profound evidence of such a connection there were numerous news articles of such connections written by leading scientists in major newspapers. The purpose of this report is going to show the two sides of this debate and then end up with what side I agree with and why.
After the record breaking 2005 hurricane season many reports came out either questioning and or debating whether global warming causes increased hurricane frequency and intensity or claiming that new studies show that it does. Basically the argument behind global warming and hurricanes is quite simple. They say that global warming has and will contuine to raise sea surface temperatures and basically
this will give me latent energy and potential to hurricanes. It's a known fact that the warmer the sea surface temperatures are the more likely it will be for tropical systems to form and to get stronger. A good example of this was Hurricane Rita. When hurricane Rita passed over a VERY warm loop current in the Central Gulf of Mexico it intensified from a Category 3 hurricane early on September 21st to a peak of a 155 knot Cat 5 by the 22nd. After that Rita weakened to a Cat 4 and eventually a cat 3 due to an Eye Wall Replacement Cycle, slightly higher shear, and cooler Sea Surface Temperatures as this system moved out of this warm eddy.(or loop current.) In general these eddy's are spin off's from the Gulf Stream.
With this being said the general trend has been rising Sea Surface Temperatures. In 2005 Nature Magazine showed a new type of index called PDI (power dissipation index) which in laymans terms is "a measure of the total kenietic energy released by tropical cyclones" This index has been correlated with an increase of sea surface temperatures. What this means is, the high the SST's are the more destructive or intense tropical cyclones will be according to this said index. The image to this correlation is right here.
So if this was correct it would should a correlation between SST's and intensity of tropical cyclones. Although this index is rather new it really does not state anything we do not already know on the subject matter.
Overall SST's have warmed about 0.6 degrees since 1900, some of that warming may be Anthropogenic and some may be due to other factors such as the AMO. From simply looking at SST anomaly charts from 04,05,06,and 07 it does show a number of factors that are interesting. (Each of these images are taken from around (7/27).
Now looking at things from the other point of view, it seems like a lot of factors are being left off with the potential increases in hurricane activity and way to much emphasis is being placed onto SST's which are only one piece of the puzzle. For one there is no sure way to know if we are MUCH more active now that there is talk of global warming, than let's say back in the 1920's or 30's. As we have only had weather satellites, since the 70's so there was no accurate way of knowing how many tropical cyclones formed in a given season before this. There are many other factors into tropical cyclone development which include but are not limited to, Shear, El-Nino and La-Nina patterns, Oceanic Heat Content Potential, AMO cycles, and last but not least the affect of African Dust on the Atlantic hurricane season. I will go into each of these indepth and then show the argument against Global Warming and the increases in hurricane activity.
A theory heavily publizied by NOAA is most likely the best argument against global warming causing increases in hurricane activity and intensity and is probably the best argument against global warming increasing Sea Surfaces Temperatures by a large margin. In General the Atlantic Multidecadal Oscillation or AMO for short is a series of long term long duration changes in the oceans SST's. There are positive and negative phases (or warm and cool phases), and each lasts about 20 to 40 years. The AMO is very influential in a variety of weather patterns including everything from rainfall in The US and Europe to hurricane activity. The AMO switched from a negative to a positive phase and from that time there has been about 50% more tropical cyclones forming in the Atlantic then when it was in it's negative phase. The graphic below shows this nicely.
To add a little to the charts there was a small increase in storms, from 88 to 89 which was supposedly thought to be the start of another active hurricane cycle, but was not realized due to a drop in activity from 1990-94 due to an El-nino event, but after this nino abated and the positive AMO cycle kicked back in 95 was a very active year.
Although there is a noticeable increase in the frequency of tropical cyclones the intensity of the storms are not really affected by this cycle, which may gives weight to the fact that SST's do have a lot to do with intensity.
Also a major factor that is overlooked in the development of tropical cyclones in a given year is the el-nino/la nina cycles. El-nino's tend to cause more wind shear in the Atlantic bason. The Reason for this is quite simply causes an increase in pacific convection and a decrease in Atlantic convection as it strongly ties into windshear with el-nino during given nino years. In a basic sense el-ninos increase the amount of wind shear in the Atlantic Ocean which causes the deeper tops of these storms to be "blown off". With this happening, the storms cannot develop well as they cannot organize in a way conducive to tropical development. Also another perplexing fact to the "global warming argument", is the same people who say global warming is making the hurricane season more active and the storms more intense also say that global warming will increase the amount of el-ninos we have. I wish they would care to explain that. Also on another note, although there has not been a great amount of research done into the sharan air layer or sarhnan dust I do feel their is an underlining index that affects the rainfall patterns in Africa that also affects the tropics season. Here is an average map that shows an index of the sahels rainfall.
So now we can analog past years to the sahel index.
1950
1955
VS:
A year such as 1977 where there are almost no cape Verde storms. Of course this is not the only factor as there are quite a few but imo there are some correlations there if you ask me.
There is some correlation between the ENSO state and the Sahel's rainfall. Looking at the top years for increased Sahel rainfall and comparing that and the latest ENSO years. It seems like the years with the highest rainfall correlate to La Nina. I've personally heard that during La nina years you do get more cape verde systems so this does correlate well.
http://www.cpc.ncep.noaa.gov/products/anal...ensoyears.shtml
Another index correlated with La Nina is the SOI which does affect large scale global wind patterns which in many cases pertains to the tropics.
The reasoning behind this is that La Nina causes a larger amount of low vertical windshear in the Atlantic basin which allows more hurricanes to form and develop.
In conclusion I find that although SST's are a piece to the puzzle to say that they are the only factor affecting the amount and intensity of the storms in any given year is bad science.
Thanks, and let me know if you have any questions or comments.
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Originally Posted by dmp
