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Hurricanes, AMO , And Sahel Droughts

September 8, 2017

By Paul Homewood




Reader Dermot Flaherty questioned the relationship of ENSO to the Atlantic hurricane season.

There are indeed many factors which affect hurricane activity. As leading hurricane expert Chris Landsea stated in his 1999 paper “Atlantic Basin Hurricanes: Indices of Climatic Changes”:

Various environmental factors including Caribbean sea level pressures and 200mb zonal winds, the stratospheric Quasi-Biennial Oscillation, the El Niño-Southern Oscillation, African West Sahel rainfall and Atlantic sea surface temperatures, are analyzed for interannual links to the Atlantic hurricane activity. All show significant, concurrent relationships to the frequency, intensity and duration of Atlantic hurricanes.

Landsea goes on:

Finally, much of the multidecadal hurricane activity can be linked to the Atlantic Multidecadal Mode – an empirical orthogonal function pattern derived from a global sea surface temperature record.


As he says, it is the AMO which probably has the biggest influence, as scientists have known for years, and as NOAA explain in their FAQ on the AMO:

During warm phases of the AMO, the numbers of tropical storms that mature into severe hurricanes is much greater than during cool phases, at least twice as many. Since the AMO switched to its warm phase around 1995, severe hurricanes have become much more frequent and this has led to a crisis in the insurance industry.



But there is an intriguing corollary which follows on from this, and that is Sahel rainfall. Again scientists have long known that, when the Atlantic is cold, the Sahel suffers from droughts.

Landsea, in another paper, “THE STRONG ASSOCIATION BETWEEN WESTERN SAHEL MONSOON RAINFALL AND INTENSE ATLANTIC HURRICANES”, published in 1991, explored these connections. This was his Abstract:

Seasonal variability of Atlantic basin tropical cyclones is examined with respect to the monsoonal rainfall over West Africa. Variations of intense hurricanes are of most interest as they are responsible for over three-quarters of United States tropical cyclone spawned destruction though they account for only one-fifth of all landfalling cyclones. Intense hurricanes have also shown a strong downward trend during the last few decades. It is these storms which show the largest concurrent association with Africa’s Western Sahel June to September rainfall for the years 1949 to 1990.

Though the Sahel is currently experiencing a multidecadal drought, the relationship between Atlantic tropical cyclones and Western Sahel rainfall is not dependent upon the similar downward trends in both data sets. A detrended analysis confirms that a strong association still exists, though reduced somewhat in variance explained. Additionally, independent data from the years 1899 to 1948 substantiate the existence of the tropical cyclone-Western Sahelian rainfall association.

The fact that the Sahel periodically experiences multidecadal wet and dry regimes suggests that the current Sahel drought which began in the late 1960’s could be a temporary condition that may end in the near future. When this occurs, the Atlantic hurricane basin — especially the Caribbean islands and the United States East Coast — will likely see a large increase in intense hurricane activity associated with abundant Sahel rainfall similar to the period of the late 1940’s through the 1960’s.

Remembering that that that was written in 1991, consider this statement closely:

Intense hurricanes have also shown a strong downward trend during the last few decades,



We only have had satellite monitoring of hurricanes since the 1970s, and alarmists often claim that an increasing trend since then is due to global warming. In fact, it is the AMO which is the key factor. As Landsea confirms, the frequency of intense hurricanes had been much greater prior to the 1960s, when the AMO was last in warm phase.

But as I mentioned earlier, the other side of the coin is the Sahel. Alarmists like to pretend a warmer climate is all bad, but nature does not work like that.

In this case, the price that some parts of the world may have to pay for hurricanes is offset by welcome wetter conditions in the Sahel.



Other scientists come to the same conclusion. Sang-Ki Lee even suggests a link with the concentration of dust.





Zhang and Delworth extend the AMO link through to the Indian monsoon as well:


Prominent multidecadal fluctuations of India summer rainfall, Sahel summer rainfall, and Atlantic Hurricane activity have been observed during the 20th century. Understanding their mechanism(s) will have enormous social and economic implications. We first use statistical analyses to show that these climate phenomena are coherently linked. Next, we use the GFDL CM2.1 climate model to show that the multidecadal variability in the Atlantic ocean can cause the observed multidecadal variations of India summer rainfall, Sahel summer rainfall and Atlantic Hurricane activity (as inferred from vertical wind shear changes). These results suggest that to interpret recent climate change we cannot ignore the important role of Atlantic multidecadal variability…..

They continue:


West central India is the core monsoon region, where the summer rainfall is highly correlated with All India Summer Rainfall [Parthasarathy et al., 1994]. Over west central India, the multidecadal wet period is in phase with the positive AMO phase (warm North Atlantic) during the middle of the 20th century (∼1926–1965); the dry periods are in phase with the negative AMO phase during both the early (∼1901–1926) and the late 20th century (∼1965–1995). The time series of west central India summer rainfall is in phase with Sahel summer rainfall



And in 2013, the Met Office’s Nick Dunstone found a possibly link between industrial pollution and hurricane activity/Sahel rainfall:

New research from the Met Office has raised the possibility that man-made aerosols, industrial pollution, may have had an impact on the number of Atlantic hurricanes.

The paper, published in Nature Geoscience, suggests aerosols may have suppressed the number of Atlantic hurricanes over the 20th Century and even controlled the decade-to-decade changes in the number of hurricanes.

Researchers found that aerosols make clouds brighter, causing them to reflect more energy from the sun back into space. This has an impact on ocean temperatures and tropical circulation patterns, effectively making conditions less favourable for hurricanes.

Dr Nick Dunstone, a Met Office climate prediction scientist and lead author of the research, said: “Industrial emissions from America and Europe over the 20th Century have cooled the North Atlantic relative to other regions of the ocean. Our research suggests that this alters tropical atmosphere circulation – making it less likely that hurricanes will form.

“Since the introduction of the clean air-acts in the 1980s, concentrations of aerosols over the North Atlantic have reduced and model results suggest that this will have contributed to recent increases in hurricane numbers. On the other hand, the reduction in aerosols has been beneficial for human health and has been linked to the recovery of Sahel rains since the devastating drought in the 1980s.”

It has long been known that North Atlantic hurricane activity has distinct long-timescale variability. Dr Doug Smith, a Met Office research fellow and co-author of the study, said: “We saw relatively quiet periods between 1900-20 and then again from 1970-80, and active periods between 1930-60 and since 1995. On average, active periods have 40% more hurricanes.”


Again, note the comments about hurricane trends:

We saw relatively quiet periods between 1900-20 and then again from 1970-80, and active periods between 1930-60 and since 1995. On average, active periods have 40% more hurricanes.



Going back to Landsea’s 1999 paper, he observed:

Accurate records of basinwide Atlantic and U.S. landfalling hurricanes extend back to the mid 1940s and the turn of the century, respectively, as a result of aircraft reconnaissance and instrumented weather stations along the U.S. coasts. Such long-term records are not exceeded elsewhere in the tropics. The Atlantic hurricanes, U.S. landfalling hurricanes and U.S. normalized damage time series are examined for interannual trends and multidecadal variability. It is found that only weak linear trends can be ascribed to the hurricane activity and that multidecadal variability is more characteristic of the region.


So to summarise:

  1. Hurricane activity, particularly the frequency of intense storms, is highly correlated with the AMO.
  2. The cold phase of the AMO from the 1960s to 1990s coincided with lower hurricane activity.
  3. The warm phase of the AMO between 1930 and 1960 was much more active, just as the current warm phase is.
  4. Increased periods of hurricane activity is highly correlated, via the AMO link, with improved rainfall in the Sahel, and possibly across a band as far as India.
  1. September 8, 2017 6:15 pm

    Just looked at BBC Weather web site – apparently

    ” It was always forecast to be an active hurricane season”

    Infallibility Factor 10 – or something like that….

  2. SteveT permalink
    September 8, 2017 10:00 pm

    What has been the solar influence over these periods? Has the sun had lots of spots, or no spots? The sun has total control over the weather on our planet.

  3. Broadlands permalink
    September 9, 2017 1:09 am

    “…and this has led to a crisis in the insurance industry.”

    And where does this industry get its “climate change forecasts”? Same place most politicians do…the media. And where does the media get its expertise? The “scientists” tell us! And where do THEY get their evidence…. from funding by politicians. And the game goes on from one “crisis” to the next.

    This was written in 1976…

    “Despite all available evidence, and the warnings from experts, and the obvious peril of a world rushing to acquire nuclear weapons as famine nears, our political leaders are reluctant to act. Like children they wait for the toothache to go away and take comfort in those who advise that nothing need be done. But part of maturity is the willingness to face problems squarely, openly, honestly, and if our world is to survive we must demand such maturity from our leaders. We cannot continue as if nothing were wrong. To do so might prove both homicidal and, eventually, suicidal.”

    “We simply cannot afford to gamble against this possibility by ignoring it. We cannot risk inaction. Those scientists who say we should ignore the evidence and the theories suggesting Earth is entering a period of climatic instability are acting irresponsibly. The indications that our climate can soon change for the worse are too strong to be reasonably ignored.”

    It could have been written yesterday? “Act Now”?

    • daver permalink
      September 9, 2017 2:38 am

      Worth recalling beeb’s ’28 Gate’ affair, a cabal of invitees especially incorporating the ‘ass’-urance industries. There is no stoop too steep these profiteering alarmists won’t descend into.

      H/t and cool beer too, to ‘omnologos’ for the sus.

  4. Dermot Flaherty permalink
    September 9, 2017 8:33 am

    Paul, Many thanks for this which I will now spend some time studying since a serious subject requires serious effort to simply get an overview.

    Serious effort seems to be beyond the capability of most of our politicians and (increasingly) the media and the growth of “groupthink” – especially around a subject such as “Climate Change” which has become almost religious in the clear overtones of “good” v “evil” in the vast majority of popular discussions on the subject – has encouraged the intellectually lazy.

    I don’t know anyone who disputes that the human race is having an effect on climate; the question is how much and indeed whether increased CO2 is a universally bad thing.
    Freeman Dyson talks sense on the subject and as you have pointed out, agricultural yields are up in a number of areas of the world.
    Having seen the world’s population treble since I was born, I would have thought that this FACT should worry people a lot more than dubious CC claims, since we all have to eat and live somewhere.

  5. Bloke down the pub permalink
    September 9, 2017 9:49 am

    Interesting that, despite the positive feedback loop described by Sang Ki-Lee, the variability is cyclical rather than linear. I wonder how this compares with the supposed positive feedbacks associated with cagw?

  6. September 10, 2017 6:23 pm

    Reblogged this on Tallbloke's Talkshop and commented:
    What really drives Atlantic hurricanes? Paul Homewood looks at some relevant research.

  7. September 10, 2017 10:32 pm

    Reblogged this on WeatherAction News and commented:
    Paul looks at done long term trends with the CO2 fairy nowhere in sIght

  8. linneamogren permalink
    September 13, 2017 2:36 pm

    There’s not much data from these isolated regions where hurricanes and tropical cyclones are developed. So any honest scientist would have to walk very carefully over that broken glass to even attempt to point to AGW as cause.

    The buildup of high pressure systems in the North Atlantic always cause instability in the southern where most hurricanes develop. Currently this has been in effect for this Atlantic hurricane season which is why it’s been active. The same process took place in the 2004-05 seasons which were very active and since that time the Atlantic has not been active with high hurricane activity until now.

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