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:
ABSTRACT
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:
- Hurricane activity, particularly the frequency of intense storms, is highly correlated with the AMO.
- The cold phase of the AMO from the 1960s to 1990s coincided with lower hurricane activity.
- The warm phase of the AMO between 1930 and 1960 was much more active, just as the current warm phase is.
- 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.
via NOT A LOT OF PEOPLE KNOW THAT
September 8, 2017 at 01:09PM
Iowa Climate Science Education 