Climate Change and Tropical Cyclone Forward Speed

26 01 2020

Dale C.S. Destin|

Many have made tropical cyclones (hurricanes, tropical storms and tropical depressions) the posterchild for climate change with respect to how they are or may be responding to a warming climate. It is a hot topic of debate that researchers are continuously investigating for answers. One of the many questions been asked is, “Are global tropical cyclones moving slower in a warning climate?”  

The track of Hurricane Irma – 2017

A recent research letter, in the Institute of Physics (IOP) Publishing, by Kelvin T.F. Chan, of the School of Atmospheric Sciences, and Guangdong Province Key Laboratory for Climate and Natural Disaster Studies, Sun Yat-sen University, Zhuhai, People’s Republic of China, weighed in, quite forcefully and persuasively, on the above question.

Time series of annual-mean global tropical-cyclone translation speed and their linear trends in periods 1949–2016 (blue) and 1970–2016 (orange).

According to Chan, the short answer to the question is no. Chan successfully show that, with the use of reliable data from 1970 to 2016, there is no significant change in the forward speed of tropical cyclones, notwithstanding the pronounced warning of the globe seen over the last half-century.

Chan’s finding virtually contradicts a 2018 paper present by James P. Kossin of the NOAA National Centers for Environmental Information, Center for Weather and Climate, Madison, WI, USA. Kossin found that there was a 10% slowdown in global tropical-cyclone translation (forward) speed over the past 68 years – 1949 to 2016. His implication was that this was related to anthropogenic climate change.

The IOP letter, by Chan, successfully shows that the findings of Kossin was based on the use of widely accepted spurious data obtained prior to the satellite era – pre 1966. Unanimously accepted, more accurate data since the satellite era – 1970 to 2016, show no significant slowdown of tropical cyclones. Understandably, prior to weather satellites in 1966, accurately tracking tropical cyclones was very difficult to impossible; hence, translation speed data before then are highly unreliable.

This finding of Chan is consistent with the findings of Knutson et al, in a very comprehensive study on the entire subject of climate change and tropical cyclones. According to Knutson, the results for the models “indicate no significant changes, and only 2 of the 10 individual… model projections show a significant change (increase).” He concluded that there was no clear consistent trend in the forward speed of tropical cyclones.

The question of how climate change is or may impact the forward speed of tropical cyclones is very important from a adoptation standpoint. A slower moving tropical cyclone has the potential for more destruction and vice versa. Naturally, the longer a tropical cyclone hangs around a particular populated area, the more the destruction is likely to be via wind, flooding rainfall and storm-surge.

Intuitively, it would be consistent to think that since a warning climate is slowing down the circulation of the tropics, it is also slowing down tropical cyclones, embedded its circulation. However, this is clearly not happening, certainly not based on the data since 1970 to 2016.

Tropical cyclones may be the posterchild for climate change; however, there is no evidence to support any changes in tropical cyclones forward speed, due to the changing climate being forced by humans.  

More broadly, there exist no evidence that there have been any changes in tropical cyclones, due to anthropogenic climate change. However, that is not to say that climate change won’t have an impact on tropical cyclones in the future.  

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Tropical Storm in May? Uncommon but not Unheard Of

5 05 2019

Dale C. S. Destin |

The first tropical disturbance for the year has formed and is currently over the Western Atlantic, near the southeast United States. Although this system is unlikely to form, it is not unheard of for tropical storms to develop in May, but it is relatively uncommon.

Looking back at the history, there have been 26 tropical storms in May, 4 of which became hurricanes – all category ones. This means, on average, one tropical storm forms every 6 to 7 years, based on data for the period 1981-2010, upon which the current climate is based.

All Tropical Storms in May on Record, 1851 to 2018

All May Tropical Storms, on Record – 1851 to 2018

The probability of a tropical storm forming in May is 15% (a slight chance), which means storms seldom form in May. The last time this happened was back in 2016 – Tropical Storm Bonnie.

Hurricanes in May are even more uncommon, if not rare. The probability of one forming in May is less than 3%, based on the full history – 1851 to 2018. This means there is a hurricane in May, on average, once every 42 years – once in a generation plus. The last one was Hurricane Alma of 1970.

One may be tempted to think that a tropical storm in May is an omen for an active (above normal) hurricane season. In a (small) poll done by me on twitter, 61% of persons said that a storm in May means an active or hyperactive season; however, this is false. The record shows the following w.r.t. seasons whenever there has been a storm in May:

  • 3 hyperactive;
  • 4 active (above Normal);
  • 11 normal and
  • 5 quiet (below normal).

In other words, whenever there was a storm in May:

  • 4% of the time the seasons were hyperactive;
  • 17% were active;
  • 48% were normal and
  • 22% were quiet.

Hence, a storm in May seems to portend a normal hurricane season, as opposed to an active or hyperactive season. So, we may, perhaps, want to wish for a storm in May.

With all the talk about climate change and tropical cyclones, one may be tempted to think that we are having more storms in May than before. However, the record does not bear that out. In the 38 years from 1981 to 2018, there have been nine tropical storms, no hurricane. By comparison, during the previous 38 years, there were eight tropical storms, two of which became hurricanes.

While tropical storms are not unheard of for the Atlantic Basin, they are unheard of for our neck of the woods – Antigua and Barbuda. We have never been impacted by a tropical cyclone (tropical depression, tropical storm or hurricane) in May.

The fortune of never having a tropical cyclone in May extends to the rest of the Eastern Caribbean. As a matter of fact, none has ever formed east of Hispaniola. Further, no hurricane has ever impacted/affected any Caribbean island in May. Clearly May is not a month to be worried about tropical storms.

Follow me for the latest on the hurricane season and all things weather and climate.

Tropical Cyclone Intensity and Climate Change

5 10 2017

Dale C. S. Destin|

This is a continuation of our series – Tropical Cyclones and Climate Change – TCs (tropical depressions, tropical storms and hurricanes). In this blog, we will look at whether climate change is having an impact on TC intensity/strength, especially with respect to wind speeds and provide you scientifically based answers.

As many Caribbean islands rebuild after the havoc caused by Hurricanes Irma and Maria, which killed over 185 persons and caused over US$150 billion in damage, many – including political leaders, have declared that these hurricanes were caused by climate change.

Even the United Nations (UN) Secretary-General seems to be peddling the notion that climate change caused Irma, Maria and the hyperactive 2017 Atlantic hurricane season.

But is climate change really to be blamed? Are these statements in harmony with the findings of the Intergovernmental Panel on Climate Change (IPCC)? What is the consensus of TC researchers?

The 2017 Atlantic hurricane season

Thus far, the season has produced 14 named storms, 8 of which became hurricanes and 5 major hurricanes – Category 3 and over. Of the 5 major hurricanes, 2 (Irma and Maria) became Category 5 – the highest category on the Saffir-Simpson Hurricane Wind Scale.2017 Atl Hurricane Season TracksRecord shows that Hurricane Irma was no ordinary Category 5 Hurricane. Far from it – it was more like a Super Category 5 Hurricane. If there were a Category 6 – it would have easily been so categorised.

Hurricanes Irma and Maria are among the strongest hurricanes to ever form over the Atlantic Basin. Irma had peak sustained winds of 295 km/h (185 mph), which makes her joint holder with three other hurricanes for the second strongest Atlantic hurricane on record, dating back to 1851. Only Hurricane Allen of 1980 was stronger with 305 km/h (190 mph) winds. As for Maria – her peak sustained winds of 280 km/h (175 mph) tied her with seven other hurricanes for the eighth strongest on record.

Super Category 5 Hurricane Irma on our doorsteps

Super Category 5 Hurricane Irma on our doorsteps – Sep 5, 2017

Have TCs become more intense?

According to the IPCC, “unlikely“. The IPCC is the UN international body designated to assess the science of climate change. It was set up in 1988 by World Meteorological Organization (WMO) and UN Environment Programme (UNEP) to provide policymakers with rigorous and balanced scientific information on climate change.

This is what the IPCC Assessment Report Five (AR5) – the latest report, says about the impact of climate change on TCs: “In summary…recent assessments indicate that it is unlikely that annual numbers of tropical storms, hurricanes and major hurricanes counts have increased over the past 100 years in the North Atlantic basin…”

So, based on the examination of many peer-reviewed scientific research papers, the IPCC concluded that there is no trend in the intensity of tropical cyclones – there is no robust data to support the notion that climate change has caused TCs to be “stronger and bigger” “with each passing hurricane season”.

There is, however, a line in the IPCC report that some may point to as evidence of climate change causing TCs to be stronger. It says: “Evidence, however, is for a virtually certain increase in the…intensity of the strongest tropical cyclones since the 1970s in that [North Atlantic] region.” Is this an endorsement of the view that climate chance is causing TCs to be stronger? Certainly not.

TC activity in the North Atlantic, like most places, go through phases – inactive and active periods that last for multiple of decades at a time, which are closely linked to the Atlantic Multidecadal Oscillation (AMO). Active phases are marked by above normal number and strength of TCs and the opposite for inactive phases.

3-Year-Average ACE

A plot of three-year-averaged Accumulated Cyclone Energy (ACE). It is a measure of the activity of a hurricane season based on strength, duration and the number of TCs. It shows the active and inactive phases of the Atlantic hurricane season. These phases are natural – NOT caused by human-induced climate change.

So, from the 1970s to the mid-1990s, the Atlantic went through an inactive phase, where there was a decline in the frequency and intensity of the number of TCs from the mid-1920s to 1960s – the previous active period before our current one.

The Atlantic is currently in an active phase which began around the mid-1990s. So, certainly, the record shows an increase in the frequency and intensity of TCs since the 1970s. But this does not mark an overall increase – it is just a part of the decadal cycle. Further, according to the IPCC’s latest report, there is low confidence that this increase is due to human-induced climate change. In other words, there is an 80% chance that this increase is not due to climate change.

So, the IPCC, and by extension, the UN is clear about the impact of climate change on TCs – it is currently “unlikely” having any effect on TC intensity. The same IPCC indicates that it is unequivocal that man is changing the climate. This is a position shared by over 99% of climate scientists and people in general. If we believe this part of the report, we must believe the part that speaks about the impact or lack thereof, at this time, of climate change on TCs.

This research position also shared by WMO and the TC research community.

Will TCs become more intense?

According the IPCC AR5, “more likely than not”. Going forward, IPCC AR5 says that climate change will “more likely than not” cause changes in TC intensities late in this current century – near year 2100.

This conclusion by the IPCC is consistent with one of the most authoritative scientific papers on the subject written by Knutson TR, McBride JL, Chan J, et al.. It says that late in this century, not now, based on models, there will be a 2 to 11% increase in wind speeds of TCs.

Empirical Data and the impact of climate change on TCs

Let’s look at the empirical data to see what they say regarding the impact of climate change on TCs. Based on the database, the following is true:


A count of the number of hurricanes – 1924 to 1969 vs. 1970 to 2016. The bracketed numbers are the yearly averages.

From the above, clearly, there is no significant difference between numbers for the period 1924-1969, when climate change was not an issue, and the period 1970-2016, when climate change became an issue. What is interesting is that the numbers for 1924-1969 would have been higher, if not for a number of TCs being missed, due to a lack of satellite technology prior to the mid-1960s.

Further, some of the most powerful hurricanes to form across the Atlantic Basin occurred when climate change was not an issue – before the mid-1980s. Hurricane Allen which still holds the record for the highest maximum sustained wind speed ever on record over the Atlantic occurred in 1980. Of the top 12 strongest hurricanes, in terms of maximum sustained winds, six occurred before the mid-1980s.

Based on the IPCC and the vast majority of the TC researchers, TCs are an unworthy poster child for climate change. By the preponderance of research papers, climate change has NOT caused TCs to be stronger. Climate change had nothing to do with the strength of Hurricanes Irma and Maria or the activity of the 2017 Atlantic hurricane season. It is all due to natural variability.

However, by late this century “more likely than not” climate change WILL cause an intensification of TCs. But this is not yet evident. This is the conclusion of IPCC AR5 – we either accept the full report or none at all. And if we accept all of it, we would not be blaming hurricane intensities on climate change, at this time.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone frequencies.

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