Extreme Weather #3 – Trends in Translation Speed of Tropical Cyclones

In #1 and #2 we looked at trends in frequency and intensity of Tropical Cyclones (TCs) and found that the IPCC 6th Assessment Report (AR6) contained good news. Not in the executive summary, and not particularly clearly even in the body of the report. But still, it is good news.

This article looks at the speed of TCs – how fast they move overall, not how fast the winds are swirling around. This is important, because as they hit land if they go more slowly there will be more rain and therefore more flooding.

This was going to be a short article, but as long time readers of Science of Doom will know, brevity was never my strong point.

Here’s the plain English summary of the report:

Translation speed of TCs has reduced over the last 70 years, leading to more flooding as TCs hit the coast.

This is bad news. The actual text, from p. 1587, is in the Notes at the end of this article.

James Kossin’s paper from 2018 is the main idea of this section of the report. Two papers are noted as questioning his conclusion. Kossin replied in 2019, confirming his original conclusion. The report essentially agrees with Kossin.

One of the lead authors of this chapter 11 on Extreme Weather is also James Kossin.

The main focus of this series of articles is the conclusions of the IPCC 6th Assessment Report, but it seems the question is still open, so read on for more analysis.

There’s a good review paper by Thomas Knutson from 2019. It has 11 authors including Knutson. We might call it a cast of luminaries. Including James Kossin. They cover lots of questions, and in the supplementary material you can see who agrees with each question. Here’s what they say about translation speed:

In summary, from a type I error avoidance perspective, a slight majority of the authors (6 of 11) had only low confidence that there has been a detectable decrease in global or western North Pacific TC translation speeds since 1949. For the other 5 authors, 4 had low to medium confidence, and 1 had medium confidence. Most authors (8 of 11) concluded that there was only low confidence that anthropogenic forcing had contributed to the observed decrease.

Alternatively, from the perspective of reducing type II errors, most authors (8 of 11) concluded that the balance of evidence suggests there has been a detectable decrease in global TC translation speeds since 1949; only 1 author concluded that the balance of evidence suggests that anthropogenic forcing has contributed to the observed decrease.

Here are some dissenting views from other papers. First the two papers cited in the IPCC report.

Uncertainties in tropical-cyclone translation speed, Lanzante 2019:

Here I raise doubt as to the veracity of that finding, because the long-term changes appear to be due primarily to a few abrupt, step-like changes, both natural and artificial, in the early part of the record. This greatly reduces the likelihood that the apparent slowdown is driven primarily by anthropogenic causes.

And from the summary:

This work has demonstrated that previously reported decreases in TCS (Tropical Cyclone Speed) are likely to be due to a combination of natural internal climate variability and abrupt changes early in the record owing to changes in measurement practices, particularly the introduction of satellite remote sensing capabilities during the 1960s in the Northern Hemisphere and in 1980 in the Southern Hemisphere, rather than indicative of a change in the climate system.

The fact that changes are step-like, and especially since most of the long-term change in TCS originates in the early part of the record, argues strongly against a dominant ACC [anthropogenic climate change] effect. However, a more subtle effect due to ACC cannot be ruled out entirely, and would require more detailed analysis to reveal.

Climate change and tropical cyclone trend, Moon 2019:

Kossin showed that tropical-cyclone translation speed (TCS) has decreased globally by 10% over the period 1949–2016 and stated that this is consistent with the expected changes in atmospheric circulation forced by anthropogenic warming.

However, we question the robustness of his conclusions for the following reasons: (1) TCSs generally increase with the latitude of the tropical cyclones and are therefore very sensitive to the bias of tropical-cyclone detection with respect to latitude; and (2) in the pre-satellite era (1949–1965), there is a high possibility that systematic biases in the detection of tropical cyclones exist in the best- track data, which could produce spurious trends in TCS.

Therefore, the slowdown of TCS stated may not be a real climate signal or it may be exaggerated.

Now another paper not cited: Are global tropical cyclones moving slower in a warming climate? KTF Chan, 2019:

Remarkably, even [though] 1970–2016 is the most pronounced warming phase in the last half-century, the corresponding relationship between global tropical-cyclone translation speed and global- mean temperature anomaly is very weak and insignificant..

..In addition, comparing to the findings inferred from period 1949 to 2016, the slowdown of tropical cyclones over water is no longer significant in 1970–2016, which is comparable to the global statistics, while the speedup of global tropical-cyclone translation speed over land becomes significant in 1970–2016 (+0.05 km h–1 yr–1; 13% increase; table 1). The relevant local rainfall totals over land are therefore expected to decrease rather than an increase in general, which contradicts what Kossin (2018a) and Hall and Kossin (2019) suggested.

I also noticed that the southern hemisphere graph (above, bottom graphic) would benefit from the comparison from 1980 – present, seeing as (my understanding) SH satellite data is not good quality before 1980. In this case, the yellow line would, like the NH, be flat.

For reference, here is Kossin 2018 (left side before, and right side after corrections):

Here is an extract from Kossin’s 2019 reply, where he provides data from the US since 1900:

The known drivers of climate and tropical-cyclone variability in the Atlantic are linked to the El Niño–Southern Oscillation, which operates on inter-annual timescales, and Atlantic multi-decadal variability (AMV), which most probably represents a convolution of internal and external regional variability. The relationship between the El Niño–Southern Oscillation and tropical-cyclone translation speed over the continental USA is weak, but the decadal variability of the tropical-cyclone translation speed time series over the continental USA is apparently linked to observed AMV (Fig. 1).

The increased tropical- cyclone translation speeds in the 1940s, which project positively onto the trends shown in Kossin, appear to be related to a warm AMV phase, whereas the suppressed tropical-cyclone translation speeds in the 1970s and 1980s appear to be related to the subsequent cool phase. The transition to the present warm phase coincides with increasing tropical-cyclone translation speeds over the continental USA.

Here’s the graphic. There’s definitely a relationship between the AMV and the estimate of translation speed. The graphic on the right attempts to remove the effect via a simple regression analysis:

When you see this graphic you think, maybe James Kossin is onto something. Clearly an important subject to study.

Taking a different approach – Global warming changes tropical cyclone translation speed, Munehiko Yamaguchi 2020. This one notes the problems of data before the satellite era and is instead a modeling study:

Because it is impossible to reproduce the observational data during the pre-satellite era with the same quality as those in the post-satellite era, we propose to analyze results of numerical simulations for the current and future climates..

..Results from the model used in this study show that the global mean TC translation speed has not slowed down in the past, but under global warming, they will in general speed up due to a poleward shift in their mean track location, although those moving into the extratropics slow down.

However, other modeling studies show a variety of results, which is not surprising given the complexity of the subject. One example is Gutmann et al 2018 who simulate changes under future global warming:

The decrease in storm translation speed is small, but statistically significant

However, modeling studies are a big subject, for a later article or ten.

Conclusion

The IPCC report says that the “translation speed” of tropical cyclones has slowed down, which is bad news. I’m not sure how confident the report should be in that conclusion. The causes of this slowdown, if it is real, are unknown.

Notes

Here is the text of the report from p. 1587:

A second metric that is argued to be comparatively less sensitive to data issues than frequency- and intensity-based metrics is TC translation speed (Kossin, 2018), which exhibits a global slowdown in the best-track data over the period 1949–2016. TC translation speed is a measure of the speed at which TCs move across the Earth’s surface, and is very closely related to local rainfall amounts (i.e., a slower translation speed causes greater local rainfall). TC translation speed also affects structural wind damage and coastal storm surge by changing the hazard event duration.

The slowdown is observed in the best-track data from all basins except the Northern Indian Ocean, and is also found in a number of regions where TCs interact directly with land.

The slowing trends identified in the best- track data by Kossin (2018) have been argued to be largely due to data heterogeneity. Moon et al. (2019) and Lanzante (2019) provide evidence that meridional TC track shifts project onto the slowing trends, and argue that these shifts are due to the introduction of satellite data.

Kossin (2019) provides evidence that the slowing trend is real by focusing on Atlantic TC track data over the contiguous USA in the 118-year period 1900–2017, which are generally considered reliable. In this period, mean TC translation speed has decreased by 17%..

.. In summary, there is mounting evidence that a variety of TC characteristics have changed over various time periods.. It is likely that TC translation speed has slowed over the USA since 1900..

References

Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change

A global slowdown of tropical-cyclone translation speed, James P. Kossin, Nature (2018)

Reply to: Moon, I.-J. et al.; Lanzante, J. R., James P. Kossin, Nature (2019)

Tropical Cyclones and Climate Change Assessment – Part I: Detection and Attribution, Thomas Knutson et al, AMS (2019)

Uncertainties in tropical-cyclone translation speed, John R. Lanzante, Nature (2019)

Climate change and tropical cyclone trend, Il-Ju Moon et al, Nature (2019)

Are global tropical cyclones moving slower in a warming climate? Kelvin TF Chan, Environ. Res. Lett. (2019)

Global warming changes tropical cyclone translation speed, Munehiko Yamaguchi et al, Nature Climate Change (2020)

Changes in Hurricanes from a 13-Yr Convection-Permitting Pseudo– Global Warming Simulation, Ethan Gutmann et al, Journal of Climate (2018)