Extreme Weather #6 – Trends in Poleward Migration of Tropical Cyclones
In #1 we looked at the trends in intensity and frequency of landfalling tropical cyclones (TCs) over 120+ years. In #2 we looked at the same metrics out over the ocean using satellite data, which is available for about the last 40 years. In #3 we looked at “translation speed” or changes in the speed at which the overall TCs are moving. In #4 we looked at trends in rainfall in TCs - missing in action from the IPCC report. And in #5 something called “intensification rates” of TCs.
This is the last trend in TCs that we’ll look at from the 6th Assessment Report.
Here’s the plain English version of the report:
Tropical Cyclones are moving away from the equator towards the poles, at the rate of about 50-60 km (30-40 miles) per decade.
The actual text, from p. 1586, is in the Notes at the end of this article.
Overall, from a global perspective, this is neutral news. If you live near the equator, it’s good news. If you live at latitudes just outside the reach of current tropical cyclonees it’s bad news.
Here’s the graphic from Kossin 2014 - LMI in the text is the location of maximum intensity, which is easy to pick out from the data and subject to less uncertainty than a metric like “where the TC starts”:
The mechanisms for this change, or attribution, are a big subject in themselves.
At the moment we are primarily considering trends, not causes, to keep things simple.
In the next article I’ll summarize all of these points and we’ll also see something interesting about the way the IPCC summarizes these points.
Notes
Text of AR6 from p. 1586 about the poleward migration of TCs tropical cyclones:
..the mean latitude where TCs reach their peak intensity – exhibits a global and regional poleward migration during the satellite period (Kossin et al., 2014).
The poleward migration can influence TC hazard exposure and risk (Kossin et al., 2016a) and is consistent with the independently observed expansion of the tropics (Lucas et al., 2014). The migration has been linked to changes in the Hadley circulation (Altman et al., 2018; Sharmila and Walsh, 2018; Studholme and Gulev, 2018).
The migration is also apparent in the mean locations where TCs exhibit eyes (Knapp et al., 2018), which is when TCs are most intense.
Part of the Northern Hemisphere poleward migration is due to basin-wide changes in TC frequency (Kossin et al., 2014, 2016b; Moon et al., 2015, 2016) and the trends, as expected, can be sensitive to the time period chosen (Tennille and Ellis, 2017; Kossin, 2018; Song and Klotzbach, 2018) and to subsetting of the data by intensity (Zhan and Wang, 2017).
The poleward migration is particularly pronounced and well-documented in the western North Pacific basin (Kossin et al., 2016a; Oey and Chou, 2016; Liang et al., 2017; Nakamura et al., 2017; Altman et al., 2018; Daloz and Camargo, 2018; J. Sun et al., 2019; T.-C. Lee et al., 2020; Yamaguchi and Maeda, 2020a; Kubota et al., 2021).
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
The poleward migration of the location of tropical cyclone maximum intensity, James Kossin et al, Nature (2014)