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Limitations of reanalyses for detecting tropical cyclone trends

Nature Climate Change volume 14, pages 143–145 (2024)Cite this article

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Matters Arising to this article was published on 20 November 2023

The Original Article was published on 27 June 2022

arising from S. S. Chand et al. Nature Climate Change https://doi.org/10.1038/s41558-022-01388-4 (2022)

Chand et al.1 detected tropical cyclones (TCs) in a large ensemble of climate reanalyses and concluded that TC activity declined on global and regional scales during the twentieth century. Here I argue that the detected decline was mostly an artificial consequence of the greatly increasing density of surface-pressure observations during this time. Observations assimilated into the climate reanalysis model serve both to increase the detection of real TCs and to tamp down TCs produced by the model but not present in nature. (This tamping down may be direct, when observations show conclusively that a cyclone is not present, or indirect if observations force the reanalysis to contain a TC and that cyclone has negative feedbacks on other, model-only cyclones.) Real TCs can be distinguished from model-only storms because the latter are not present in ensemble-mean data. I show that the reduction over time in model-only storms is highly correlated with a measure of the probability that storms were detected by observations, and point out that neither the real storms in the ensemble mean nor the individual ensemble member storms reported in Chand et al. show any strong trends during times of plentiful observations, which also coincided with the period of most rapid global warming. (An exception is the Atlantic region, in which many metrics of TC activity show increasing activity.) I argue that TCs directly detected in twentieth-century climate analyses are unsuitable for deducing long-term trends.

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Fig. 1: Excess OWZ counts.
Fig. 2: Predicted versus actual excess OWZ counts.

Data availability

The raw ensemble data from the NOAA-CIRES Twentieth Century Reanalysis Project version 3, was obtained by correspondence with Gilbert Compo (gilbert.p.compo@noaa.gov). Files containing the OWZ counts and some other variables, processed from the raw ensemble data, can be downloaded at http://texmex.mit.edu/pub/emanuel/DataCodes/data_codes.zip.

Code availability

MATLAB scripts for analysing the data and producing the plots presented here can be downloaded from http://texmex.mit.edu/pub/emanuel/DataCodes/data_codes.zip.

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Acknowledgements

This research received support by the generosity of E. and W. Schmidt by recommendation of Schmidt Futures as part of its Virtual Earth System Research Institute (VESRI). Support for the Twentieth Century Reanalysis Project version 3 dataset is provided by the US Department of Energy, Office of Science Biological and Environmental Research (BER), by the National Oceanic and Atmospheric Administration Climate Program Office, and by the NOAA Earth System Research Laboratory Physical Sciences Laboratory. I am grateful to G. Compo, L. Slivinski and C. McColl of NOAA for assistance in accessing the NOAA 20CR3 data and for helpful advice on an earlier draft of this comment. I thank S. Chand and his co-authors for their forbearance and cooperation in this exchange.

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  1. Lorenz Center, Massachusetts Institute of Technology, Cambridge, MA, USA

    Kerry Emanuel

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  1. Kerry Emanuel
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Correspondence to Kerry Emanuel.

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Nature Climate Change thanks Michael Montgomery, Jill Trepanier and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figs. 1–6, Tables 1 and 2, and Discussion.

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Emanuel, K. Limitations of reanalyses for detecting tropical cyclone trends. Nat. Clim. Chang. 14, 143–145 (2024). https://doi.org/10.1038/s41558-023-01879-y

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