# Statistical Decomposition of the Recent Increase in the Intensity of Tropical Storms

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

- (a)
- is the increase in the MH ratio driven by changes in the numerator (the number of cat35 fixes), the denominator (the number of cat15 fixes), or both?
- (b)
- is the increase in the MH ratio driven by any particular strength categories, or is it driven by several strength categories?
- (c)
- is the increase driven by any particular regions, or is it the result of similar changes in all regions?

## 2. Dataset and Increase in the MH Ratio

## 3. Results

#### 3.1. Numerator vs. Denominator

#### 3.2. Category of Storm Fixes

#### 3.3. Regional Analysis

## 4. Quantitative Analysis

#### 4.1. Decomposition by Numerator and Denominator

#### 4.2. Decomposition by Category of Storm

#### 4.3. Decomposition by Region

#### 4.4. Regional Analysis of Ratios

#### 4.5. Discussion

- (1)
- The upward trend in the global MH ratio is mainly driven by a downward trend in the number of cat1 fixes in basins other than the North Atlantic.
- (2)
- A secondary and less important driver is the upward trend in the number of cat34 fixes in the North Atlantic. Although this driver is less important than the decrease in the cat1 fixes, without it the trend in the MH ratio is no longer significant.
- (3)
- Of the other five basins, only one shows a significant upward trend in the basin-level ratio (while noting that the trends in individual basins are subject to a greater extent to observational errors)
- (4)
- The small upward trend in global total cat35 fixes shown in Figure 2a does not reflect a uniform global increase in cat35 fixes, but is driven by the large increase in the North Atlantic, an increase in the Southern Indian Ocean, and decreases in the three Pacific basins.

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Appendix A

**Figure A1.**(

**a**–

**f**) show the number of cat1 fixes (black time series), with a linear trend (black line), and lines showing 95% confidence intervals on the trend slope, for 6 basins.

**Figure A2.**(

**a**–

**f**) show the number of cat34 fixes (black time series), with a linear trend (black line), and lines showing 95% confidence intervals on the trend slope for 6 basins.

## References

- Emanuel, K. The dependence of hurricane intensity of climate. Nature
**1987**, 326, 483–485. [Google Scholar] [CrossRef] - Sobel, A.; Camargo, S.; Hall, T.; Lee, C.; Tippett, M.; Wing, A. Human influence on tropical cyclone intensity. Science
**2016**, 353, 242–246. [Google Scholar] [CrossRef] [PubMed][Green Version] - Knutson, T.; Camargo, S.; Chan, J.; Emanuel, K.; Ho, C.; Kossin, J.; Mohapatra, M.; Satoh, M.; Sugi, M.; Walsh, K.; et al. Tropical cyclones and climate change assessment: Part II: Projected response to anthropogenic warming. Bull. Am. Meteorol. Soc.
**2020**, 101, E303–E322. [Google Scholar] [CrossRef] - Landsea, C.; Harper, B.; Hoarau, K.; Knaff, J. Can we detect trends in extreme tropical cyclones? Science
**2006**, 313, 452–454. [Google Scholar] [CrossRef] [PubMed][Green Version] - Goldenberg, S.; Landsea, C.; Mestas-Nunez, A.; Gray, B. The Recent Increase in Atlantic Hurricane Activity: Causes and Implications. Science
**2001**, 293, 474–479. [Google Scholar] [CrossRef] [PubMed][Green Version] - Elsner, J.; Kossin, J.; Jagger, T. The increasing intensity of the strongest tropical cyclones. Nature
**2008**, 455, 92–95. [Google Scholar] [CrossRef] [PubMed] - Knutson, T.; Camargo, S.; Chan, J.; Emanuel, K.; Ho, C.; Kossin, J.; Mohapatra, M.; Satoh, M.; Sugi, M.; Walsh, K.; et al. Tropical cyclones and climate change assessment: Part 1: Detection and attribution. Bull. Am. Meteorol. Soc.
**2019**, 100, 1987–2007. [Google Scholar] [CrossRef][Green Version] - Kossin, J.; Knapp, K.; Olander, T.; Velden, C. Global Increase in major tropical cyclone exceedance probability over the past four decades. Proc. Natl. Acad. Sci. USA
**2020**, 117, 11975–11980. [Google Scholar] [CrossRef] [PubMed] - Kossin, J.; Olander, T.; Knapp, K. Trend analysis with a new global record of tropical cyclone intensity. J. Clim.
**2013**, 26, 9960–9976. [Google Scholar] [CrossRef] - Knapp, K.R.; Kossin, J.P. New global tropical cyclone data from ISCCP B1 geostationary satellite observations. J. Appl. Remote Sens.
**2007**, 1, 013505. [Google Scholar] - Knapp, K.; Kruk, K.; Levinson, D.; Diamond, H.; Neumann, C. The international best track archive for climate stewardship (IBTrACS): Unifying tropical cyclone best track data. Bull. Am. Meteorol. Soc.
**2010**, 91, 363–376. [Google Scholar] [CrossRef] - Holland, G.; Bruyere, C. Recent intense hurricane response to global climate change. Clim. Dyn.
**2014**, 42, 617–627. [Google Scholar] [CrossRef][Green Version] - Dunstone, N.J.; Smith, D.M.; Booth, B.B.; Hermanson, L.; Eade, R. Anthropogenic aerosol forcing of Atlantic tropical storms. Nat. Geosci.
**2013**, 6, 534–539. [Google Scholar] [CrossRef] - Murikami, H.; Delworth, T.L.; Cooke, W.F.; Zhao, M.; Xiang, B.; Hsu, P.C. Detected climatic change in global distribution of tropical cyclones. Proc. Natl. Acad. Sci. USA
**2020**, 117, 10706–10714. [Google Scholar] [CrossRef] [PubMed] - Yan, X.; Zhang, R.; Knutson, T. The role of Atlantic overturning circulation in the recent decline of Atlantic major hurricane frequency. Nat. Commun.
**2017**, 8, 1695. [Google Scholar] [CrossRef] [PubMed][Green Version] - Pausata, F.S.; Camargo, S.J. Tropical cyclone activity affected by volcanically induced ITCZ shifts. Proc. Natl. Acad. Sci. USA
**2019**, 116, 7732–7737. [Google Scholar] [CrossRef] [PubMed][Green Version]

**Figure 1.**(

**a**) shows numbers of cat15 (red), cat12 (black) and cat35 (blue) fixes from the ADT-HURSAT dataset, and (

**b**) shows the ratio of cat35 to cat15 fixes (major hurricane (MH) ratio, black time series), with a fitted linear trend (black line), with lines showing 95% confidence limits (red). The linear trend is fitted using ordinary least squares (OLS) regression. The trend change is 7 percentage points over 39 years, and is significant with a p-value of 0.01. In the calculations, 1980 is treated as a missing value, and in the graph it is filled in using linear interpolation between 1979 and 1981.

**Figure 2.**(

**a**) shows numbers of cat35 fixes (black time series) (as in Figure 1a), with a linear trend (black line), and 95% confidence intervals trend (red lines). (

**b**) shows cat12 in the same format; (

**c**) shows cat 15 in the same format, and (

**d**) shows the ratio of cat35 to cat12 in the same format.

**Figure 3.**(

**a**) shows the number of cat1 fixes (black time series), with a linear trend (black line), and lines showing 95% confidences on the trend slope; (

**b**–

**e**) show cat2, cat3, cat4 and cat5 fixes in the same format; (

**f**) shows time series of all five categories: cat1 (black), cat2 (red), cat3 (blue), cat4 (green), cat5 (purple).

**Table 1.**Trend slopes and significance levels for global trends in numbers of tropical cyclone fixes. The first column (Trend) shows trend slope values expressed as numbers of fixes per year. The second column (Significance) shows the p-value for significance of the trend.

Trend | Significance | |
---|---|---|

cat1 | −1.95 | 0.004 |

cat2 | −0.20 | 0.54 |

cat3 | 0.28 | 0.45 |

cat4 | 0.08 | 0.86 |

cat5 | −0.14 | 0.18 |

cat12 | −2.15 | 0.02 |

cat15 | −1.94 | 0.23 |

cat35 | 0.21 | 0.79 |

cat1 | cat2 | cat3 | cat4 | cat5 | Total | |
---|---|---|---|---|---|---|

NA | −12 | −12 | 31 | 23 | 3 | 33 |

EP | 32 | 6 | −7 | −7 | −1 | 23 |

WP | 18 | 5 | 2 | −16 | −12 | −3 |

SP | 23 | 8 | −10 | −5 | −1 | 15 |

SI | 14 | 3 | 5 | 10 | 1 | 33 |

NI | 2 | −1 | 0 | 0 | 0 | 1 |

total | 77 | 9 | 21 | 5 | −10 | 102 |

cat1 | cat2 | cat3 | cat4 | cat5 | Total | |
---|---|---|---|---|---|---|

NA | 12 | 12 | 17 | 13 | 1 | 55 |

EP | −32 | −6 | −4 | −4 | −1 | −47 |

WP | −18 | −4 | 1 | −9 | −6 | −36 |

SP | −22 | −8 | −6 | −3 | 0 | −39 |

SI | −14 | −3 | 3 | 6 | 1 | −7 |

NI | −2 | 1 | 0 | 0 | 0 | −1 |

total | −76 | −8 | 11 | 3 | −5 | −75 |

**Table 4.**Fitted trends for various cat35/cat15 fixes ratios. The first row (Trend) shows trend values expressed as change in the ratio over 39 years. The second row (Significance) shows the p-value for significance of the trend. The first column (ALL) is for all regions combined. The second column (-NA) is for all regions excluding the North Atlantic.

ALL | -NA | NA | EP | WP | SP | SI | NI | |
---|---|---|---|---|---|---|---|---|

Trend | 0.07 | 0.05 | 0.31 | 0.04 | 0.00 | 0.12 | 0.23 | −0.04 |

Significance | 0.0099 | 0.0910 | 0.0036 | 0.4782 | 0.9735 | 0.1576 | 0.0003 | 0.7083 |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Jewson, S.; Lewis, N.
Statistical Decomposition of the Recent Increase in the Intensity of Tropical Storms. *Oceans* **2020**, *1*, 311-325.
https://doi.org/10.3390/oceans1040021

**AMA Style**

Jewson S, Lewis N.
Statistical Decomposition of the Recent Increase in the Intensity of Tropical Storms. *Oceans*. 2020; 1(4):311-325.
https://doi.org/10.3390/oceans1040021

**Chicago/Turabian Style**

Jewson, Stephen, and Nicholas Lewis.
2020. "Statistical Decomposition of the Recent Increase in the Intensity of Tropical Storms" *Oceans* 1, no. 4: 311-325.
https://doi.org/10.3390/oceans1040021