Recent evidence has suggested that an increase in global TC precipitation rates is detectable in historical rain data archives48, though these studies are limited in spatiotemporal extent. To investigate if these trends exist in the climate scale or are rather the result of natural variability in sub-climate timescales, the mean and extreme values of TC precipitation are considered at a yearly basis over a 40-year period. We ask the question: “When considering the distribution of rainfall rate values, do we observe an increase in the mean consistent with expectations of Clausius–Clapeyron to super Clausius-Clapeyron scaling—expected in kilometer- and sub-daily scale processes49 —and do we see an elongation of the tail indicating greater extreme values?” To probe this question, the mean (\(⟨R⟩\)), 90th (\({R}_{90}\)), and 99th (\({R}_{99}\)) percentile of precipitation rates over the life cycle of a TC (rainy grid cells only) were calculated, organized by basin and intensity classifications, then averaged annually. To overcome the fundamental variability in TC events, values are smoothed over a five-year window and trends were tested using robust linear fitting, which helps limit the influence of outliers. The results of this analysis can be seen in Fig. 1, where trends in precipitation rates are reported as annual percent changes (APC) measured by the fitted model. In general, most regions, categories, and precipitation rates are seeing significantly increasing trends over the study period. These are reflected by significant increases globally across intensity basins, with average APC in \(⟨R⟩\), \({R}_{90}\), and \({R}_{99}\) of 0.32 ± 0.03%/year, 0.42 ± 0.04%/year and 0.41 ± 0.04%/year, respectively, in the All TCs category and between 0.42–0.70%/year in Strong and Very Strong TCs. However, when considering the “global” category, it is important to note that WP basin has > 30% of global TC occurrences of all basins (Table 1) and will inevitably influence trends greater than other basins—for example the NI basin, which includes less than 10% of TC events per year. This influence will also exist in the results presented in Figs. 2, 3, and 4.
Table S1
Tropical cyclone (TC) occurrences from 1980–2019 by basin and intensity. Percentages are per intensity classification (i.e. all percentages in the All Tropical Cyclone (ATC) category add to 100%) and the numbers in parenthesis are the totals. WTC, STC, and VSTC stand for Weak, Strong, and Very Strong TCs, respectively.
|
EP
|
NA
|
NI
|
SI
|
SP
|
WP
|
ATC
|
19% (827)
|
14% (618)
|
7% (311)
|
17% (745)
|
11% (474)
|
31% (1,340)
|
WTC
|
18% (433)
|
14% (356)
|
10% (249)
|
17% (427)
|
12% (287)
|
29% (712)
|
STC
|
21% (260)
|
16% (197)
|
3% (42)
|
18% (215)
|
11% (140)
|
30% (364)
|
VSTC
|
21% (134)
|
10% (65)
|
3% (20)
|
17% (108)
|
8% (54)
|
41% (265)
|
Given the consistent increases in precipitation rates, one may assume that that the mean volume of precipitated water per event has also increased. To determine this, TC volume is measured per storm and averaged yearly. TC volumes are calculated by multiplying each rainy grid cell by the grid cell area (latitude-dependent), then summing over the storm’s lifetime (see the Methods section for specifics on grid size and TC extent). The time series of mean annual TC precipitation volumes (\(⟨V⟩\), hereafter referred to as “volume(s)”) are plotted in Fig. 2, separated by category and basin. Again, five-year smoothing was applied to the time series and trends were tested using robust linear fitting. The time series at the top-right of Fig. 2 provide a summarizing view of global \(⟨V⟩\) trends: increasing at a rate of 0.28 ± 0.10% per year, though not without a strong oscillation in the time series. Indeed, most categories have either seen an increasing trend or no trend, though no basin has seen increasing trends across all intensity categories. Of particular interest are the following results: 1) A sharp increase in volumes in the SI (1.26 ± 0.05%) and NA (1.53 ± 0.06%) basins driven by Weak and Strong TCs; 2) A modest increase in volumes from Very Strong TCs in the SP basin (APC = 0.87 ± 0.18%); and 3) A negative trend in the volumes of Strong and Very Strong TCs in the WP basin of -0.44 ± 0.16% and − 0.42 ± 0.12%, respectively, the latter of which seemingly causing a negative trend in global Very Strong TC volumes (APC=-0.40 ± 0.09%). Overall, Weak TCs have seen increases in four of six basins, Strong TCs have changed in three of six, with two positive trends and the other negative, and Very Strong TCs have an increasing and decreasing trend in one basin each. This culminates into the All TCs category having increased in three out of six basins.
Trends in \(⟨V⟩\) could be interpreted as being a result of increases in TC duration and not necessarily precipitation intensity. Increases in global SST values have been linked to slowing of TC translation speeds50 and slower decay following landfall50 both increasing TC duration values. We normalize the \(⟨V⟩\) quantity by its duration in hours to produce hourly \(⟨V⟩\), hereafter symbolized as \(⟨hV⟩\). The results of this analysis, shown in Fig. 3, is that across categories there have only been increasing or non-significant (no) trends. Only one basin/intensity pair has a no trend in \(⟨hV⟩\) and a positive trend in \(⟨V⟩\), which indicates an increase in volume from an increase in duration rather than changes in precipitation. All other time series with positive trends in \(⟨V⟩\) (max: 1.24%±0.06 per year) have also experienced an increase in \(⟨hV⟩\). Simply put, in most cases an increase in TC precipitation volume is at least partially a result of increasing precipitation rates and not solely duration. Increasing trends in Very Strong TC \(⟨hV⟩\) in the global and WP categories hint that the corresponding decreasing trends in \(⟨V⟩\) are a result of a decrease in event duration. Further analysis into TC durations confirms these results, with decreasing trends of -0.56 ± 0.11% and − 0.32 ± 0.09% for WP and global Very Strong TC duration, respectively. Other notable negative trends in duration (Table 2) occur in global Strong TCs (-0.51 ± 0.08%/year) and All TCs in the NI (-0.43 ± 0.16%/year) and WP (-0.44 ± 0.12%/year) basins, while increasing trends occur in NA (0.78 ± 0.07%/year) and SI (0.53 ± 0.08%/year). These results are calculated solely from track data.
Table S2
The annual percent change in TC duration over the 1980–2019 period. Bolded numbers indicate statistically significant trends at α = 0.05.
|
EP
|
NA
|
NI
|
SI
|
SP
|
WP
|
Global
|
ATC
|
0.28 + 0.13%
|
0.78 + 0.07%
|
-0.43 ± 0.16%
|
0.53 + 0.08%
|
0.12 ± 0.14%
|
-0.44 ± 0.12%
|
-0.00 ± 0.08%
|
WTC
|
0.66 ± 0.08%
|
0.71 ± 0.07%
|
-0.87 ± 0.19%
|
0.86 ± 0.04%
|
-0.30 ± 0.19%
|
-0.29 ± 0.16%
|
0.02 ± 0.08%
|
STC
|
0.30 ± 0.13%
|
0.21 ± 0.12%
|
0.03 ± 0.39%
|
-0.32 ± 0.07%
|
-0.79 + 0.33%
|
1.23 ± 0.14%
|
-0.51 ± 0.08%
|
VSTC
|
0.07 ± 0.16%
|
0.46 ± 0.20%
|
-0.03 ± 0.14%
|
-0.46 ± 0.16%
|
0.60 ± 0.14%
|
-0.56 ± 0.11%
|
-0.32 ± 0.09%
|
Considering the observed increases in rainfall rates and volumes, we now consider if changes are occurring over human population centers, i.e. do we see greater TC precipitation volumes over land? To calculate this metric (\(\sum {V}_{land}\), where land indicates the landfalling component of TC volume and the summation sign indicates a yearly accumulation rather than an average), precipitation volumes are decomposed into their fractions over oceans and land and accumulated annually. As can be seen in Fig. 4a, increasing trends are the most common of all trends, where they are recorded in 16 out of 28 (57%) basins, while only Weak TCs in the WP and Strong TCs in the SP have seen decreasing trends. Most positive trends can be seen in the Very Strong TC category, where they’re recorded in four out of six basins and globally (2.08 ± 0.05%/year). Weak TCs are also changing in four out of six basins, but only three of the trends are positive. Still, these increases are enough for the global trend to be positive (0.30 ± 0.12%/year). Strong TCs are increasing in two basins but aren’t enough to produce a significant trend in the global category. Overall, \(\sum {V}_{land}\) in All TCs is increasing globally (0.70 ± 0.11%/year) and in four out of six basins.
As values of \(\sum {V}_{land}\) are inevitably linked to the number of landfalling TCs in a year and the duration they remain over land, trends in \(\sum {V}_{land}\) were correlated against those of TC duration over land and landfalling frequency (Table 3, 4). Consistent with expectations, most categories correlate with landfalling frequency at α = 0.05: this is true in 20 out of 28 (71%) of the time series and 100% of the Very Strong TC timeseries. Likewise, \(\sum {TCV}_{y,land}\) correlates with duration over land in 17 of 28 (61%) of the time series. Curiously, time series of global All TCs, Weak TCs, and Strong TCs are uncorrelated with TC frequency.
Table 3
Correlation coefficient of \(\sum {V}_{land}\) and annual frequency of landfalling TCs. Bolded numbers indicate statistically significant trends at α = 0.05.
|
EP
|
NA
|
NI
|
SI
|
SP
|
WP
|
Global
|
ATC
|
0.50
|
0.43
|
0.84
|
0.71
|
0.17
|
0.44
|
-0.06
|
WTC
|
0.76
|
0.06
|
0.90
|
0.29
|
0.35
|
0.67
|
-0.23
|
STC
|
0.20
|
0.22
|
0.67
|
0.73
|
0.58
|
0.38
|
0.13
|
VSTC
|
0.61
|
0.87
|
0.68
|
0.82
|
0.48
|
0.57
|
0.73
|
Table 4
Correlation coefficient of \(\sum {V}_{land}\) and TC duration over land. Bolded numbers indicate statistically significant trends at α = 0.05.
|
EP
|
NA
|
NI
|
SI
|
SP
|
WP
|
Global
|
ATC
|
0.60
|
0.85
|
-0.30
|
0.33
|
0.46
|
0.39
|
0.78
|
WTC
|
0.23
|
0.67
|
-0.04
|
0.46
|
0.57
|
0.39
|
0.70
|
STC
|
0.34
|
0.78
|
0.24
|
0.27
|
0.43
|
0.56
|
0.79
|
VSTC
|
0.21
|
0.25
|
0.24
|
0.23
|
0.27
|
0.07
|
0.74
|
To investigate how precipitation intensity over land—independent of frequency and duration—has changed over time, a new variable was calculated: hourly mean precipitation volume over land (\(⟨h{V}_{land}⟩\)), which in essence is \(\sum {V}_{land}\) normalized by its temporal and frequency components. Or it can also be thought of as the over land component of \(⟨hV⟩\). In the simplest terms, it is the average precipitation volume a TC drops over land in one hour. As shown in Fig. 4b, most basins—as well as the global category—have seen an increase in \(⟨h{V}_{land}⟩\) over the period, indicating an increase in precipitation volumes over land independent of frequency or duration, except for in the SI basin where \(\sum {V}_{land}\) is also stagnant. Increases are significant, with APC of 1.90 ± 0.01%/year, 1.16 ± 0.12%/year, 1.96 ± 0.02%/year, and 1.01 ± 0.05%/year recorded in the EP, NA, SP, and globally, respectively.
The trends in \(\sum {V}_{land}\) presented in Fig. 4a can be considered concurrently with Fig. 5, where trends in \(\sum {V}_{land}\) are tested spatially over major river basins using the Kendall rank correlation metric (useful for basins for years with no activity; see Materials & Methods for more information)—though only for the “All TCs” category. Basins are only considered if over half the years on record include a nonzero value of \(\sum {V}_{land}\), which meant that five-year smoothing could not be used as it was for Figs. 1–4. In summary, every major river watershed recording significant changes are located within the North American continent and the Indian subcontinent and have witnessed increases in \(\sum {V}_{land}\). This helps illustrate the positive trends in \(\sum {V}_{land}\) uncovered in Fig. 4 for the NA, EP, and NI basins. East Asia and Australia have seen mixed insignificant trends, consistent with the no trend results for the WP and SI basin, though at odds with the increasing trend found for the SP basin. This can be explained by differences in the trend detection tool. For reference, Figure S2a and S2b shows where TC precipitation is located and its contribution to climatology. Between the insignificant trends detected in Fig. 5a and the lack of appreciable rainfall from TCs shown in Figure S2, the trends in the Arabian Peninsula, East Africa, South America, and the Maritime Continent basins are ignored in this figure and in further discussion.
The variation in trends of TC precipitation is significant: one cannot extrapolate changes in activity in one basin and apply it to another, even for basins close in proximity (example: SI and SP). In order to give an overview of what changes in TC activity have occurred within each basin, we use the following criteria to grade changes of the \(⟨R⟩\), \({R}_{90}\), and \({R}_{99}\), \(⟨V⟩\), and \(\sum {V}_{land}\) variables in Table 5:
-
No changes: >50% of the category is not seeing a significant change.
-
Mixed changes: ≥50% of the category is seeing a significant change, but the trends are mixed between increasing and decreasing.
-
Significantly increased: ≥50% and < 100% of the category is seeing a significant positive change.
-
Very significantly increased: 100% of the category has seen a significant positive change.
Percentages are calculated based on the number of intensity categories out of four (All, Weak, Strong, and Very Strong) that are seeing a significant trend for each variable. Though not independent of the other categories, the All TCs classification is included in the grading to ensure the Strong and Very Strong categories are not overrepresented against Weak TCs, which are the most frequent TC grade. In most cases, its inclusion does not change results. Note that the \(⟨hV⟩\) and \(⟨h{V}_{land}⟩\)variables are not considered, as they are closely related to TC precipitation rates.