Abstract
Theory1 and numerical modelling2 suggest that tropical cyclones (TCs) will strengthen with rising ocean temperatures. Even though models have reached broad agreement on projected TC intensification3,4,5, observed trends in TC intensity remain inconclusive and under active debate6,7,8,9,10 in all ocean basins except the North Atlantic, where aircraft reconnaissance data greatly reduce uncertainties11. The conventional satellite-based estimates are not accurate enough to ascertain the trend in TC intensity6,11, suffering from contamination by heavy rain, clouds, breaking waves and spray12. Here we show that weak TCs (that is, tropical storms to category-1 TCs based on the Saffir–Simpson scale) have intensified in all ocean basins during the period 1991–2020, based on huge amounts of highly accurate ocean current data derived from surface drifters. These drifters have submerged ‘holy sock’ drogues at 15 m depth to reduce biases induced by processes at the air–sea interface and thereby accurately measure near-surface currents, even under the most destructive TCs. The ocean current speeds show a robust upward trend of ~4.0 cm s−1 per decade globally, corresponding to a positive trend of 1.8 m s−1 per decade in the TC intensity. Our analysis further indicates that globally TCs have strengthened across the entirety of the intensity distribution. These results serve as a historical baseline that is crucial for assessing model physics, simulations and projections given the failure of state-of-the-art climate models in fully replicating these trends13.
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Data availability
The 6-hourly positions and upper-ocean current velocities of drifters are obtained from https://www.aoml.noaa.gov/phod/gdp/interpolated/data/all.php. TC occurrence also with 6 h temporal resolution is acquired from the best track data from the Joint Typhoon Warning Center (https://www.metoc.navy.mil/jtwc/jtwc.html?best-tracks) for the Western Pacific Ocean, the Indian Ocean and the Southern Hemisphere, and the National Hurricane Center and Central Pacific Hurricane Center (https://www.nhc.noaa.gov/data/#hudat) for the Atlantic and Northeast and Central Pacific Oceans. Daily SST is from the NOAA 1/4° Optimum Interpolation Sea Surface Temperature (OISST), and it is downloaded from https://www.ncei.noaa.gov/data/sea-surface-temperature-optimum-interpolation/v2.1/access/avhrr/). The hourly current and wind data from the TAO/TRITON, RAMA and PIRATA buoy arrays are downloaded from https://www.pmel.noaa.gov/tao/drupal/disdel/. The JRA-55 Reanalysis dataset is downloaded from https://jra.kishou.go.jp/JRA-55/index_en.html. The IAP monthly ocean temperature analysis data are downloaded from ftp://www.ocean.iap.ac.cn/cheng/CZ16_v3_IAP_Temperature _gridded_1month_netcdf. Source data are provided with this paper.
Code availability
Analysis and figure generation were performed using MATLAB. The code and scripts of the two main methods and four figures in the paper are available from Zenodo: https://doi.org/10.5281/zenodo.7013352.
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Acknowledgements
G.W. and L.W. were supported by the National Key R&D Program of China (grant no. 2019YFC1510100) and the National Natural Science Foundation of China (grant no. 41976003).
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G.W. initiated the idea, designed the study and interpreted the results. L.W. processed the data and performed the analyses. All the authors developed the idea and wrote the paper. W.M. and S.-P.X. discussed the results and commented on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Buoy current and wind observations under TC conditions.
a, Relationship of observed wind speed and ageostrophic current speed considering latitudes under TC conditions (blue dots). Linear regression (red line) is also plotted between the binned averages of the observed wind speeds and current speeds (red dots), with \({V}_{0}\) in the regression equation being coupling between the ageostrophic current speed \(V\) and latitude \(\phi \) as \({V}_{0}=V\times \sqrt{{\rm{\sin }}\left|\phi \right|}\), and the slope of the fitted line (along with the 95% margin of error and the p-value of the t-test) is reported in the bottom right corner. The binned averages of the current speeds are calculated from the ageostrophic current speeds in every 0.1 m s−1 bin, and the corresponding observed wind speeds are taken to calculate the binned averages of the observed wind speeds. b, Comparison of the binned averages of the theoretical wind speeds estimated from the ageostrophic currents and observed wind speeds (red dots). The binned averages of the observed wind speeds are calculated from the observed wind speeds in every 1 m s−1 bin, and the corresponding theoretical wind speeds are taken to calculate the binned averages of the theoretical wind speeds. c, Distribution of buoys with current and wind observations in the TC-coordinate system (dots), with the purple dots indicating that the maximum wind speeds of the corresponding TCs are larger than 35 kt. The results are based on observations from the TAO/TRITON, RAMA, and PIRATA buoy arrays.
Extended Data Fig. 2 Winds of weak TCs derived from drifter current measurements based on typhoon wind field model of Batts.
a, The difference of spatial averages between the theoretical wind fields and composite wind fields for each five-consecutive-year period from 1991 to 2020. The length of the error bar for each period is twice the standard deviation divided by the square root of the effective number of observations during that period (i.e., twice the standard error of the mean). The effective number of observations is approximated as the number of observations that are separated by at least 500 km in distance or at least 10 days in time. b, The theoretical and composite wind fields for each five-consecutive-year period during 1991–2020, and differences between the two winds (theoretical winds minus composite winds). c, The composite wind fields for the periods of 1991–2005 and 2006–2020, and change of the wind fields between the two periods (2006–2020 minus 1991–2005).
Extended Data Fig. 3 PDFs of the maximum sustained wind under global weak TCs.
for 1991–2005 (blue) and 2006–2020 (red) derived from (a) all drifter observations, and (b) the same as (a) but with the results for 2006–2020 obtained with a random sampling method. Specifically, the random sampling procedure was repeated 10,000 times. Each time, 25,538 observations (i.e., the number available for the period of 1991–2005) were drawn from the entire 59,643 observations during 2006–2020, and a PDF of TC intensities was computed. The red curve shows the average PDF using the obtained 10,000 individual PDFs, with the error bars indicating the standard deviations of the 10,000 realizations. Bin size is 3 m s−1.
Extended Data Fig. 4 Drifter data quantity statistics and mean near-surface ocean current fields under weak TCs.
(a1, b1, c1, d1) are numbers of drifter records within \({r < 7R}_{{\rm{\max }}}\) for the period of 1991–2005, and (a2, b2, c2, d2) are the same as (a1, b1, c1, d1) but for 2006–2020, respectively. Global mean current fields, (e1, f1, g1) are for 1991–2005 and (e2, f2, g2) are for 2006–2020, and (e3, f3, g3) are changes of the mean currents between the two periods (2006–2020 minus 1991–2005). a1, a2 are for weak TCs range from 17 to 42 m s−1. b1, b2, e1-e3 are for weak TCs range from 17 to 32 m s−1, c1, c2, f1-f3 are for weak TCs whose LMI is 17 to 32 m s−1, d1, d2, g1-g3 are for weak TCs whose LMI is 17 to 42 m s−1, respectively.
Extended Data Fig. 5 Evolution of the drifter measured ocean current speeds within r < 7Rmax under weak TCs.
from 1991 to 2020. The color shading and solid lines represent the current speeds at different distances from the corresponding TC centers along the cross-track direction, and the dashed lines are the associated current speed trends. The lower and upper limits of all the lines are 18 and 45 cm s−1, respectively.
Extended Data Fig. 6 Translation speed (m s−1) of weak TCs during the period of 1991–2020.
The red and blue lines are the average during the periods of 1991–2005 and 2006–2020, respectively.
Extended Data Fig. 7 Linear trends in the large-scale atmospheric environment and upper-ocean thermal condition during 1991–2020.
a, Potential intensity. b, Relative humidity at 600 hPa. c, Upper-ocean thermal stratification (represented by the difference of water temperature between ocean surface and 75-m depth, i.e., SST - T75m). d, Mixed layer depth (MLD, depth where the density change reaches a threshold value of 0.03 kg m−3 relative to the surface). The results in a and b are based on the JRA-55 reanalysis, and in c and d are calculated using the IAP ocean analysis data. Red dots denote regions where the linear trend is significant at the 0.05 level. Note this figure is produced by MATLAB.
Extended Data Fig. 8 Evolution of the near-surface ocean current speeds derived from drifters under TCs.
for individual basins and the globe. a is for weak TCs range from 17 to 32 m s−1, b is for weak TCs whose LMI is 17 to 32 m s−1, c is the same as b but LMI is 17 to 42 m s−1, and d is based on drifter data under strong TCs (Category-2–5) over NWP. The dashed lines indicate the fitted linear trends of the curves, and the estimated trends (along with the 95% margin of errors) and the p-values of the t-test for the trends are also reported. In each panel, the length of the error bar for each year is twice the standard deviation divided by the square root of the effective number of observations in that year. Discontinuity points along the curves in b and c represent missing values due to insufficient data.
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Wang, G., Wu, L., Mei, W. et al. Ocean currents show global intensification of weak tropical cyclones. Nature 611, 496–500 (2022). https://doi.org/10.1038/s41586-022-05326-4
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DOI: https://doi.org/10.1038/s41586-022-05326-4
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