Introduction

Tropical cyclones (TCs) are critical for transferring moisture from the tropics to mid-latitudes and high latitudes1,2 and from the ocean to land3,4, thereby influencing weather patterns in tropical and subtropical regions worldwide [e.g. ref. 5] and playing a crucial role in the atmospheric branch of the global hydrological cycle3,4,6,7. While relevant in these roles, the contribution of TCs to poleward water vapour transport from tropical to extratropical regions across the mid-latitudes globally is substantially smaller than that of atmospheric rivers8. Nonetheless, intense convective activity during TCs induces a vast amount of water vapour that is redistributed across the atmosphere and can produce notable precipitation totals in terrestrial regions6,7, providing moisture to areas that may experience dry conditions9. However, impacts of TCs are associated with substantial environmental disturbances as well as socioeconomic and human losses caused by extreme compound events, such as strong winds, heavy rainfall, storm surges, flash flooding, and landslide10,11.

Atmospheric moisture availability is crucial for TC genesis and development12. Indeed, modelling and theoretical studies have demonstrated that an increase (decrease) in mid-level water vapour content enhances (inhibits) TC intensification13,14. Furthermore, understanding the origin of moisture for precipitation produced by TCs is key to significantly assisting in risk analysis and mitigating the often strong associated impacts. Recently, Pérez-Alarcón et al.15,16,17 revealed that precipitating moisture was predominantly highest from sources close to the TC positions. Likewise, large-scale atmospheric circulation patterns in each region drove the moisture towards the TC locations.

Improving our knowledge of moisture transport is crucial for investigating extreme precipitation events18,19. Anomalous moisture transport caused by changes in large-scale circulation is a key factor in long-term changes in extreme precipitation20. Thus, TCs contribute significantly to onshore moisture transport. Because moisture and heat fluxes from the warm sea surface are the principal fuels for TCs13, previous studies have been mainly focused on the TC water budget21,22,23,24,25,26, the origin of moisture for TCs precipitation15,16,17,27, the contribution of TCs to monthly and annual precipitation totals in tropical and subtropical latitudes6,7 or the TC-related onshore moisture transport3,4. For example, moisture convergence is the main contributor to TC rainfall24,26; TCs are responsible for approximately 14–19% of the net onshore moisture driven towards North America3 and their precipitation represents a significant fraction of the annual precipitation totals at regional, continental and even global scales4,6. Additionally, part of the moisture gained by TCs from the ocean evaporation is brought about by the storm itself22, so TCs can induce anomalous moisture fluxes towards their locations. However, few studies [e.g. refs. 28,29,30,31] have investigated anomalies in moisture transport by TCs, which are necessary to understand the role of TCs in regional and global hydrological cycles. Extreme precipitation and moisture flux are positively correlated3. On this basis, the spatial pattern of anomalous moisture convergence tends to agree with the regions of anomalous cyclonic circulation in the tropical northeastern Pacific Ocean28. In particular, a climatological analysis of vertically integrated water vapour transport from hurricanes Matthew (2016) and Florence (2018) revealed the critical role of anomalous moisture flow from the Atlantic Ocean in producing extreme rainfall in North and South Carolina30.

Based on these previous studies and large-scale water and energy budgets, it remains unclear what happens with atmospheric moisture before it arrives at TC locations, which areas experience more evaporation than precipitation during TCs and vice versa, and whether the long-term tendencies of these anomalous moisture fluxes are linked to global warming. Therefore, this knowledge gap and the projected increase in atmospheric water vapour content at a rate of ~6–7% per degree of sea surface warming (on the basis of the Clausius–Clapeyron equation)32,33,34 warrant further examination of the anomalous moisture fluxes induced by TCs and their impacts on regional and global hydrological cycles.

The extensive application of Lagrangian moisture tracking models35,36 to investigate moisture source-sink relationships has been adopted in the last decade for examining anomalous moisture transport37,38 and TCs15,16,17,27. Here, we performed a global analysis to identify areas where TCs induced anomalous surface freshwater flux (TC-induced evaporation minus TC-induced precipitation) between 1980 and 2018. We focused on quantifying the anomalies in moisture fluxes induced by TCs at the regional and global scales by applying a Lagrangian tracking method using global outputs from the FLEXible PARTicle dispersion model (FLEXPART)39. More details on the FLEXPART simulations and moisture tracking approach are available in the ‘Methods’ section.

Results

Anomalous surface freshwater flux

TCs require substantial moisture availability to form and intensify12,13 and consequently produce precipitation along their trajectories27,40,41,42. On this basis, TCs should induce anomalous moisture fluxes. Therefore, we examined the anomalies in TC-induced evaporation-precipitation patterns by applying a Lagrangian moisture tracking approach43,44 to the pathways of atmospheric parcels within the outer radius of TCs from the global outputs of the FLEXPART model39 fed by the ERA-Interim reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF)45. For each TC position, we first computed the TC-related surface freshwater flux (E-P) pattern (see the ‘Methods’ section) and then estimated the annual surface freshwater flux and the corresponding anomalies over the study period for the same TC dates (Fig. 1).

Fig. 1: Annual TC-related surface freshwater flux.
figure 1

a Surface freshwater flux (in mm) linked to tropical cyclones (TCs). b Climatological surface freshwater fluxes for the same dates of TCs. Greenish shade represents areas where the evaporation exceeds precipitation (positive surface freshwater flux) and bluish shade areas where the precipitation is higher than the evaporation (negative surface freshwater flux). c Significative surface freshwater flux anomalies (computing (a) − (b)) during TCs. The level of significance at 95% was calculated using the Wilcoxon signed-rank test. d Surface freshwater fluxes anomalies in (%) during TCs. The percentage was computed as the difference between the TC-related surface freshwater flux and the climatological value, divided by the latter. The contour lines are schematic representations of areas of positive and negative surface freshwater flux anomalies and the corresponding change during TCs, and the arrows illustrate the changes in the surface freshwater flux during TCs relative to the climatological value. Supplementary Table 1 shows a comprehensive description of all annotations shown in panel (d). The analysis was performed using the FLEXible PARTicle dispersion model (FLEXPART) outputs fed by the European Centre for Medium-Range Weather Forecast ERA-Interim reanalysis from 1980 to 2018.

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Contrasting the spatial distribution of the annual surface freshwater flux linked to TCs (Fig. 1a) and the climatological pattern for the same TC time step during the study period (Fig. 1b), TCs generally induced more evaporation and precipitation than the climatological values in the same areas (also in agreement with the annual surface freshwater flux field, Supplementary Fig. 1). As expected, during TC days, the widespread increase in evaporation (~>50%, Fig. 1d) implies anomalous positive surface freshwater flux areas larger than negative ones over the tropics because the increased precipitation associated with TCs is more regionally concentrated (Fig. 1c). The outstanding value of the large maximum of positive freshwater flux anomalies resulted from the relevance of TCs to moisture transport. These highly evaporative regions agree with previous findings that focused on the origin of moisture for the precipitation of TCs15,16,17. Meanwhile, the pattern of sink regions for TC days coincided with climatological areas of higher precipitation than evaporation (Fig. 1a and Supplementary Fig. 1), namely the Intertropical Convergence Zone (ITCZ) in both hemispheres46, the Western North Pacific Monsoon trough region, and southeastern Asia47. The nuclei of change (~50–80%, Fig. 1d) in the negative surface freshwater flux anomalies are located in the western North Pacific Ocean (WNP) and northeast Pacific Ocean (NEPAC), which are intrinsically related to the higher TC frequency in these basins, accounting for ~31%48 and ~20%49 of the annual global TCs, respectively. Interestingly, based on a simple inspection of Fig. 1c, d, positive surface freshwater flux anomalies over southern Asia indicate a reduction in precipitation over this region during TCs by comparing analogous dates during the study period. Overall, the spatial pattern of the TC-related surface freshwater flux (Fig. 1 and Supplemenyary Fig. 2) revealed that the impact of TCs on the hydrological cycle is more significant in the Northern Hemisphere than in the Southern Hemisphere, which is likely related to the substantially larger number of TCs formed in the Northern Hemisphere (Supplementary Figs. 3 and 4).

Due to the impact of El Niño-Southern Oscillation (ENSO) on the basin-scale TC frequency, we also examined the possible influence of ENSO on the anomalous moisture transport induced by TCs from 1980 to 2018 (Fig. 2). The negative surface freshwater flux (precipitation>evaporation) induced by TCs was noticeably higher in absolute value during El Niño years than during La Niña years over southern Asia and along the average position of the ITCZ in the Pacific and eastern Indian Oceans, and lower over the Gulf of Mexico, southern United States and the Atlantic and western Indian Oceans ITCZ region during TC season (dark blue shaded area in Fig. 2d). Similarly, the positive surface freshwater flux during El Niño exceeded that during La Niña over large sectors of both the northern and southern Pacific Ocean, Wharton and Perth basins, Arabian Sea, southern Bay of Bengal, Caribbean Sea, and western North Atlantic (dark green areas in Fig. 2d). Meanwhile, it decreased in the Atlantic Ocean north and south of the ITCZ, central Indian Ocean south of the ITCZ, and Somali Basin in the western Indian Ocean (lime-green areas in Fig. 2d).

Fig. 2: Influence of ENSO on the annual TC-related surface freshwater flux.
figure 2

Surface freshwater flux (in mm), linked to tropical cyclones during a warm (El Niño) and b cold (La Niña) phases of ENSO. Greenish-shaded regions represent areas where evaporation exceeds precipitation (positive surface freshwater flux), whereas bluish regions indicate areas where precipitation is higher than evaporation (negative surface freshwater flux). c Significant differences in surface freshwater flux during El Niño and La Niña years (computing (a)−(b)). Significance was calculated at the 95% level using the Wilcoxon signed-rank test. d Schematic representation of areas of positive and negative surface freshwater flux differences (from (c)) and the corresponding change during El Niño years; arrows illustrate the changes in the surface freshwater flux during El Niño years relative to those during La Niña years. Supplementary Table 1 shows a comprehensive description of all annotations shown in (d). The analysis was performed using FLEXible PARTicle dispersion model (FLEXPART) outputs fed by the European Centre for Medium-Range Weather Forecast ERA-Interim reanalysis from 1980 to 2018.

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Surface freshwater flux trend

TCs have marked year-to-year variability at basin and global scales (Supplementary Fig. 4). Therefore, we also examined the annual trend in TC-related surface freshwater flux values from 1980 to 2018 (Fig. 3). By matching areas with statistically significant trends (p < 0.05) in the TC-related surface freshwater flux shown in Fig. 3 with the moisture source and sinks regions displayed in Fig. 1a, we detected an overall reduction of the TC-related surface freshwater flux across the globe, especially in the South Indian and Pacific Ocean basins. The sink regions over the eastern tropical Pacific Ocean and the Philippine Sea exhibited the highest decrease, dropping by up to 70–90 and 40–60 mm year−1, respectively. Meanwhile, the surface freshwater flux over source regions has similarly decreased over the last four decades, exhibiting the highest drop over the Wharton and Perth basins in the South Indian Ocean at a rate of ~40–90 mm year−1.

Fig. 3: Annual trend in TC-related surface freshwater flux.
figure 3

Filled areas show statistically significant surface freshwater flux tendencies (95% level of significance) between 1980 and 2018. A positive trend indicates a reduction in the surface freshwater flux over sink regions and negative values represent a decreasing trend in the surface freshwater flux over source regions. TCs sink and source regions are clearly identified in Fig. 1a.

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The sensitivity of TC-related surface freshwater flux to sea surface temperature (SST) is also investigated. The annual mean SST was computed from the Daily Optimum Interpolation Sea Surface Temperature dataset50 within the region of higher TC activity in each basin (see ‘Methods’ and Supplementary Fig. 5) for the dates of TC occurrence. By matching the spatial patterns of SST trends and the Spearman correlation coefficients between SST- and TC-related surface freshwater flux with the spatial distribution of TC-related surface freshwater flux trends (Fig. 3), two features stand out: SST exhibits a basin-wide statistically significant increasing trends and inversely correlates with the TC-related surface freshwater flux. It is worth noting that positive correlation coefficients coincide with sink regions (Fig. 1a), and thereby, TC-related surface freshwater flux decreases with increasing SST. The former feature provides evidence of global warming, reaching noticeable values of 0.035–0.04 °C year−1 in NEPAC and South Indian Ocean (SIO) basins (Supplementary Fig. 6a), and the second reveals a closer relationship between SST and TC-related surface freshwater flux. The sign of Spearman correlation coefficients coincides with that from TC-related surface freshwater flux trends (Supplementary Fig. 6b). Overall, an increase in SST leads to a decrease in TC-related surface freshwater flux, which suggests that TC-induced precipitation is evolving to become similar to TC-induced evaporation. To examine this hypothesis, we computed the differences between the annual time series of basin and global scales lifetime accumulated evaporation and precipitation within the TC outer radius from the ERA-Interim reanalysis and Multi-Source Weighted-Ensemble Precipitation version 2 database (MSWEP V2)51. While the lifetime accumulated TC-induced evaporation minus precipitation within the cyclone outer radius (Supplementary Fig. 7) significantly increases in North Atlantic ocean (NATL) in agreement with Hallam et al.52, it exhibits an overall decrease in the remaining basins and globally, being statistically significant for the South Pacific and WNP basins and at global scale. Although the lifetime accumulated precipitation (evaporation) is highly dependent on TC frequency (Supplementary Fig. 4) and duration (Supplementary Fig. 8), it is an important metric for the influence of TCs on global water budgets53. While TC lifetime significantly increases in NATL, it decreases in WNP. Globally we did not detect any statistically significant trend in TC lifetime, thus TC activity in WNP seems to have a modulating role on the influence of TCs on the global hydrological cycle. In fact, WNP achieves ~30% and ~35% of annual global TC frequency and 6-hourly track points, respectively. The global trends of the differences between lifetime accumulated TC-induced evaporation and precipitation within the outer radius highlight the marked influence of TC frequency in the TC-related freshwater flux, as discussed above.

The overall decreasing trend in the TC-related surface freshwater flux is also found by computing its annual value. During the last four decades we found decreasing trends for all basins, except the NEPAC (Supplementary Fig. 9), globally a statistically significant trend of −40 mm year−1 is found (Fig. 4). We have concluded that for a 1 °C warming of SST, the TC-related surface freshwater flux reduces by 86% compared to its 1980 value. On average, global mean SST has increased by ~0.02 °C year−1 since 1980 (Supplementary Fig. 10a) and is inversely correlated with the TC-related surface freshwater flux (r = −0.65). Similarly, average TC size increases by 6 km with 1 °C of SST warming and exhibits a statistically significant growth in the last four decades of 0.84 km year−1 (Supplementary Fig. 10b). Previously, Pérez-Alarcón et al.17 found a positive correlation between TC size and moisture uptake for TC precipitation. In line with this, the TC-related surface freshwater flux decreases with increasing TC outer radius by 8 mm year−1 (p < 0.1), indicating an overall increase in TC-related precipitation. Similarly, previous studies15,16,17 pointed out that strong TCs tend to gain more moisture for generating the associated precipitation than weak TCs. In fact, a stronger TC usually produces a higher rain rate54. Additionally, although we found a non-statistically significant relationship between TC-related surface freshwater flux and the accumulated cyclone energy (ACE) during the study period, it was statistically significant (1.18 mm day−1 10-4 kt−2, p < 0.05) after 1990, when global ACE significantly decreased (Supplementary Fig. 10d), supporting the global reduction in the TC-related surface freshwater flux. Overall, the annual average of the Oceanic Niño Index and the interannual variability of SST, frequency, size and lifetime accumulated TC-induced evaporation and precipitation explain ~80.5% of the annual variability of TC-related surface freshwater flux.

Fig. 4: Annual global TC-related surface freshwater flux.
figure 4

The green line denotes the annual global TC-related surface freshwater flux (SFWF) and the dashed red line shows its statistically significant (p<0.05) decreasing linear trend. The analysis was performed using FLEXible PARTicle dispersion model (FLEXPART) outputs fed by the European Centre for Medium-Range Weather Forecast ERA-Interim reanalysis from 1980 to 2018.

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Discussion

This study investigated and quantified anomalous moisture fluxes during TCs globally and, therefore, their impacts on the global hydrological cycle. By applying a Lagrangian moisture tracking method43,44 to many atmospheric parcel trajectories from the outputs of the Lagrangian FLEXPART model39, we computed the anomalies in the surface freshwater flux during the TCs from 1980 to 2018 and the grid-to-grid trend in the surface freshwater flux budget. Although the Lagrangian approach neglects liquid water and ice content in the atmosphere, the mixing of air parcels, and the evaporation of precipitating hydrometeors43,44, this work represents the current state-of-the-art on the impacts of TCs on the global hydrological cycle.

The regions with positive anomalous surface freshwater fluxes induced by TC (Fig. 1) mostly agreed with the main moisture sources for the precipitation associated with them for each major ocean basin15,16,17. It is worth noting that Pérez-Alarcón et al.15,16,17 identified some regions (e.g. Philippine Sea, South China Sea and eastern tropical Pacific Ocean) as moisture sources for the precipitation within the TCs outer radius that we identified here as sink regions. That means the evaporation from these regions contributes to the precipitation produced by TCs; however, in terms of the water budget, the TC-induced evaporation in these areas is lesser than the induced precipitation. Overall, TCs induced higher evaporation than the climatological value for analogous dates of TC occurrence and similarly induced precipitation above climatological values over the sink regions (negative surface freshwater flux). Despite the role of TCs in the annual rainfall amounts over Southern Asia6, we detected a statistically significant (p < 0.05) decrease in the absolute value of the negative surface freshwater flux in these regions during TCs on the same dates over the study period. This contradictory result underlines the fact that most TCs occur during the summer and autumn monsoon seasons, two periods with particularly high values of moisture transport from the ocean (and associated precipitation over land), independent of TCs occurrence. Previously, Chen et al.55 noted that TC activity and seasonal monsoon climate may contribute in opposite manners to total rainfall in southern Asia, leading to complex interannual variability. Most recently, Chen et al.56 detected that the early onset of the South China Sea summer monsoon after the 1990s favoured positive heating anomalies over South China, strengthening the East Asia Summer Monsoon and monsoon-related precipitation.

The spatial distribution of TC-related surface freshwater flux (Fig. 1 and Supplementary Fig. 2) reveals that TC frequency controls the impact of TCs on the global hydrological cycle. Large-scale and regional thermodynamic and dynamic conditions govern TC activity [e.g. ref. 57] and thus influence TC-related moisture transport patterns. The highest impact of TCs on the global water budget detected during the peak Northern Hemisphere TC activity in August and September (Supplementary Fig. 2) is probably closely related to the highest SSTs, which play a critical role in regulating global atmospheric circulation58, and the maximum northward equatorial position of tropical rain belts59. Although Sobel et al.60 emphasised that global heat or moisture budgets are not useful for explaining the global number of TCs, seasonal TC frequency largely controls TC-related contributions to the global hydrological cycle.

The overall influence of ENSO on the annual TC-related surface freshwater flux, as shown in Fig. 2, is linked to the impact of ENSO on TC frequency in each basin. In the Pacific Ocean, TC activity was enhanced during El Niño years61,62, whereas the NATL and Australian regions exhibited an overall reduction in the number of TCs60,62. The increase in anomalous moisture transport in the Pacific Ocean during El Niño events can also be attributed to convective anomalies over the western and central equatorial Pacific63. Conversely, the reduction in surface freshwater flux in the NATL basin is a response to the decrease in TC activity due to eastward displaced deep convection in the tropical Pacific, which enhances wind shear in the Atlantic Ocean62, the reduction in moist convection64 and the weakening of northeasterly trade in the tropical Atlantic Ocean65. Nonetheless, the warm phase of ENSO intensifies the Caribbean Low-Level jet66,67, explaining the intensification of the TC-related surface freshwater flux over the Caribbean Sea in the NATL basin (Fig. 2). El Niño also enhances the negative surface freshwater flux over South Asia owing to the abnormal integrated vapour transport associated with westerly winds from the Indian Ocean and northern to southern China68. It also correlates negatively with TC-induced precipitation in the southern US69, which explains the reduction in anomalous moisture flux in this region. The surface freshwater flux differences in the Indian Ocean between the warm and cold phases of ENSO were noticeably weaker than those in the remaining basins. El Niño slightly increased the moisture supply from the Arabian Sea and southern Bay of Bengal (Fig. 2); however, this response was not significant in terms of changes in TC frequency62. TCs activity over the SIO was also substantially influenced by ENSO, which enhanced (suppressed) the number of TCs west of 75°E during El Niño (La Niña) years and exhibited an opposite tendency east of 75°E60,62. The surface freshwater flux differences in the SIO exhibited eastward intensification and westward weakening (Fig. 2c, d), in line with the moisture source and transport patterns associated with TCs in that region16. Overall, the changes in large-scale mechanisms induced by ENSO influence the TC-related surface freshwater flux patterns in each basin by controlling the annual TC count. Nonetheless, more in-depth regional studies are needed to further investigate the modulatory role of ENSO in the anomalous surface freshwater flux in each basin.

Meanwhile, the overall statistically significant decreasing trend in the TC-related surface freshwater flux (Fig. 3) indicates that TC-induced precipitation has increased over the source areas or evaporation has increased over the sink regions, as revealed in Supplementary Fig. 9. Chauvin et al.26 pointed out that moisture support from evaporation decreases with increasing TC-rainfall intensities; however, this behaviour is not captured when evaluating the lifetime accumulated TC-induced evaporation and precipitation (Supplementary Fig. 7). Additionally, our results suggest that the reduction in the TC-related surface freshwater flux also results from a slight decrease in TC frequency (Supplementary Fig. 4) and lifetime (Supplementary Fig. 8) in the last two decades and to a significant rise in SST (Supplementary Fig. 10a) over the study period. If this relationship remains invariable, we can hypothesise that in a warmer climate the global TC-induced precipitation will be higher than the TC-induced evaporation. Therefore, the increasing low-level moisture availability at a rate of 6–7% per degree of SST rising using the Clausius–Clapeyron relationship32,33,34 in response to increasing evaporation due to global warming70 can compensate for the reduction in TC-induced evaporation in regions far from the TC circulation and support the additional moisture required for excess TC-induced precipitation. Previous studies [e.g. ref. 71] have detected tropical precipitation change rates higher than that predicted from the Clausius–Clapeyron relationship. They argued that these super Clausius–Clapeyron rates of changes (rates of change larger than those predicted according to the Clausius–Clapeyron relationship) are linked to enhanced TC dynamics, which will compensate for latent heat release from TC precipitation, favouring rainfall intensity itself. As previously noted, surface evaporation from the TC underlying surface is lesser than TC-induced precipitation within the outer radius (Supplementary Fig. 9), confirming the role of the moisture flux convergence in supporting moisture for TCs from external sources26,27,72. The contribution of moisture flux convergence in the water budget is additive and should lead to supper Clausius–Clapeyron rates of changes25,26,73. This previous relationship supports the rate of changes of the TC-induced surface freshwater flux with increasing SST, which projects that in a warming world the TC-induced precipitation, including those that occurred outer of TC circulation during moisture transport towards TC locations, will be higher than the total TC-induced evaporation. Therefore, the water vapour deficit should be supplied by the increasing low-level moisture availability with SST rising. However, care must be taken in interpreting the decreasing TC-related surface freshwater flux with rising SST, firstly because the rate of changes were computed based on the annual value in 1980, and second, because the role of TCs in future changes in the hydrological cycle is not entirely clear within the scope of an inevitably warming world. We also acknowledge that a comprehensive assessment of the relationship between TCs and SSTs is rather more complex, as it must include other components not considered in our analysis. TC-related surface freshwater flux can modify the upper-ocean responses through its effects on sea surface salinity, causing sea surface freshening and enhancing stratification, which reduces oceanic mixing and SST cooling, resulting in a positive feedback to TC intensification [e.g. refs. 74,75]. Meanwhile, TC-induced surface wind stress has the opposite effect, leading to SST cooling, resulting in a negative feedback on the TC intensity through its impact on air-sea enthalpy fluxes [e.g. refs. 76,77,78,79,80]. Overall, the feedback from TCs to SST is controlled by a balance between the momentum flux of the storm and turbulent mixing in the upper ocean, and their relative magnitudes [e.g. ref. 75]. This relationship should be further investigated under global warming to understand the full impact of TCs on the global water cycle under climate change.

Additionally, based on the robustness of future reductions in the global frequency of TCs in response to global warming81,82, a reduction in surface freshwater flux induced by TCs is expected in the future. However, the results of Pérez-Alarcón et al.15,16,17 revealed that intense TCs tend to gain more moisture to produce precipitation and therefore induce stronger moisture transport than weak TCs. Additionally, several studies [e.g. refs. 81,83] have indicated an increase in the number of intense TCs. Based on these previous findings, moisture transport related to the growth of intense TCs under global warming can compensate for the reduction in surface freshwater flux caused by the projected decrease in global TC count. Given the profound impact of global warming on TC frequency and intensity81,82,84, low-level moisture availability, column-integrated moisture32,33,34, TC-related rainfall85 and atmospheric circulation patterns86, it is necessary to investigate the role of TC in hydrological cycles under climate change. This topic will be addressed in future studies.

In summary, the role of TCs in the hydrological cycle is strongly modulated by seasonal TC activity. This study confirms that TCs globally induce more evaporation and precipitation from moisture source and sink regions, respectively. Nonetheless, we provided evidence of a statistically significant globally decrease in TC-related surface freshwater flux, which could be attributed to a slight reduction of global TC frequency and lifetime in the last two decades. Additionally, we detected that TC-induced surface freshwater flux decreases approximately 86% per °C of SST warming. However, it should be noted that this rate of changes was estimated by comparing with TC-related surface freshwater flux in 1980. Overall, this study quantified the impact of TCs on global water budgets.

Methods

TC data

The observed TC trajectories for the North Atlantic and NE Pacific were obtained from the HURDAT2 dataset87 provided by the US National Hurricane Center. The US Joint Typhoon Warning Center provided information on TCs in the remaining basins. These datasets contain the records at 6-h intervals (synoptic times), although HURDAT2 includes entries at non-synoptic times to indicate intensity maxima or landfalling. There have been approximately 100 years of TC records in some basins [e.g. ref. 87]; however, historical TC data have had the highest quality for climatological analysis since the beginning of satellite observations in the 1980s88. The TCSize dataset89 provided the outer radii of all the TCs within the study period.

FLEXPART model simulations

The FLEXible PARTicle dispersion model (FLEXPART) model39 is a Lagrangian model for tracking atmospheric moisture along air parcel trajectories. It was fed by the 6-h European Centre for Medium-Range Weather Forecast ERA-Interim reanalysis45 with 61 vertical levels and 1° × 1° grid spacing in latitude and longitude. ERA-Interim data were available from 1979 to August 2019. Therefore, the study period was from 1980 to 2018.

Despite the coarse grid spacing of ERA-Interim data, the FLEXPART model forced with this reanalysis has been widely used to investigate the source-sinks relationship associated with different weather systems, i.e., atmospheric rivers90, extratropical91, subtropical92, and tropical cyclones15,16,17,27. Likewise, Fernández-Alvarez et al.93 recently detected no significant differences between source-sink patterns from FLEXPART outputs forced with ERA-Interim and ERA5 reanalysis. Additionally, we acknowledge that several authors [e.g. refs. 26,94] have addressed the inability of ERA-Interim to capture extreme precipitation associated with TCs. Nonetheless, Pérez-Alarcón et al.27 highlighted that TC-related precipitation estimated from the Lagrangian approach, using ERA-Interim as input data for the FLEXPART model, fits well with observations.

On this basis, the model version and simulation design were the same as those used by Pérez-Alarcón et al.15,16,17 to identify the moisture sources for precipitation produced by TCs during their three well-known stages of development (genesis, lifetime maximum intensity, and dissipation). FLEXPART requires three-dimensional (temperature and specific humidity, horizontal and vertical wind components) and two-dimensional (total cloud cover, 10 m horizontal wind components, surface pressure, large-scale and convective precipitation, east/west and north/south surface stress, 2 m temperature and dew point temperature, topography, land-sea mask and subgrid standard deviation of topography, and sensible heat flux) fields as input data. To account for subgrid convective transport and turbulence in the planetary boundary layer, FLEXPART uses the convection parameterisation scheme proposed by Emanuel and Živković-Rothman95 and solves the Langevin equations for Gaussian turbulence96, respectively.

During the simulations, FLEXPART divided the atmosphere into approximately two million air parcels of equal mass that were uniformly distributed, and the three-dimensional wind field advected air parcels throughout the atmosphere. We obtained the model outputs at a 6-h time step containing the information of each parcel, that is, position in latitude, longitude, and specific humidity.

Quantification of the anomalous moisture uptake

To estimate the water budget linked to the TCs, we backtracked the air parcels residing within the area delimited by the outer radius of the TCs at each system location for up to 10 days (240 h), which is considered the average time spent by moisture in the atmosphere from evaporation to precipitation97. Along the parcel pathways, the changes in specific humidity (q, expressed in g kg−1) with time (dt = 6 h) respond to moisture increases and decreases by evaporation (e) and precipitation (p), respectively, as illustrated by the Lagrangian water budget equation43,44 (Eq. 1).

$$\left(e-p\right)=m\frac{{dq}}{{dt}}$$
(1)

where m is the parcel mass (expressed in kg) and the left term (e-p) represents the freshwater flux of the parcel. By solving Eq. (1) for all parcels and amassing (e-p) over all N parcels residing in the atmospheric column over an area A (1° × 1°), we estimated the surface freshwater flux (E-P) as follows:

$$(E-P)\approx \frac{\sum (e-p)}{A}$$
(2)

Backward analysis revealed the origin of the moisture in air masses during TCs events. That is, TCs induce net evaporation when (E-P > 0) and net precipitation when (E-P < 0). Therefore, regions in which evaporation (precipitation) exceeds precipitation (evaporation) can be considered moisture sources (sinks).

For each TC position, the climatological surface freshwater flux (including TC’s surface freshwater flux) was computed by averaging the surface freshwater flux for the same location and time step (month, day, and hour) of the TC over the study period. Thus, we verified whether the predominant evaporation (precipitation) areas during the TCs differed from the climatology by estimating surface freshwater flux anomalies. The surface freshwater flux anomalies for the TC time were obtained as the difference between the surface freshwater flux associated with the TC and the climatological flux.

TC-induced precipitation and evaporation within the outer radius

To account for the lifetime accumulated TC-induced precipitation (evaporation), we summed over the TC lifetime the total precipitation (evaporation) within a circle centred on the moving TC. All precipitation (evaporation) events within the TC outer radius are attributable to TCs. Previously, Lavender and McBride53 and Hallam et al.52 applied a similar approach to compute TC-related precipitation totals. TC-induced precipitation was extracted from the Multi-Source Weighted-Ensemble Precipitation V2 (MSWEP) database51. The MSWEP has high spatial (0.1° × 0.1° grid spacing) and temporal (3 h) resolutions and covers a long period (1979–present). It integrates observations from different data sources, e.g. surface rainfall gauge stations, satellite and reanalysis data. Additionally, it uses global streamflow observations at 13,762 stations for bias correction. Meanwhile, we computed TC-induced evaporation from the ERA-Interim reanalysis.

Annual mean sea surface temperature

As we are interested in the sea surface temperature (SST) during the TC occurrence, we used the US National Oceanic and Atmospheric Administration Daily Optimum Interpolation Sea Surface Temperature (OISST) dataset50 to compute the annual mean SST at basin and global scale. This dataset merges observations from different platforms, such as satellites, ships and buoys, into a regular global grid of 0.25° × 0.25° in latitude and longitude. This dataset is available from September 1981 to the present. Thus, the analysis that included SST data was performed from 1982 to 2018. For each basin, we average the SST within the limited area by the highest TC activity (Supplementary Fig. 5) during TC occurrence, while for the global mean SST we average the SST of all the basins during TC dates.

Classification of ENSO years

To classify a TC season under El Niño La Niña, we followed the procedure of the US Climate Prediction Center based on the Ocean Niño Index (ONI, 3-month running mean of sea surface temperature anomalies in Niño 3.4 region). A year was classified as El Niño (La Niña) if the ONI remained higher (lower) than 0.5°C (−0.5 °C) for at least 5 consecutive months. If neither of these conditions was satisfied, the year was classified as an ENSO-neutral year. This approach was previously applied by Pérez-Alarcón et al.97 and Colbert and Soden98 to investigate the impact of the ENSO on the trajectories and moisture sources of the North Atlantic TCs formed in the main development region.