Introduction

Drought stands as one of the most devastating hydroclimatic hazards, unleashing a multiplicity of hydrological, agricultural, ecological, and socioeconomic consequences across large areas1,2,3. The severity and occurrence of droughts are influenced by a multitude of factors, encompassing human activities linked to land and water resource management4,5, as well as warming processes that elevate land evaporation and induce plant water stress6,7,8,9. However, it is unequivocally the deficit in precipitation that serves as the paramount force propelling the initiation and intensification of droughts10,11.

Precipitation deficits under long-term normal conditions are influenced by a diverse array of physical processes, varying across temporal scales, geographical regions, and seasons12,13,14. The interplay of dynamic and thermodynamic processes governs precipitation, evaporation, humidity transport, and vertical air motion across both oceans and continents15. The genesis of precipitation deficits is linked to overarching atmospheric circulation phenomena, such as the North Atlantic Oscillation, El Niño-Southern Oscillation, and others16,17,18. These large-scale patterns contribute to drought events on a regional scale. Additionally, the frequent occurrence of anticyclonic weather patterns at regional scale, such as blocking or anticyclonic ridges, play a role in causing regional droughts19,20. Thermodynamic processes can also influence over precipitation deficits21, particularly through anomalies in land water and energy conditions that impact evapotranspiration22,23. The relative importance of atmospheric circulation and thermodynamic processes delineate the origins of land precipitation, which may stem from either oceanic or continental sources24.

The majority of continental moisture originates from the ocean, transported through various mechanisms such as atmospheric rivers or low-level jets25,26,27,28. Once on the continent, the process of land moisture recycling comes into effect, where a single droplet of water can undergo multiple precipitation-evaporation cycles before ultimately returning to the ocean. In fact, the contribution of continental moisture accounts for over 60% of precipitation over large regions worldwide29. Remarkably, on a global scale, the contribution of continental sources to land precipitation outweighs that of oceanic sources24.

In the broader context, precipitation deficits can arise due to a spectrum of anomalies affecting the chain of moisture transport and recycling. Currently, there is an ongoing debate about the relative importance of circulation mechanisms versus land-atmosphere feedbacks in triggering and amplifying drought events22,23,30,31,32, and how this may change in response to climate change24,33,34. Some studies suggest that dynamic processes, such as anomalies in horizontal and vertical air movements, are the primary drivers of precipitation deficits and will continue to dominate under changing climate conditions. In contrast, other research emphasizes the role of land conditions, particularly soil moisture availability and land cover (e.g., vegetation type and density), and their influence on the boundary layer as key factors in explaining drought changes. Given the pivotal role of land-atmosphere fluxes in moisture recycling, it can be surmised that precipitation deficits influenced by land-atmosphere feedback are likely to be driven by continental moisture sources22. In this scenario, soil moisture and evapotranspiration anomalies could emerge as substantial contributors to the development and intensification of drought22,34,35. However, acknowledging the oceans as the primary source of moisture12, alterations in the transport dynamics from oceanic to continental regions could exert a profound impact on the evolution of meteorological drought conditions.

The oceanic source is tied to a prevailing advective component, tightly interwoven with atmospheric circulation and the conveyance of moisture from oceans to continents36. In contrast, the continental source may arise from the deficiency of recycling within the same region (where water evaporates and precipitates over the same area). Additionally, it can be influenced by the transport of moisture from other continental regions22. Regardless of the specific mechanisms, precipitation deficits of continental origin are fundamentally driven by land-atmosphere interactions37.

The impact of moisture transport deficits from both oceanic and terrestrial sources on precipitation deficits is observed to have highly localized effects in coastal and continental regions, but a substantial part of the Earth’s surface experiences undetermined influences from both sources38. Consequently, anomalies in moisture transport may play a noteworthy role in the progression of drought events, and an evaluation of the contribution of continental and oceanic moisture sources becomes crucial in delineating the relative significance of circulation processes versus land-atmosphere feedbacks in the genesis of drought events. In fact, during prolonged drought events, feedback processes play a significant role22. This warrants thorough investigation to clarify the intricate dynamics underlying the development of drought conditions.

Overall drought severity is influenced by various drivers and interactions39, some of which are highly complex. These interactions give rise to different types of drought: meteorological, agricultural, hydrological, and ecological3. The assessment of each drought type requires different variables. This study focuses on meteorological droughts, which are driven by precipitation deficits. These deficits often trigger other types of drought and play a crucial role in determining the overall severity of drought conditions.

We have conducted an assessment to elucidate the extent to which the major global meteorological drought events spanning the last few decades have been influenced by precipitation deficits in oceanic and continental moisture sources, contributing to unraveling the origins and progression of meteorological droughts on a global scale. The spatial patterns of the relative importance of precipitation deficits from continental or oceanic sources mirror the contributions of oceanic and continental components to total precipitation38. However, a distinct paradigm emerges at the scale of meteorological drought events, encompassing mixed oceanic and continental precipitation deficits, which are particularly evident in the most severe and prolonged events. Remarkably, these events, characterized by deficits from both sources, constitute a category that would remain unaccounted for without recognizing the dual influence. It underscores the pivotal role of both processes in generating the most severe and impactful meteorological droughts, emphasizing the intricate interplay of oceanic and land-based factors in shaping the socioeconomically and environmentally detrimental outcomes of these events.

Results and discussion

Role of continental and oceanic moisture sources

The majority of dry months result from precipitation deficits with a mixed origin (35.3%), followed in relevance by moderate continental (31.07%) and moderate oceanic (26%) (Fig. 1). Globally, notable variations exist in the prevalent moisture deficit origin that leads to drought38. Nevertheless, most world regions exhibit a predominance of precipitation deficits originating from either moderate continental and oceanic sources or from mixed origins. For instance, in many regions, including Central North America, Central America, Europe, North of South America, East China, and Southern Africa, precipitation deficits leading to dry conditions predominantly stem from mixed sources.

Fig. 1: Influence of continental and oceanic sources.
figure 1

A Percentage of dry months linked to various sources of precipitation deficits. B Dominant source, determined by the mode, causing dry conditions at the grid scale. C Percentage of the oceanic contribution to dry conditions, categorized by different regions based on their types of origin. The vertical lines of each plotted box plot illustrate the 10th, 25th, 75th and 90th quantiles, respectively. The interquartile spread is represented by the range between the 25th and 75th quantiles.

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The recorded meteorological drought events throughout the study period reveals distinct relative contributions of continental and oceanic sources, especially for events affecting expansive areas over extended durations (e.g., Supplementary Fig. 1). The spatial pattern of major global meteorological drought events recorded between 1980 and 2018 and categorized by their duration and magnitude, illustrates a prevalence of either continental or oceanic origins based on the significance of these precipitation sources in the respective regions (Fig. 2A). For instance, drought events in North and Central Asia, Australia, or Central South America are frequently associated with a substantial contribution of precipitation deficits of continental origin. Conversely, drought events in Northwest North America, the British and Irish Islands, or Japan exhibit a high percentage of precipitation deficits originating from an oceanic source, particularly evident in low-magnitude drought events in these regions. However, despite discernible spatial patterns, a majority of global meteorological drought events demonstrate contributions from both continental and oceanic sources of precipitation deficits. An intriguing pattern emerges, where the severity of the meteorological drought event is affected by the contribution of pure continental or oceanic sources. In low-severity meteorological drought events, there is a higher contribution from either pure continental or oceanic sources. As drought events become more severe, the contribution from one of the two sources significantly decreases. (Fig. 2B).

Fig. 2: Response of drought events to the source of precipitation deficits.
figure 2

A Spatial distribution of identified drought events based on total precipitation. Each circle represents a drought event, with its size indicating the severity and colour reflecting the oceanic and continental contribution. B The relationship between the severity of the drought event and the contribution of the source to the drought event. C Box-plots illustrating the distribution of severity and duration of drought events based on the dominant source of the drought event. The vertical lines of each plotted box plot illustrate the 10th, 25th, 75th and 90th quantiles, respectively. The interquartile spread is represented by the range between the 25th and 75th quantiles.

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Extreme meteorological drought events tend to exhibit a slightly higher contribution of continental precipitation deficits, aligning with the dominance of continental precipitation found at the global scale24,29 and underscoring the importance of recycling processes in explaining dry conditions in large world regions21,29 and additionally, emphasizing the probable significance of drought conditions propagating between continental areas due to anomalies in the moisture transport chain22,40. However, the most severe and prolonged meteorological drought events, characterized by high severity, necessitates a mixed origin of precipitation deficits to occur. Consequently, meteorological drought events with a mixed origin consistently display significantly greater severity and duration compared to events predominantly driven by precipitation deficits of extreme or moderate oceanic or continental origin (see Fig. 2C).

It could be hypothesized that the dominance of a mixed origin in the most extreme meteorological drought events could be driven by the spatial fusion of drought events recorded in different regions of the world, ultimately generating a drought event characterized by high spatial extension and severity. However, this is not the case, as the most extreme meteorological drought events demonstrate that precipitation deficits during their development typically exhibit a mixed origin. Only in a small percentage of cases do these deficits have a moderately continental or oceanic origin, and very few months show deficits with a purely continental or oceanic origin (Fig. 3). This observation strongly supports the conclusion that the development of extreme meteorological drought events in the vast majority of world regions requires precipitation deficits from both continental and oceanic sources (see Supplementary Tables 1 and 2).

Fig. 3: Evolution of the dominant origin of the precipitation deficits for each month over the duration of drought events characterized by a duration exceeding 12 months.
figure 3

The colour of the circles represents the dominant category throughout each drought event.

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Temporal consistency

While some previous studies have proposed an increasing significance of land-atmospheric feedbacks in the development and intensification of drought events under climate change conditions21,22,34, and others suggest a reinforcement of the importance of oceanic moisture sources in causing continental precipitation24,41, our findings do not indicate substantial changes in the dominant mixed contribution of precipitation deficits from continental and oceanic origins to meteorological drought events. The results of a comparison conducted between the periods 1980–1999 and 2000–2018 at the grid scale, and in most cases, does not show any relevant change between the two periods at the grid cell (Fig. 4a, b) and drought event scales (Fig. 4c). For all categories, except EC, more than 50% of the pixels in the category during the 1980–1999 period maintained the same category in the 2000–2018 period. Most changes involved only a shift to an adjacent category. To simplify the interpretation of changes, only three categories were used: C (continental), MIX (mixed), and O (oceanic). EC and MC were combined into the new C category, while EO and MO were grouped into the new O category. These results do not show clear global pattern in changes, indicating that deficits of continental origin played a more significant role during the 2000–2018 period than during 1980–1999 in certain regions (e.g., Scandinavia or central North America), while in others, it played a lesser role (e.g., central Europe, central Africa, and India). However, it is noteworthy that most regions exhibit no discernible changes and dominance of mixed precipitation sources in the development of dry conditions. On the event scale, while it is not feasible to conduct a spatial analysis for comparing drought events between subperiods, a consistent conclusion can be drawn: there appear to be no discernible differences in the severity of events based on the dominant source between the two subperiods, with a comparable contribution of the source to the meteorological drought events.

Fig. 4: Changes in the influence of moisture sources on droughts.
figure 4

a Contingency table that displays drought categories, comparing the periods 1980–1999 and 2000–2018. For the total surface area in each category in 1980–1999, the table indicates the percentage of surface area in each category in 2000–2018. The diagonal grid boxes indicate no changes. b The global map illustrates a simplified change matrix, highlighting areas that have transitioned between oceanic, continental, and mixed origins over the two analyzed periods. c Box-plots showing the drought severity (in SPI units) and the contribution of the source to the drought event for the subperiods 1980–1999 and 2000–2018 considering the dominant source of each drought event. The vertical lines of each plotted box plot illustrate the 10th, 25th, 75th and 90th quantiles, respectively. The interquartile spread is represented by the range between the 25th and 75th quantiles.

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Relevance of the results

While a more in-depth analysis is necessary to thoroughly investigate the major atmospheric moisture transport mechanisms and land-atmosphere processes that contribute to continental and oceanic precipitation deficits, our study provides valuable insights into the occurrence of dry conditions worldwide. In particular, it sheds light on the origins of the most severe meteorological drought events recorded globally over the past few decades. Given the role played by land-atmosphere feedbacks, the relationship between precipitation and droughts is bidirectional over land. A decrease in precipitation initiates droughts, and, secondarily, decreases in soil moisture can contribute to further declines in precipitation, intensifying drought at the local or regional scale22,23. Additionally, these conditions may propagate to other continental regions through deficits in the transport of moisture40. In such a scenario, one might expect deficits of continental origin to be the primary component of precipitation affected and the one guiding drought evolution over continents. However, our work’s results indicate that most drought events are influenced and guided by decreases in both components of precipitation. Consequently, the drought category characterized by a mixed origin of precipitation deficits (MIX) emerges as the most common during the evolution of meteorological droughts of particular severity.

When interpreting these results, it is essential to note that our analysis did not consider the atmospheric mechanisms responsible for the initiation of droughts, nor did we conduct a spatial analysis of their trajectory for specific events, following the approach of previous studies42,43,44. Nevertheless, our results should be interpreted as indicating that the atmospheric mechanisms giving rise to droughts and marking their evolution generally affect both the oceanic and continental components of precipitation.

A decrease in soil evaporation can have several effects, such as reducing local cloud formation and the potential moisture to be transported, negatively impacting local and downwind precipitation, respectively45. Simultaneously, it can induce an increase in surface temperature and trigger various other effects. Under certain conditions, reduced evaporation can be significant enough to induce self-intensification or even self-propagation of droughts21,23,46, as well as heatwaves47,48,49. However, it’s crucial to consider that land-atmosphere feedback requires specific atmospheric conditions to impose itself. Anticyclonic conditions and stability are optimal for land-atmosphere feedback to be influential35,50,51. These specific conditions can disrupt the continental evaporation-precipitation cycle and also impact the movement of cyclonic systems capable of transporting water from oceans to continents19,52. During drought events, both sources of precipitation are diminished, explaining the observed dominance of the mixed origin of precipitation deficits in generating the most severe drought events over the last decades. In other words, meteorological droughts guided by only one component are not capable of becoming extensive, severe droughts.

These results carry significant implications for enhancing the understanding of the drivers behind meteorological droughts worldwide, particularly the origins of the main precipitation deficits that lead to droughts. Future research should focus on the specific origins of major drought episodes recorded globally to distill the physical mechanisms triggering these precipitation deficits, whether of continental or oceanic origin. To achieve this, it is crucial to combine precipitation data with other variables, such as atmospheric evaporative demand, which significantly influences land evapotranspiration and drought severity. This approach will be essential for understanding the origin, intensification, and propagation of droughts on a global scale.

Data and methods

The data concerning the monthly oceanic and continental components of total precipitation over continental areas was generated from Nieto and Gimeno53,54,55 and it is available at: https://doi.org/10.5281/zenodo.4550857. This dataset was created using a Lagrangian approach employing global input data from the ERA-Interim reanalysis56 at 1° in latitude and longitude, spanning 61 atmospheric levels vertically, covering the period from 1980 to 2018. Approximately 2 million particles (air masses) were simulated backward in time using the FLEXible PARTicle dispersion model (FLEXPART) v9.057,58. The use of Lagrangian models, such as FLEXPART, represents a notable advancement in studying the role of continental evaporation in the water and energy balance, facilitating the development of both regional and global studies. This approach enables the partitioning of precipitation over land into two fractions based on their source: oceanic and continental. The dataset utilized in this study has been previously employed in studies by Gimeno et al. 24, Sorí et al. 42 and Gimeno-Sotelo et al.38, and it has undergone validation in prior work by Nieto and Gimeno53.

Three monthly precipitation series, representing the total precipitation over land and its oceanic and continental components, were employed to create three versions of the Standardized Precipitation Index (SPI)59 at a 3-month time scale for each grid point over land. SPI has become the most widely used meteorological drought index and is widely recommended by the World Meteorological Organization for drought quantification and monitoring60,61. The three SPI versions generated in this study assess drought conditions by independently considering the total received precipitation, as well as drought conditions by independently considering the oceanic and continental inputs.

To identify drought events, total precipitation SPI was utilized. A grid cell was considered to be affected by a drought in a specific month when the 3-month SPI dropped below -1.28. This threshold corresponds to the 10% probability, representing the maximum drought severity expected in a 10-year period62. Following the identification of all grid cells affected by dry conditions from 1980 to 2018, a 3D segmentation algorithm was applied to delineate specific drought events based on the contiguity of drought-affected pixels in both space and time. The algorithm utilizes an explicit 3D (longitude, latitude, time) structure-based method, in which drought events are defined by a spatially and temporally coherent set of grid cells displaying standardized precipitation below the −1.28 SPI threshold63. This method extracts coherent space-time structures that define particular drought events, for which different characteristics can be defined, including the total duration, total severity (measured by integrating the SPI values over time and region that characterize the entire drought event), and also the drought centroid, summarizing the location of the drought event throughout its trajectory. This approach has been followed by several recent studies to generate drought catalogs and characterize drought events from a multidimensional perspective64,65,66,67.

The application of the 3D segmentation algorithm resulted in the creation of a catalog comprising over 9000 drought events affecting various world regions throughout the analysis period. For subsequent analysis and to prevent small drought events from dominating, we selected events with a duration equal to or exceeding four months, retaining 1157 drought events. The total duration but also the severity (integrating the SPI values over the whole duration of the event) were quantified for each drought event. To illustrate the detailed monthly evolution of the contribution of continental and oceanic precipitation deficits to drought severity, we conducted a more in-depth analysis of events lasting more than 12 months, reducing the number of events to 66. A qualitative classification system based on the relevance of oceanic and continental SPI in the development of a drought event was employed. This classification comprised five categories: Extremely Continental (EC), Moderately Continental (MC), Mixed (MIX), Moderately Oceanic (MO), and Extremely Oceanic (EO) (see Table S3). The MIX category referred to situations where the anomalies in the contribution of oceanic and continental deficits to dry conditions were almost equal. MC denoted situations where the drought event was primarily guided by deficits in continental precipitation, although oceanic precipitation also exhibited below-average conditions. EC referred to scenarios in which the drought event was entirely guided by the deficit of precipitation of continental origin. MO and EO were equivalent to MC and EC, respectively, but with the roles of continental and oceanic precipitation deficits reversed. This classification was applied to: i) grid cells and months classified as dry based on SPI values for oceanic and continental components, ii) each individual drought event by averaging continental and oceanic SPI throughout the entire event, and iii) each month of the drought event based on the average continental and oceanic SPI for that specific month.

In addition to the qualitative classification of droughts, a quantitative approach was employed. Negative values indicate cases where the role of oceanic precipitation deficit is higher than the role of continental deficit in generating dry conditions. Conversely, positive values represent the opposite scenario, where the role of continental precipitation deficit is more pronounced than the role of oceanic deficit, following this equation:

$${Contribution}=\frac{\left({SPIcontinental}-{SPIoceanic}\right)}{\left({SPIcontinental}+{SPIoceanic}\right)}$$

A summary of the methodology used in this study is showed in Figure S2.