The global atmospheric water transport from the net evaporation to the net precipitation regions has been traced using Lagrangian trajectories.
A matrix has been constructed by selecting various group of trajectories based on their surface starting (net evaporation) and ending (net precipitation) positions to show the connectivity of the 3-D atmospheric water transport within and between the three major ocean basins and the global landmass. The analysis reveals that a major portion of the net evaporated water precipitates back into the same region, namely 67 % for the Indian Ocean, 64 % for the Atlantic Ocean, 85 % for the Pacific Ocean and 72 % for the global landmass. It has also been calculated that 58 % of the net terrestrial precipitation was sourced from land evaporation. The net evaporation from the subtropical regions of the Indian, Atlantic and Pacific oceans is found to be the primary source of atmospheric water for precipitation over the Intertropical Convergence Zone (ITCZ) in the corresponding basins. The net evaporated waters from the subtropical and western Indian Ocean were traced as the source for precipitation over the South Asian and eastern African landmass, while Atlantic Ocean waters are responsible for rainfall over North Asia and western Africa. Atlantic storm tracks were identified as the carrier of atmospheric water that precipitates over Europe, while the Pacific storm tracks were responsible for North American, eastern Asian and Australian precipitation. The bulk of South and Central American precipitation is found to have its source in the tropical Atlantic Ocean. The land-to-land atmospheric water transport is pronounced over the Amazon basin, western coast of South America, Congo basin, northeastern Asia, Canada and Greenland. The ocean-to-land and land-to-ocean water transport through the atmosphere was computed to be
The hydrologic cycle traces the continuous movement of the water in the Earth system. The atmospheric hydrological cycle starts from the evaporation regions and ends in the precipitation regions. Generally, evaporation tends to exceed precipitation over the ocean, while for land the opposite holds true.
A consequence of this excess precipitation over land is that this surplus water eventually discharges into the ocean by the rivers, completing the atmospheric branch of the water cycle. The hydrological cycle is believed to strengthen in a future warmer climate. The Clausius–Clapeyron (CC) thermodynamic relation indicates that for every 1
Given these diverse roles of the hydrologic cycle within the Earth system, it is important to disentangle and understand its different parts. Previous studies were able to provide an estimate of the water storage in the reservoirs and also the net exchange of water between them using the surface water budgets
The mass conserving Lagrangian trajectory model TRACMASS v7.0
The horizontal water transports through the model grid box faces are obtained by multiplying the air transports with its water content. The vertical water transport field is then obtained from an atmospheric water-mass conservation equation
The mass conserving ability of TRACMASS (i.e. mass transport of a trajectory is conserved throughout its journey) has made it possible to compute Lagrangian stream functions from the simulated trajectories. The Lagrangian stream function is a useful tool to understand atmospheric and oceanic circulation pathways and has been used in previous studies extensively
The atmospheric water transports were computed using the surface pressure, specific humidity, specific cloud liquid and ice water content and horizontal wind velocities from the ERA-Interim reanalysis
Spaghetti plot of few selected atmospheric water trajectories for the month of January 2016. The selected ocean basins are represented by different shadings of gray and defined as the Indian Ocean (IO), Pacific Ocean (PO) and Atlantic Ocean (AO). Note that the Arctic Ocean is included in the Atlantic. The global landmass is taken as one single entity. The atmospheric water transport within and between the ocean basins and land has been calculated based on these defined sectors. The representative trajectories associated with these intra- and inter-basin water transport are labelled with different colours. The black dots indicate the starting points and the red points represent the ending points of the atmospheric water trajectories.
To understand the 3-D global atmospheric water transport, the Lagrangian trajectories were started over the entire surface of the globe when evaporation exceeded precipitation, and followed until they reach back to the surface, which occurs when precipitation exceeded evaporation. These water trajectories were started at the surface every 6 h during 2016, where
Annual mean
The atmospheric water is always on the move through space and in time within the climate system. In order to grasp the full characteristics of the atmospheric water circulation, it is thus necessary to reduce its dimensionality. The geographical connection of the atmospheric water transports within and between the ocean basins and the global landmass has been established by tracing the atmospheric water from the evaporation-dominated to the precipitation-dominated regions (Fig.
The Lagrangian meridional overturning stream function within and between the ocean basins and land. This has been undertaken by grouping the trajectories according to their starting and ending locations. The starting and ending points of the atmospheric trajectories are defined as per the sectors presented in Fig.
Atmospheric freshwater transport within and between the ocean basins and land. The rows represent net evaporative (atmospheric water source) sectors and the columns represent the net precipitation (atmospheric water sink) regions. Units are in Sv (
Same as Fig.
The vertically integrated horizontal water flux (shaded; Sv m
The average residence time (days) of the atmospheric waters mapped on their net evaporative points within and between the three ocean basins and land. Note that this has been mapped where the net evaporation exceeds a monthly mean value of 0.2 mm d
The results show that the net evaporation from the subtropical Atlantic, Pacific and Indian Ocean is the major source of water for net precipitation over the Intertropical Convergence Zone (ITCZ) in their respective basins (Fig.
The easterly (Figs.
A simplified schematic of the annual mean global atmospheric water transports from both the surface water budget and Lagrangian perspectives is presented in Fig.
A sketch of the atmospheric water exchange between the global ocean and land. The top panel shows the surface water budget understanding of the hydrologic cycle, while the bottom panel elaborates the intricacies of the water movement that can be obtained using a Lagrangian framework. The upward and downward arrows represent net evaporation and net precipitation transport, respectively. Note, the numbers presented here are the crudely estimated transports from Table
The strength of the hydrological cycle in the present study is stronger than previous estimates such as
One of the most striking and robust features of climate change is the acceleration of the atmospheric water cycle branch, which is associated with the temperature increase of the lower troposphere. In order to gain a detailed understanding of the future atmospheric water cycle and its importance, one should know the intricacies of the present climate water cycle in the atmosphere. Although earlier studies were able to provide a quantification of the global atmospheric water cycle, they missed detailed and important information which is essential to explain variations in continental water availability and near surface ocean salinity asymmetries. For instance, the global ocean-to-ocean, ocean-to-land, land-to-ocean and land-to-land water transport through the atmosphere were not extensively studied previously. Thus, the global picture of the atmospheric water movement was incomplete. These shortcomings were overcome in the present study using a novel Lagrangian framework, and presented a complete synthesised and quantitative view of the atmospheric water cycle. This Lagrangian methodology used in the present study made it possible to trace the atmospheric water transport from the net evaporation to the net precipitation regions within and between the different ocean basins and land. Earlier studies focused more on the regional or basin-scale surface water budget analysis
The Eulerian/Lagrangian moisture tracking models that have been used in earlier studies were focused, in particular, on isolated aspects of the atmospheric hydrologic cycle, e.g. only ocean-to-river basin transport, land-to-land transport or some extreme precipitation events
In a warmer climate, the atmospheric water transport is expected to be enhanced, which has far-reaching consequences. An extension of the present study could be to repeat a similar investigative strategy for future climate scenarios and identify how the atmospheric water transport within and between ocean basins and the landmass will change with respect to the present climate. The results could provide a detailed understanding of the future ocean salinity asymmetries, as the ocean salinity is closely tied to the surface evaporation and precipitation, which are the starting and ending points of the atmospheric water transport. Note that observational evidence of the oceanic salinity change already indirectly indicates a strengthening of the atmospheric branch of the water cycle
The Lagrangian trajectory model TRACMASS v7.0 can be freely downloaded from
The supplement related to this article is available online at:
DD and KD conceptualised the study. DD collected all the necessary data sets and employed the trajectory model TRACMASS. The outputs from the TRACMASS were analysed by DD with the programming help from AAC. The results were then discussed elaborately between all the authors. The paper is written by DD with inputs from all the co-authors. AAC was responsible for the inclusion of the cloud liquid and ice water into the updated version of the TRACMASS.
The contact author has declared that none of the authors has any competing interests.
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The authors wish to acknowledge Peter Lundberg and Sara Berglund for their constructive comments. This work has been financially supported by the Swedish Research Council through grant agreement no. 2019-03574. The TRACMASS model integrations and the Lagrangian trajectory computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at the National Supercomputer Centre (NSC) partially funded by the Swedish Research Council through grant agreement no. 2018-05973.
This research has been supported by the Vetenskapsrådet (grant no. 2019-03574). The article processing charges for this open-access publication were covered by Stockholm University.
This paper was edited by Niko Wanders and reviewed by Dominik Schumacher and Ruud van der Ent.