Water plays a crucial role in modulating Earth’s climate by regulating the transfer of energy between Earth’s surface, the atmosphere, and space. Although previous studies have offered a general global overview of water circulation between the oceans and land, this traditional two-region model does not take into account the considerable precipitation that occurs over tropical coastal regions, including the Indonesian maritime continent, the Indian subcontinent, and the Bay of Bengal. To fully understand Earth’s energy budget, the role these boundary regions play in transferring water between the oceans and land needs to be clarified.
Here Ogino et al. reexamine the role of precipitation along tropical coasts in the global water cycle. The team estimated evaporation, precipitation, and the amount of water vapor transport relative to the distance from the coastline in the tropics and the northern and southern extratropics for the 30-year period from 1981 to 2010.
The results show that about half of the water vapor transported inland from the oceans precipitates out along a coastal region, which the team assumes to stretch from 200 kilometers inland to 300 kilometers seaward, and that this effect is most pronounced in the tropics. The researchers also found that the landward transport of water vapor is not symmetric relative to the coastline; instead, it peaks about 300 kilometers offshore and decreases more rapidly landward than seaward. To account for this rapid dehydration of the atmosphere along tropical coasts, the authors propose that the traditional two-region model of ocean-land water circulation should be updated to a three-region concept that separately defines this crucial coastal region.
Because of the magnitude of the freshwater contribution to coastal waters via precipitation, this research may offer additional insights into the distribution of salinity in the ocean, which influences the density gradients that drive circulation. In fact, the team estimates that precipitation along tropical coastlines contributes as much freshwater to oceans as global river discharge. The research also points to the importance of understanding how changes in sea level will impact the lengths and shapes of coastlines and how these differences could, in turn, affect the circulation of water around the globe. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075760, 2017)
—Terri Cook, Freelance Writer