More than three-fourths of the global water cycle consists of the annual rainfall and evaporation freshwater exchange between the ocean and atmosphere. The water cycle is expected to intensify in a warmer climate, with shifting large-scale rainfall and drought patterns. Ocean salinity variations in recent decades provide a clear indicator of such changes and offer a key index for monitoring future climate variability related to the hydrologic cycle. In this sense, the ocean behaves like a rain gauge. This simple idea will also contribute to resolving the major discrepancies among rainfall climatologies. Addressing these problems requires a full understanding of complex upper ocean processes such as mixing and advection that balance the net freshwater flux at the surface. The global surface salinity measurement system, including both in situ instruments and satellites, along with regional upper ocean process studies, will soon be in place to advance these studies.

Lagerloef, G., R. Schmitt, J. Schanze, and H.-Y. Kao. 2010. The ocean and the global water cycle. Oceanography 23(4):82–93, https://doi.org/10.5670/oceanog.2010.07.

Alley, R.B., J. Marotzke, W.D. Nordhaus, J.T. Overpeck, D.M. Peteet, R.A. Pielke Jr., R.T. Pierrehumbert, P.B. Rhines, T.F. Stocker, L.D. Talley, and J.M. Wallace. 2003. Abrupt climate change. Science 299(5615):2,005–2,010. [CrossRef]

Baumgartner, A., and E. Reichel. 1975. The World Water Balance. Elsevier, New York, 179 pp.

Boyer, T.P., S. Levitus, J.I. Antonov, R.A. Locarnini, and H.E. Garcia. 2005. Linear trends in salinity for the world ocean, 1955–1998. Geophysical Research Letters 32, L01604. [CrossRef]

Brewer, P., W.S. Broecker, W.J. Jenkins, P.B. Rhines, C.G. Rooth, J.H. Swift, T. Takahashi, and R.T. Williams. 1983. A climatic freshening of the deep Atlantic north of 50°N over the past 20 years. Science 222(4629):1,237–1,239.

Broecker, W.S. 1987. Commentary: Unpleasant surprises in the greenhouse? Nature 372:123–126. [CrossRef]

Cable, J.E., G.C. Bugna, W.C. Burnett, and J.P. Chanton. 1996. Application of 222Rn and CH4 for assessment of groundwater discharge to the coastal ocean. Limnology and Oceanography 41(6):1,347–1,353.

Curry, R., R.R. Dickson, and I. Yashayaev. 2003. A change in the freshwater balance of the Atlantic over the past four decades. Nature 426(6968):826–829. [CrossRef]

Dai, A., and K.E. Trenberth. 2002. Estimates of freshwater discharge from continents: Latitudinal and seasonal variations. Journal of Hydrometeorology 3:660–687. [CrossRef]

Dai, A., T. Qian, K.E. Trenberth, and J.D. Milliman. 2009. Changes in continental freshwater discharge from 1948 to 2004. Journal of Climate 22:2,773–2,792.

Dickson, R.R., I. Yashayaev, J. Meincke, W. Turrell, S. Dye, and J. Holfor. 2002. Rapid freshening of the deep North Atlantic over the past four decades. Nature 416:832–837. [CrossRef]

Dohan, K., and N. Maximenko. 2010. Monitoring ocean currents with satellite sensors. Oceanography 23(4):94–103. [CrossRef]

Durack, P.J., and S.E. Wijffels. 2010. Fifty year trends in global ocean salinities and their relationship to broad-scale warming. Journal of Climate 23:4,342–4,362. [CrossRef]

Emile-Geay, J., M. Cane, N. Naik, R. Seager, A. Clement, and A. van Geen. 2003. Warren revisited: Atmospheric freshwater fluxes and “Why is no deep water formed in the North Pacific.” Journal of Geophysical Research 108, C6. [CrossRef]

Freeland, H., K. Denman, C.S. Wong, F. Whitney, and R. Jacques. 1997. Evidence of change in the winter mixed layer in the Northeast Pacific Ocean. Deep-Sea Research Part I 44(12):2,117–2,129.

Gordon, A., and C.F. Giulivi. 2008. Sea surface salinity trends over 50 years within the subtropical North Atlantic. Oceanography 21(1):20–29. Available online at: http://www.tos.org/oceanography/issues/issue_archive/21_1.html (accessed September 23, 2010).

Hakkinen, S., and P.B. Rhines. 2009. Shifting surface currents in the northern North Atlantic Ocean. Journal of Geophysical Research 114, C04005. [CrossRef]

Haupt, B.J., and D. Seidov. 2007. Strengths and weaknesses of the global ocean conveyor: Inter-basin freshwater disparities as the major control. Progress In Oceanography 73(3–4):358–369. [CrossRef]

Hilburn, K. 2009. The Passive Microwave Water Cycle Product. Remote Sensing Systems (REMSS) Technical Report 072409, 30 pp. Available online at: ftp://ftp.remss.com/water_cycle (accessed September 14, 2010).

Holliday, N.P., S.L. Hughes, S. Bacon, A. Beszczynska-Möller, B. Hansen, A. Lavin, H. Loeng, K.A. Mork, S. Østerhus, T. Sherwin, and W. Walczowski. 2008. Reversal of the 1960s to 1990s freshening trend in the northeast North Atlantic and Nordic Seas. Geophysical Research Letters 35, L03614. [CrossRef]

Lagerloef, G., and J. Font. 2010. SMOS and Aquarius/SAC-D missions: The Era of Spaceborne salinity measurements is about to begin. Chapter 3 in Oceanography from Space. V. Barale, J.F.R. Gower, and L. Alberotanza, eds, Springer. [CrossRef]

Lagerloef, G., J. Boutin, J. Carton, Y. Chao, T. Delcroix, J. Font, J. Lilly, N. Reul, R. Schmitt, and S. Riser. 2010. Resolving the global surface salinity field and variations by blending satellite and in situ observations. In Proceedings of OceanObs’09: Sustained Ocean Observations and Information for Society (Vol. 2). Venice, Italy, September 21–25, 2009, J. Hall, D.E. Harrison, and D. Stammer, eds, ESA Publication WPP-306.

Lagerloef, G., F. Colomb, D. LeVine, F. Wentz, S. Yueh, C. Ruf, J. Lilly, J. Gunn, Y. Chao, A. deCharon, and others. 2008. The Aquarius/SAC-D mission: Designed to meet the salinity remote-sensing challenge. Oceanography 21(1):68–81. Available online at: http://www.tos.org/oceanography/issues/issue_archive/21_1.html (accessed September 23, 2010).

Large, W., and S. Yeager. 2009. The global climatology of an interannually varying air–sea flux data set. Climate Dynamics 33:341–364. [CrossRef]

Lee, T., T. Awaji, M. Balmaseda, E. Greiner, and D. Stammer. 2009. Ocean state estimation in climate research. Oceanography 22(3):160–167. Available online at: http://www.tos.org/oceanography/issues/issue_archive/22_3.html (accessed September 23, 2010).

Milly, P.C.D., R.T. Wetherald, K.A. Dunne, and T.L. Delworth. 2002. Increasing risk of great floods in a changing climate. Nature 415:514–517. [CrossRef]

Roemmich, D., and the Argo Steering Team. 2009. Argo: The challenge of continuing 10 years of progress. Oceanography 22(3):46–55. Available online at: http://www.tos.org/oceanography/issues/issue_archive/22_3.html (accessed September 23, 2010).

Schanze, J., R. Schmitt, and L. Yu. In press. The global oceanic freshwater cycle: A state-of-the-art quantification. Journal of Marine Research.

Schmitt, R.W. 1995. The ocean component of the global water cycle. Pp. 1,395–1,409 in US National Report to International Union of Geodesy and Geophysics, 1991–1994, Supplement to Reviews of Geophysics.

Schmitt, R.W. 2008. Salinity and the global water cycle. Oceanography 21(1):12–19. Available online at: http://www.tos.org/oceanography/issues/issue_archive/21_1.html (accessed September 23, 2010).

Schmitt, R.W., P.S. Bogden, and C.E. Dorman. 1989. Evaporation minus precipitation and density fluxes for the North Atlantic. Journal of Physical Oceanography 9(9):1,208–1,221.

Schneider, T., P.A. O’Gorman, and X. Levine. 2010. Water vapor and the dynamics of climate changes. Reviews of Geophysics 48, RG3001. [CrossRef]

Stammer, D., K. Ueyoshi, A. Köhl, W.B. Large, S. Josey, and C. Wunsch. 2004. Estimating air-sea fluxes of heat, freshwater and momentum through global ocean data assimilation. Journal of Geophysical Research 109, C05023. [CrossRef]

Warren, B.A. 1983. Why is no deep water formed in the North Pacific? Journal of Marine Research 41(2):327–347. [CrossRef]

Weyl, P.K. 1968. The role of the oceans in climatic change: A theory of the ice ages. Meteorological Monograph 8:37–62.

Wijffels, S.E. 2001. Ocean transport of fresh water. Pp. 475–488 in Ocean Circulation and Climate. G. Siedler, J. Church, and J. Gould, eds, Academic Press. [CrossRef]

Wijffels, S.E., R.W. Schmitt, H.L. Bryden, and A. Stigebrandt. 1992. Transport of freshwater by the oceans. Journal of Physical Oceanography 22:155–162.

Wüst, G. 1936. Oberflächensalzgehalt, Verdunstung und Niederschlag auf dem Weltmeere. Länderkundliche Forschung, Festschrift Norbert Krebs, 347–359.

Yu, L., and R.A. Weller. 2007. Objectively analyzed air-sea heat fluxes for the global ice-free oceans (1981–2005). Bulletin of the American Meteorological Society 88:527–539.

This is an open access article made available under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution, and reproduction in any medium or format as long as users cite the materials appropriately, provide a link to the Creative Commons license, and indicate the changes that were made to the original content. Images, animations, videos, or other third-party material used in articles are included in the Creative Commons license unless indicated otherwise in a credit line to the material. If the material is not included in the article’s Creative Commons license, users will need to obtain permission directly from the license holder to reproduce the material.