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Subtropical coral reveals abrupt early-twentieth-century freshening in the western North Pacific Ocean

¹ MARUM–Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany
² Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8567, Japan
³ Alfred Wegener Institute for Polar and Marine Research, 27570 Bremerhaven, Germany
⁴ Department of Atmospheric Physics, Faculty of Physics, University of Bucharest, 77125 Bucharest, Romania
⁵ Graduate School of Frontier Sciences, University of Tokyo, Tokyo 164-8639, Japan
⁶ Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan

Correspondence: *E-mail: tfelis{at}uni-bremen.de

Instrumental climate observations provide robust records of global land and ocean temperatures during the twentieth century. Unlike for temperature, continuous salinity observations in the surface ocean are scarce prior to 1970, and the magnitude of salinity changes during the twentieth century is largely unknown. Surface ocean salinity is a major component in climate dynamics, as it influences ocean circulation and water mass formation. Here we present an annually resolved reconstruction of salinity variations in the surface waters of the western subtropical North Pacific Ocean since 1873, based on bimonthly records of ¹⁸O, Sr/Ca, and U/Ca in a coral from the Ogasawara Islands. The reconstruction indicates that an abrupt regime shift toward fresher surface ocean conditions occurred between 1905 and 1910. Observational atmospheric data suggest that the abrupt freshening was associated with a weakening of the winds that drive the Kuroshio Current system and the associated subtropical gyre circulation. We note that the abrupt early-twentieth-century freshening in the western subtropical North Pacific precedes abrupt climate change in the northern North Atlantic by a few years. The potential for abrupt regime shifts in surface ocean salinity should be considered in climate predictions for the coming decades.