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Bottom water warming in the North Pacific Ocean


Observations of changes in the properties of ocean waters have been restricted to surface1 or intermediate-depth waters2,3, because the detection of change in bottom water is extremely difficult owing to the small magnitude of the expected signals. Nevertheless, temporal changes in the properties of such deep waters across an ocean basin are of particular interest, as they can be used to constrain the transport of water at the bottom of the ocean and to detect changes in the global thermohaline circulation. Here we present a comparison of a trans-Pacific survey completed in 1985 (refs 4, 5) and its repetition in 1999 (ref. 6). We find that the deepest waters of the North Pacific Ocean have warmed significantly across the entire width of the ocean basin. Our observations imply that changes in water properties are now detectable in water masses that have long been insulated from heat exchange with the atmosphere.

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Figure 1: The location of stations occupied for WHP Line P1 revisit in 1999.
Figure 2: The difference in potential temperature (a) and salinity (b) between the two surveys of line P1 in 1999 and 1985, plotted against depth.
Figure 3: The difference in salinity between 1999 and 1985 for line P1, as in Fig. 2b, but plotted with the density parameter σ4 as the vertical axis.
Figure 4: The difference in area between the 1999 and 1985 surveys in the areas along WHP Line P1 occupied by different potential temperatures.

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We thank the officers and crew of the three research vessels (RV Kaiyo-maru of JFA, RV Mirai of JAMSTEC, and CCGS John P. Tully) and all technical assistants. The Canadian contribution was funded by the Strategic Science Fund of the Department of Fisheries and Oceans, and the Japanese contributions were funded by the Promotional Foundation for Science and Technology of the Science and Technology Agency of Japan (now known as the Ministry of Education, Culture, Sports, Science and Technology).

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Correspondence to Masao Fukasawa or Howard Freeland.

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Fukasawa, M., Freeland, H., Perkin, R. et al. Bottom water warming in the North Pacific Ocean. Nature 427, 825–827 (2004).

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