North Pacific Intermediate Water response to a modern climate warming shift

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C11, 3349, doi:10.1029/2003JC001987, 2003 North Pacific Intermediate Water response to a modern climate warming shift Guillermo Auad Climate Research Division, Scripps Institution of Oceanography, La Jolla, California, USA James P. Kennett Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, Santa Barbara, California, USA Arthur J. Miller Climate Research Division, Scripps Institution of Oceanography, La Jolla, California, USA Abstract Oceanic observations and an isopycnal ocean model simulation are used to investigate the response of North Pacific Intermediate Water (NPIW) to atmospheric forcing associated with the well-known 1976–1977 climate regime shift to a warm regime. The model reproduces numerous features of NPIW including distribution, depth, temperature, and salinity. Changes in NPIW associated with the climate shift in the California coastal region were strongly influenced by an anomalous poleward flow at depth (300–1100 m). This current transports old, high salinity, low oxygen intermediate waters from the northern tropics to the midlatitudes. For depths below the mixed layer, the model reproduces observed changes in salinity, nitrates, and, to some extent, oxygen, thus suggesting that advective/diffusive processes are dominant in determining their concentrations below 300 m, isolated from the surface effects of direct atmospheric forcing and biological processes. These changes are structurally similar to those induced by much larger, abrupt climate changes at the end of the last glacial episode. Received 30 May 2003; accepted 12 September 2003; published 13 November 2003.