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North Pacific mode water controls extratropical-to-tropical heat and mass exchange. Analysis and model simulations show that decadal temperature variations in this subtropical North West Pacific water mass are forced remotely by Atlantic Multidecadal Variability.

North Pacific subtropical mode water, a vertically homogeneous thermocline water mass, occupies the entire subtropical Western Pacific Ocean. It transports mass, heat and nutrients from the surface to the subsurface ocean, providing memory of climate variability(1-11). Decadal variability of the mode water temperature has been attributed to the Pacific Decadal Oscillation (PDO)(2,3,12,13), but this is based on short data records. Here, using long records of observations, we show that decadal-to-multidecadal variability of the mode water mean temperature is instead controlled by the Atlantic Multidecadal Variability (AMV). During an AMV-positive phase, warm sea surface temperatures (SSTs) in the North Atlantic Ocean weaken the subtropical North Pacific westerlies; the associated anomalous easterlies in the subtropical west Pacific(4,5) drive a northward Ekman transport of warm water to the mode water formation area. Subduction of the warm water increases mode water temperature, influencing Northwestern Pacific upper ocean heat content and fish catches. A long pre-industrial model simulation with multiple AMV cycles and a pacemaker experiment support this mechanism-the AMV forcing alone can drive decadal variability of the mode water. Thus, the AMV provides important memory for prediction of decadal climate and ecosystem variability in the Pacific Ocean.