Parallel decadal variability of inferred water temperatures for Northern and Southern Hemisphere intermediate water masses

We use a novel proxy (growth rates of long-lived deep water fish, orange roughy) to reconstruct inferred water temperatures of intermediate water masses in both Northern and Southern Hemispheres since the mid-1800s. The data are consistent with instrumental records showing long-term warming in the Northern Hemisphere but also indicate decadal variability of intermediate depth temperatures that is coherent across the two hemispheres. This variability correlates with the dominant subpolar annular mode in each hemisphere and implies a bihemispheric oceanic response to external forcing that influences the properties of intermediate depth water masses. Key Points Growth rates of deep-sea fish proxy intermediate depth water temperature Long-term warming in North Atlantic but variable temperatures in South Pacific Parallel variability implies hemispheres respond simultaneously to forcing