Are observed decadal changes in intermediate water masses a signature of anthropogenic climate change?

Recent observations have shown relatively large changes in the temperature and salinity of intermediate water masses in the ocean on decadal timescale s. We compare the observed changes with simulations of the coupled climate model HadCM3. In simulations driven by anthropogenic forcing, we see signif icant changes in intermediate waters in the Southern Ocean which are simila r in both pattern and magnitude to the observations. Our results suggest th at the observed changes are most likely to be a signal of anthropogenic cli mate change. The strong signal in the Southern Ocean, which is detectable i n the model from the 1980s, is in marked contrast with the intermediate wat ers of the Northern hemisphere oceans, where internal climate variability i s large and a signal of anthropogenic climate change is not detectable in t he model until 2020 at the earliest. Our results suggest that intermediate waters, particularly those of the Southern hemisphere, are a potentially se nsitive indicator of anthropogenic climate change, and could be an importan t part of a climate monitoring network.