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 timescales. We compare the observed changes with simulations of the coupled climate model HadCM3. In simulations driven by anthropogenic forcing, we see significant changes in intermediate waters in the Southern Ocean which are similar in both pattern and magnitude to the observations. Our results suggest that the observed changes are most likely to be a signal of anthropogenic climate change. The strong signal in the Southern Ocean, which is detectable in the model from the 1980s, is in marked contrast with the intermediate waters of the Northern hemisphere oceans, where internal climate variability is large and a signal of anthropogenic climate change is not detectable in the model until 2020 at the earliest. Our results suggest that intermediate waters, particularly those of the Southern hemisphere, are a potentially sensitive indicator of anthropogenic climate change, and could be an important part of a climate monitoring network.