Detecting changes in the meridional overturning circulation at 26°N in the Atlantic from observations

We present an observation-based detection variable for the meridional overturning circulation (MOC) at 26°N in the Atlantic. The detection variable is derived by projecting a climate signal onto a pattern of natural variability (Hasselmann, 1979; Hegerl et al., 1996; Baehr et al., 2007). For the MOC, this method so far not been tested solely relying on observations. Here, we test the method relying solely on existing observations of the zonal density gradient at 26°N. For the fixed spatial pattern of natural variability, we use observations available from the hydrographic occupations of the zonal transect in 1957 and 2004. For the climate signal, we use observations from the RAPID/MOCHA array. We find that the method can not only be meaningful applied to the observations, but also yields similar results with respect to the magnitude of variability. This opens the prospect of timely and reliable detection of MOC changes at 26°N in the Atlantic based on the currently implemented observing system.