Global-Scale Decadal Hyper Modes

This article discusses the idea of stochastic global-scale multi-decadal Hyper Modes. The chapter will first illustrate the spatial pattern and time-scales of the leading mode of global-scale multi-decadal climate variability in observations and model simulations. It is further shown that the main characteristic of the leading mode of global-scale multi-decadal variability can be simulated in the absence of any ocean dynamics and by atmospheric teleconnections only. In a simple stochastic model for the Hyper Modes, local (basin-wide) modes of variability act mostly on interannual time-scales forced by atmospheric white noise. Most of these modes have a weak link to the tropical regions via atmospheric teleconnections, which on longer (decadal) time-scales become more dominant. The tropical link provides a synchronization of the local modes to near global multi-decadal Hyper Modes. The persistence of higher latitudes SST variability with an exponentially decreasing mixing into the deeper oceans leads to a power spectrum with a low-frequency tail and different slope compared to a red noise power spectrum. The main elements of this stochastic model do not involve any ocean dynamics beyond local vertical mixing.