Investigating the Atlantic Meridional Overturning Circulation using boundary information

The Atlantic Meridional Overturning Circulation (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a global coupled model pre-industrial control run we outline a method for decomposing the overturning circulation as the sum of (near surface) Ekman, (external mode) bottom velocity, eastern and western boundary density components, as a function of latitude. This method proves a useful low-dimensional characterisation of the full 3-D overturning circulation, exploiting the relatively flat isopycnal structure in the interior and along the eastern boundary. The decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning and the relative role of eastern versus western contributions on different timescales.

Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total transport. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and the importance of boundary density components at decadal time-scales. Also clear is the increasing contribution of the eastern boundary component in the Southern Hemisphere with increasing time-scale, and the role of the bottom component at 30°N on all time-scales.