Rethinking the definitions of modes of decadal climate variability

The role of external (radiative) forcing factors and internal unforced modes of (ocean) variability in the instrumental global temperature record is still debated. While recent findings point towards a larger contribution from changes in external forcing in general, the definition of indices which aim at describing low-frequency North Pacific and North Atlantic could be improved in particular.

Most commonly, the Atlantic Multidecadal Variability (AMV) and the Pacific Decadal Variability (PDV) index are used to approximate the observed decadal or multi-decadal variability. All current definitions of the AMV contain a substantial fraction of radiatively forced signal. If instead, an index definition that is centered over the AMOC-slowdown region is used - normalized against Northern Hemisphere SSTs - the forced component can be largely eliminated. This new index, which we have introduced in Haustein et al. (2019) and coined North Atlantic Variability Index (NAVI), is likely a good indicator for the true AMOC slowdown or variability for that matter.

The PDV is defined by the leading pattern (EOF) of the North Pacific SST anomalies, normalized against global SSTs. As with AMV, artefacts from external forcing variations cannot be completely eliminated. The same is true for indices such as the Multivariate ENSO index (MEI) which use more than one component to describe Pacific ocean variability. In order to obtain an index that sensibly characterizes ENSO-related variations, both in North and Central Pacific, we suggest to use the leading pattern (EOF) of the North and Central Pacific SSTs, but utilize a 30-year sliding base period of the underlying SST anomalies. This way, the remaining external signal is robustly removed. The same has been proposed by Eric Webb for an updated MEI definition.

We discuss the repercussion that these novel formulations have for simulating the global surface temperature in a state-of-the-art two-box response model framework.