M2 Decadal and Multidecadal Variability

Topic

Decadal and multidecadal variability in the climate system manifests as low-frequency modes that modulate mean climate conditions over entire decades—unlike the El Niño-Southern Oscillation, which modulates these same mean conditions in cycles lasting only a few years. The most significant low-frequency modes of this decadal and multidecadal variability are the Pacific Decadal Oscillation, with a dominant period of twenty to thirty years, and the Atlantic Multidecadal Oscillation, with a period of sixty to eighty years. Unlike the El Niño-Southern Oscillation—a well-defined, internally coupled oscillator—these low-frequency modes are not well-defined, purely internal oscillators; rather, they are patterns of variability emerging from the interaction between the deep ocean and the atmosphere, and are possibly also modulated by external forcing, such as volcanic activity, solar forcing, or aerosols.

The Pacific Decadal Oscillation can be understood as the dominant mode of sea-surface temperature variability in the extratropical North Pacific, and it arises physically from two distinct mechanisms. The first mechanism is a stochastic integration process: the surface ocean of the North Pacific integrates—much like a time-accumulated average—the high-frequency atmospheric forcing it receives daily. This is analogous to how a flywheel absorbs rapid, random impulses and converts them into much slower, smoother motion. On its own, this stochastic integration produces sea-surface temperature variability in which slow, low-frequency fluctuations possess far more energy than rapid, high-frequency fluctuations—a pattern known as "red noise"—without the Pacific Decadal Oscillation requiring any internal oscillator of its own to generate such low-frequency variability. The second mechanism driving the Pacific Decadal Oscillation involves low-frequency ocean-atmosphere feedbacks, mediated by the North Pacific subtropical gyre circulation and by slow, westward-propagating oceanic modes—similar to those that redistribute the thermocline during the El Niño-Southern Oscillation, but operating here on a much slower timescale. These low-frequency ocean-atmosphere feedbacks complement the previously described stochastic integration process; together, they shape the full spatial and temporal pattern of the Pacific Decadal Oscillation.

The Atlantic Multidecadal Oscillation is linked to multidecadal variations in the strength of the Atlantic Meridional Overturning Circulation—the very circulation that transports heat into the North Atlantic: a stronger circulation warms the North Atlantic, while a weaker one cools it. This warming or cooling of the North Atlantic driven by the Atlantic Multidecadal Oscillation has global teleconnections, modulating precipitation over the Sahel, Atlantic hurricane activity, Asian monsoons, and Arctic temperatures.

Future projections suggest that the Atlantic Meridional Overturning Circulation will weaken due to anthropogenic warming, potentially fundamentally altering the Atlantic Multidecadal Oscillation and its associated global teleconnections. Disentangling the component of the observed Atlantic Multidecadal Oscillation attributable to internal variability from the component resulting from aerosol or greenhouse gas forcing remains an active challenge in climate attribution—much like the challenge of distinguishing internally generated low-frequency variability from potential external forcing in the Pacific Decadal Oscillation described earlier.

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