Decadal and Multidecadal Variability

Slow fluctuations of the ocean and atmosphere over decades that shape long-lived climate regimes and can mask or reinforce greenhouse warming for a time.

Definition

Decadal and multidecadal variability is internally generated climate fluctuation on timescales of about ten to many tens of years, often associated with slow changes in ocean circulation and heat storage.

Scope

This topic covers the lowest-frequency internal variability of the climate system, with timescales of roughly a decade to many decades. It treats patterns such as the Pacific Decadal Oscillation and Atlantic Multidecadal Variability, the ocean-driven mechanisms that may sustain them, their footprints on regional temperature, rainfall, and ecosystems, and their importance for interpreting hiatus periods and accelerations in the observed warming trend.

Core questions

• What mechanisms generate variability on decadal timescales?
• How do patterns such as the Pacific Decadal Oscillation affect regional climate?
• How does decadal variability modulate the global warming trend?
• How predictable is the climate on decadal timescales?

Key theories

Ocean memory and slow modes

The large heat capacity and slow circulation of the ocean store and release heat over years to decades, providing the memory that sustains low-frequency climate variability.

Modulation of the forced trend

Decadal variability can temporarily add to or subtract from greenhouse warming, producing apparent accelerations and slowdowns that complicate short-term trend interpretation.

Mechanisms

The ocean's large heat capacity and the slow adjustment of its circulation let anomalies persist and re-emerge over years to decades, producing basin-scale patterns such as the Pacific Decadal Oscillation and Atlantic Multidecadal Variability. These patterns shift the distribution of warm and cool surface waters, altering atmospheric circulation and regional climate, and because they redistribute heat between ocean and atmosphere they can mask or amplify the underlying forced warming for periods of a decade or more.

Clinical relevance

Decadal variability governs multi-year regimes in droughts, fisheries, hurricane activity, and regional temperature, and it must be accounted for when judging whether a short-term trend reflects forced change or natural fluctuation.

Evidence & guidelines

The IPCC Sixth Assessment Report assesses that internal decadal variability contributed to the slower surface warming of the early 2000s and that distinguishing it from forced change requires multidecadal records.

History

Multidecadal swings in fisheries and temperature had long been noticed regionally, and Mantua and colleagues formalized the Pacific Decadal Oscillation in 1997; the early-2000s slowdown in surface warming sharpened scientific attention on how ocean-driven decadal variability modulates the forced trend.

Debates

Forced versus internal origin of multidecadal Atlantic variability

Whether Atlantic Multidecadal Variability arises mainly from internal ocean dynamics or is substantially driven by external aerosol and volcanic forcing is actively debated.

Key figures

• Nathan Mantua
• John Wallace
• Clara Deser
• Thomas Delworth

Seminal works

• mantua1997
• hartmann2016

Frequently asked questions

What is the Pacific Decadal Oscillation?

It is a long-lived pattern of North Pacific sea-surface temperature variability that shifts between warm and cool phases over one to several decades, affecting climate and ecosystems.

Can decadal variability hide global warming?

Temporarily, yes; phases that store extra heat in the deep ocean can slow surface warming for a decade or so, while opposite phases can accelerate it, but the long-term forced trend persists.