Decadal Climate Variability

The physical Earth System consists of the atmosphere, the hydrosphere (oceans and rivers), the cryosphere (sea- and land-ice and snow), and the biosphere (land surface-vegetation). Interactions within and among these Earth System components, forced by solar radiation, generates the Earth’s climate. These interactions and variations in the Earth System’s radiative forcings produce variations in climate state, characterized by precipitation, temperature, winds, storminess, etc.. Timescales of such variations are determined by characteristic properties of the Earth System components, such as thermal and mechanical inertia, and adjustment timescales of each component’s mass, momentum, and energy. Because air is lighter and can store less heat per unit mass than land, ice, and water, it has faster characteristic adjustment timescales than the others. Characteristic adjustment timescales of the atmosphere and the oceans also depend on the Earth’s rotation speed and thus on latitude. Typical adjustment timescales are days to weeks for the atmosphere, weeks to years for the biosphere, months to decades for the ice, and months to centuries for the oceans. Therefore, perturbations to the atmosphere can produce climate variability at days to weeks timescales whereas perturbations to the slower Earth System components can produce climate variability at months to centuries timescales.

Radiative forcings on and within the Earth System also vary. Solar radiation at the top of the atmosphere is known to vary at years to millennia timescales; variations or changes in atmospheric constituents, such as ozone, carbon dioxide, methane, and aerosols, can also cause radiative forcings within the Earth System to vary or change, possibly causing climate variations or climate change.

Known manifestations of societal impacts of decade-to-century scale climate variations go back at least 2400 years before the present. Meton, a water resources engineer in Athens around 400 B.C., speculated about the possible influence of solar variability, as he observed in sunspot variability, on rainfall and, consequently, on the Athenian water supply. In the 19th and 20th centuries, a large body of literature developed on observed relationships between climate variability in various parts of the world and solar and lunar influences primarily at decade-to-century timescales. In the last two decades, as our knowledge of the Earth System has expanded, other mechanisms of decade-to-century scale variability have been proposed in which known attributes of Earth System components generate such variability without or with accompanying variability in external forcings on the Earth System.

Some of the better-known examples of decade-to-century scale climate variability are the “The Little Ice Age” in Europe and North America from the mid-17th century to the early 18th century, the 1930s “Dust Bowl” droughts in the US, the long-running droughts in the Sahel region of Africa in the last two decades, and long-term variability and changes in the North Atlantic Oscillation which influences weather and climate in eastern North America and Europe. “The Little Ice Age” is believed to have been caused by a relative minimum in solar radiation whereas the “Dust Bowl”, the Sahel droughts, the North Atlantic Oscillation variability, and other such long-term climate events are believed to be caused by ocean-atmosphere-land interactions, especially at tropical latitudes. Therefore, CRCES scientists are heavily engaged in research on such interactions in the central North Pacific (Pacific Decadal Oscillation-PDO), the West Pacific Warm Pool (WPWP) region, the tropical-subtropical Atlantic Ocean (Tropical Atlantic Gradient-TAG), and the decadal variability of the El Niño-Southern Oscillation (ENSO).

Below are descriptions and figures for each of the four DCVs mentioned above.  Each figure shows monthly indices from January 1900 through December 2018.