E6 OceanAtmosphereCryosphere Couplings

Topic

The climate system is fundamentally coupled: the ocean, the atmosphere, and the cryosphere continuously exchange energy, momentum, and mass with one another, and no single component of this coupled system can be fully understood without the other two. The most widely studied example of this ocean-atmosphere coupling is the El Niño-Southern Oscillation, in which a small perturbation in wind stress along the equator triggers a massive reorganization of the entire coupled system on a hemispheric scale.

A classic model of this ocean-atmosphere coupling, formulated in 1987—along with the simplified versions developed subsequently—captures the essence of the El Niño-Southern Oscillation through two mutually reinforcing relationships: surface winds respond to the sea-surface temperature gradient along the equator, and sea-surface temperature, in turn, responds to the ocean circulation driven by those very surface winds. When surface winds and sea-surface temperatures reinforce each other rather than stabilizing, this mutual feedback instability can produce a self-sustaining cycle with a period of three to seven years—precisely the cycle that characterizes the El Niño-Southern Oscillation.

The cryosphere—the third component of the coupled climate system mentioned earlier—acts as a signal amplifier at high latitudes: sea ice reflects nearly eighty percent of the incoming solar radiation, whereas the open ocean exposed when the sea ice disappears reflects only about seven percent of that same solar radiation. Due to the vast difference in the fraction of solar radiation reflected by sea ice versus open ocean, the loss of sea ice triggers a powerful local positive feedback loop: as more sea ice is lost, more low-reflectivity open ocean is exposed; this open ocean absorbs far more solar radiation than the sea ice did, further warming the region and leading to even greater sea ice loss.

On timescales far longer than the three-to-seven-year cycle of the El Niño-Southern Oscillation or the local sea-ice feedback, the deep ocean acts as a thermal buffer for the coupled climate system, owing to its immense heat-storage capacity. Because the deep ocean absorbs and releases heat much more slowly than the atmosphere, the ocean surface, or the cryosphere, it delays the coupled climate system's full response to any disturbance by several decades. Thus, the three components of the coupled climate system introduced at the outset—the ocean, the atmosphere, and the cryosphere—exchange energy, momentum, and mass across three distinct timescales: the coupling between the ocean and the atmosphere generates cycles lasting a few years, such as the El Niño-Southern Oscillation; the cryosphere amplifies that signal locally almost immediately; and the deep ocean delays and dampens it over the course of decades.

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