M4 Oscillations and Climatic Variability

Topic

Oceanic climate oscillations are modes of coupled ocean-atmosphere variability that modulate climate—both regionally and globally—on timescales ranging from years to decades. The El Niño-Southern Oscillation is the dominant mode of this variability on an interannual scale: during an El Niño event, the equatorial Pacific thermocline flattens—deepening in the east and shoaling in the west—while sea surface temperatures in the eastern Pacific rise two to four degrees Celsius above average and trade winds weaken. This entire reconfiguration generates precipitation and temperature anomalies that are felt, via teleconnections, in regions far removed from the Pacific itself.

The Bjerknes instability describes the very mechanism that amplifies the El Niño-Southern Oscillation; it is a self-reinforcing loop: an initial disturbance that warms sea surface temperatures in the central Pacific causes trade winds to weaken; this weakening reduces equatorial upwelling, which in turn causes sea surface temperatures to warm even further, closing the loop back to the initial temperature state in a positive feedback cycle that intensifies with each repetition.

This cycle is regulated by two types of oceanic waves that adjust the depth of the thermocline: oceanic Kelvin waves, which propagate this adjustment eastward at speeds of two to three meters per second, and equatorial Rossby waves, which propagate the adjustment in the opposite direction—westward—at a mere half-meter per second. It is the combination of these vastly different speeds that causes the complete El Niño-Southern Oscillation cycle to repeat every three to seven years. The Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation are modes of variability operating on multi-decadal timescales; their very existence modulates the intensity of the El Niño-Southern Oscillation and the climate of their respective ocean basins at a pace far slower than that of the interannual cycle itself. This low-frequency ocean-atmosphere coupling also involves heat stored in the deep ocean, which acts as a form of long-term memory for the entire climate system.

The expected behavior of these oceanic climate oscillations involves an irregular alternation between El Niño and La Niña episodes—occurring at intervals of three to seven years—with global impacts on precipitation, temperature, and sea level across virtually the entire planet.

Thus, from the flattening of the thermocline—with which this document begins—to the irregular alternation between El Niño and La Niña—with which it concludes—and encompassing the Bjerknes feedback loop and the decadal modulation exerted by the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation, oceanic climate oscillations drive an ocean-atmosphere coupling that distributes the same temperature and precipitation anomalies across the globe for years on end—anomalies that originated merely as a perturbation in sea surface temperature in the central Pacific.

ID:2119