E10 Atmosphere-Ocean Interaction

Topic

Ocean-atmosphere interaction is the most important coupling mechanism within the climate system on timescales ranging from days to decades: the ocean transfers heat, moisture, and momentum to—or extracts them from—the atmosphere through turbulent fluxes occurring right at the interface between the two. Sea surface temperature modifies the atmospheric convection occurring above it from below: high sea surface temperatures promote air convergence and convection, whereas low temperatures stabilize the overlying atmosphere and inhibit such convection. This dependency is so strong that the Intertropical Convergence Zone—the band where winds from both hemispheres meet—closely tracks the region of highest sea surface temperature.

The El Niño-Southern Oscillation phenomenon is the dominant mode of this ocean-atmosphere interaction on an interannual scale: small perturbations in the trade winds blowing across the tropical Pacific can grow into a full-blown El Niño or La Niña event, driven by the Bjerknes feedback. This instability operates as a self-reinforcing loop: trade winds alter sea surface temperature; that temperature, in turn, modifies atmospheric convection in the manner previously described; and that convection feeds back to the trade winds that initiated it, either strengthening or weakening them depending on the direction of the initial perturbation. When this loop intensifies sufficiently, it causes warming of two to four degrees Celsius in the eastern Pacific, triggering climate teleconnections that span the globe. The complete El Niño-Southern Oscillation cycle has a period of three to seven years and can be described using mathematical models of coupled ocean-atmosphere oscillators. Tropical cyclones are another manifestation of this same ocean-atmosphere interaction, albeit on a much smaller and faster scale than El Niño-Southern Oscillation: they extract energy from the warm ocean via the same wind-induced surface heat exchange mechanism that drives their intensification—whereby evaporation, enhanced by the storm's own strong winds, supplies the energy fueling convection within the spiral bands and the eyewall. Ultimately, the potential intensity of a tropical cyclone depends on the equivalent potential temperature of the air masses entering at its base—a value determined by the sea surface temperature where that air originated; the higher this equivalent potential temperature, the higher the thermodynamic limit of intensity the cyclone can reach.

Thus, ranging from the years-long cycle required for an El Niño-Southern Oscillation event to develop to the mere days a tropical cyclone needs to intensify, the underlying mechanism of ocean-atmosphere interaction remains the same: sea surface temperature alters convection, that convection modifies the winds, and those winds in turn alter the sea surface temperature—forming feedback loops that close on vastly different timescales yet share the same fundamental coupling between ocean and atmosphere.

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