E7 Ocean - Atmosphere Interaction

Topic

Ocean-atmosphere interaction is the thermal, mechanical, and chemical coupling between these two fluids sharing a common interface. Heat fluxes—latent, sensible, and radiative—transfer energy between the ocean and the atmosphere at rates dependent on the temperature and humidity differences between the air and the sea. On average, the ocean receives approximately 168 watts per square meter of net solar radiation and loses an equal amount distributed across three pathways: about 88 watts per square meter as latent heat, about 10 watts per square meter as sensible heat, and about 66 watts per square meter as net longwave radiation. However, this balance varies enormously by region and season, meaning no single point in the ocean experiences exactly the same heat exchange with the overlying atmosphere.

The momentum flux transferred from the wind to the ocean—known as wind stress—drives both surface circulation and waves. This stress depends on air density, a drag coefficient specific to the sea surface, and the square of the wind speed measured at a height of ten meters; consequently, doubling the wind speed quadruples the stress exerted on the water.

The feedback between sea surface temperature and wind is a bidirectional coupling mechanism that forms a closed loop: warmer sea surface temperatures induce increased atmospheric convection, which modifies the wind field; this modified wind field, in turn, alters both ocean circulation and the sea surface temperature that initiated the loop, thereby perpetuating the coupling indefinitely as long as underlying conditions continue to fuel it. This feedback is critical for both the El Niño-Southern Oscillation and the Madden-Julian Oscillation; on much slower timescales—ranging from decades to centuries—that same coupling involves the heat stored in the deep ocean, which acts as a long-term memory for the entire coupled system.

Oceanic evaporation is by far the largest source of atmospheric water vapor—accounting for nearly eighty-five percent of the total—and depends on air density, a transfer coefficient, wind speed at a height of ten meters, and the difference between the saturation humidity at the sea surface temperature and the actual humidity of the air. This evaporation rate varies between one and three millimeters per day in the tropics and along the paths of warm currents; it is precisely this humidity differential that causes drier air over warm seas to evaporate far more water than air that is already near saturation.

Furthermore, the expected behavior of this entire ocean-atmosphere interaction varies with latitude: in the tropics, sea surface temperature and surface wind are negatively correlated—meaning winds are weaker where the sea is warmest, as active convection consumes the wind's kinetic energy—causing the tropics and mid-latitudes to respond in opposite ways to the same sea surface temperature; in the mid-latitudes, by contrast, this correlation becomes positive. Thus, from the heat fluxes that open this document to the correlation between sea surface temperature and wind that concludes it, ocean-atmosphere interaction reveals the same underlying coupling at work across scales ranging from the watts per square meter of the daily energy balance to the centuries-long memory of heat stored in the deep ocean.

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