Why We Observe the Ocean

Air-Sea Exchanges of Heat, Momentum, and Freshwater

Annual Mean Heat Flux (from Ocean to Atmosphere) in 2010.

• The ocean, which stores the bulk of the sun’s energy absorbed by the planet, communicates with the atmosphere via exchanges across the ocean surface. These air-sea fluxes need to be quantified in order to identify changes in forcing functions driving ocean and atmospheric conditions, and to elucidate oceanic influences on the global water cycle.

• Ocean-atmosphere exchange of heat, water and momentum drive the circulation of the ocean, which redistributes heat (e.g., from the tropics to the poles), thereby influencing global and regional climate. Ocean-atmosphere exchange of heat and freshwater alters the density of the surface water; cooling of the ocean makes the ocean denser, evaporation makes the ocean saltier and, hence, denser, while heating of the ocean and precipitation have the opposite effect. These air-sea exchanges start a cascade of phenomena in which dense surface water sinks thereby feeding the large-scale meridional overturning circulation. On the other hand, surface winds distort the sea surface and drive the large-scale upper ocean circulation, which also transports heat and freshwater. Surface water blown away from the shore in coastal regions causes upwelling of nutrients that influences fisheries.

Time series of annual-mean globally averaged evaporation and precipitation over the ocean. Evaporation (E) from the WHOI OAFlux (Objectively Analyzed air-sea Fluxes) project. Precipitation (P) from the NASA GPCP (Global Precipitation Climatology Project).

• Heat taken up by the ocean surface layer in the summer is released back to the atmosphere in the winter; evaporation cools the ocean and warms the atmosphere (via subsequent heat release caused by condensation to precipitable liquid water), with the net effect of mitigating seasonal extremes in atmospheric temperature.

• Evaporation over the ocean exceeds precipitation over the ocean, resulting in net transport of moisture to land. As the ocean warms, the hydrological cycle is expected to intensify, with consequences for rain and snowfall over land. Via this mechanism, the oceans capture large amounts of the sun’s energy and transfer it to land.

• Ocean-atmosphere exchange of carbon dioxide by diffusion across the ocean surface results in ocean uptake of between a third and a half of all anthropogenic CO2 emissions.