DownwellingProzesse

DownwellingProzesse denotes a class of oceanographic processes that transport surface water downward, producing downwelling in marine environments. The term is used in some texts to describe the physical mechanisms that cause surface water to become submerged and incorporated into deeper water masses, often near coasts or at frontal zones.

The primary mechanisms are wind-driven and density-driven. Wind stress at the surface can induce Ekman transport that drives water toward the coast or into convergent regions, generating downward motion. In regions of strong surface convergence, buoyant water is compacted and sinks. Density differences associated with temperature and salinity can create unstable or stratified layers, leading to subduction or vertical mixing that carries surface water downward. Internal wave breaking and turbulent mixing also contribute to localized downwelling, particularly near topographic features such as shelf breaks and continental slopes.

Spatially, coastal downwelling occurs along shorelines where onshore Ekman transport dominates, while open-ocean downwelling occurs at frontal zones or within subtropical gyres where convergent geostrophic flows drive sinking. Downwelling is a key component of the global meridional overturning circulation, helping ventilate deep ocean layers with oxygen and redistributing heat and carbon.

Observation and modeling rely on instruments such as Argo floats, CTD casts, and satellite altimetry, supplemented by numerical ocean models to estimate vertical fluxes. Because vertical velocities are typically small and transient, direct measurement is challenging, and inferences rely on tracers and indirect methods.

See also: downwelling, upwelling, Ekman transport, thermohaline circulation, ocean ventilation.