OCEAN CIRCULATION
Ocean circulation is a key regulator of climate by storing and transporting heat, carbon, nutrients and freshwater all around the world . Complex and diverse mechanisms interact with one another to produce this circulation and define its properties.
Ocean circulation can be conceptually divided into two main components: a fast and energetic wind-driven surface circulation, and a slow and large density-driven circulation which dominates the deep sea.
Wind-driven circulation is by far the most dynamic. Blowing wind produces currents at the surface of the ocean which are oriented at 90° to its direction (on its right in the Northern Hemisphere and on its left in the Southern Hemisphere) due to the Earth rotation. As a consequence, it creates zones of convergence or divergence of ocean currents at the point where they meet. Divergence of currents will create an upwelling phase (interior waters reach the surface) and convergence a downwelling phase (surface waters sink in the interior ocean), linking surface and interior waters.
The slow and deep circulation is largely driven by water density, and thus its temperature and salinity. It acts on the ocean as a whole and has a major influence on the abyssal properties where wind-driven circulation has no effect. However, this circulation is slow and generates weak currents, it is therefore more difficult to observe: a single drop of water travels 1,000 years to close the global overturning circulation.
The large-scale circulation is relatively stable on long timescales. At some very specific locations – mainly in the Northern Atlantic and around Antarctica – surface waters become denser and sink to the depths. Densification occurs due to both cooling surface waters and increasing salinity, the latter as a result of the removal of freshwater and the formation of ice. Surface waters are then pulled up to replace the sinking ones. How waters upwell from depths to the surface is still unclear. As stated above, zones of divergence of waters are of critical importance for these phenomena but near-seafloor turbulence also plays a major role. These mechanisms are still poorly understood and their spatial variability remains largely unknown.
Oceanic circulation is very sensitive to the global freshwater flux. This flux can be described as the difference between [Evaporation + Sea Ice Formation], which enhances salinity, and [Precipitation + Runoff + Ice melt], which decreases salinity. Global warming will undoubtedly lead to more ice melting in the poles and thus larger additions of freshwaters in the ocean at high latitudes. This input of freshwater, by decreasing surface water density near the poles, could limit downwelling, prevent deep waters formation, slowing down global circulation.
Such a process could have tremendous consequences for our societies. It would mean less carbon and heat uptake by the ocean and thus higher rates of both carbon and heat in the atmosphere. It could potentially accelerate global warming and enhance its negative effects.
More generally, it is important to note that interactions between oceanic circulation and climate are still poorly understood: more observations, an increased understanding and reliable numerical models of oceanic circulation are needed at different space and time scales. Such progress could dramatically improve IPCC global climate projections.
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