Skip to main content

Ocean ventilation: oxygen pathways from the ocean's surface to the ocean's interior

As land-dwelling animals, we breathe plentiful oxygen from the atmosphere, except perhaps if we are climbing a high mountain summit where oxygen is more scarce. But ocean-dwelling animals are often not as fortunate as we are when it comes to oxygen abundance.  

To begin with, oxygen as a gas is NOT easily dissolved in water. Though a liter of seawater weighs nearly 1000 times more than a liter of air, that very same liter of water contains about 40 times less oxygen than one liter of air (0.008 g versus 0.3g). 

But things can get much worse than this, because below the surface layer, oxygen can litterally go down to zero. At 300 m depth, vast expanses of the ocean contain a very small percentage of the amount of  oxygen these same waters contained when they were last found at the sea surface, in direct contact with the atmosphere. This percentage gets very close to zero in the northern Indian Ocean and the eastern equatorial Pacific, and actually reaches zero in the Black Sea.

You may ask yourself why oxygen can get so scarce in the ocean interior. The ocean’s surface mixed layer is well oxygenated because of photosynthesis and gas exchange with the atmosphere. But below the sunlit surface layer, there is no photosynthesis and no direct gas exchange with the atmosphere. Oxygen must then be carried away by ocean currents from the regions of the ocean where subtropical mode waters, intermediate waters, or deep waters are formed, towards the ocean interior. The main regions of the surface ocean that export recently ventilated and oxygenated waters to the ocean interior are shown below.   

Waters of a given density (isopycnal) within the permanent thermocline of the subtropical gyres are ventilated at the latitude where that isopycnal intersects the base of the surface mixed layer during the winter. At the end of each winter, waters near the base of the mixed layer subduct into the ocean interior, and then begin to slowly diffuse equatorward. This ventilation process is particularly intense in the high latitude range of the subtropical gyres, where subtropical mode waters (STMW) and subantarctic mode waters (SAMW) are formed This process ventilates subtropical gyres to depths of about 500 to 900m.  

In the Southern Ocean, a surface convergence zone between about 50°S and 60°S causes local sinking of surface waters and the formation of Antarctic Intermediate Water (AAIW) which ventilates water depths between about 700 and 1200m. The Southern Ocean is also home to the formation of the densest water mass of the world ocean, Antarctic Bottom Water (AABW), formed in areas of intense sea ice freezing and brine rejection around the continent of Antarctica. AABW sinks all the way to the bottom of the three major ocean basins and propagates northward.

In the northern hemisphere, ventilation processes vary quite a lot between the three major ocean basins. The formation of North Pacific Intermediate Water (NPIW) in the mixed water region of the Oyashio and Kuroshio ventilates the upper 300 to 700 meters of the North Pacific subpolar gyre. In the North Atlantic, to the north of the Denmark-Faroe-Shetland ridge, deep convection in the Greenland-Iceland-Norwegian Sea (GINSEA) is the source of Denmark Strait Overflow Water (DSOW) that spills over the Denmark Strait sill (625 m deep) and then descends to the bottom of the North Atlantic. Some of this dense water also flows northward and propagates towards the Arctic Ocean. The Labrador Sea, and to a lesser extent the Irminger Sea, is another important site of deep water formation in the North Atlantic, with convection events reaching up to 2000 m depth. This leads to the formation of North Atlantic Deep Water (NADW) which then propagates southward. The ultimate fate of oxygen-rich NADW is to either 1) upwell into a surface divergence zone of the Antarctic Circumpolar Current; 2) ventilate the Indian Ocean; 3) ventilate the Pacific Ocean.

While ventilation provides new oxygen to the ocean interior, oxygen consumption processes (animal respiration, bacterial degradation of organic matter) remove oxygen from the water column. The rate of oxygen consumption is greatest near the ocean surface, and decreases exponentially with depth. Observed oxygen concentrations in the ocean reflect a fine balance between the ventilation supply mechanisms illustrated here and the oxygen consumption mechanisms. Some ocean regions have very high oxygen consumption rates, such as the Eastern Boundary Upwelling Systems off Peru-Chile or Namibia, and this is where we tend to find the most intense and thick oxygen minimum zones (OMZs).

During the very long journey from its formation region in the northern North Atlantic, NADW remains isolated from the atmosphere and its oxygen content becomes progressively lower as we get further away from its source region. In the Indian and Pacific Oceans, a mixture of NADW and predominantly AABW propagates northwards. No deep water formation takes place in the Pacific and Indian Ocean,  so that they are less well ventilated than the North Atlantic and have lower oxygen content, as can be seen in the South to North sections of the three major ocean basins shown below. 
Projections from Earth System Models that simulate global warming from rising CO2 levels indicate that ventilation processes will likely become more sluggish in the next decades. Ocean scientists believe this would lead to lower oxygen levels in the ocean interior. Recent studies suggest that such reduced ventilation may already be underway. The quicker and the more aggressively we reduce our CO2 emissions, the better it will be from the point of view of oxygen availability in the ocean interior!

Suggested further reading:

Bopp, L., Resplandy, L., Orr, J.C., Doney, S.C., Dunne, J.P., Gehlen, M., Halloran, P., Heinze, C., Ilyina, T., Seferian, R. and Tjiputra, J., Vichi, M., 2013. Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models, Biogeosciences, 10, 6225-6245.

Breitburg, D., Levin, L. A., Oschlies, A., Grégoire, M., Chavez, F. P., Conley, D. J., Garçon, V., Gilbert, D., Gutiérrez, D., Isensee, K., Jacinto, G. S., Limburg, K. E., Montes, I., Naqvi, S. W. A., Pitcher, G. C., Rabalais, N. N., Roman, M. R., Rose, K. A., Seibel, B. A., Telszewski, M., Yasuhara, M. & Zhang, J. 2018. Declining oxygen in the global ocean and coastal waters. Science, 359, 11p.

Gilbert, D., 2017. Oceans lose oxygen. Nature, 542: 303-304.

Gilbert, D., N.N. Rabalais, R.J. Díaz, and J. Zhang, 2010. Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean. Biogeosciences, 7: 2283-2296.

Schmidtko, S., Stramma, L. Visbeck, M. 2017. Decline in global oceanic oxygen content during the past five decades. Nature, 542, 335.


Post a Comment

Comment moderation is turned on for this blog: your respectful and civil comments are welcome, whether you agree or disagree with my views. / La modération des commentaires est en vigueur sur ce blog: vos commentaires respectueux et civilisés sont bienvenus, que vous soyez d'accord ou en désaccord avec moi. / La moderación de comentarios está activada para este blog: sus comentarios respetuosos y civiles son bienvenidos, ya sea que esté de acuerdo o en desacuerdo con mis puntos de vista.

Popular posts from this blog

Climate projections to 2100 for Toronto (Ontario, Canada)

U pdated 2020-02-10.   In June 2019,  Environment and Climate Change Canada ( ECCC ) launched a new Canadian climate data portal: . Through this portal, decision makers in the private sector, municipalities, provincial and federal departments are now better equipped to make informed decisions about future development options all across Canada, taking into account projections of future climate change. In this post, my goal is simply to illustrate the types of climate data that are available for thousands of municipalities by taking the example of Canada's largest city: Toronto. A second example is provided for the city that I live in: Rimouski (in French) . I encourage decision makers to explore what information has in store for the communities they live in. Are there any takers of this challenge for Vancouver, Halifax, Calgary or Tuktoyaktuk? For the City of Toronto (43.7417°N, 79.3733°W), I present plots of historical ( 1950-2005) and plaus

CO2 emissions from all-electric, plug-in hybrid, hybrid and conventional vehicles in the USA

Updated 2019-06-02.  One of the main reasons explaining the rise in popularity of electric vehicles (EVs) is that they do not directly require the burning of fossil fuels in order to take us from point A to point B. In other words, EVs have the potential of helping us reduce CO2 emissions that are responsible for human-caused global heating. However, if the power grid from which we charge EVs requires the burning of fossil fuels (coal, natural gas, oil) in order to generate electricity, are we better off in terms of CO2 emissions to the atmosphere?  It depends. Online tool to estimate annual CO2 emissions Using grid electricity is not always the only choice for EVs; a growing number of people install solar panels on their house's roof and store excess energy in home battery storage systems. This enables them to recharge their electric cars with 100% renewable energy regardless of which state they live in. But for those who must rely on grid electricity, the U.S. Department

Points de Lagrange - places de "stationnement" spatiales

Les points de Lagrange 1 du système Soleil-Terre, au nombre de cinq, sont des endroits où l'effet combiné de la force de gravité exercée par la Terre et par le Soleil est tel que si on y plaçait un corps de très faible masse, ce corps pourrait constamment se maintenir à la même position relativement à la Terre et au Soleil. Cette situation est illustrée dans le graphique ci-bas où les cercles verts identifiés par les chiffres 1 à 5 montrent la position des points de l'espace qui ont la même vitesse de rotation angulaire que la Terre (en bleu) autour du Soleil (en jaune). Source: Anynobody CC BY-SA 3.0, Wikimedia Commons   L'oeuvre de deux hommes: Euler et Lagrange Les trois premiers points (1, 2, 3), tous situés sur la ligne joignant la Terre au Soleil, furent découverts par le mathématicien et physicien suisse du XVIIIème siècle Leonhard Euler (1707-1783). Les deux derniers points (4 et 5) furent quant à eux découverts par le mathématicien et physicien  Jose