olelog

The Sea contains a much higher percentage of oxygen (34 %) than the atmosphere (ca. 21 %).
Most of the oxygen is of course bound to hydrogen in water molecules. A smaller part is bound in molecules of other substances (like calcium carbonate), and only part of the oxygen is freely available for respiration as oxygen gas dissolved in the water. Oxygen occurs as a by-product of photosynthesis in plants (plankton in the open sea). Oxygen is also dissolved at the interface between the sea surface and the atmosphere.
Most of the oxygen-rich ocean water and the animal life which uses oxygen for respiration is therefore found near the surface. As one descends into the depths, the amount of dissolved oxygen in the water drops rapidly, and so does animal life. In a zone occurring at depths of about 200 to 1,000 metres, depending on local circumstances, oxygen saturation in seawater in the ocean is at its lowest. This zone is called the Oxygen Minimum Zone (sometime referred to as the shadow zone). From the oxygen minimum layer downward the amount of dissolved oxygen increases initially, and another decrease occurs near the bottom.

Please note that oxygen profiles like the one shown here vary from location to location. Just now I would like to stress that the occurrence as such of a minimum oxygen zone is a natural phenomenon due to destruction of dissolved oxygen by respiration (something more or less like this CH₂O + O₂ = CO₂ + H₂O). I’ll come back to the degree of oxygen depletion later.

Surface ocean waters generally have oxygen concentrations close to equilibrium with the Earth's atmosphere. In general, colder waters hold more oxygen than warmer waters. This is obvious in the following image of annual mean sea surface dissolved oxygen for the World Ocean. Data from the World Ocean Atlas 2001.

Dissolved oxygen is measured in units as millilitres O₂ per litre (ml/l), millimoles O₂ per litre (mmol/l), milligrams O₂ per litre (mg/l) and moles O₂ m^-3. For example, in freshwater under atmospheric pressure at 20°C, O₂ saturation is 9.1 mg/l. (more about the unit mole at this Wikipedia page). 1 mol of oxygen (O₂) molecules weighs approximately 32 grams. For example an oxygen concentration in surface water of 0.27 mol O₂m^-3 approximates 6 millilitre per litre.

In the oxygen minimum layers the concentration of dissolved oxygen can approach zero, a condition called suboxic. Important mobile macroorganisms are stressed or die under hypoxic conditions; that is, when oxygen concentrations drop below ~60 to 120 mmol kg^-1 (3). Hypoxia occurs at different oxygen concentrations among various species of macroorganisms, so the threshold is not precise. Water lacking dissolved oxygen (0% saturation) is termed anoxic.

As I said above colder waters hold more oxygen than warmer waters. (Salinity is another important factor with differences between fresh water and sea water). The global ocean has warmed substantially over the past 50 years. So what happens during global warming. You would expect reduced oxygen levels and the oxygen minimum zones to expand. Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies.

According to the study Expanding Oxygen-Minimum Zones in the Tropical Oceans by Stramma et al. published in Science of 2 may 2008 the oxygen minimum zones of tropical oceans are expanding, restricting habitats for fish and other marine life, the researchers found that oxygen levels here have declined significantly over the past fifty years. The study was concentrated on the eastern tropical Atlantic and the equatorial Pacific simply because these areas have the best historical data, the situation may in fact be worse (or better?) in other parts of the world’s oceans. The expected impacts on subtropical and subpolar regions are larger than in the Tropics. Long-term oxygen changes have been observed and reported in the subpolar and subtropical regions.

And now back to the eastern tropical Atlantic and the equatorial Pacific during the past 50 years. The data reveal a vertical expansion of the intermediate-depth low-oxygen zones. The oxygen decrease in the 300- to 700-m layer is 0.09 to 0.34 micromoles per kilogram per year. In the tropical North Atlantic the vertical extent of the layer with oxygen concentrations of <90 mmol kg^-1 increased 85%, from a thickness of 370 m in 1960 to 690 m in 2006. The tropical ocean oxygen minimum zones in the central and eastern tropical Atlantic and equatorial Pacific Oceans appear to have expanded and intensified during the past 50 years.

Oceanic dissolved oxygen concentrations have varied widely in the geologic past. The anoxic ocean at the end of the Permian (251 million years ago) is associated with elevated atmospheric CO₂ and massive terrestrial and oceanic extinctions.

The trends have fundamental implications for marine ecosystems and thereby fisheries.


• http://www.sciencemag.org/cgi/content/abstract/320/5876/655
• http://news.nationalgeographic.com/news/2008/05/080501-dead-zones.html
• http://www.terradaily.com/reports/Oxygen_depletion_threatens_ocean_habitats_study_999.html
• http://www.scientificblogging.com/news_releases/the_decline_in_ocean_oxygen

In my latest post I mentioned that the Madden-Julian Oscillation, or in short MJO might serve as tool to extend the weather forecasts beyond the usual 5 days.

The MJO is a cyclical wave in Earth’s atmosphere - a cyclical pattern of slow, eastward-moving waves of clouds, rainfall and large-scale atmospheric circulation anomalies that can strongly influence long-term weather patterns around the world. It is a 30-to-90-day cycle, and it spans nearly half of Earth's equator, primarily over the Indian Ocean and western Pacific. The MJO affects precipitation over the tropical monsoon regions. It affects the winter jet stream and atmospheric circulation in the Pacific/North America region, causing anomalies that can lead to extreme rainfall events. It can also change summer rainfall patterns in Mexico and South America and may trigger El Niño events.

Fortunately it now seems possible to predict the MJO itself according to an article in Science of 14 December 2007, titled A Madden-Julian Oscillation Event Realistically Simulated by a Global Cloud-Resolving Model. The results of the study demonstrate the potential making of month-long MJO predictions when global cloudresolving models with realistic initial conditions are used. The predictions are based on infrared satellite images of clouds. It is indeed expected that weather forecasts beyond 10 days could be improved if the MJO representations in global weather prediction models were more realistic.

Another article in the same issue of Science titled Deep Ocean Impact of a Madden-Julian Oscillation Observed by Argo Floats shows that these atmospheric waves also affect the oceans - and to a greater depth than previously known. The authors used a data set of unprecedented size obtained from autonomous, free-drifting instruments, called Argo floats, to show that the surface wind stress associated with the MJO can force eastward-propagating oceanic Kelvin waves that extend to a depth of at least 1500 meters and that have amplitudes of as much as six times those of annual-cycle Kelvin waves. These amplitudes are significantly greater than those predicted by ocean models, so that the MJO could affect a much larger volume of the Pacific Ocean than just the ocean surface.

Argo floats are described here. and the use of these floats in oceanic studies mentioned in an editorial in Science of 15 December 2006. I wrote about oceanic Kelvin waves here.

Oceanic equatorial Kelvin waves in the central and eastern Pacific are forced by MJO wind stress anomalies. The MJO can also influence the deep ocean in high latitudes and has has a direct impact on the ocean biosphere, with implications for the fishing industry.

• http://www.sciencemag.org/cgi/content/short/318/5857/1763
• http://www.sciencemag.org/cgi/content/short/318/5857/1765
• http://www.sciencemag.org/cgi/content/summary/314/5806/1657
• http://www.argo.ucsd.edu/

After the record-breaking hurricane season in 2005 it was suspected that global warming is leading to stronger and more frequent tropical storms (hurricanes, typhoons, and cyclones). Satellites have only given us sufficient data over the last twenty years or so, but nevertheless recently documented trends in the existing records of hurricane intensity and their relationship to increasing sea surface temperatures suggest that hurricane intensity may be increasing due to global warming. It seems however that the Atlantic is more vulnerable to climate change than the Pacific and the Indian Ocean.

According to a paper published on 29 November 2007 in the Bulletin of the American Meteorological Society warmer ocean water is only part of the story. Wind and precipitation over the ocean also play their role. The atmosphere and the ocean interact with each other in a distinct way creating a special basin wide north south circulation pattern, called Meridional Mode (meaning along longitudinal meridians and thus north/south and south/north directed).

In a study published in February 2007, Kossin and his co-authors created a record of hurricane data that accounted for the significant improvement in storm detection that followed the advent of weather satellites. An analysis of this recalibrated data showed that hurricanes have become stronger and more frequent in the Atlantic Ocean over the last two decades. The increasing trend, however, is harder to identify in the world's other oceans. The other oceanic basins have their own modes of ocean-atmosphere variability.

Because higher sea surface temperatures in the Atlantic act in concert with the Atlantic Meridional Mode, Vimont and Kossin suggest that Atlantic hurricanes will be more sensitive to climate changes than storms in other ocean basins.

• http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FBAMS-88-11-1767
• http://www.news.wisc.edu/14493
• http://www.physorg.com/news115574276.html
• http://www.sciencedaily.com/releases/2007/11/071129183753.htm
• http://www.terradaily.com/reports/Recipe_For_A_Storm_The_Ingredients_For_More_Powerful_Atlantic_Hurricanes_999.html
• http://www.agu.org/pubs/crossref/2007/2007GL029683.shtml
• http://www.aos.wisc.edu/~dvimont/Research/


Meridional: In meteorology, a flow, average, or functional variation taken in a direction that is parallel to a line of longitude; along a meridian; northerly or southerly; as opposed to zonal.

Meridian: An imaginary great circle on the earth's surface passing through the North and South geographic poles. All points on the same meridian have the same longitude.




climate change, oceanography, hurricanes

In the global carbon cycle the sea absorbs a proportion of the atmospheric CO₂ but also releases CO₂ into the atmosphere again. About half of the man made emission of CO₂ is absorbed naturally by the oceans. It is therefor important to understand how the exchange of CO₂ between the ocean and the atmosphere functions with regard to a world that is warming up. A newly available study shows that the ocean was able to store more CO₂ during the ice age than it can today.

To find out how the situation has changed compared to the last ice age, researchers studied mud from the sub-Arctic Pacific Ocean lying approximately one metre below the present sea bed and about 20,000 years old, and thus from the ice age. The Wisconsin (in North America), Devensian (in the British Isles), Midlandian (in Ireland), Würm (in the Alps), and Weichsel (in northern central Europe) glaciations, which ended around 10,000 BC, are the world's most recent glaciations (periods during which the ice caps extend towards the equator). The maximum extent was reached about 18,000 years ago.

Foraminifera, microscopic floating organisms that live in shells, in Earth's oceans hold clues to global climate change. See Image.

Tiny single-celled organisms with limestone shells known as foraminifera were selected from this mud under a microscope and afterwards measured with mass spectrometers. These foraminifers locked in the carbon isotope signature of the seawater of their day. The research team has now been able to measure the ¹⁴C content precisely. This enabled them to show that the water in the ocean depths exchanged less CO₂ with the atmosphere than at present. They found unusually clear evidence that this water captured more CO₂ from the atmosphere than the water at the present day. The latest research results show that the oceans are generally able to fix more CO₂ when they are cold. Or said the other way round: Oceans that warm up as a result of climate change release more CO₂ into the atmosphere. The more CO₂ we emit to the atmosphere, the more CO₂ the oceans also release to the atmosphere.

Reference: Galbraith et al. Carbon dioxide release from the North Pacific abyss during the last deglaciation, Nature, 449, 890-894. Abstract.

• http://www.sciencedaily.com/releases/2007/10/071023163953.htm
  

Many of us are used to think of ocean (surface) currents as either cold or warm, and moving in the same direction all the year round. Warm ocean currents are corridors of warm water moving from the tropics poleward. Cold ocean currents are corridors of cold water moving from higher latitudes toward the equator.

As the major ocean (surface) currents are wind-driven currents, the currents obviously must behave differently where the monsoons, or seasonal winds, rule the waves - like in the northern part of the Indian Ocean, where the surface circulation change seasonally, in response to the monsoons.

The most spectacular seasonal change is the reversal of the Somali Current, off east Africa. During the North-East Monsoon the Somali Current flows to the south-west, while during the South-West Monsoon it is a major western boundary current, comparable with the Gulf Stream and the Kuroshio Current. Typical for western boundary currents is their high flow velocity. The Somali current is no exception - it can manifest velocities in excess of 3.5 m s^-1.


During the northern summer months and the southwest monsoon the coastal waters move northeastward with surface velocities reaching up to 14 km per hour. At longitude 6°–10° N (off Somalia), the northeastward Somali flow turns eastward as the Monsoon Current. With the monsoon's reversal to the northeast in September, the current begins to weaken until, in the winter, it disappears entirely, to be replaced by a slow southwestward drift. During the southwest monsoon a two gyre system develops in the region - the Great Whirl between 5-10°N with clockwise rotation and a secondary eddy towards its south. This two gyre system is stable until August or September, when the southern gyre propagates northward and merges with the Great Whirl.

The Somali Current is also known as the East Africa Coast Current.