Global warming is 'affecting' Ocean Circulations - Seeker's Thoughts

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Global warming is 'affecting' Ocean Circulations


According to the conclusion of a new paper, today in Science Advances. Based on observations combined with models, the authors claim that from 1990 to 2013, the energy of the currents increased by some 15% per decade. 


The ocean’s great continent- wrapping currents, each one moving as much water as the world’s rivers combined, can rightly be considered the planet’s circulatory system. And this circulation, it appears, has started to thump faster: for nearly 25 years the currents have been rapidly speeding up, partly because of global warming.

An oceanographer Susan Wijffels, at the Woods Hole Oceanographic Institution. Says, this is a really huge increase and it's going to stimulate a lot of other work. If the acceleration is real, it could affect jet streams, weather patterns, and the amount of heat stored in the ocean’s depth.



What is Ocean Current?
An ocean current is any more or less permanent or continuous, directed movement of ocean water that flows in one of the Earth’s oceans.
The currents are generated from the forces acting upon the water like the earth’s rotation, the wind, the temperature, and salinity differences and the gravitation of the moon.
The depth contours, the shoreline, and other currents influence the current’s direction and strength. 
Ocean currents can flow for thousands of kilometers.
They are very important in determining the climates of the continents, especially those regions bordering on the ocean. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude.
                                          
Distribution of Ocean Current
Maps of the general circulation at the sea surface were originally constructed from a vast amount of data obtained from inspecting the residual drift of ships after cpurse direction and speed are accounted for in a process called dead reckoning.
This information is collected by satellite-tracked surface drifters at sea at present. The pattern is nearly entirely that of wind-driven circulation.
At the surface, aspects of wind-driven circulation cause the gyres (large anticyclonic current cells that spiral about a central point) to displace their centres westward, forming strong western boundary currents against the eastern coasts of the continents, such as Gulf strea, North Atlantic, Norway Current in the Atlantic Ocean and the Kuroshio – North Pacific Current in the Pacific Ocean.
In the Southern Hemisphere the counterclockwise circulation of the gyres creates strong eastern boundary currents against the western coasts of continents, such as the Peru (Humboldt) Current off South America, the Benguela Current off western Africa, and the Western Australia Current.

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The Southern Hemisphere currents are also influenced by the powerful, eastward-flowing, circumpolar Antarctic Current. It is a very deep, cold and relatively slow current, but it carries a vast mass of water, about twice the volume of the Gulf Stream.
The Peru and Benguela currents draw water from this Antarctic current and, hence, are cold. The Northen Hemisphere lacks continuous open water bordering the Arctic and so has no corresponding powerful circumpolar current, but there are small old currents flowing south through the Bering strait to form the Oya and Anandyr currents off eastern Russia and the California Current off Western North America; others flow south around Greenland to form the cold Labrador and East Greenland currents.
The Kuroshio North pacific and Gulf Stream – North Atlantic – Norway currents move warmer water into the Arctic Ocean via the Bering, Cape, and West Spitsbergen currents.
In the tropics the great clockwise and counterclockwise gyres flow westward as the Pacific North and South Equatorial currents, Atlantic North and South Equatorial currents, and the Indian South Equatorial Current. Because of the alternating monsoon climate of the northern Indian Ocean, the current in the northern Indian Ocean and the Arabian Sea alternates. Between these massive currents are narrow eastward-flowing countercurrents.
Other smaller current systems found in certain enclosed seas or ocean areas are less affected by wind-driven circulation and more influenced by the direction of water inflow. Such currents are found in the Tasmanian Sea, where the southward-flowing East Australian Current generates counterclockwise circulation, in the northwestern Pacific, where the eastward-flowing Kuroshio–North Pacific current causes counterclockwise circulation in the Alaska Current and Aleutian Current (or Subarctic Current), in the Bay of Bengal, and in the Arabian Sea.
 
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Deep-ocean circulation consists mainly of thermohaline circulation. The currents are inferred from the distribution of seawater properties, which trace the spreading of specific water masses. The distribution of density is also used to estimate the deep currents. Direct observations of subsurface currents are made by deploying current meters from bottom-anchored moorings and by setting out neutral buoyant instruments whose drift at depth is tracked acoustically.




Causes of Ocean Currents
Ocean currents can be caused by wind, density differences in water masses caused by temperature salinity variations, gravity, and events such as earthquakes or storms.
Surface currents in the ocean are driven by global wind systems that are fuelled by energy from the sun. patterns of surface currents are determined by wind direction, Coriolis forces from the Earth's rotation, and the position of landofmrs that interact with the currents. Surface wind-driven currents generate upwelling currents in conjunction with landforms, creating deepwater currents.
Currents may also be caused by density differences in water masses due to temperature (thermo
) and salinity (haline)variation via a process known as thermonaline circulation. These currents move water masses through the deep ocean-taking nutrients, oxygen, and heat with them.
Occasional events such as huge storms and underwater earthquakes can also trigger serious ocean currents, moving masses of water inland when they reach shallow water and coastlines. Earthquake may also trigger rapid downslope movement of water- saturated sediments, creating strong turbidity currents.
Finally, when a current that is moving over a broad area is forced into confined space, it may become very strong. On the ocean floor, water masses forced through a narrow opening in a ridge system or flowing around a seamount may create currents that are far greater than in the surrounding water affect the distribution and abundance of organisms as well as the scientists and their equipment seeking to study these organisms.



 How Global Warming is affecting ocean circulation?
 several Oceanographers have suspected that climate warming is affecting ocean circulation, but so far, observation hasn’t shown a trend. The Kuroshio current, running up Eastern Asia, has seemed stable, whereas the Agulhas, flowing along Africa’s eastern coast has broadened, fracturing into meandering eddies. 
The Atlantic Ocean’s Gulf stream may be weakening as Arctic melt slows its driver, the sinking of salty water in the North Atlantic, whereas currents in the Pacific Ocean have seen a strong uptick. 
No sustained, direct measurements of currents around the world are available, however. Instead, a team of oceanographers turned to so-called reanalyses, which combined observations of the ocean and atmosphere with computer models to fill in the gaps and produce a global picture. The approach is tricky to use for time spans of decades: Changes in observations, for example when new satellites come online, can cause unknown biases.
The team combined five different reanalyses of coean circulation, hoping their differing methods could reveal a true trend. From each one, they extracted the ocean’s kinetic energy month by month, at coasts scale the would ignore the turbulence of eddies and storms. And each showed a distinct rise starting around 1990.


How real it was?
A look at data from the Argo array, a fleet of nearly 4000 robotic floats deployed around the world, provided the best test. The floats have been bobbing up and down in the ocean’s uppermost 2000 meters for the past 15 years, measuring temperature and salinity. 

They don’t track velocity through the water column. But their data do indicate where winds have piled up water, helping create differences in pressure that drive large-scale flows. By combining those data with the floats’ own current-borne trajectories, investigators can reconstruct overall currents and their speed.
The data set, compiled by oceanographer Alison Gray of the University of Washington, Seattle, covers only 6 years, from 2005 to 2010, but Hu found that it reveals an even clearer global speedup than the reanalysis models. “The evidence in the Argo data is absolutely astonishing,” says Eleanor Frajka-Williams, an oceanographer at the United Kingdom’s National Oceanography Centre, who was not part of the study.

ocean winds, which drive most currents, have steadily increased over the past 3 decades. And  there’s good evidence that human activity has contributed to that strengthening. For example, in the Southern Hemisphere, ozone depletion and greenhouse warming have altered atmospheric circulation to push the Southern Ocean’s famed westerly winds to the south, perhaps causing a slight strengthening and spreading of the Antarctic Circumpolar Current. Meanwhile, heat from the warming tropical Atlantic has goosed the Walker Circulation, an equatorial pattern that drives the Pacific trade winds.
natural fluctuations can’t be ruled out, a climate scientist at the Alfred Wegener Institute. Over the past few decades, long-term cooling-off western North America has caused Pacific winds to pick up—and that cooling may reflect natural oscillations in the ocean’s state. Other researchers doubt these cycles exist. Either way, the oscillations could be responsible for at most one-third of the wind speedup.
The ocean acceleration could have globe-spanning effects. Stronger tropical currents could carry more warm water to higher latitudes, for example. Because carbon dioxide (CO2) is less soluble in warm water, that could slow the ocean’s uptake of CO2 from the atmosphere. The high-latitude warming may also be shifting weather patterns. At the same time,  by reaching deep into the ocean, the acceleration could boost the storage of heat in the depths, helping slow the warming on the land


                       


      Conclusion
Oceanographers will likely fan out to test the study’s findings. Perhaps the strongest confirmation could come from updated data from the Argo floats, due out later this year. Still, it will probably take another decade of observations to be sure the trend is real and driven by global warming,. “This study does highlight how ill-prepared we are to truly diagnose what’s going on.”




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