Thursday, 23 July 2015

Spices, Power and Rock – The Pacific Ocean

As the song goes, “the winds blow over the ocean”. What do the oceans do to you? What does the biggest of all oceans, the Pacific does for you? Spicing, Powering & Rocking!

(from Encyclopadia Britannica)

Avant Propose 

 [adapted from [1]]:

"Spicing the Indian Ocean: The Indonesian Throughflow is the only low-latitude inter-ocean current between world oceans, linking the Pacific to the Indian Ocean through boundary currents along the western shores of the Pacific. The Indonesian Throughflow sources the Indian Ocean with warm, less salty water from the North Pacific Ocean via the Mindanao Current and water from the South Pacific Ocean via the New Guinea Coastal Current. These Pacific western boundary currents are important players in the global climate system since the closing of the Panama Isthmus during the Pliocene and thus cutting the exchange between tropical Pacific and tropical Atlantic."

"Powering winter storms: In winter, the warm subtropical currents in the along the western boundary of Pacific Basin supply enormous amounts of heat and moisture into the atmosphere, feeding the North Pacific storm tracks. As cold and dry sub-polar air comes into contact with the warm water carried poleward by the subtropical western boundary currents, heat and moisture to fuel the storms are extracted from the surface water."

"Rocking the globe: The Pacific is the home of the El Nino, an inter-annual ocean-atmosphere oscillation, which severely disrupts global weather patterns worldwide. El Nino affects ecosystems, agriculture, tropical cyclones, drought, bushfires, floods and other weather events worldwide. This El Nino/Southern Oscillation (ENSO) is the most prominent source of global climate variability. The low-latitude western boundary currents in the Pacific are transporting mass into the equatorial Pacific. The western boundary currents critically influence the Western Pacific Warm Pool (a region of sea surface temperatures warmer than 28.5 C). In consequence, they affect the life cycle of the El Nino/Southern Oscillation, the East Asian monsoon and the Indian/Southeast Asian monsoon."

Western Boundary Currents - what?

Three events, same special agent of change: Western Boundary Currents. These currents are a common marine agent with manifold impacts on the global ocean circulation and climate. The warming of the western coast of all continents is the best-known feature that these currents trigger. However, there are many more features as the examples of Pacific Ocean show. Anyhow, how are these boundary currents set-up? That is the story to tell here, because the impacts of the western boundary currents on the global ocean circulation and climate variability are manifold.

Take three: winds, continents and rotation

The winds stir the surface layer of the sea, driving waves, currents and mixing. Water flows at the surface of the ocean mainly are to the West in tropical zones and the East in temperate regions. Closer to polar regions the circulation pattern gets a bit different. In the south, the Circumpolar Current circles eastward the Antarctic continent. In the north, the continents hinder such a mighty flow of water around the globe.

Surface currents in the North Atlantic (model) -Credit: 
Erik Behrens (distributed via
The continents shaping the ocean basins of Atlantic Ocean, Indian Ocean, and Pacific Ocean hinder ocean currents flowing around the globe, with the Antarctic Circumpolar Current in the Southern Polar Ocean being the sole exception. Otherwise, ocean currents arrange as big gyres. The gyres in the tropical-temperate zones of the oceans show gentle, broad flows towards the equator in the east and swift, narrow and poleward currents in the west. The currents in the west, the Gulf Stream, for example, flow quite close to the continent before they get unstable and turn eastward into the open ocean.

The rapid spinning of planet Earth causes the different water flows in eastern and western parts of the oceans. The easterly winds in the tropical regions, the westerly winds in the temperate regions, and the confinement of the ocean between north-south boundaries together alone would not cause this east-west asymmetry. The interplay of the Earth's rotation, the winds, and the continents causes the narrow western boundary currents and the broad eastern flow of the ocean surface waters in the subtropical gyre.

The earth rotation affects in a different manner a blob of water at the pole or the equator. A blob of water exactly at the pole rotates around its vertical axis. A blob of water at the equator is swung around Earth without being rotated around its vertical axis. Between both places, the situation changes smoothly from one extreme to the other. Thus due to the rotation of the earth, blobs of water experience a torque that depends on their latitude.

Blobs of water situated at the same latitude have the same amount of torque that stems from the rotation of the Earth but the contribution to their torque that is arising from other sources may vary. Blobs of surface water acquire additional torque from the winds blowing over the sea when these winds vary in strength or direction over the blob.

If a blob of water driven by the wind moves straight east-west, thus along a latitude, then little happens to it beyond the contribution to its torque that stems from the wind. However, if the same blob of water moves poleward (or towards the equator), then the internal repartition of its torque has to adjust. Adjustment is done by changing the shape of the blog. Changing the blob's form requires that water flows within the blob change. Finally, these changes result in the east-west asymmetry of ocean currents, with narrow western boundary currents and the broad eastern return-flows. The western boundary currents have a second dynamic function. The torque added by the wind has to be dissipated somewhere in the ocean. Dissipation happens through the high lateral shear in the western boundary currents and bottom friction.

The German research vessel METEOR
off the coast  of Brazil
Credit: MARUM -
Center for Marine Environmental Sciences
(distributed via
[adapted from [1]]: “Much of modern wind-driven ocean circulation theory was derived from a quest to understand these ocean currents. Knowledge of the effect of the Earth’s rotation on western boundary currents, and of the wind driven water flow [Ekman transport], led to ground-breaking advances: that wind stress is a driving agent of ocean currents, but it is the horizontal change rather than the absolute strength that is important; that latitudinal gradients in the effect of Earth’s rotation on the horizontal motion cause a flow intensification towards the west of the ocean basins; and that ocean wind-forced ocean currents include the equatorial current system, the low-latitude and subtropical western boundary currents.”

Name them, the Western Boundary Currents, some

Within the North Atlantic Ocean, the Gulf Stream along the East Coast of the USA is a well-known western boundary current. The Gulf Stream is part of the subtropical gyre of the North Atlantic Ocean. Its counterpart in the subtropical gyre of the South Atlantic Ocean is the Brazil Current. Between both gyres flow the equatorial currents in the tropical belt. These currents link the two oceanic gyres along the continental borders. In the Atlantic Ocean, that link transports surface water northward along the American continent. Further in the south the subtropical gyre of the South Atlantic Ocean ties with the Antarctic Circumpolar Current and through the Agulhas Current with the subtropical gyre of the Indian Ocean.

Within the subtropical gyres of the Pacific Ocean, the western boundary currents are the Kuroshio Current in the Northern Pacific Ocean, and the East Australian Current in the Southern Pacific Ocean. The Pacific tropical belt western boundary currents include the Mindanao Current northern of the Equator and the New Guinea Coastal Current south of the Equator. The circulation system of the equatorial Pacific connects the currents in both hemispheres.

Within the Indian Ocean, the Somalia Current along the east-coats of Africa is a western boundary current. That current changes the direction of its flow with the seasonal switch of the Monsoon winds. The surface flow of New Guinea Coastal Current in the Pacific Ocean exhibits the same annual feature. The New Guinea Coastal Current changes its direction with the monsoon season. The related under current, the New Guinea Coastal Undercurrent that is flowing in the depth of the sea, flows poleward all the year.

Five Simple Ocean Gyres
(from Wikipedia)
[1] Dunxin Hu, Lixin Wu, Wenju Cai, Alex Sen Gupta, Alexandre Ganachaud, Bo Qiu, Arnold L. Gordon, Xiaopei Lin, Zhaohui Chen, Shijian Hu, Guojian Wang, QingyeWang, Janet Sprintall, Tangdong Qu, Yuji Kashino, FanWang & William S. Kessler, Pacific western boundary currents and their roles in climate, Nature 522, 299–308 (18 June 2015) doi:10.1038/nature145

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