The Subtropical Gyres

As discussed in Section 3.4. current flow itt the subtropical gyres is related to the overly mg an li eye Ionic w ind systems, which blow around the subtropical high pressure regions. The centres of the atmospheric and oceanic gyres are not. however, coincident: the centres of the atmospheric gyres tend to he displaced towards the eastern side of the oceans, while the centres ol (he oceanic gyres lend to be displaced lowards the western side. As a result, the Currents dial flow along the western sides of oceans the western boundary currents - are characteristically last, intense, deep and narrow, whiie those that How along the eastern sides - the eastern boundary currents - are characteristically slow, w ide, shallow and diffuse

The western houndaiy current in the North Atlantic is the Gull Stream; its counterpart in the North Pacific is the Kurushio, Both of these currents arc typically only some 100km w ide and in places have surface velocities it) excess of 2 m s_l. By contrast, the Canary Current and the California Currenl are over 1000 km wide and generally ha\e surface velocities less than 0.25 m S"In the South Atlantic and the South Pacific, (he difference between the western boundary currents (the Brazil Current and the Bust Australian Current, respectively! and the eastern boundary currents (the Be tig tie la Current and (he Humboldt, or Peru. Current) is not so marked. This may be because the South Atlantic and South Pacific arc open to the Southern Ocean so that their gyres are strongly influenced by the Antarctic C ire unipolar Current: furthermore; in the case of the South Pacific there is no continuous barrier along its western side. By contrast, the southern Indian Ocean has the powerful Agulhas Current flowing south along its western boundary: the northern Indian Ocean has u seasonal western boundary current in the form of the Somali Current (both of these w ill be discussed in Chapter 5).

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