258
D. H. CUSRING
nearer the equator turn into the north-east and south-east trades, must
blow along the coast, northerly in the northern hemisphere and
southerly in the southern hemisphere. As the wind blows parallel to the
coast towards the equator, the water is moved offshore by Coriolis
force and, because the movement does not extend far in depth, water
is drawn from below near the coast.
As the water is blown offshore at an angle to the coast it is moved
towards the equator. At about 100 km offshore there is often a convergence where the water sinks and, a little further offshore, a divergence, generating a secondary source of upwelling above the thermocline (Hart and Currie, 1960). Sverdrup (1938) called this region the
dynamic boundary; in this area, the flow towards the equator is
intensified and Sverdrup (1938) called it a convection current. There
are often large swirls in fixed positions, between local upwelling systems
(Reid et al., 1958). The current flowing towards the equator at the
surface is compensated by a countercurrent below about 200 m flowing
towards the pole (Gunther, 1936; Reid et al., 1958; Hart and Cunie,
1960). Further, there may be narrow surface countercurrents generated
inshore during the period of upwelling (Reid et al., 1958; Wyrtki, 1963).
The depth of the whole upwelling system is shallow, less than 200 m,
and its offshore boundary is imprecise.
Wooster and Reid (1963) make the generalization that upwelling is
found in the eastern boundary currents of the subtropical anticyclone.
Upwelling could occur in the western boundary currents because the
wind blows parallel to the coast and Coriolis force would carry the
water offshore, if the existing currents were not so swift and stable ; in
other words, the forces generating upwelling are local and weak, whereas those generating the western boundary currents are strong and spread
across the whole ocean. The Somali Current in the Arabian Sea is a
special case because it tilts when it swings eastwards t o the south of
Socotra Island (Warren et al., 1966) and cool water upwells on the coast
south of Cape Gardafui, the Horn of Africa. It is interesting that off
the Malabar coast an upwelling is generated at the end of the south-west
monsoon as the current tilts (Darbyshire, 1967). In higher latitudes,
the circulation tends to be cyclonic (for example in the Alaska gyral)
and when the wind blows parallel to the coast and towards the pole on
western coasts, the water is moved towards the coast by Coriolis force
and so there is a tendency for the water to sink. On eastern coasts,
under such conditions, upwelling can occur, as happened off Nova
Scotia in September during a period of south-westerly winds (Hachey,
1936-37). When the wind blows parallel to the coast and towards the
cquator on eastern coasts there should be sinking, as Longard and Banks
D. H. CUSRING
nearer the equator turn into the north-east and south-east trades, must
blow along the coast, northerly in the northern hemisphere and
southerly in the southern hemisphere. As the wind blows parallel to the
coast towards the equator, the water is moved offshore by Coriolis
force and, because the movement does not extend far in depth, water
is drawn from below near the coast.
As the water is blown offshore at an angle to the coast it is moved
towards the equator. At about 100 km offshore there is often a convergence where the water sinks and, a little further offshore, a divergence, generating a secondary source of upwelling above the thermocline (Hart and Currie, 1960). Sverdrup (1938) called this region the
dynamic boundary; in this area, the flow towards the equator is
intensified and Sverdrup (1938) called it a convection current. There
are often large swirls in fixed positions, between local upwelling systems
(Reid et al., 1958). The current flowing towards the equator at the
surface is compensated by a countercurrent below about 200 m flowing
towards the pole (Gunther, 1936; Reid et al., 1958; Hart and Cunie,
1960). Further, there may be narrow surface countercurrents generated
inshore during the period of upwelling (Reid et al., 1958; Wyrtki, 1963).
The depth of the whole upwelling system is shallow, less than 200 m,
and its offshore boundary is imprecise.
Wooster and Reid (1963) make the generalization that upwelling is
found in the eastern boundary currents of the subtropical anticyclone.
Upwelling could occur in the western boundary currents because the
wind blows parallel to the coast and Coriolis force would carry the
water offshore, if the existing currents were not so swift and stable ; in
other words, the forces generating upwelling are local and weak, whereas those generating the western boundary currents are strong and spread
across the whole ocean. The Somali Current in the Arabian Sea is a
special case because it tilts when it swings eastwards t o the south of
Socotra Island (Warren et al., 1966) and cool water upwells on the coast
south of Cape Gardafui, the Horn of Africa. It is interesting that off
the Malabar coast an upwelling is generated at the end of the south-west
monsoon as the current tilts (Darbyshire, 1967). In higher latitudes,
the circulation tends to be cyclonic (for example in the Alaska gyral)
and when the wind blows parallel to the coast and towards the pole on
western coasts, the water is moved towards the coast by Coriolis force
and so there is a tendency for the water to sink. On eastern coasts,
under such conditions, upwelling can occur, as happened off Nova
Scotia in September during a period of south-westerly winds (Hachey,
1936-37). When the wind blows parallel to the coast and towards the
cquator on eastern coasts there should be sinking, as Longard and Banks