436
Chapter 11: The Pacific Ocean
3 × 10
5 km
2 . The southwest coast of the South Island is especially steep-to, the 1000-m
contour lying only a few kilometers from the shore along the entire Fjordland coast.
The only extensive areas of shelf within the 200-m isobath around New Zealand are to
the west of Cook Strait, and the adjacent coasts of North and South Island, and also
the Snares Shelf beyond Stewart Island, off the south coast of South Island. On the
north coast of North Island, off the Hauraki Gulf and Bay of Plenty, the shelf width is
significant, though the area involved is smaller than to the west of Cook Strait. The shelf
on the east side of South Island widens somewhat off the Canterbury Bight (45
S), from
where the deeper topography (>1000 m) of the Chatham Rise extends eastward.
Tidal currents and mixing are not as relatively important as in regions where the shelf
is wider; the M 2 tide is a coastally trapped Kelvin wave that travels counterclockwise
around New Zealand. It is only in Cook Strait that important tidal currents are generated,
and here they may attain 4–5 m sec
−1 . A recent study of internal tide dissipation on
the northeast shelf showed that the energy associated with the internal tide was greater
than that of barotropic tides or internal shear and that it generated significant vertical
diffusivity at the nutricline. This flux, of 7 × 10
−4 m
2 sec
−1 , is sufficient to support annual
new production on the shelf of 100 g C m
−2 (Sharples et al., 2001).
Defining Characteristics of Regional Oceanography
New Zealand lies between subtropical weather patterns on the north coast of North Island
and the frequent passage of cyclones generated in the Tasman across the high topography
of South Island; this conjuncture generates extremely high rainfall on the Southern Alps
and heavy discharge of freshwater along the west coast. The consequences of the location
of New Zealand, lying across the wind and current regimes of the Southern Ocean,
have been reviewed recently by Sharples and Greig (1998). As he points out, for this
province we have access to only a relatively limited number of regional studies of coastal
ocean circulation. Much of the following account of both the physical and the biological
oceanography of New Zealand waters is drawn from Heath (1981, 1985), Heath and
Bradford (1980), Vincent et al. (1991), and Butler et al. (1992).
The geographical location of New Zealand and the relatively shallow topography of
the Campbell Plateau and the Chatham Rise strongly modify the zonal arrangement
of the circumpolar frontal systems of the Southern Ocean, and determine circulation
around New Zealand. Sharples comments that only here, off New Zealand, does shallow
topography intercept a western boundary current in midocean.
The circulation around New Zealand over the shelf and just beyond is complex,
reflecting the fact that two major currents encounter its western topography: (i) the flow
of the retroflected East Australian Current along the Tasman Front meets the North
Island at 33–35
S and (ii) that the gyral circulation of the southern Tasman Sea, together
with the subtropical convergence zone beyond, is constrained by the Snares Plateau at
about 45–47
S.
Each of these flows, on encountering the shelf edge, becomes locked to its topography
and passes around New Zealand as the East Auckland and Southland Currents, respectively. Thus, the main flow along the Tasman Front passes as a high-velocity stream down
the east coast of North Island to about 43
S, off the Banks Peninsula on South Island.
Here it meets the frontal jet of the Subtropical Convergence that, after encountering the
Snares Plateau, has passed around South Island, beyond the shelf edge but locked to its
topography; this conjuncture—at the margin of the NEWZ Province—is very dynamic,
with frequent cross-frontal exchange of subtropical and subantarctic surface water bodies.
Here, the distinction between subtropical and subantarctic water masses is usually clear
because the interleaving water parcels are typified by their origins: mixed-layer depth,
chlorophyll profiles, and nitrate levels in the surface water are all clearly different. Finally,
Chapter 11: The Pacific Ocean
3 × 10
5 km
2 . The southwest coast of the South Island is especially steep-to, the 1000-m
contour lying only a few kilometers from the shore along the entire Fjordland coast.
The only extensive areas of shelf within the 200-m isobath around New Zealand are to
the west of Cook Strait, and the adjacent coasts of North and South Island, and also
the Snares Shelf beyond Stewart Island, off the south coast of South Island. On the
north coast of North Island, off the Hauraki Gulf and Bay of Plenty, the shelf width is
significant, though the area involved is smaller than to the west of Cook Strait. The shelf
on the east side of South Island widens somewhat off the Canterbury Bight (45
S), from
where the deeper topography (>1000 m) of the Chatham Rise extends eastward.
Tidal currents and mixing are not as relatively important as in regions where the shelf
is wider; the M 2 tide is a coastally trapped Kelvin wave that travels counterclockwise
around New Zealand. It is only in Cook Strait that important tidal currents are generated,
and here they may attain 4–5 m sec
−1 . A recent study of internal tide dissipation on
the northeast shelf showed that the energy associated with the internal tide was greater
than that of barotropic tides or internal shear and that it generated significant vertical
diffusivity at the nutricline. This flux, of 7 × 10
−4 m
2 sec
−1 , is sufficient to support annual
new production on the shelf of 100 g C m
−2 (Sharples et al., 2001).
Defining Characteristics of Regional Oceanography
New Zealand lies between subtropical weather patterns on the north coast of North Island
and the frequent passage of cyclones generated in the Tasman across the high topography
of South Island; this conjuncture generates extremely high rainfall on the Southern Alps
and heavy discharge of freshwater along the west coast. The consequences of the location
of New Zealand, lying across the wind and current regimes of the Southern Ocean,
have been reviewed recently by Sharples and Greig (1998). As he points out, for this
province we have access to only a relatively limited number of regional studies of coastal
ocean circulation. Much of the following account of both the physical and the biological
oceanography of New Zealand waters is drawn from Heath (1981, 1985), Heath and
Bradford (1980), Vincent et al. (1991), and Butler et al. (1992).
The geographical location of New Zealand and the relatively shallow topography of
the Campbell Plateau and the Chatham Rise strongly modify the zonal arrangement
of the circumpolar frontal systems of the Southern Ocean, and determine circulation
around New Zealand. Sharples comments that only here, off New Zealand, does shallow
topography intercept a western boundary current in midocean.
The circulation around New Zealand over the shelf and just beyond is complex,
reflecting the fact that two major currents encounter its western topography: (i) the flow
of the retroflected East Australian Current along the Tasman Front meets the North
Island at 33–35
S and (ii) that the gyral circulation of the southern Tasman Sea, together
with the subtropical convergence zone beyond, is constrained by the Snares Plateau at
about 45–47
S.
Each of these flows, on encountering the shelf edge, becomes locked to its topography
and passes around New Zealand as the East Auckland and Southland Currents, respectively. Thus, the main flow along the Tasman Front passes as a high-velocity stream down
the east coast of North Island to about 43
S, off the Banks Peninsula on South Island.
Here it meets the frontal jet of the Subtropical Convergence that, after encountering the
Snares Plateau, has passed around South Island, beyond the shelf edge but locked to its
topography; this conjuncture—at the margin of the NEWZ Province—is very dynamic,
with frequent cross-frontal exchange of subtropical and subantarctic surface water bodies.
Here, the distinction between subtropical and subantarctic water masses is usually clear
because the interleaving water parcels are typified by their origins: mixed-layer depth,
chlorophyll profiles, and nitrate levels in the surface water are all clearly different. Finally,