354
Chapter 11: The Pacific Ocean
Tasman Sea Province (TASM)
Extent of the Province
The Tasman Sea Province (TASM) lies between the Subtropical Convergence Zone across
about 45
S and the Tasman Front to the south of the Coral Sea at about 32
S and is
enclosed by the coasts of Australia and New Zealand. This is a relatively small province,
but because of its unique characteristics it is useful to recognize the Tasman Sea as an
entity.
There is no shelf in this province: the eastern Tasman Sea coast of Australia and
Tasmania is in AUSE and the western coastline of New Zealand is allocated to the New
Zealand Coastal Province (NEWZ). Shelf-edge fronts, where they are found, represent the
landward boundary of the province along these two coasts: they are especially prominent
along the Westland upwelling strip of South Island.
Defining Characteristics of Regional Oceanography
To place this province in the Westerlies Biome, when it lies entirely equatorward of
the Subtropical Front, is somewhat at odds with the definition offered of this biome. I
have chosen to place it here for pragmatic reasons, because—as a consequence of the
arrangements of the coastline of Tasmania and Australia in relation to the circumpolar
Southern Ocean—westerly winds do extend over most of the Tasman Sea. During austral
winter, persistent high pressure over Australia induces strong westerlies, continuous with
those over the Southern Ocean, across the entire province so that, in this sector alone, the
STCZ does not have easterlies on its equatorward flank. In austral summer, the northern
part of the province lies below northeasterly winds induced by the persistent low-pressure
belt over northern Australia.
The Tasman is a windy sea, although it lies at only about 40
of latitude, so the
seasonal cycle of winter wind mixing and summer stratification is strong. Wind stress and
thermal convection drive the mixed layer down to 300 m in the southern half, though the
regional mean depth of winter mixing is only about 125 m. This delivers end-of-winter
mixed-layer nitrate in the range 20–40 M. Incursions of subantarctic water in meanders
isolated from the Subtropical frontal zone to the south have slightly higher (40–50 M)
values and those of subtropical water are nitrate-deficient.
Because this is a complex region of interaction between the general anticyclonic gyral
flow and the instabilities induced in it along the topographic boundaries, the second
principal characteristic of the Tasman Sea is a profusion of mesoscale eddy fields. The
southern limit of the province is the edge of the eddy field associated with the South
Subtropical Convergence that passes south of Tasmania and then around the Snares
Bank, south of New Zealand. You may find old figures that show it passing diagonally
across the Tasman Sea and around northern New Zealand: but modern data (Garner,
1967; Heath, 1981) show this to be incorrect. The eddy field associated with the northern
edge of the convergence adds to the mesoscale activity of the central Tasman Sea, and
the location of the convergence forces the general anticyclonic flow of the Tasman Sea
to encounter the Westland coast of South Island where flow diverges north and south
(see NEWZ).
The retroflection of the East Australia Current, the poleward western boundary current
down the east coast of Australia, occurs near Sydney (32–34
S), whence it flows eastward
across the Tasman Sea as a meandering frontal jet in which the individual eddies may
be very large. Warm Coral Sea water lies to the north, and cool Tasman Sea water to
the south of the Tasman Front that coincides with a zone of seasonal Ekman suction
where upwelling reaches 2–4 × 10
−5 m sec
−1 during winter, though vertical motion in
summer is weaker (Andrews et al., 1980; Comiso et al., 1993). Meanders within the
Chapter 11: The Pacific Ocean
Tasman Sea Province (TASM)
Extent of the Province
The Tasman Sea Province (TASM) lies between the Subtropical Convergence Zone across
about 45
S and the Tasman Front to the south of the Coral Sea at about 32
S and is
enclosed by the coasts of Australia and New Zealand. This is a relatively small province,
but because of its unique characteristics it is useful to recognize the Tasman Sea as an
entity.
There is no shelf in this province: the eastern Tasman Sea coast of Australia and
Tasmania is in AUSE and the western coastline of New Zealand is allocated to the New
Zealand Coastal Province (NEWZ). Shelf-edge fronts, where they are found, represent the
landward boundary of the province along these two coasts: they are especially prominent
along the Westland upwelling strip of South Island.
Defining Characteristics of Regional Oceanography
To place this province in the Westerlies Biome, when it lies entirely equatorward of
the Subtropical Front, is somewhat at odds with the definition offered of this biome. I
have chosen to place it here for pragmatic reasons, because—as a consequence of the
arrangements of the coastline of Tasmania and Australia in relation to the circumpolar
Southern Ocean—westerly winds do extend over most of the Tasman Sea. During austral
winter, persistent high pressure over Australia induces strong westerlies, continuous with
those over the Southern Ocean, across the entire province so that, in this sector alone, the
STCZ does not have easterlies on its equatorward flank. In austral summer, the northern
part of the province lies below northeasterly winds induced by the persistent low-pressure
belt over northern Australia.
The Tasman is a windy sea, although it lies at only about 40
of latitude, so the
seasonal cycle of winter wind mixing and summer stratification is strong. Wind stress and
thermal convection drive the mixed layer down to 300 m in the southern half, though the
regional mean depth of winter mixing is only about 125 m. This delivers end-of-winter
mixed-layer nitrate in the range 20–40 M. Incursions of subantarctic water in meanders
isolated from the Subtropical frontal zone to the south have slightly higher (40–50 M)
values and those of subtropical water are nitrate-deficient.
Because this is a complex region of interaction between the general anticyclonic gyral
flow and the instabilities induced in it along the topographic boundaries, the second
principal characteristic of the Tasman Sea is a profusion of mesoscale eddy fields. The
southern limit of the province is the edge of the eddy field associated with the South
Subtropical Convergence that passes south of Tasmania and then around the Snares
Bank, south of New Zealand. You may find old figures that show it passing diagonally
across the Tasman Sea and around northern New Zealand: but modern data (Garner,
1967; Heath, 1981) show this to be incorrect. The eddy field associated with the northern
edge of the convergence adds to the mesoscale activity of the central Tasman Sea, and
the location of the convergence forces the general anticyclonic flow of the Tasman Sea
to encounter the Westland coast of South Island where flow diverges north and south
(see NEWZ).
The retroflection of the East Australia Current, the poleward western boundary current
down the east coast of Australia, occurs near Sydney (32–34
S), whence it flows eastward
across the Tasman Sea as a meandering frontal jet in which the individual eddies may
be very large. Warm Coral Sea water lies to the north, and cool Tasman Sea water to
the south of the Tasman Front that coincides with a zone of seasonal Ekman suction
where upwelling reaches 2–4 × 10
−5 m sec
−1 during winter, though vertical motion in
summer is weaker (Andrews et al., 1980; Comiso et al., 1993). Meanders within the