Pacific Westerly Winds Biome
355
Tasman Front shed equatorward cyclonic meanders and poleward anticyclonic meanders,
and these in turn shed warm and cold-core rings. The climatological origins of the
eddies are topographically determined by the highest points of Lord Howe Rise and the
West Norfolk Ridge (Hamilton, 1992), and hence preferred locations can be mapped
for cold and warm features along the front. The actual location of the front as it passes
across the Tasman Sea varies from 30
S to as much as 38
S (Mulhearn, 1987) before it
rounds Cape North (35
S) to enter the poleward flow of the east New Zealand coastal
currents.
The retroflection of the East Australian Current has major consequences for the
Tasman Sea apart from bounding it to the north by the Tasman Front. The meandering
flow along the front generates a field of Rossby waves that pass westward toward the
Australian coast and induce the shedding of very large warm eddies: two or three are shed
per year, and a standing population of six to eight in the Tasman Sea is normal. Within
these eddies the seasonal cycles of mixing and stabilization are imposed on their initial
structure. The surface thermal signature and shallow thermocline of Coral Sea water is
lost during the first winter of the existence of an eddy, and in subsequent summers the
surface mixed layer established over the whole Tasman Sea passes across the subsurface
warm core of the eddy. A further source of eddies are the episodic irruptions of warm
water of Leeuwin Current origin, transported along the south coast of Australia and
occasionally penetrating the Bass Straits.
Regional Response of the Pelagic Ecosystem
The southern Tasman Sea exhibits a typical spring bloom (Harris et al., 1987) so that
chlorophyll biomass in the south-central region is maximal in late September–early
October, but regular seasonality does not extend across the whole region; the integrated
chlorophyll data are, in this province, somewhat misleading because the effect of the
southwest region dominates the whole (see Color plate 17). Monthly chlorophyll images
indicate that higher surface chlorophyll occurs patchily across the whole southern Tasman
Sea through austral summer and autumn associated with frontogenesis and other bloominducing processes; however, overall, this is the period when lowest surface chlorophyll
biomass is observed in satellite data.
The spring bloom has been best described to the east of Tasmania, where it is a
typical but rather uncertain process. Although strong westerlies may induce a regional
mixed layer of almost 300 m, the actual depths achieved are dependent on the nature
of the subsurface water mass, which has strong interannual variability caused by shifts
in the location of the STCZ, and as mesoscale eddies come and go. There is a 40-day
periodicity in wind stress in the westerly wind field and blooms occur intermittently
whenever the westerlies slacken sufficiently to permit stabilization of the upper water
column. The bloom and onset of nutrient depletion varies between years by as much as
4 months at a fixed station and is usually about 1 month earlier inshore than in the open
Tasman Sea.
Austral winter chlorophyll values of < 05 mg chl m
−3 rise to 20–25 mg chl m
−3 at
the peak of the early summer bloom, usually between August and October, and then
decline in late austral summer and early autumn from January to March. Nutrients
become wholly depleted by about February, when the entire province is dominated
by productivity and chlorophyll biomass that is sustained during austral summer by
dynamic processes in the well-developed frontal region of the SSTC (see Color plate
17). A weak regional autumn bloom is evident in some images. All this, Murphy et al.
(2001) suggest, is consistent with production being colimited by light and nitrate. Warmcore eddies associated with the retroflection of the East Australian Current may develop
phytoplankton blooms later than in the surrounding water mass; one such, observed
355
Tasman Front shed equatorward cyclonic meanders and poleward anticyclonic meanders,
and these in turn shed warm and cold-core rings. The climatological origins of the
eddies are topographically determined by the highest points of Lord Howe Rise and the
West Norfolk Ridge (Hamilton, 1992), and hence preferred locations can be mapped
for cold and warm features along the front. The actual location of the front as it passes
across the Tasman Sea varies from 30
S to as much as 38
S (Mulhearn, 1987) before it
rounds Cape North (35
S) to enter the poleward flow of the east New Zealand coastal
currents.
The retroflection of the East Australian Current has major consequences for the
Tasman Sea apart from bounding it to the north by the Tasman Front. The meandering
flow along the front generates a field of Rossby waves that pass westward toward the
Australian coast and induce the shedding of very large warm eddies: two or three are shed
per year, and a standing population of six to eight in the Tasman Sea is normal. Within
these eddies the seasonal cycles of mixing and stabilization are imposed on their initial
structure. The surface thermal signature and shallow thermocline of Coral Sea water is
lost during the first winter of the existence of an eddy, and in subsequent summers the
surface mixed layer established over the whole Tasman Sea passes across the subsurface
warm core of the eddy. A further source of eddies are the episodic irruptions of warm
water of Leeuwin Current origin, transported along the south coast of Australia and
occasionally penetrating the Bass Straits.
Regional Response of the Pelagic Ecosystem
The southern Tasman Sea exhibits a typical spring bloom (Harris et al., 1987) so that
chlorophyll biomass in the south-central region is maximal in late September–early
October, but regular seasonality does not extend across the whole region; the integrated
chlorophyll data are, in this province, somewhat misleading because the effect of the
southwest region dominates the whole (see Color plate 17). Monthly chlorophyll images
indicate that higher surface chlorophyll occurs patchily across the whole southern Tasman
Sea through austral summer and autumn associated with frontogenesis and other bloominducing processes; however, overall, this is the period when lowest surface chlorophyll
biomass is observed in satellite data.
The spring bloom has been best described to the east of Tasmania, where it is a
typical but rather uncertain process. Although strong westerlies may induce a regional
mixed layer of almost 300 m, the actual depths achieved are dependent on the nature
of the subsurface water mass, which has strong interannual variability caused by shifts
in the location of the STCZ, and as mesoscale eddies come and go. There is a 40-day
periodicity in wind stress in the westerly wind field and blooms occur intermittently
whenever the westerlies slacken sufficiently to permit stabilization of the upper water
column. The bloom and onset of nutrient depletion varies between years by as much as
4 months at a fixed station and is usually about 1 month earlier inshore than in the open
Tasman Sea.
Austral winter chlorophyll values of < 05 mg chl m
−3 rise to 20–25 mg chl m
−3 at
the peak of the early summer bloom, usually between August and October, and then
decline in late austral summer and early autumn from January to March. Nutrients
become wholly depleted by about February, when the entire province is dominated
by productivity and chlorophyll biomass that is sustained during austral summer by
dynamic processes in the well-developed frontal region of the SSTC (see Color plate
17). A weak regional autumn bloom is evident in some images. All this, Murphy et al.
(2001) suggest, is consistent with production being colimited by light and nitrate. Warmcore eddies associated with the retroflection of the East Australian Current may develop
phytoplankton blooms later than in the surrounding water mass; one such, observed