Indian Ocean Coastal Biome
321
from May to September (Quadfasel and Cresswell, 1992). Precipitation exceeds evaporation (by 15 cm per month) during the wet season, whereas evaporation dominates (by
5 cm per month) during the dry season of boreal winter. The low salinity of coastal waters
during the wet season induces a cross-shore pressure gradient and so forces a westward
jet current at the coast, reinforced by the surface wind stress. This coastal jet forms part
of the origin of the South Equatorial Current that is at its most northerly position in
boreal summer. During boreal winter, when wind stress is toward the east, flow in the
same direction is induced along the whole coast from the equator (off central Sumatra)
into the western Timor Sea. The current reversal between seasons is influenced not only
by local winds but also by distant forcing of strong westerly winds in the central Indian
Ocean that induce eastward-propagating equatorial Kelvin waves.
Associated with the season of strongest westward flow of the Java Coastal Current
under the forcing of the Southeast Monsoon, a period of significant coastal upwelling
occurs from May to September along the coasts of Java and Sumbawa (Wyrtki, 1962,
1973b). This is a deep upwelling so that 200-m temperatures in the center of the feature
fall to 12
C compared with 18–20
C at the same depth in the open ocean between Java
and western Australia. The region of enhanced nutrient availability is larger than that off
Arabia.
Western Australia The Leeuwin Current is anomalous among all other eastern boundary currents in that it flows poleward, despite the equatorward wind stress over the eastern
Indian Ocean and the general equatorward flow in the eastern limb of the subtropical
gyre further offshore. It is a surface flow of warm, low-salinity tropical water above an
equatorward undercurrent, with the level of no motion between the two flows lying at
200–300 m. It has significant seasonality, flowing south along the entire coastline more
consistently during austral autumn and winter. The anomalous poleward flow is forced
by the strength of the onshore geopotential anomaly caused by the poleward pressure
gradient in the eastern Indian Ocean—this is sufficient to overcome equatorward wind
stress (Church et al., 1989). In the coastal zone, the eastward component of flow arising
from the alongshore pressure gradient creates a downwelling situation in which isopycnals
slope down toward the coast (Thompson, 1987).
The source waters for the Leeuwin Current are off the northwest Australian coast
over the Sahul shelf, which is within the influence of the monsoon systems of Java and
Sumatra; thus, a rainy monsoon period in austral summer (January–March) intervenes
in an otherwise very dry area having exceptionally strong irradiance at the sea surface.
Flow is maximal during the monsoon period and the velocity core of the current lies
over the shelf edge; in austral winter it moves farther out to sea, forced by changes in
local wind stress rather than changes in the longshore pressure gradient (Church et al.,
1989; Smith et al., 1991). Satellite thermal imagery shows that the highly meandering
flow extends at least 250–350 km offshore and frequently includes prominent warm
filaments with strong vorticity. South of about 32
S, eddying extends even farther into
the equatorward flow of cool, high-salinity water of the subtropical gyre, sometimes
referred to as the West Australian Current. The temperature gradient between Leeuwin
flow and the offshore gyral water becomes stronger progressively southward, despite the
entrainment of subtropical water.
Great Australian Bight The coastal boundary of the southern coast of Australia,
between Cape Leeuwin in the west and Tasmania in the east, is under the influence of
Leeuwin Current water, which is topographically locked to the continental edge as it
rounds the Cape Leeuwin and proceeds eastward along the southern Australian shelf.
Nevertheless, maximal flow follows the shelf break of the Great Australian Bight as far
as 130
E (Rochford, 1986) and is maximal in austral winter (May–October). After July,
321
from May to September (Quadfasel and Cresswell, 1992). Precipitation exceeds evaporation (by 15 cm per month) during the wet season, whereas evaporation dominates (by
5 cm per month) during the dry season of boreal winter. The low salinity of coastal waters
during the wet season induces a cross-shore pressure gradient and so forces a westward
jet current at the coast, reinforced by the surface wind stress. This coastal jet forms part
of the origin of the South Equatorial Current that is at its most northerly position in
boreal summer. During boreal winter, when wind stress is toward the east, flow in the
same direction is induced along the whole coast from the equator (off central Sumatra)
into the western Timor Sea. The current reversal between seasons is influenced not only
by local winds but also by distant forcing of strong westerly winds in the central Indian
Ocean that induce eastward-propagating equatorial Kelvin waves.
Associated with the season of strongest westward flow of the Java Coastal Current
under the forcing of the Southeast Monsoon, a period of significant coastal upwelling
occurs from May to September along the coasts of Java and Sumbawa (Wyrtki, 1962,
1973b). This is a deep upwelling so that 200-m temperatures in the center of the feature
fall to 12
C compared with 18–20
C at the same depth in the open ocean between Java
and western Australia. The region of enhanced nutrient availability is larger than that off
Arabia.
Western Australia The Leeuwin Current is anomalous among all other eastern boundary currents in that it flows poleward, despite the equatorward wind stress over the eastern
Indian Ocean and the general equatorward flow in the eastern limb of the subtropical
gyre further offshore. It is a surface flow of warm, low-salinity tropical water above an
equatorward undercurrent, with the level of no motion between the two flows lying at
200–300 m. It has significant seasonality, flowing south along the entire coastline more
consistently during austral autumn and winter. The anomalous poleward flow is forced
by the strength of the onshore geopotential anomaly caused by the poleward pressure
gradient in the eastern Indian Ocean—this is sufficient to overcome equatorward wind
stress (Church et al., 1989). In the coastal zone, the eastward component of flow arising
from the alongshore pressure gradient creates a downwelling situation in which isopycnals
slope down toward the coast (Thompson, 1987).
The source waters for the Leeuwin Current are off the northwest Australian coast
over the Sahul shelf, which is within the influence of the monsoon systems of Java and
Sumatra; thus, a rainy monsoon period in austral summer (January–March) intervenes
in an otherwise very dry area having exceptionally strong irradiance at the sea surface.
Flow is maximal during the monsoon period and the velocity core of the current lies
over the shelf edge; in austral winter it moves farther out to sea, forced by changes in
local wind stress rather than changes in the longshore pressure gradient (Church et al.,
1989; Smith et al., 1991). Satellite thermal imagery shows that the highly meandering
flow extends at least 250–350 km offshore and frequently includes prominent warm
filaments with strong vorticity. South of about 32
S, eddying extends even farther into
the equatorward flow of cool, high-salinity water of the subtropical gyre, sometimes
referred to as the West Australian Current. The temperature gradient between Leeuwin
flow and the offshore gyral water becomes stronger progressively southward, despite the
entrainment of subtropical water.
Great Australian Bight The coastal boundary of the southern coast of Australia,
between Cape Leeuwin in the west and Tasmania in the east, is under the influence of
Leeuwin Current water, which is topographically locked to the continental edge as it
rounds the Cape Leeuwin and proceeds eastward along the southern Australian shelf.
Nevertheless, maximal flow follows the shelf break of the Great Australian Bight as far
as 130
E (Rochford, 1986) and is maximal in austral winter (May–October). After July,