424
Chapter 11: The Pacific Ocean
ranks second among 50 shelf areas (for all seas) in dissipation of tidal energy per unit
length of coastline (Hunter and Sharp, 1983; Forbes, 1984). Tidal mixing is especially
strong around headlands in the Gulf of Carpentaria and also along the whole south coast
of New Guinea. Fronts between tidally mixed and thermally stratified areas may develop
only in austral summer, during the southerly monsoon, when wind stress is lowest in the
eastern archipelago and when thermal stratification can occur over the shallow shelves.
Defining Characteristics of Regional Oceanography
This province is wholly under the influence of the seasonally reversing monsoon winds
and of the circulation thereby generated, which has been sufficiently noted already
(see ARCH, MONS) and will not be reviewed again here in detail. It suffices to say that
current flow between the Indian and Pacific Oceans generally reverses with the monsoon
winds whose exact direction differs across the province: in the South China Sea it is
seasonally NE-SW but in Indonesia closer to NW-SE. So one must be aware that both
the relative meridional and zonal wind stress differs progressively across the province,
because of the curvature of pressure contours around the main atmospheric systems. It
must also be emphasized that in thinking about the oceanography of this province, you
should also remember that it spans the equator and should expect the consequences of
seasonality in the two hemispheres to be observable.
The source document for this region remains the splendid regional synthesis of Wyrtki
(1961), from which I have drawn much of the following account, but I am indebted also to the
recent reviews of Arief (1998) and of Church and Craig (1998) for additional information.
Over much of the province an Ekman layer of 30–50 m exists at all seasons, and only
in the northern part of the South China Sea (north of Hainan) and in the seas north of
Australia does this deepen in boreal winter to 70–90 m; in the former case, this is due to
wind mixing (see China Sea Coastal Province), and in the latter caused by the seasonal
alternation between upwelling and relaxation in the Banda and Flores Seas (see ARCH).
A simple comparison between potential mixed-layer depths and the depth of water over
the shelves shows that much of the southern Sunda Shelf between Borneo and Sumatra
must be mixed to the bottom, and here we can expect active development of shelf sea
fronts. The temperature of the mixed layer thus varies more at the northern and southern
ends of the province, whereas salinity is more variable seasonally in the central regions,
though actual seasonality is not uniform, following rainfall and river effluent patterns.
In the Gulf of Carpentaria, winter mixing is complete and in the austral summer season
only the central gulf is stratified because tidal currents are sufficiently strong to mix the
bottom 30 m of the water column (Church and Craig, 1998). There are significant consequences of this mixing for the benthic fauna (see later discussion). Circulation in the gulf
changes with the monsoons, clockwise in the NE and counterclockwise in the SE trades,
although the coastal jets do not readily mix with the higher-salinity central gulf water.
The mixed layer is almost universally nutrient-depleted, except in river plumes, and
lies above a nutricline that is coincident with the thermocline; Wyrtki commented that
the input of nutrients from rivers is, or was before serious clear-cut logging, remarkably
low and that flux of nutrients into the mixed layer seems to occur mostly in tidally mixed
areas or where seasonal upwelling of subthermocline water occurs. In such places, concern
is currently expressed at the amount of nitrogen and phosphorus (and also, in some
places, the biological oxygen demand of the discharge) entering some of the embayments.
The Gulf of Thailand is the best example of this, or at least it is the most investigated;
here, there are several episodic blooms during the year without a very clear seasonality,
and the frequency of these is said to have significantly increased in recent decades and
to be associated with the discharge of nitrate from the major rivers entering the gulf.
As already noted (see ARCH) some coastal upwelling occurs here, as we would expect
from the complex and fractal geography of the coastlines; there must be many places where
Chapter 11: The Pacific Ocean
ranks second among 50 shelf areas (for all seas) in dissipation of tidal energy per unit
length of coastline (Hunter and Sharp, 1983; Forbes, 1984). Tidal mixing is especially
strong around headlands in the Gulf of Carpentaria and also along the whole south coast
of New Guinea. Fronts between tidally mixed and thermally stratified areas may develop
only in austral summer, during the southerly monsoon, when wind stress is lowest in the
eastern archipelago and when thermal stratification can occur over the shallow shelves.
Defining Characteristics of Regional Oceanography
This province is wholly under the influence of the seasonally reversing monsoon winds
and of the circulation thereby generated, which has been sufficiently noted already
(see ARCH, MONS) and will not be reviewed again here in detail. It suffices to say that
current flow between the Indian and Pacific Oceans generally reverses with the monsoon
winds whose exact direction differs across the province: in the South China Sea it is
seasonally NE-SW but in Indonesia closer to NW-SE. So one must be aware that both
the relative meridional and zonal wind stress differs progressively across the province,
because of the curvature of pressure contours around the main atmospheric systems. It
must also be emphasized that in thinking about the oceanography of this province, you
should also remember that it spans the equator and should expect the consequences of
seasonality in the two hemispheres to be observable.
The source document for this region remains the splendid regional synthesis of Wyrtki
(1961), from which I have drawn much of the following account, but I am indebted also to the
recent reviews of Arief (1998) and of Church and Craig (1998) for additional information.
Over much of the province an Ekman layer of 30–50 m exists at all seasons, and only
in the northern part of the South China Sea (north of Hainan) and in the seas north of
Australia does this deepen in boreal winter to 70–90 m; in the former case, this is due to
wind mixing (see China Sea Coastal Province), and in the latter caused by the seasonal
alternation between upwelling and relaxation in the Banda and Flores Seas (see ARCH).
A simple comparison between potential mixed-layer depths and the depth of water over
the shelves shows that much of the southern Sunda Shelf between Borneo and Sumatra
must be mixed to the bottom, and here we can expect active development of shelf sea
fronts. The temperature of the mixed layer thus varies more at the northern and southern
ends of the province, whereas salinity is more variable seasonally in the central regions,
though actual seasonality is not uniform, following rainfall and river effluent patterns.
In the Gulf of Carpentaria, winter mixing is complete and in the austral summer season
only the central gulf is stratified because tidal currents are sufficiently strong to mix the
bottom 30 m of the water column (Church and Craig, 1998). There are significant consequences of this mixing for the benthic fauna (see later discussion). Circulation in the gulf
changes with the monsoons, clockwise in the NE and counterclockwise in the SE trades,
although the coastal jets do not readily mix with the higher-salinity central gulf water.
The mixed layer is almost universally nutrient-depleted, except in river plumes, and
lies above a nutricline that is coincident with the thermocline; Wyrtki commented that
the input of nutrients from rivers is, or was before serious clear-cut logging, remarkably
low and that flux of nutrients into the mixed layer seems to occur mostly in tidally mixed
areas or where seasonal upwelling of subthermocline water occurs. In such places, concern
is currently expressed at the amount of nitrogen and phosphorus (and also, in some
places, the biological oxygen demand of the discharge) entering some of the embayments.
The Gulf of Thailand is the best example of this, or at least it is the most investigated;
here, there are several episodic blooms during the year without a very clear seasonality,
and the frequency of these is said to have significantly increased in recent decades and
to be associated with the discharge of nitrate from the major rivers entering the gulf.
As already noted (see ARCH) some coastal upwelling occurs here, as we would expect
from the complex and fractal geography of the coastlines; there must be many places where