Additional factors contributing to recent
changes in sea level include the melting of nonpolar glaciers, changes in terrestrial water storage,
and changes in the mass of the Greenland and
Antarctic ice sheets, all stemming from climate
change during the twentieth century and the aftermath of the last ice age.
The geographical distribution of sea-level change
is principally determined by modified surface
momentum, heat and freshwater fluxes with consequent alterations to the ocean density structure and
ocean circulation. However, confidence in the
regional distribution of sea-level rise is low as there
is little similarity between results from different
models (Church et al., 2001; Gregory et al., 2001).
1.2.7.2 Regional impacts of climate
variability and change – links to shelf and
inshore circulations
Properties of shelf waters and shelf circulation patterns have a rich spectrum of variability. This
results from the multiplicity of forcing mechanisms
for the shelf and coastal region. First, there is often
a coastal discharge of water carrying nutrients and
contaminants. This discharge is most often of fresh
(light) water from rivers but can also be of dense
water. Examples of the latter are the high-salinity
water flowing out of the Mediterranean Sea
(Candela, Chapter 5.7) and cold/saline water
formed by air–sea–ice interaction on the Arctic
and Antarctic shelves.
Second, atmospheric forcing of the coastal wave
guide, primarily through along-shore wind stress,
but also through across-shelf winds and surface
heat and freshwater fluxes, is also an important
driver of shelf circulation. Understanding the
mechanisms of wind-induced circulation on the
shelf, particularly through coastal trapped wave
theory (see for example Brink et al., 1987), has
progressed significantly over the last two decades.
The combined impacts of wind-induced upwelling
and buoyancy effects can lead to complex threedimensional, time-variable patterns both on and
off the shelf. These circulations lead to considerable shelf/deep ocean exchange and remain a
challenge to both observers and modellers
(Hickey, 1998). Variability in both the coastal and
atmospheric forcing is itself related to seasonal,
interannual and decadal variability in climate.
Third, offshore ocean phenomena are a significant driver of circulation patterns on the shelf
and in the near-coastal region. These phenomena
include ocean swell, tides (including the generation
of baroclinic tides over topographic features), deep
ocean eddies impacting the shelf, seasonal, interannual and decadal variability in the offshore region
and, of course, the mean offshore conditions. Often
the water properties on the shelf (temperatures,
salinities, nutrient concentrations) are largely determined by the offshore conditions, but the processes
of exchange between the deep ocean, the shelf and
the near-shore region are poorly understood. The
offshore influence is especially strong when the
shelf is narrow and/or there is an energetic offshore
current such as a western boundary current. An
overview of the dynamics of offshore forcing of
shelf regions is given by Brink (1998) and a number of examples of offshore forcing of shelf conditions can be found in the volume on the Global
Coastal Ocean (Brink and Robinson, 1998).
One of the classic examples of offshore forcing
of shelf circulation is the generation of instabilities
on the inshore edge of the Gulf Stream (Boicourt
et al., 1998; Brink, 1998). These instabilities are
the dominant source of variability on the outer
shelf and reach 40 km inshore from the shelf break.
They also drive nutrient-rich cold water, upwelled
from below the Gulf Stream, onto the shelf. These
vigorous motions are a major contributor to the
exchange of oceanic and shelf waters and provide
the main nutrient supply for the continental shelf
(Lee et al., 1991). Similarly, the East Australian
Current is sometimes the dominant factor driving
shelf circulation off eastern Australia. This can be
through a rapid flushing of the shelf as the current
spills onto the shelf when the East Australian
Current or one of its eddies impinges on the shelf
(Huyer et al., 1988).
Another example of offshore forcing comes from
the west coast of North and South America. Kelvin
waves travelling eastwards in the equatorial waveguide encounter the shelf, refracting energy into
northward and southward travelling disturbances in
the coastal wave guide. These disturbances are
observed to travel thousands of kilometres (Brink,
1998) directly impacting ecosystems.
There are a number of documented examples
where higher trophic level shelf productivity
appears to be influenced by offshore conditions.
For example, Pierce and Phillips (1988) demonstrated that there is a strong link between coastal
sea level at Freemantle (western Australia) and the
1.2 Ocean Processes and Climate Phenomena
29
Clarke, Church and Gould
changes in sea level include the melting of nonpolar glaciers, changes in terrestrial water storage,
and changes in the mass of the Greenland and
Antarctic ice sheets, all stemming from climate
change during the twentieth century and the aftermath of the last ice age.
The geographical distribution of sea-level change
is principally determined by modified surface
momentum, heat and freshwater fluxes with consequent alterations to the ocean density structure and
ocean circulation. However, confidence in the
regional distribution of sea-level rise is low as there
is little similarity between results from different
models (Church et al., 2001; Gregory et al., 2001).
1.2.7.2 Regional impacts of climate
variability and change – links to shelf and
inshore circulations
Properties of shelf waters and shelf circulation patterns have a rich spectrum of variability. This
results from the multiplicity of forcing mechanisms
for the shelf and coastal region. First, there is often
a coastal discharge of water carrying nutrients and
contaminants. This discharge is most often of fresh
(light) water from rivers but can also be of dense
water. Examples of the latter are the high-salinity
water flowing out of the Mediterranean Sea
(Candela, Chapter 5.7) and cold/saline water
formed by air–sea–ice interaction on the Arctic
and Antarctic shelves.
Second, atmospheric forcing of the coastal wave
guide, primarily through along-shore wind stress,
but also through across-shelf winds and surface
heat and freshwater fluxes, is also an important
driver of shelf circulation. Understanding the
mechanisms of wind-induced circulation on the
shelf, particularly through coastal trapped wave
theory (see for example Brink et al., 1987), has
progressed significantly over the last two decades.
The combined impacts of wind-induced upwelling
and buoyancy effects can lead to complex threedimensional, time-variable patterns both on and
off the shelf. These circulations lead to considerable shelf/deep ocean exchange and remain a
challenge to both observers and modellers
(Hickey, 1998). Variability in both the coastal and
atmospheric forcing is itself related to seasonal,
interannual and decadal variability in climate.
Third, offshore ocean phenomena are a significant driver of circulation patterns on the shelf
and in the near-coastal region. These phenomena
include ocean swell, tides (including the generation
of baroclinic tides over topographic features), deep
ocean eddies impacting the shelf, seasonal, interannual and decadal variability in the offshore region
and, of course, the mean offshore conditions. Often
the water properties on the shelf (temperatures,
salinities, nutrient concentrations) are largely determined by the offshore conditions, but the processes
of exchange between the deep ocean, the shelf and
the near-shore region are poorly understood. The
offshore influence is especially strong when the
shelf is narrow and/or there is an energetic offshore
current such as a western boundary current. An
overview of the dynamics of offshore forcing of
shelf regions is given by Brink (1998) and a number of examples of offshore forcing of shelf conditions can be found in the volume on the Global
Coastal Ocean (Brink and Robinson, 1998).
One of the classic examples of offshore forcing
of shelf circulation is the generation of instabilities
on the inshore edge of the Gulf Stream (Boicourt
et al., 1998; Brink, 1998). These instabilities are
the dominant source of variability on the outer
shelf and reach 40 km inshore from the shelf break.
They also drive nutrient-rich cold water, upwelled
from below the Gulf Stream, onto the shelf. These
vigorous motions are a major contributor to the
exchange of oceanic and shelf waters and provide
the main nutrient supply for the continental shelf
(Lee et al., 1991). Similarly, the East Australian
Current is sometimes the dominant factor driving
shelf circulation off eastern Australia. This can be
through a rapid flushing of the shelf as the current
spills onto the shelf when the East Australian
Current or one of its eddies impinges on the shelf
(Huyer et al., 1988).
Another example of offshore forcing comes from
the west coast of North and South America. Kelvin
waves travelling eastwards in the equatorial waveguide encounter the shelf, refracting energy into
northward and southward travelling disturbances in
the coastal wave guide. These disturbances are
observed to travel thousands of kilometres (Brink,
1998) directly impacting ecosystems.
There are a number of documented examples
where higher trophic level shelf productivity
appears to be influenced by offshore conditions.
For example, Pierce and Phillips (1988) demonstrated that there is a strong link between coastal
sea level at Freemantle (western Australia) and the
1.2 Ocean Processes and Climate Phenomena
29
Clarke, Church and Gould