124
Chapter 8: Longer Term Responses: From Seasons to Centuries
anomalies across North America, warm and cool phases of each mutually reinforcing
their effects. There are large-scale, low-frequency, and sometimes very rapid changes in
the distribution of atmospheric pressure over the North Pacific that are, in turn, reflected
in ocean properties and circulation. Oceanic ecosystems respond on similar time and
space scales to variations in physical conditions. Perhaps the best known response is that
of the fish and birds of the eastern boundary currents of the Pacific, reviewed recently by
Chavez et al. (2003). I shall discuss some of these phenomena in detail in Chapter 11, so
here it is only necessary to note that the relative abundance of sardines and anchovies,
and the relative productivity of the planktonic ecosystem, changed in our instrumental
and sedimentary records throughout the 20th century. In the historical and archaeological
records this phenomenon has been traced back several centuries and, in sediment cores
off California, for two millennia.
A general relationship between long-term changes in SST and pelagic productivity
has been proposed by Kamykowski and Zentara (2005). The very long SST time series
now available was made nitrate-sensitive by subtracting “nitrate-depletion temperatures”
associated with SST in water of salinity >28 ppt; this enabled estimates to be made of
effects of decadal-scale SST changes on the relative nitrate supply to the surface. In periods
of anomalously warmer SST, some regions of the ocean are predicted to have suffered
nitrate loss in a manner that is consistent with long-term fishery production data and of
blooms of dinoflagellates, better able to access subsurface nitrate supplies than nonmotile
phytoplankters.
The North Atlantic Oscillation (NAO) Index is quantified by the departure from the
mean pressure gradient between the Azores high-pressure and the Iceland low-pressure
cells during winter. Winter westerly winds intensify and storm tracks shift northward
during positive phases of the NAO, leading to milder conditions in the eastern part of
the ocean and heavier weather toward the north. The converse occurs during negative
phases.
Using sea level and meteorological records, it has been possible to reconstruct a
record for the Gulf Stream transport and the strength of the midlatitude westerly winds
extending back to 1850 from which long-term trends of the NAO can been identified.
Planque and Taylor (1998) review the connections between the two phases of the NOA,
the position of the Gulf Stream, changes in stratification and hence in the timing of
the spring bloom and in the subsequent zooplankton response. Because the north wall
of the Gulf Stream is constrained to lie below the line of zero Ekman pumping, where
no wind-driven divergence or convergence occurs at the sea surface, its position may be
predicted from the value of the NAO index (Taylor et al., 1998). When this takes low
values, the north wall of the Gulf Stream shifts to the south, and southward flux of the
Labrador Current is increased; in the Norwegian Sea, under these conditions of wind
forcing, flow of arctic water along the eastern coast of Greenland is enhanced.
A trend toward increasingly positive NAO values was sustained from 1920 to about
1950 and was renewed after 1964, so that in the early 1990s the NAO took stronger
positive values than at any time in the previous 175 years (Fig. 8.3 and Dickson, 2003).
This trend represented a major and sustained climate change that is imposed on the
year-to-year consequences of variability in the value of the NAO that may have significant
regional consequences (e.g., Color plate 10).
From 1950 to 1964, mean sea surface temperatures increased progressively in parts
of the North Atlantic, and this induced major shifts in the distribution of fish species:
for example, the penetration of the boreal seas by cod (Gadus atlantica), which built
up stocks progressively further north along the coast of western Greenland (Cushing
and Dickson, 1966). Northerly winds were progressively strengthened along the seaboard
of western Europe (Dickson et al., 1988), so that upwelling on the Portuguese coast
became unusually strong. Northerly winter winds created a high-salinity anomaly east of
Chapter 8: Longer Term Responses: From Seasons to Centuries
anomalies across North America, warm and cool phases of each mutually reinforcing
their effects. There are large-scale, low-frequency, and sometimes very rapid changes in
the distribution of atmospheric pressure over the North Pacific that are, in turn, reflected
in ocean properties and circulation. Oceanic ecosystems respond on similar time and
space scales to variations in physical conditions. Perhaps the best known response is that
of the fish and birds of the eastern boundary currents of the Pacific, reviewed recently by
Chavez et al. (2003). I shall discuss some of these phenomena in detail in Chapter 11, so
here it is only necessary to note that the relative abundance of sardines and anchovies,
and the relative productivity of the planktonic ecosystem, changed in our instrumental
and sedimentary records throughout the 20th century. In the historical and archaeological
records this phenomenon has been traced back several centuries and, in sediment cores
off California, for two millennia.
A general relationship between long-term changes in SST and pelagic productivity
has been proposed by Kamykowski and Zentara (2005). The very long SST time series
now available was made nitrate-sensitive by subtracting “nitrate-depletion temperatures”
associated with SST in water of salinity >28 ppt; this enabled estimates to be made of
effects of decadal-scale SST changes on the relative nitrate supply to the surface. In periods
of anomalously warmer SST, some regions of the ocean are predicted to have suffered
nitrate loss in a manner that is consistent with long-term fishery production data and of
blooms of dinoflagellates, better able to access subsurface nitrate supplies than nonmotile
phytoplankters.
The North Atlantic Oscillation (NAO) Index is quantified by the departure from the
mean pressure gradient between the Azores high-pressure and the Iceland low-pressure
cells during winter. Winter westerly winds intensify and storm tracks shift northward
during positive phases of the NAO, leading to milder conditions in the eastern part of
the ocean and heavier weather toward the north. The converse occurs during negative
phases.
Using sea level and meteorological records, it has been possible to reconstruct a
record for the Gulf Stream transport and the strength of the midlatitude westerly winds
extending back to 1850 from which long-term trends of the NAO can been identified.
Planque and Taylor (1998) review the connections between the two phases of the NOA,
the position of the Gulf Stream, changes in stratification and hence in the timing of
the spring bloom and in the subsequent zooplankton response. Because the north wall
of the Gulf Stream is constrained to lie below the line of zero Ekman pumping, where
no wind-driven divergence or convergence occurs at the sea surface, its position may be
predicted from the value of the NAO index (Taylor et al., 1998). When this takes low
values, the north wall of the Gulf Stream shifts to the south, and southward flux of the
Labrador Current is increased; in the Norwegian Sea, under these conditions of wind
forcing, flow of arctic water along the eastern coast of Greenland is enhanced.
A trend toward increasingly positive NAO values was sustained from 1920 to about
1950 and was renewed after 1964, so that in the early 1990s the NAO took stronger
positive values than at any time in the previous 175 years (Fig. 8.3 and Dickson, 2003).
This trend represented a major and sustained climate change that is imposed on the
year-to-year consequences of variability in the value of the NAO that may have significant
regional consequences (e.g., Color plate 10).
From 1950 to 1964, mean sea surface temperatures increased progressively in parts
of the North Atlantic, and this induced major shifts in the distribution of fish species:
for example, the penetration of the boreal seas by cod (Gadus atlantica), which built
up stocks progressively further north along the coast of western Greenland (Cushing
and Dickson, 1966). Northerly winds were progressively strengthened along the seaboard
of western Europe (Dickson et al., 1988), so that upwelling on the Portuguese coast
became unusually strong. Northerly winter winds created a high-salinity anomaly east of