122
Chapter 8: Longer Term Responses: From Seasons to Centuries
Binet (1997) has reviewed fisheries responses in the Canary Current. Here, two periods
(in the early 1970s and the late 1980s) when coastwise wind stress was anomalously
strong resulted in strengthening of the upwelling cells and a very significant extension
southward of the temperate sardine Sardina pilchardus. This led to a threefold increase
in its abundance, at the expense of the home range of the tropical sardines S. aurita
and S. maderensis. During these wind-stress anomalies, S. pilchardus extended its range
even south of Senegal, onto the Arguin Bank (see GUIN province). We may reasonably
suppose that changes in organisms at such relatively high trophic levels as sardines
must reflect very profound changes in the planktonic ecosystem on which their growth
depends.
Obviously, the seasonal evolution of the atmospheric circulation pattern is not identical
from year to year, associated with changes in oceanic circulation. It may be useful to
identify some of the characteristic and recurrent aspects of these changes. Consider,
for example, the consequences of variability in the seasonal march of the atmospheric
Intertropical Convergence Zone (ITCZ), which follows the apparent movement of the sun,
seasonally, from about 2
S to about 9
N in the Atlantic Ocean and between appropriate
latitudes in the other oceans. The date on which its poleward movement begins is variable
between years, as is the northing attained during each boreal summer. In this ocean, in
some years, the ITCZ reaches only to 8
N, whereas in others it reaches to 10
N and
the annual displacement is matched by the northing achieved by the ITCZ in other
oceans. The amplitude of the between-year differences may appear to be minor, but
are associated with distant changes in weather patterns that are very significant—for
example, the continental wind and rainfall fields over sub-Saharan Africa, and trade
wind stress on the sea surface (Delacluse et al., 1994). The effect of the location of the
ITCZ on the surface temperature anomalies in the eastern tropical Atlantic is complex
but observable. The anomalously warm summer of 1984 in the Gulf of Guinea was
caused by unusually intense trade-wind stress during the previous summer and fall,
leading to an unusually deep mixed layer of warm water in the western basin. On the
seasonal relaxation of the trades, this mass of warm water surged eastward to form
an unusually deep and warm mixed layer in the Gulf of Guiana that persisted for
many months (Citeau et al., 1988; Carton and Huang, 1994). Between-year variability in
the location of the ITCZ, the principal convergence zone in the planetary atmospheric
circulation, is associated with major anomalies in SST, of scale 3000 to 4000 km, and
observable for periods of up to 12 months. Such anomalies may be coherent over even
longer periods, spanning more than a decade, and are characteristic of the variability of
planetary weather systems as discussed, for example, by Cushing and Dickson (1966) or
by Cushing (1982).
We may expect to observe significant effects of these anomalies on pelagic ecology
over large areas of the warm oceans. The curl of the wind stress in the ITCZ is associated
with Ekman suction in the North Equatorial Counter Currents of both the Pacific and
the Atlantic Oceans and, hence, with a zonal band of enhanced chlorophyll in each ocean.
Two of the provinces of the Trade Wind biome are partially defined in relation to this
process so that their actual locations will be sensitive to annual variation in the meridional
position of the ITCZ. Strong El Niño events, like that of 1982–1983, may generate sufficiently energetic, poleward Kelvin waves along the continental margin that these establish
conditions for the radiation westward of Rossby waves at mid-latitudes (35–40
N). These
then advance very slowly across the North Pacific and eventually (a decade after the
original ENSO event) modify the latitude of the axial flow of the Kuroshio extension.
The consequent advection of anomalously warm water to the northeast Pacific has the
same amplitude and extent as that which occurs by atmospheric forcing during an ENSO
event (Jacobs et al., 1994).
Chapter 8: Longer Term Responses: From Seasons to Centuries
Binet (1997) has reviewed fisheries responses in the Canary Current. Here, two periods
(in the early 1970s and the late 1980s) when coastwise wind stress was anomalously
strong resulted in strengthening of the upwelling cells and a very significant extension
southward of the temperate sardine Sardina pilchardus. This led to a threefold increase
in its abundance, at the expense of the home range of the tropical sardines S. aurita
and S. maderensis. During these wind-stress anomalies, S. pilchardus extended its range
even south of Senegal, onto the Arguin Bank (see GUIN province). We may reasonably
suppose that changes in organisms at such relatively high trophic levels as sardines
must reflect very profound changes in the planktonic ecosystem on which their growth
depends.
Obviously, the seasonal evolution of the atmospheric circulation pattern is not identical
from year to year, associated with changes in oceanic circulation. It may be useful to
identify some of the characteristic and recurrent aspects of these changes. Consider,
for example, the consequences of variability in the seasonal march of the atmospheric
Intertropical Convergence Zone (ITCZ), which follows the apparent movement of the sun,
seasonally, from about 2
S to about 9
N in the Atlantic Ocean and between appropriate
latitudes in the other oceans. The date on which its poleward movement begins is variable
between years, as is the northing attained during each boreal summer. In this ocean, in
some years, the ITCZ reaches only to 8
N, whereas in others it reaches to 10
N and
the annual displacement is matched by the northing achieved by the ITCZ in other
oceans. The amplitude of the between-year differences may appear to be minor, but
are associated with distant changes in weather patterns that are very significant—for
example, the continental wind and rainfall fields over sub-Saharan Africa, and trade
wind stress on the sea surface (Delacluse et al., 1994). The effect of the location of the
ITCZ on the surface temperature anomalies in the eastern tropical Atlantic is complex
but observable. The anomalously warm summer of 1984 in the Gulf of Guinea was
caused by unusually intense trade-wind stress during the previous summer and fall,
leading to an unusually deep mixed layer of warm water in the western basin. On the
seasonal relaxation of the trades, this mass of warm water surged eastward to form
an unusually deep and warm mixed layer in the Gulf of Guiana that persisted for
many months (Citeau et al., 1988; Carton and Huang, 1994). Between-year variability in
the location of the ITCZ, the principal convergence zone in the planetary atmospheric
circulation, is associated with major anomalies in SST, of scale 3000 to 4000 km, and
observable for periods of up to 12 months. Such anomalies may be coherent over even
longer periods, spanning more than a decade, and are characteristic of the variability of
planetary weather systems as discussed, for example, by Cushing and Dickson (1966) or
by Cushing (1982).
We may expect to observe significant effects of these anomalies on pelagic ecology
over large areas of the warm oceans. The curl of the wind stress in the ITCZ is associated
with Ekman suction in the North Equatorial Counter Currents of both the Pacific and
the Atlantic Oceans and, hence, with a zonal band of enhanced chlorophyll in each ocean.
Two of the provinces of the Trade Wind biome are partially defined in relation to this
process so that their actual locations will be sensitive to annual variation in the meridional
position of the ITCZ. Strong El Niño events, like that of 1982–1983, may generate sufficiently energetic, poleward Kelvin waves along the continental margin that these establish
conditions for the radiation westward of Rossby waves at mid-latitudes (35–40
N). These
then advance very slowly across the North Pacific and eventually (a decade after the
original ENSO event) modify the latitude of the axial flow of the Kuroshio extension.
The consequent advection of anomalously warm water to the northeast Pacific has the
same amplitude and extent as that which occurs by atmospheric forcing during an ENSO
event (Jacobs et al., 1994).