Multidecadal Trends and Changes
123
Multidecadal Trends and Changes
El Niño–Southern Oscillation events occur against a background of longer, decadal
and secular-scale change. While emphasizing that they are integral components of the
planetary weather system, I shall discuss two examples: the Pacific Decadal Oscillation
(PDO), and the North Atlantic Oscillation (NAO). Both involve major and sustained
changes in conditions across whole ocean basins of the Northern Hemisphere. We must
assume that comparable changes are sustained in the Southern Hemisphere, but these
are yet to be identified. The NAO is better understood than the PDO, and a simple
meteorological index suffices to indicate its state, whereas the more recently recognized
PDO is usually indexed by the resultant sea surface temperature changes. The two phases
of the PDO are characterized by anomalously warm and cool sea surface temperatures
along the western coast of North America, although these are accompanied by sealevel pressure and wind anomalies that are large-scale, low-frequency, and occur rapidly.
These changes are, in turn, reflected in ocean properties and circulation pattern and, not
surprisingly, the oceanic ecosystems respond on similar time and space scales (Francis
et al., 1998). During the 20th century, the warm phase dominated during the periods
1900–1915, 1922–1945, 1958–1961, and 1976–1998.
Thus, during the winter of 1976–1977 the entire North Pacific weather system underwent a change of state. The Aleutian low-pressure cell intensified, thus shifting storm
tracks further south than normal and from then until the 1990s the tropical ocean
remained in a quasi-permanent warm mode. Full development of El Niño events was
more frequent during this period so that until 1995 the mean return interval was less
than 2 years, and in only a single year (1988–1989) was there full development of the
trade winds (Graham, 1994). During this period, upwelling in the California Current
was often constrained by a cap of light, warm water, and winter zooplankton biomass
there progressively declined by about 80% from the 1970s to the 1990s (Roemmich and
McGowan, 1995). In the central North Pacific, the carrying capacity of the ecosystem
significantly increased, with integrated chlorophyll almost doubling from 1975 to 1985
(Venrick et al., 1994). This signaled that the central North Pacific was undergoing a major
ecological change, close to what general ecologists have called “catastrophic ecosystem
shifts” (e.g., Scheffer et al., 2001), representing a very rapid step change from one set of
dominant organisms to another, or from one dominant process to another. In this case,
both of these changes occurred. The mixed layer of the central gyre became more strongly
stratified than previously, and the vertical flux of nutrients from below the nutricline was
accordingly constrained.
These changes represent a phylogenetic, or domain, shift from a photoautotroph community dominated by larger eukaryotes to one dominated by very numerous, very small
cells of the domain Bacteria, and by bundles of filaments of nitrogen-fixing cyanobacteria,
inedible to most herbivores. Such domain shifts, as Karl et al. (2001) remark, will echo
up the food chain. Selection for protistan “herbivores” over metazoan herbivores must
have occurred and will have involved another fundamental shift in ecosystem structure
and function, toward a more complex food web. Carbon will have tended to be retained
within the system, rather than being exported via herbivorous crustaceans and their fish
predators.
But this is just the most recent example of a phase shift of the PDO. During the
20th century, several such events were sustained, each for 20–30 years; a “cool” phase
dominated until 1924 and from 1947 to 1976, and a “warm” phase in the intervening
years. Within these periods were embedded many El Niño events, each of 6–18 months
duration, whose effects are somewhat spatially distinct: PDO phase changes are most
clearly expressed in high latitudes, whereas those of ENSO events are centered in the
tropical ocean. Both ENSO and the PDO are associated with rather similar climate
123
Multidecadal Trends and Changes
El Niño–Southern Oscillation events occur against a background of longer, decadal
and secular-scale change. While emphasizing that they are integral components of the
planetary weather system, I shall discuss two examples: the Pacific Decadal Oscillation
(PDO), and the North Atlantic Oscillation (NAO). Both involve major and sustained
changes in conditions across whole ocean basins of the Northern Hemisphere. We must
assume that comparable changes are sustained in the Southern Hemisphere, but these
are yet to be identified. The NAO is better understood than the PDO, and a simple
meteorological index suffices to indicate its state, whereas the more recently recognized
PDO is usually indexed by the resultant sea surface temperature changes. The two phases
of the PDO are characterized by anomalously warm and cool sea surface temperatures
along the western coast of North America, although these are accompanied by sealevel pressure and wind anomalies that are large-scale, low-frequency, and occur rapidly.
These changes are, in turn, reflected in ocean properties and circulation pattern and, not
surprisingly, the oceanic ecosystems respond on similar time and space scales (Francis
et al., 1998). During the 20th century, the warm phase dominated during the periods
1900–1915, 1922–1945, 1958–1961, and 1976–1998.
Thus, during the winter of 1976–1977 the entire North Pacific weather system underwent a change of state. The Aleutian low-pressure cell intensified, thus shifting storm
tracks further south than normal and from then until the 1990s the tropical ocean
remained in a quasi-permanent warm mode. Full development of El Niño events was
more frequent during this period so that until 1995 the mean return interval was less
than 2 years, and in only a single year (1988–1989) was there full development of the
trade winds (Graham, 1994). During this period, upwelling in the California Current
was often constrained by a cap of light, warm water, and winter zooplankton biomass
there progressively declined by about 80% from the 1970s to the 1990s (Roemmich and
McGowan, 1995). In the central North Pacific, the carrying capacity of the ecosystem
significantly increased, with integrated chlorophyll almost doubling from 1975 to 1985
(Venrick et al., 1994). This signaled that the central North Pacific was undergoing a major
ecological change, close to what general ecologists have called “catastrophic ecosystem
shifts” (e.g., Scheffer et al., 2001), representing a very rapid step change from one set of
dominant organisms to another, or from one dominant process to another. In this case,
both of these changes occurred. The mixed layer of the central gyre became more strongly
stratified than previously, and the vertical flux of nutrients from below the nutricline was
accordingly constrained.
These changes represent a phylogenetic, or domain, shift from a photoautotroph community dominated by larger eukaryotes to one dominated by very numerous, very small
cells of the domain Bacteria, and by bundles of filaments of nitrogen-fixing cyanobacteria,
inedible to most herbivores. Such domain shifts, as Karl et al. (2001) remark, will echo
up the food chain. Selection for protistan “herbivores” over metazoan herbivores must
have occurred and will have involved another fundamental shift in ecosystem structure
and function, toward a more complex food web. Carbon will have tended to be retained
within the system, rather than being exported via herbivorous crustaceans and their fish
predators.
But this is just the most recent example of a phase shift of the PDO. During the
20th century, several such events were sustained, each for 20–30 years; a “cool” phase
dominated until 1924 and from 1947 to 1976, and a “warm” phase in the intervening
years. Within these periods were embedded many El Niño events, each of 6–18 months
duration, whose effects are somewhat spatially distinct: PDO phase changes are most
clearly expressed in high latitudes, whereas those of ENSO events are centered in the
tropical ocean. Both ENSO and the PDO are associated with rather similar climate