120
Chapter 8: Longer Term Responses: From Seasons to Centuries
of planetary waves across the Pacific Ocean, modified by orbital and solar constants.
From 1803 to 1987, El Niño had a mean return time of 3.7 years during periods of high
solar constant and 3.2 years when the solar constant was low. Extreme return intervals
ranged from 1 to 8 years. As I shall discuss later, in recent decades the return interval has
shortened significantly. The SO is, in truth, the dominant interannual climate signal on
the global scale, as Philander and Rasmusson pointed out in 1985. If we assume that the
relaxed state of the tropical weather patterns is the existence of a well-developed trade
wind belt in both hemispheres, then a strong zonal pressure gradient across the tropical
Indo-Pacific, associated with a high value of the SOI, is also the norm. A Niño event is
foreshadowed by a period of anomalous southwesterly winds in the Tasman Sea and by
a weakening of the trades in the western Indo-Pacific. It is important to understand how
the reinforcement of this process is induced by feedback mechanisms: for instance, the
warming of the surface waters of the western Pacific caused by the previous weakening
of the trades acts to enhance trade-wind anomalies even further. This reinforces, rather
than weakens, the contingent surface warming. The event develops by the progressive
extension eastward of outbursts of rainy, westerly winds initiated between a succession
of pairs of tropical cyclonic cells, north and south of the equator in the western Pacific.
These bursts of eastward wind stress transport surface water to the east and induce
the propagation of Kelvin waves along the equatorial wave guide. Consequently, the
thermocline tilts across the entire Pacific basin, deepening in the east even as it shallows
in the west. There are also smaller scale changes in thermocline depth along the equator
associated with the passage of each Kelvin wave: deepening equatorward and shoaling
poleward. Initially, flow of the Equatorial Undercurrent accelerates as Kelvin waves pass
along it, but then it weakens in response to reduced zonal pressure gradient across the
Pacific basin. When they encounter the continent, the Kelvin waves propagate poleward
as coastally trapped waves along the western coast of the Americas. A fully developed
ENSO event is characterized by weakening of coastal upwelling on the western coasts
of the Americas and along the equator so that the surface waters of the eastern tropical
ocean are warm, oligotrophic, and overlie a deep thermocline. Coastal upwelling does
continue in these circumstances, but the unusually warm water brought to the surface
will not be nitrate-rich. The biological consequences of all this are well described by
Arntz (1986). In contrast, in the mid-ocean portion of the subtropical gyres (at 20–25
N),
an increase in overall productivity may occur, together with significant changes in the
functioning of the pelagic ecosystem (Table 8.1) that are readily observed in regional
satellite images. Ryan et al. (2002) comment that the 1998 transition from El Niño to
La Niña conditions was the first to have been observed by modern satellite imagery and
offer striking examples of the extent of the large-scale blooms that are induced by the
rapid and extreme shoaling of the thermocline that occurs during the transition period
(see Color plate 4, discussed later). This is an important phenomenon to which I shall
refer again in this and in following chapters.
Anomalous conditions may also occur in the trade-wind zone of the Atlantic during
some Niño events, as noted previously. This was observed in both 1968 and 1984 (Hisard,
1980; Tomczak and Godfrey, 1994). In both instances, the expected coastal upwelling
did not occur during boreal summer in the Gulf of Guinea, and heavy coastal rainfall
occurred in the winter dry season. The whole of the eastern part of the ocean had an
anomalous deep, warm mixed layer. In the same year, cold anomalies occurred in the
northwest Atlantic near Newfoundland. Recently, the response of the Atlantic to El Niño
events has been more satisfactorily analyzed (Bakun, 1996; Binet, 1997). During these
events, the anomalous low-pressure region (warm, wet, rising air) over central America
strengthens the Atlantic trade-wind system, and the westward slope of the sea surface
is enhanced, as is thermocline uplift in the east, leading to anomalous cool sea surface
temperature (SST) in the eastern tropical Atlantic. When the event relaxes, high pressure
Chapter 8: Longer Term Responses: From Seasons to Centuries
of planetary waves across the Pacific Ocean, modified by orbital and solar constants.
From 1803 to 1987, El Niño had a mean return time of 3.7 years during periods of high
solar constant and 3.2 years when the solar constant was low. Extreme return intervals
ranged from 1 to 8 years. As I shall discuss later, in recent decades the return interval has
shortened significantly. The SO is, in truth, the dominant interannual climate signal on
the global scale, as Philander and Rasmusson pointed out in 1985. If we assume that the
relaxed state of the tropical weather patterns is the existence of a well-developed trade
wind belt in both hemispheres, then a strong zonal pressure gradient across the tropical
Indo-Pacific, associated with a high value of the SOI, is also the norm. A Niño event is
foreshadowed by a period of anomalous southwesterly winds in the Tasman Sea and by
a weakening of the trades in the western Indo-Pacific. It is important to understand how
the reinforcement of this process is induced by feedback mechanisms: for instance, the
warming of the surface waters of the western Pacific caused by the previous weakening
of the trades acts to enhance trade-wind anomalies even further. This reinforces, rather
than weakens, the contingent surface warming. The event develops by the progressive
extension eastward of outbursts of rainy, westerly winds initiated between a succession
of pairs of tropical cyclonic cells, north and south of the equator in the western Pacific.
These bursts of eastward wind stress transport surface water to the east and induce
the propagation of Kelvin waves along the equatorial wave guide. Consequently, the
thermocline tilts across the entire Pacific basin, deepening in the east even as it shallows
in the west. There are also smaller scale changes in thermocline depth along the equator
associated with the passage of each Kelvin wave: deepening equatorward and shoaling
poleward. Initially, flow of the Equatorial Undercurrent accelerates as Kelvin waves pass
along it, but then it weakens in response to reduced zonal pressure gradient across the
Pacific basin. When they encounter the continent, the Kelvin waves propagate poleward
as coastally trapped waves along the western coast of the Americas. A fully developed
ENSO event is characterized by weakening of coastal upwelling on the western coasts
of the Americas and along the equator so that the surface waters of the eastern tropical
ocean are warm, oligotrophic, and overlie a deep thermocline. Coastal upwelling does
continue in these circumstances, but the unusually warm water brought to the surface
will not be nitrate-rich. The biological consequences of all this are well described by
Arntz (1986). In contrast, in the mid-ocean portion of the subtropical gyres (at 20–25
N),
an increase in overall productivity may occur, together with significant changes in the
functioning of the pelagic ecosystem (Table 8.1) that are readily observed in regional
satellite images. Ryan et al. (2002) comment that the 1998 transition from El Niño to
La Niña conditions was the first to have been observed by modern satellite imagery and
offer striking examples of the extent of the large-scale blooms that are induced by the
rapid and extreme shoaling of the thermocline that occurs during the transition period
(see Color plate 4, discussed later). This is an important phenomenon to which I shall
refer again in this and in following chapters.
Anomalous conditions may also occur in the trade-wind zone of the Atlantic during
some Niño events, as noted previously. This was observed in both 1968 and 1984 (Hisard,
1980; Tomczak and Godfrey, 1994). In both instances, the expected coastal upwelling
did not occur during boreal summer in the Gulf of Guinea, and heavy coastal rainfall
occurred in the winter dry season. The whole of the eastern part of the ocean had an
anomalous deep, warm mixed layer. In the same year, cold anomalies occurred in the
northwest Atlantic near Newfoundland. Recently, the response of the Atlantic to El Niño
events has been more satisfactorily analyzed (Bakun, 1996; Binet, 1997). During these
events, the anomalous low-pressure region (warm, wet, rising air) over central America
strengthens the Atlantic trade-wind system, and the westward slope of the sea surface
is enhanced, as is thermocline uplift in the east, leading to anomalous cool sea surface
temperature (SST) in the eastern tropical Atlantic. When the event relaxes, high pressure