1. What Causes El Niños and La Niñas?There are actually two parts to
this question: (1) Why do these anomalies arise? That is, what causes
an El Niño or La Niña to become established? and (2) What causes the
reversals, or oscillations, between the two extremes?The Role of
Positive FeedbackAn answer to the first question was proposed more than
40 years ago in a pioneering study by Jacob Bjerknes, and has been
essentially confirmed by more recent analysis and data. Unlike some
other types of climate change, both anomalies arise without any external
forcing. That is, they do not result from a nonclimatic event such as
an asteroid impact, a volcanic eruption, or a change in the Sun’s
radiation. Rather, they result from processes internal to the
ocean–atmosphere system. They are, in fact, classic examples of a
positive feedback mechanism, in which relatively small changes are
continually reinforced and amplified.Consider, for example, the La Niña,
where the strengthening trades push more warm water westward. In the
western Pacific, the growing pool of warm water promotes more uplift,
precipitation, and lower pressure, Lower pressure in the west
intensifies the pressure gradient and strengthens the trades, which in
turn lead to further transport of warm water. At the same time, export
of warm water cools the eastern Pacific, enhances upwelling, and raises
the thermocline. All of these processes promote further growth of the
anomaly. Similar reasoning, but in reverse; explains why El Niños
develop. In this case warmer water in the eastern Pacific weakens the
trades, which leads to less heat export by westward-moving water. With
less heat exported, warmth in the eastern Pacific intensifies, further
reducing the pressure gradient and reducing export of heat. Thus, we see
that once the system begins to drift toward either condition, there
will be a tendency for the anomaly to intensify and become firmly
established. This line of reasoning favors two very different states. In
the El Niño state, there is a thick pool of anomalously warm water in
the eastern Pacific lying above a relatively deep thermocline. At the
other extreme, the eastern Pacific is anomalously cold, the thermocline
is shallow, and the trades are strong.Breakdown of El Niños and La
NiñasIf positive feedback explains the appearance of El Niños and La
Niñas, what leads to their breakdown? That is, once established, why
don’t they persist indefinitely? Why is there oscillation from one state
to the other? Much less is known with certainty about this phenomenon,
and the topic remains a matter of intense interest within the scientific
community. However, there is little doubt that for oscillations to
arise naturally in the system, delayed negative feedback processes must
be occurring that are out of phase with the positive feedbacks described
above, That is, one or more restorative processes must be lagging
behind processes driving the system toward the extreme states. As an El
Niño or La Niña builds, the delayed restoring processes also grow and
eventually become large enough to overwhelm the amplifying processes.
When that happens, the system moves away from the extreme state (El Niño
or La Niña) toward more normal conditions. But the positive feedbacks
are still in play, and if they are large enough, the system moves to the
other extreme. So, in general terms, an oscillation between the
extremes arises from the joint effects of positive and delayed negative
feedbacks.A number of candidate processes have been proposed as delayed
negative feedbacks. One possibility is that during an El Niño Rossby
waves generated in the east-central Pacific travel westward and are
reflected backward from the western ocean boundary as small waves on the
ocean surface. These so-called Kelvin waves bring a deeper thermocline
to the eastern Pacific, counteracting the El Niño. Similar waves form
with La Niña, but they bring a shallower thermocline to the eastern
Pacific, and thus similarly move the system toward more normal
conditions Another possibility, termed the recharge oscillator, contends
that the buildup and discharge of warm water from the tropical Pacific
gives rise to the oscillation. The idea is that while an El Niño builds,
the entire tropical Pacific experiences a gradual “recharge” of heat as
the thermocline deepens. Some time during the El Niño proper, the
excess warmth is discharged to higher latitudes, the thermocline becomes
shallower, and the system moves toward the other extreme. This
institutes another cycle of recharge, followed by another flushing of
heat to the extratropics (the middle and high latitudes) with the
ensuing El Niño.Another possibility is that during an El Niño the
central Pacific atmosphere warms because of enhanced convection and
condensation, and this in turn generates low-pressure cells on either
side of the equator. Initially these cyclones amplify the El Niño, but
eventually they pump cold water eastward and thereby destroy the event
they helped construct. Still other mechanisms have been suggested as
important in the oscillations, including forcing by disturbances
unrelated to the ENSO. No single process has been shown to be
responsible for ENSO events, and many experts believe that multiple
processes are involved to varying degrees from one episode to the
next.Support for all these ideas comes from computer models that are
able to reproduce ENSO-like cycles. But there is no agreement about what
controls the period of oscillation, why it averages about four years
instead of some other period, or even why the period of oscillation is
variable, with ENSOs appearing every two to seven years. Answers to
these and other questions about ENSO events are important both for
understanding our present climate and for anticipating the nature and
impact of future climate changes.What role does positive feedback play
in La Niñas? Get solution
2. What Causes El Niños and La Niñas?There are actually two parts to this question: (1) Why do these anomalies arise? That is, what causes an El Niño or La Niña to become established? and (2) What causes the reversals, or oscillations, between the two extremes?The Role of Positive FeedbackAn answer to the first question was proposed more than 40 years ago in a pioneering study by Jacob Bjerknes, and has been essentially confirmed by more recent analysis and data. Unlike some other types of climate change, both anomalies arise without any external forcing. That is, they do not result from a nonclimatic event such as an asteroid impact, a volcanic eruption, or a change in the Sun’s radiation. Rather, they result from processes internal to the ocean–atmosphere system. They are, in fact, classic examples of a positive feedback mechanism, in which relatively small changes are continually reinforced and amplified.Consider, for example, the La Niña, where the strengthening trades push more warm water westward. In the western Pacific, the growing pool of warm water promotes more uplift, precipitation, and lower pressure, Lower pressure in the west intensifies the pressure gradient and strengthens the trades, which in turn lead to further transport of warm water. At the same time, export of warm water cools the eastern Pacific, enhances upwelling, and raises the thermocline. All of these processes promote further growth of the anomaly. Similar reasoning, but in reverse; explains why El Niños develop. In this case warmer water in the eastern Pacific weakens the trades, which leads to less heat export by westward-moving water. With less heat exported, warmth in the eastern Pacific intensifies, further reducing the pressure gradient and reducing export of heat. Thus, we see that once the system begins to drift toward either condition, there will be a tendency for the anomaly to intensify and become firmly established. This line of reasoning favors two very different states. In the El Niño state, there is a thick pool of anomalously warm water in the eastern Pacific lying above a relatively deep thermocline. At the other extreme, the eastern Pacific is anomalously cold, the thermocline is shallow, and the trades are strong.Breakdown of El Niños and La NiñasIf positive feedback explains the appearance of El Niños and La Niñas, what leads to their breakdown? That is, once established, why don’t they persist indefinitely? Why is there oscillation from one state to the other? Much less is known with certainty about this phenomenon, and the topic remains a matter of intense interest within the scientific community. However, there is little doubt that for oscillations to arise naturally in the system, delayed negative feedback processes must be occurring that are out of phase with the positive feedbacks described above, That is, one or more restorative processes must be lagging behind processes driving the system toward the extreme states. As an El Niño or La Niña builds, the delayed restoring processes also grow and eventually become large enough to overwhelm the amplifying processes. When that happens, the system moves away from the extreme state (El Niño or La Niña) toward more normal conditions. But the positive feedbacks are still in play, and if they are large enough, the system moves to the other extreme. So, in general terms, an oscillation between the extremes arises from the joint effects of positive and delayed negative feedbacks.A number of candidate processes have been proposed as delayed negative feedbacks. One possibility is that during an El Niño Rossby waves generated in the east-central Pacific travel westward and are reflected backward from the western ocean boundary as small waves on the ocean surface. These so-called Kelvin waves bring a deeper thermocline to the eastern Pacific, counteracting the El Niño. Similar waves form with La Niña, but they bring a shallower thermocline to the eastern Pacific, and thus similarly move the system toward more normal conditions Another possibility, termed the recharge oscillator, contends that the buildup and discharge of warm water from the tropical Pacific gives rise to the oscillation. The idea is that while an El Niño builds, the entire tropical Pacific experiences a gradual “recharge” of heat as the thermocline deepens. Some time during the El Niño proper, the excess warmth is discharged to higher latitudes, the thermocline becomes shallower, and the system moves toward the other extreme. This institutes another cycle of recharge, followed by another flushing of heat to the extratropics (the middle and high latitudes) with the ensuing El Niño.Another possibility is that during an El Niño the central Pacific atmosphere warms because of enhanced convection and condensation, and this in turn generates low-pressure cells on either side of the equator. Initially these cyclones amplify the El Niño, but eventually they pump cold water eastward and thereby destroy the event they helped construct. Still other mechanisms have been suggested as important in the oscillations, including forcing by disturbances unrelated to the ENSO. No single process has been shown to be responsible for ENSO events, and many experts believe that multiple processes are involved to varying degrees from one episode to the next.Support for all these ideas comes from computer models that are able to reproduce ENSO-like cycles. But there is no agreement about what controls the period of oscillation, why it averages about four years instead of some other period, or even why the period of oscillation is variable, with ENSOs appearing every two to seven years. Answers to these and other questions about ENSO events are important both for understanding our present climate and for anticipating the nature and impact of future climate changes.If you wanted to prove that a process is not involved in the development of El Niños or La Niñas, how might you go about that? What tools would you use? Get solution
2. What Causes El Niños and La Niñas?There are actually two parts to this question: (1) Why do these anomalies arise? That is, what causes an El Niño or La Niña to become established? and (2) What causes the reversals, or oscillations, between the two extremes?The Role of Positive FeedbackAn answer to the first question was proposed more than 40 years ago in a pioneering study by Jacob Bjerknes, and has been essentially confirmed by more recent analysis and data. Unlike some other types of climate change, both anomalies arise without any external forcing. That is, they do not result from a nonclimatic event such as an asteroid impact, a volcanic eruption, or a change in the Sun’s radiation. Rather, they result from processes internal to the ocean–atmosphere system. They are, in fact, classic examples of a positive feedback mechanism, in which relatively small changes are continually reinforced and amplified.Consider, for example, the La Niña, where the strengthening trades push more warm water westward. In the western Pacific, the growing pool of warm water promotes more uplift, precipitation, and lower pressure, Lower pressure in the west intensifies the pressure gradient and strengthens the trades, which in turn lead to further transport of warm water. At the same time, export of warm water cools the eastern Pacific, enhances upwelling, and raises the thermocline. All of these processes promote further growth of the anomaly. Similar reasoning, but in reverse; explains why El Niños develop. In this case warmer water in the eastern Pacific weakens the trades, which leads to less heat export by westward-moving water. With less heat exported, warmth in the eastern Pacific intensifies, further reducing the pressure gradient and reducing export of heat. Thus, we see that once the system begins to drift toward either condition, there will be a tendency for the anomaly to intensify and become firmly established. This line of reasoning favors two very different states. In the El Niño state, there is a thick pool of anomalously warm water in the eastern Pacific lying above a relatively deep thermocline. At the other extreme, the eastern Pacific is anomalously cold, the thermocline is shallow, and the trades are strong.Breakdown of El Niños and La NiñasIf positive feedback explains the appearance of El Niños and La Niñas, what leads to their breakdown? That is, once established, why don’t they persist indefinitely? Why is there oscillation from one state to the other? Much less is known with certainty about this phenomenon, and the topic remains a matter of intense interest within the scientific community. However, there is little doubt that for oscillations to arise naturally in the system, delayed negative feedback processes must be occurring that are out of phase with the positive feedbacks described above, That is, one or more restorative processes must be lagging behind processes driving the system toward the extreme states. As an El Niño or La Niña builds, the delayed restoring processes also grow and eventually become large enough to overwhelm the amplifying processes. When that happens, the system moves away from the extreme state (El Niño or La Niña) toward more normal conditions. But the positive feedbacks are still in play, and if they are large enough, the system moves to the other extreme. So, in general terms, an oscillation between the extremes arises from the joint effects of positive and delayed negative feedbacks.A number of candidate processes have been proposed as delayed negative feedbacks. One possibility is that during an El Niño Rossby waves generated in the east-central Pacific travel westward and are reflected backward from the western ocean boundary as small waves on the ocean surface. These so-called Kelvin waves bring a deeper thermocline to the eastern Pacific, counteracting the El Niño. Similar waves form with La Niña, but they bring a shallower thermocline to the eastern Pacific, and thus similarly move the system toward more normal conditions Another possibility, termed the recharge oscillator, contends that the buildup and discharge of warm water from the tropical Pacific gives rise to the oscillation. The idea is that while an El Niño builds, the entire tropical Pacific experiences a gradual “recharge” of heat as the thermocline deepens. Some time during the El Niño proper, the excess warmth is discharged to higher latitudes, the thermocline becomes shallower, and the system moves toward the other extreme. This institutes another cycle of recharge, followed by another flushing of heat to the extratropics (the middle and high latitudes) with the ensuing El Niño.Another possibility is that during an El Niño the central Pacific atmosphere warms because of enhanced convection and condensation, and this in turn generates low-pressure cells on either side of the equator. Initially these cyclones amplify the El Niño, but eventually they pump cold water eastward and thereby destroy the event they helped construct. Still other mechanisms have been suggested as important in the oscillations, including forcing by disturbances unrelated to the ENSO. No single process has been shown to be responsible for ENSO events, and many experts believe that multiple processes are involved to varying degrees from one episode to the next.Support for all these ideas comes from computer models that are able to reproduce ENSO-like cycles. But there is no agreement about what controls the period of oscillation, why it averages about four years instead of some other period, or even why the period of oscillation is variable, with ENSOs appearing every two to seven years. Answers to these and other questions about ENSO events are important both for understanding our present climate and for anticipating the nature and impact of future climate changes.If you wanted to prove that a process is not involved in the development of El Niños or La Niñas, how might you go about that? What tools would you use? Get solution
