Present ocean circulation
41
Quaternary. During this period an alternation of warmer and colder episodes,
called glaciation cycles, is prominent. The cyclicity is manifest in the broad peaks
present in Fig. 2.8 between roughly 1 kyr and 1 Myr. The three peaks at about 20
kyr, 40 kyr and 400 kyr reflect variations in Earth’s orbit. These orbital variations
represent, respectively, variations in precession, obliquity and eccentricity, three
parameters that are used to describe Earth’s orbit.
Within these glaciation cycles there are higher frequency oscillations prominent in the North Atlantic paleoclimatic records. These are the Bond cycle, with
a near-periodicity of 6–7 kyr, and the Dansgaard-Oeschger cycles providing the
peak at around 1–2.5 kyr in Fig. 2.8. Rapid changes in temperature, of up one half
of the amplitude of a typical glacial-interglacial temperature difference, occur in
a Dansgaard-Oeschger cycle. Progressive cooling through several of these cycles
followed by an abrupt warming defines the Bond cycle. In North Atlantic sediment cores, the coldest part of each Bond cycle is marked by a so-called Heinrich
layer that is rich in ice-rafted debris.
In summary, climate variations range from the large-amplitude climatic transitions of the past millennia to small-amplitude fluctuations on shorter time scales.
Several frequencies of variability are clearly related to forcing mechanisms. However, variability can also arise through processes internal in the climate system
giving rise to frequencies that are not directly related to the temporal variability of
the forcing. Examples are the 3–7 day synoptic variability of midlatitude weather
which arises through instability of the zonal mean circulation of atmosphere, and
the El Ni˜ no variability whose frequency is set by the coupled interaction and intrinsic processes in the equatorial Pacific and atmosphere. Internal variability on
longer time scales can occur through instabilities of states in the slower components of the climate system, such as the ocean and the ice caps. Hence, even if the
external, i.e., solar, forcing was steady, the climate system would display variability on many time scales. It is the response of this highly complex climate system
to relatively small time-dependent variations in the forcing, which is recorded in
the sediment cores, ice cores and instrumental data.
Changes in the ocean circulation can influence climate substantially through the
impact on the meridional heat transport. This can affect mean global temperature
and precipitation, as well as their distribution in space and time. Subtle changes
in the North Atlantic surface circulation and interactions with the overlying atmosphere are thought to be involved in the interannual and interdecadal climate
variability as observed in the instrumental record of the last century. Changes in
the circulation may also occur on a global scale, involving a transition to different
large-scale patterns. Such changes may have been involved in the large-amplitude
climate variations of the past, like the Dansgaard-Oeschger cycles.
This is enough motivation to try to understand the ocean circulation in more
detail! In the following chapters of part I we make a first step by (i) identification
of the characteristic time scales of the different processes and (ii) presenting the
41
Quaternary. During this period an alternation of warmer and colder episodes,
called glaciation cycles, is prominent. The cyclicity is manifest in the broad peaks
present in Fig. 2.8 between roughly 1 kyr and 1 Myr. The three peaks at about 20
kyr, 40 kyr and 400 kyr reflect variations in Earth’s orbit. These orbital variations
represent, respectively, variations in precession, obliquity and eccentricity, three
parameters that are used to describe Earth’s orbit.
Within these glaciation cycles there are higher frequency oscillations prominent in the North Atlantic paleoclimatic records. These are the Bond cycle, with
a near-periodicity of 6–7 kyr, and the Dansgaard-Oeschger cycles providing the
peak at around 1–2.5 kyr in Fig. 2.8. Rapid changes in temperature, of up one half
of the amplitude of a typical glacial-interglacial temperature difference, occur in
a Dansgaard-Oeschger cycle. Progressive cooling through several of these cycles
followed by an abrupt warming defines the Bond cycle. In North Atlantic sediment cores, the coldest part of each Bond cycle is marked by a so-called Heinrich
layer that is rich in ice-rafted debris.
In summary, climate variations range from the large-amplitude climatic transitions of the past millennia to small-amplitude fluctuations on shorter time scales.
Several frequencies of variability are clearly related to forcing mechanisms. However, variability can also arise through processes internal in the climate system
giving rise to frequencies that are not directly related to the temporal variability of
the forcing. Examples are the 3–7 day synoptic variability of midlatitude weather
which arises through instability of the zonal mean circulation of atmosphere, and
the El Ni˜ no variability whose frequency is set by the coupled interaction and intrinsic processes in the equatorial Pacific and atmosphere. Internal variability on
longer time scales can occur through instabilities of states in the slower components of the climate system, such as the ocean and the ice caps. Hence, even if the
external, i.e., solar, forcing was steady, the climate system would display variability on many time scales. It is the response of this highly complex climate system
to relatively small time-dependent variations in the forcing, which is recorded in
the sediment cores, ice cores and instrumental data.
Changes in the ocean circulation can influence climate substantially through the
impact on the meridional heat transport. This can affect mean global temperature
and precipitation, as well as their distribution in space and time. Subtle changes
in the North Atlantic surface circulation and interactions with the overlying atmosphere are thought to be involved in the interannual and interdecadal climate
variability as observed in the instrumental record of the last century. Changes in
the circulation may also occur on a global scale, involving a transition to different
large-scale patterns. Such changes may have been involved in the large-amplitude
climate variations of the past, like the Dansgaard-Oeschger cycles.
This is enough motivation to try to understand the ocean circulation in more
detail! In the following chapters of part I we make a first step by (i) identification
of the characteristic time scales of the different processes and (ii) presenting the