40
DYNAMICAL OCEANOGRAPHY
Between the two sharp lines at 1 day and 1 year lies the synoptic variability of
midlatitude weather systems, concentrated at 3–7 days, as well as intraseasonal
variability, i.e. variability that occurs on the time scale of 1–3 months. The latter is also called low-frequency atmospheric variability, a name that refers to the
fact that this variability has longer periods than the life cycle of weather systems.
Immediately to the left of the seasonal cycle in Fig. 2.8 lies interannual, i.e. yearto-year, variability. An important component of this variability is the El Ni˜ no variability in the Tropical Pacific. About every four years, over a period of about one
year, the sea-surface temperature (SST) in the Eastern Tropical Pacific increases
by a few degrees. This is associated with variations (the Southern Oscillation) in
the tropical Pacific surface winds, the trade winds. The ENSO (El Ni˜ no/Southern
Oscillation) phenomenon arises through large-scale interaction between the equatorial Pacific and the global atmosphere. The last strong El Ni˜ no wasin1997and
the pattern of the sea surface temperature anomalies is plotted in Fig. 2.9. The
temperature of the eastern Pacific was in December 1997 about 6 ◦ C higher than
normal.
Figure 2.9. Sea surface temperature anomaly with respect to a long term mean for December
1997, at the maximum of the 1997-1998 El Nino (from Dijkstra and Burgers (2002)).
The energy in the spectrum at interdecadal time scales is likely due to the climate system’s internal processes: each spectral component can be associated, at
least tentatively, with a mode of interannual or interdecadal variability. Interdecadal variability is present in global records, instrumental as well as paleoclimatic. The leftmost part of Fig. 2.8 represents paleoclimatic variability. The information summarized there comes exclusively from proxy indicators of climate.
These include coral records and tree rings for the historic past, as well as marinesediment and ice-core records for the last two million years of Earth history, the
DYNAMICAL OCEANOGRAPHY
Between the two sharp lines at 1 day and 1 year lies the synoptic variability of
midlatitude weather systems, concentrated at 3–7 days, as well as intraseasonal
variability, i.e. variability that occurs on the time scale of 1–3 months. The latter is also called low-frequency atmospheric variability, a name that refers to the
fact that this variability has longer periods than the life cycle of weather systems.
Immediately to the left of the seasonal cycle in Fig. 2.8 lies interannual, i.e. yearto-year, variability. An important component of this variability is the El Ni˜ no variability in the Tropical Pacific. About every four years, over a period of about one
year, the sea-surface temperature (SST) in the Eastern Tropical Pacific increases
by a few degrees. This is associated with variations (the Southern Oscillation) in
the tropical Pacific surface winds, the trade winds. The ENSO (El Ni˜ no/Southern
Oscillation) phenomenon arises through large-scale interaction between the equatorial Pacific and the global atmosphere. The last strong El Ni˜ no wasin1997and
the pattern of the sea surface temperature anomalies is plotted in Fig. 2.9. The
temperature of the eastern Pacific was in December 1997 about 6 ◦ C higher than
normal.
Figure 2.9. Sea surface temperature anomaly with respect to a long term mean for December
1997, at the maximum of the 1997-1998 El Nino (from Dijkstra and Burgers (2002)).
The energy in the spectrum at interdecadal time scales is likely due to the climate system’s internal processes: each spectral component can be associated, at
least tentatively, with a mode of interannual or interdecadal variability. Interdecadal variability is present in global records, instrumental as well as paleoclimatic. The leftmost part of Fig. 2.8 represents paleoclimatic variability. The information summarized there comes exclusively from proxy indicators of climate.
These include coral records and tree rings for the historic past, as well as marinesediment and ice-core records for the last two million years of Earth history, the