Geography Reference
In-Depth Information
ENSO (for El Nino—Southern Oscillation). It is a dramatic example of interannual
climate variability associated with atmosphere-ocean coupling.
The leading theoretical model for ENSO is the “delayed oscillator” model. In
this model the sea surface temperature anomaly, T , in the eastern Pacific satisfies
an equation of the form
dT
dt
=
bT (t)
−
cT (t
−
τ)
where b and c are positive constants and τ is a time delay determined by the adjust-
ment time for the equatorial ocean. The first term on the right represents a positive
feedback associated with changes in the Darwin to Tahiti pressure difference. This
term represents the atmosphere-ocean coupling through which an initial weaken-
ing of the wind causes an increase of the SST, which induces a further weakening
of the wind, and hence a further increase in the SST. The second term provides
a negative feedback due to the adjustment in the thermocline depth (and hence
the ocean temperature) caused by propagating equatorial waves (see Section 11.4)
in the ocean that are excited by the SST changes. The time delay in the negative
feedback term is determined by the time that it takes for wave energy excited
by air-sea interaction in the eastern Pacific to propagate to the western boundary
of the ocean, undergo reflection, and propagate back to the region of origin. For
realistic parameters the delayed oscillator model leads to ENSO oscillations in the
period range of 3-4 years. This highly simplified model qualitatively accounts for
the average characteristics of an ENSO cycle, but cannot, however, account for the
observed irregularity of ENSO.
In addition to its profound effects in the equatorial region, ENSO is associated
with a wide range of interannual climate anomalies in the extratropics. Thus, the
development of models that show skill in predicting ENSO several months in
advance is of considerable practical importance.
11.1.7
Equatorial Intraseasonal Oscillation
In addition to the interannual variability associated with El Nino, the equatorial
circulation has an important intraseasonal oscillation, which occurs on a timescale
of 30-60 days and is often referred to as the Madden-Julian oscillation (MJO) in
honor of the meteorologists who first described it. The structure of the equatorial
intraseasonal oscillation is shown schematically in Fig. 11.12, which shows the
time development of the oscillation in the form of longitude-height sections along
the equator, with time increasing at an interval of about 10 days for each panel from
top to bottom. The circulations in Fig. 11.12 are intended to represent anomalies
from the time-mean equatorial circulation.
The oscillation originates with development of a surface low-pressure anomaly
over the Indian Ocean, accompanied by enhanced boundary layer moisture
