| Abstract: Enormous strides have been made towards the goal of
operational predictions of seasonal and interannual climate fluctuations,
especially as regards the phenomenon El Niño. To initialize models, measurements
are available from an impressive array of instruments that monitor the
tropical Pacific continually; coupled general circulation models of the
ocean and atmosphere are already capable of reproducing many aspects of
the earth's climate, its seasonal cycle, and the Southern Oscillation.
These achievements crown the studies, over the past few decades, that describe,
explain and simulate the atmospheric response to sea-surface temperature
variations, the oceanic response to different types of wind fluctuations,
and the broad spectrum of coupled ocean-atmosphere modes that results from
interations between the two media. Those modes, which are involved not
only in the Southern Oscillation but also in the seasonal cycle and the
climatology, differ primarily as regards the main mechanisms that determine
sea-surface temperature variations in the central and eastern tropical
Pacific: advection by surface currents, and vertical movements of the thermocline
induced by either local winds or, in the case of the delayed oscillator
mode, by non-local winds. The observed Southern Oscillation appears to
be a hybrid mode that changes from one episode to the next so that El Niño
can evolve in a variety of ways -- advection and nonlocally generated thermocline
displacements are important to different degrees on different occasions.
The extent to which random disturbances, such as westerly wind bursts over
the western equatorial Pacific, influence El Niño depends on whether the
southern oscillation is self-sustaining or damped. Attention is now turning
to the factors that determine this aspect of the Southern Oscillation,
its decadal modulation whih causes it to be more energetic in some decades
than others. Those factors include interactions between the tropics and
extratropics that affect the mean depth of the thermocline, and the intensity
of the climatological trade winds. |