Thursday, 2 May 2002: 10:00 AM
ENSO-Induced Air-Sea Interaction in the Maritime Continent
A
seasonally varying feedback between SST, winds and rainfall in the Maritime
Continent is identified, which explains the growth, persistence and coherence
of the local anomalies during the dry season and their decay or change
in sign once the wet season commences. During the dry season (JUne-October)
anomalous surface easterlies (westerlies), remotely driven by warm (cold)
SSTs in the eastern Pacific during El Niño (La Niña), act
to increase (decrease) local wind speed, cooling (warming) the ocean surrounding
Indonesia and thereby increasing (decreasing) the SST gradient across the
Pacific. Hence, local rainfall and the strength of the Walker circulation
are further reduced (enhanced). Once the wet season commences (December)
and the climatological surface winds across Indonesia shift from southeasterly
to northwesterly, the anomalous surface easterlies (westerlies) act to
reduce (increase) the wind speed. The initial cold (warm) SST anomaly is
damped, reducing the negative (positive) rainfall anomalies and surface
easterlies (westerlies). This ENSO-induced air-sea interaction also gives
rise to a SST "dipole" in the Indian Ocean, which is apparent only during
the dry season and is surmised to be largely driven by surface heat flux
variations. Biennial variations in the Indonesian region may also arise
by this interaction.
A similar local feedback between SST anomalies, rainfall, and surface winds in the Maritime Continent is identified in a atmospheric GCM coupled 1-D ocean mixed layer model. This coupled model does not possess ENSO variability. However, without the slowly-evolving remote forcing provided by ENSO, the feedback by itself is not sufficient to produce the Indian Ocean dipole, biennial variations, or local anomalies that persist longer than a few months.
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