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Internal Atmospheric Variability, Pacific Meridional Mode and ENSO
Ping Chang
Texas A&M University
Abstract
Much of the literature on the study of ENSO has been directed towards
understanding the dynamic aspects of the tropical coupled system. The
current theory of ENSO perceives it as strictly a tropical phenomenon.
A few recent studies, however, have linked extra-tropical internal
atmospheric variability to ENSO. In this talk, we will attempt to
further our understanding of this linkage. We will present intriguing
evidence that a significant number of El Nino events over the past
four decades are preceded by a distinctive warming at the sea-surface
accompanied by a southwesterly wind anomaly in the vicinity of the
Intertropical Convergence Zone during the boreal spring. This
phenomenon, known as the Meridional Mode (MM), is shown to be inherent
to the thermodynamic coupling between the atmosphere and ocean. It
acts as an effective conduit for extratropical atmospheric influence
on ENSO. We will present modeling evidence that this conduit plays a
vital role in the onset and the seasonal phase-locking behavior of
ENSO. Finally, we will demonstrate how improving model skills in
simulating and predicting the MM may lead to improved skill in
forecasting ENSO, particularly with respect to the spring
predictability barrier.
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