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The Role of Intraseasonal Atmospheric Forcing in the
El Nino-Southern Oscillation
Paul Roundy
NOAA
Abstract
The atmospheric Madden-Julian Oscillation (MJO) has long been
implicated as a major source of the wind stress anomalies that trigger
Kelvin waves in the ocean, allowing it to influence the development of
El Nino. However, many periods of MJO activity are not followed by El
Nino development. I will show by means of case studies and
compositing techniques that the way the MJO forces the ocean changes
with the basic state, and that this forcing can produce long-term
trends toward or away from El Nino conditions. Trends toward El Nino
tend to develop when westerly wind anomalies that are first associated
with the MJO follow Kelvin waves across the Pacific, allowing the
waves to amplify and raise East Pacific sea surface temperature (SST).
Trends away from El Nino tend to develop when MJO-induced wind bursts
are followed by surges in the trade winds along the trajectories of
the Kelvin waves. These surges cause the Kelvin waves to attenuate
and thus not increase East Pacific SST. The trade surges cause cold
water to upwell across the East, resulting in the trend toward La
Nina.
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