The Role of Intraseasonal Atmospheric Forcing in the El Nino-Southern Oscillation

Paul Roundy

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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