The relationship between the Madden Julian Oscillation and Kelvin waves in the Pacific Ocean during El Nino development

Paul Roundy

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Data from the Tropical Atmosphere Ocean (TAO) buoy array, including dynamic height of the ocean surface, surface wind, currents, and ocean temperature will be used along with NOAA interpolated outgoing longwave radiation data to reveal details about how the MJO spawns Kelvin waves in the equatorial Pacific. Though the MJO is widely accepted as the principal source of the wind stress that is responsible for spawning the Kelvin waves, many questions about how this happens remain largely unanswered. The answers have been particularly difficult to find because the MJO is characterized by periods of between 40 and 50 days, whereas the Kelvin waves have measured periods of around 70 days in the East Pacific. This frequency difference implies that a strongly nonlinear process is necessary to relate the two processes. The relationship is further complicated because both processes have differing phase speeds, allowing the MJO to weaken and switch phase during Kelvin wave development.

This presentation will show by means of a multiple linear regression model and direct observation that the frequency discrepancy is caused by changes in phase speed of the Kelvin waves over the course of El Nino development, when the waves tend to be most active across the basin. High amplitude waves tend to cluster in time during periods of El Nino development, and the earlier waves propagate more quickly than the later waves. The later waves take longer to cross the basin. This lengthening transit time allows the waves have periods between 40-50 days in the West Pacific (characteristic of the forcing) and around 70 days in the East.

The regression model also reveals an apparent air-sea interaction by which the zonal current anomalies of breaking Kelvin waves modulate SST along and east of regions of strong zonal SST gradients. The winds accelerate in response to the resulting SST anomalies and continue to amplify the waves even after direct forcing by the MJO diminishes and moves eastward. This process is only observed during the early development periods of ENSO warm events, and it appears to amplify waves that break and form fronts as they impact the upward-sloping thermocline at the east side of the warm pool. These fronts move the warm-pool eastward during development of El Nino.

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13 Apr, 2004
2 PM/ DSRC 1D 403
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