14D.5 Synoptic-scale disturbances embedded in the Madden-Julian Oscillation compared under the different ENSO phases

Thursday, 1 May 2008: 11:15 AM
Palms I (Wyndham Orlando Resort)
Ayako Seiki, Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan; and Y. N. Takayabu, H. Kubota, H. Yamada, K. Yoneyama, R. Shirooka, and M. Yoshizaki

Behaviors of internal disturbances embedded in the Madden-Julian Oscillation (MJO) over the western Pacific warm pool during boreal winter of 2000 and 2001 under the different El Niño Southern Oscillation (ENSO) phases were examined using satellite, reanalysis, and stationary data.

In the case in 2000, after La Niña, several cyclonic disturbances developed and moved northwestward over the western Pacific. On the other hand, during the El Niño development phase in 2001, convective regions kept propagating eastward along the equator and activated with a twin cyclonic structure around the central Pacific, resulting in westerly wind bursts.

The SST distributions in two periods were relatively similar which do not suggest direct cause for difference in behaviors of synoptic disturbances. However, there was a distinct difference in the moisture fields which facilitate active convections. In 2000, relatively high moisture field spread meridionally but was confined to the western Pacific, while in 2001, moisture is confined to a narrow band along the equator extended to the date line. The narrower moist band than that estimated from SST distribution in 2001 was related to the stronger northeasterly trade winds and could inhibit the synoptic disturbances from propagating northward. In addition, the differences in environmental wind fields influenced on the eddy kinetic energy budget. Shallow environmental westerlies over the equatorial western Pacific in 2001 caused shallow energy conversion from the basic fields and could prevent initial cyclonic disturbances from the extreme development. Energy converted from active synoptic convection was also necessary but not sufficient for that development.

Abrupt shifts from westerlies to easterlies in the upper troposphere around the days of MJO convective maxima, which have been known as the impact of MJO arrival, was found in stationary data even at the higher latitudes around 7°N in 2000 but not in 2001. These results indicated that large-scale wind pattern, responsible for the behaviors of synoptic disturbances within the MJO, brought about the modulation of the MJO wind structures by ENSO.

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