A possible mechanism for ENSO turnabouts

 

Bin Wang*, Renguang Wu, Roger Lukas, and Soon-Il An*

 

School of Ocean and Earth Science and Technology, University of Hawaii,

Honolulu, HI 96822, USA

 

Submitted to J. Climate

July 8, 1999

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Corresponding authorís address: Dr. Bin Wang, Department of Meteorology and International Pacific Research Center, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822. E-mail: bwang@soest.hawaii.edu

* Member of the International Pacific Research Center (IPRC). IPRC is sponsored in part by the Frontier Research System for Global Change.

 

 

Abstract

 

The reversal of warming or cooling trends of ENSO cycles occurs most frequently toward the end of the calendar year. A possible mechanism is advanced based on observational evidence and model results. Prior to the peaks of major warm (cold) episodes, an anomalous surface anticyclone (cyclone) establishes rapidly over the Philippine Sea. To the south of the anticyclone (cyclone), anomalous easterlies (westerlies) prevail in the equatorial western Pacific and, by forcing oceanic equatorial upwelling (downwelling) Kelvin waves, provides a negative feedback to the eastern Pacific warming (cooling). In the western North Pacific, the coherent variations in the surface winds and SST suggest a local positive feedback between the Philippine Sea anomalous anticyclone (cyclone) and negative (positive) SST anomalies. This positive feedback plays critical roles in intensification and maintenance of the wind and SST anomalies in the Philippine Sea. This positive feedback also depends on the existence of mean northeasterly trades and the central Pacific warming during the boreal cold season. Therefore, the proposed negative feedback favors the turnabouts of ENSO cycles in boreal winter.

This hypothesis was tested using a modified Cane-Zebiak coupled model in which the original atmosphere model was replaced by an empirical atmosphere model derived from the singular value decomposition of observations. The empirical atmospheric model contains significant seasonally-dependent wind anomalies in the western Pacific. The model results indicate that the annual variation of the anomalous western Pacific winds favors reversal of the warming and cooling trends toward the end of the calendar year. Without this process, the coupled model fails to reproduce the preferred occurrence of the cold peaks in boreal winter.