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A Proposed Mechanism for the Asymmetric Duration of El Niño and La Niña

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  • 1 Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado
  • | 2 Central Research Institute of Electric Power Industry, Abiko, Japan
  • | 3 Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado
  • | 4 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Japan
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Abstract

El Niño and La Niña exhibit significant asymmetry not only in their spatial structure but also in their duration. Most El Niños terminate rapidly after maturing near the end of the calendar year, whereas many La Niñas persist into the following year and often reintensify in boreal winter. Through atmospheric general circulation model experiments, it is shown that the nonlinear response of atmospheric deep convection to the polarity of equatorial Pacific sea surface temperature anomalies causes an asymmetric evolution of surface wind anomalies over the far western Pacific around the mature phase of El Niño and La Niña. Because of the eastward displacement of precipitation anomalies in the equatorial Pacific during El Niño compared to La Niña, surface winds in the western Pacific are more affected by SST forcing outside the equatorial Pacific, which acts to terminate the Pacific event.

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Corresponding author address: Dr. Yuko M. Okumura, Climate and Global Dynamics Division, NCAR, P.O. Box 3000, Boulder, CO 80307. E-mail: yukoo@ucar.edu

Abstract

El Niño and La Niña exhibit significant asymmetry not only in their spatial structure but also in their duration. Most El Niños terminate rapidly after maturing near the end of the calendar year, whereas many La Niñas persist into the following year and often reintensify in boreal winter. Through atmospheric general circulation model experiments, it is shown that the nonlinear response of atmospheric deep convection to the polarity of equatorial Pacific sea surface temperature anomalies causes an asymmetric evolution of surface wind anomalies over the far western Pacific around the mature phase of El Niño and La Niña. Because of the eastward displacement of precipitation anomalies in the equatorial Pacific during El Niño compared to La Niña, surface winds in the western Pacific are more affected by SST forcing outside the equatorial Pacific, which acts to terminate the Pacific event.

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Corresponding author address: Dr. Yuko M. Okumura, Climate and Global Dynamics Division, NCAR, P.O. Box 3000, Boulder, CO 80307. E-mail: yukoo@ucar.edu
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