Climate Systems Interaction Group

Sea Level Pressure Anomalies in the Western Pacific during El Niño: Why are they there?

Xuan Ji, J. David Neelin and C. Roberto Mechoso, 2015:

J. Clim., 28, 8860-8872.
Paper (4.9 MB).
Supplemental Info (3.5 MB).

Abstract Although sea level pressure (SLP) anomalies in the western Pacific have long been recognized as an integral part of the classic Southern Oscillation pattern associated with El Niño/Southern Oscillation (ENSO), there is an unresolved question regarding the dynamics that maintain these anomalies. Traditional studies of the ENSO response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies. However, the SLP anomalies in the western Pacific are spatially separated from the baroclinic signal in the NCEP-NCAR reanalysis, CMIP5 models, and an intermediate complexity model [a Quasi-equilibrium Tropical Circulation Model (QTCM)]. Separation of ENSO SLP anomalies in the tropical Pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western Pacific. To demonstrate the roles of baroclinic and barotropic modes in ENSO teleconnections within the tropics, a series of QTCM experiments is performed, where anomalies in the interactions between baroclinic and barotropic modes are suppressed over increasingly wider latitudinal bands in the tropical Pacific. If this suppression is done in the 15N-15S band, the pressure signals in the western Pacific are only partly removed, whereas if it is done in the 30N-30S band, the anomalies in the western Pacific are almost entirely removed. This suggests the following pathway: interactions with SST anomalies create the baroclinic response in the central and eastern Pacific, but baroclinic-barotropic interactions, arising substantially in the subtropical Pacific, generate a barotropic response that yields the SLP anomalies in the western Pacific.

Citation Ji, X., J. D. Neelin and C. R. Mechoso, 2015: Sea Level Pressure Anomalies in the Western Pacific during El Niño: Why are they there? J. Clim., 28, 8860-8872.

Acknowledgments. We thank Joyce Meyerson for assistance with graphics. This work was supported in part by National Science Foundation grants AGS-1102838 and AGS-1041477, National Oceanic and Atmospheric Administration grants NA11OAR4310099 and NA14OAR4310274, and a scholarship awarded by the Chinese Scholarship Council to support XJ's PhD study at University of California, Los Angeles. We thank Hui Su and Matt Munnich for unpublished initial QTCM and NCEP analysis (2004) related to this problem. We also thank Xin Qu for his comments. JDN would like to acknowledge the role of Wallace et al. (1998) Plate 8 in which the mismatch of SLP and tropospheric temperature patterns led to the puzzle analyzed here.

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