El Niño–Southern Oscillation Sea Level Pressure Anomalies in the Western Pacific: Why Are They There?*

Abstract

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 15°N–15°S band, the pressure signals in the western Pacific are only partly removed, whereas if it is done in the 30°N–30°S 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.