Xuan Ji, J. David Neelin and C. Roberto Mechoso, 2014:
J. Clim., submitted 10/14.
Preprint (4.6 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 ENSO, there is an unresolved question regarding the dynamics that maintain these. 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 well 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 the SLP anomalies into their baroclinic and barotropic components indicates that while the baroclinic components are fundamental contributors to ENSO anomalies in the central and eastern Pacific (coincident with the temperature anomalies), the barotropic components provide the primary contributions 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.
Citation Ji, X., J. D. Neelin and C. R. Mechoso, 2014: Sea Level Pressure Anomalies in the Western Pacific during El Niño: Why are they there? J. Climate, submitted 10/14.