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

Wednesday, 17 December 2014
Xuan Ji, J David Neelin and Carlos R Mechoso, University of California Los Angeles, Los Angeles, CA, United States
Although sea level pressure (SLP) anomalies in the Western Pacific have long been known as part of the classic Southern Oscillation pattern associated with ENSO, there is an unresolved question regarding the dynamics that maintains these. Contrary to traditional studies that assume a single deep baroclinic mode for ENSO response in the tropics, the SLP anomalies in the western Pacific are spatially well separated from the baroclinic signal associated with the tropospheric temperature anomalies in NCEP-NCAR reanalysis, CMIP5 models, and an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)]. Separation of the SLP into its baroclinic and barotropic components indicates that the baroclinic mode SLP contributions are in the central and eastern Pacific (coincident with the temperature anomalies), whereas the western Pacific SLP anomalies arise primarily from barotropic mode contributions.

In a set of QTCM experiments, we suppress anomalies in baroclinic-barotropic interaction terms, i.e., anomalies of the barotropic Rossby wave source, over successively wider latitudinal bands in the tropical Pacific, to diagnose their effects on the SLP anomalies in the western Pacific associated with ENSO. In the 15°N-15°S experiment, the pressure signals in the western Pacific are only partly suppressed, whereas in the 30°N-30°S suppression experiment, the anomalies in the western Pacific are almost entirely removed. This suggests the following pathway: interactions with SST anomalies create the baroclinic mode signal in the central and Eastern Pacific, but baroclinic-barotropic interactions, arising substantially in the subtropical Pacific, create a barotropic response that is yields the SLP anomaly pattern in the western Pacific.