The Vertical Structure and Development of the ENSO Anomaly Mode during 1979–1989

Abstract

The latest two Pacific basinwide warm episodes (1982?83 and 1986?87) exhibit some common features in their development and vertical structure. These features are examined by multivariate empirical orthogonal function analysis of the interannual variability of the ocean-atmosphere system along equatorial Indian and Pacific oceans. The updraft and downdraft branches of the anomalous Walker circulation originate over the western Pacific and the eastern Indian Ocean, respectively. The early development of basinwide warming is characterized by the strengthening of a cross-equatorial low-level southerly component over the eastern Pacific and the enhancement of convection and boundary-layer westerlies over the western Pacific. The structure of the ENSO anomaly mode changes from the cold to the warm phase of the Southern Oscillation. This is attributed to its eastward migration and the intrinsic longitudinal dependence of the vertical structure. The latter results from the east?west contrast of the air?sea interaction processes. Over Indonesia and the western Pacific, the land?sea thermal contrast and high SST maintain a semipermanent convective action center, whose intensity is sensitively modulated by small SST fluctuation. Since moist static ability is small, the surface pressure responds sensitively to the heating, so that the anomalous low pressure and associated zonal wind convergence in the boundary layer are in phase with the enhanced convection. In contrast, over the central-eastern Pacific, large SST gradient-induced pressure gradient force drives boundary-layer flows whose beta convergence determines atmospheric heating, while the feedback of the free atmosphere to boundary-layer flows is weak due to large static stability. The enhanced convection is thus nearly in phase with anomalous boundary-layer westerlies, positive zonal SST gradient, and negative zonal surface pressure gradient. It is possible that an individual ENSO event may result from different combinations of various sets of coupled processes, especially with regard to those that work in the eastern Pacific cold tongue and those in the western Pacific warm pool.