Processes of Short-Term Climatic Variability in the Baroclinic Structure of the Interior Western Tropical North Pacific

Abstract

A realistic simulation of short-term climatic variability in the relative dynamic height of the interior western tropical North Pacific (4°?22°N, 127°E?180°) for 9 years (1966?74) was conducted using a two-layer, wind-driven, baroclinic long-wave model. Results of this model were compared with bimonthly maps of observed dynamic height (0/400 db) for the region, constructed by White and Hasunuma from all available temperature/depth observations for the same period. The interannual rms differences about the mean annual cycle of the model dynamic height had a spatial pattern over the region of interest that was nearly identical to that observed, with maximum values (i.e., 3.5 and 4.5 dyn cm, respectively) in the vicinity of the North Equatorial Ridge at 15°N and minimum values (i.e., 2.0 and 3.5 dyn cm, respectively) in the vicinity of the North Equatorial Ridge at 15°N and minimum values (i.e., 2.0 and 3.5 dyn cm, respectively) in the Countercurrent Trough at 7°N. Model frequency spectra were identical in shape to those observed (i.e., increasing linearly toward low frequency at the fourth power of frequency for periods of 6 months to 2 years), with the model spectral magnitude slightly less than observed. Empirical orthogonal function (EOF) analysis of 9 years of bimonthly maps of observed and model dynamic height finds the first three functions explaining approximately 75% of the total short-term climatic variance of each parameter, most of which was associated with two ENSO (El Niño/Southern Oscillation) events, occurring in 1968?69 and 1972?73. In both the observed, and to a lesser extent, the model data sets, dynamic height decreased radically during these ENSO events; the maximum decrease was at 15°N in the vicinity of the North Equatorial Ridge, less near the equator. Cross-correlation of the model EOFs highly correlated, with 48% of observed variance explained by the model. The dynamics of short-term climatic variability in the western interior tropical North Pacific is dominated by steady equilibrium processes. Frequency cross-spectra between long-wave model dynamic height and that produced by an equilibrium model of Sverdrup reveals that at periods of 1 year and longer, the two models produce identical time sequences for the latitude range 4°?12°N, the transient effects became important, so that at 21°N only 64% of the long-wave model variability could be explained by the equilibrium model at a period of one year. An important transient effect is produced by baroclinic long waves propagating information from east to west at speeds ranging from 100 cm s?1 at 4°N to 10 cm s?1 at 22°N. In the latitude band 10?15°N, where maximum short-term climatic variance occurred in both observed and model data, 50% of the total model variance was associated with baroclinic long wave propagation of information from east of 180°. Of the percent of model variance due to local wind forcing in this latitude band, approximately 25% was due to resonant wind forcing, 75% due to off-resonant wind forcing.