The spatial and temporal signatures of low-frequency Pacific variability

Several standard techniques have been previously used for extracting decadal-scale variability from Pacific Sea Surface Temperature (SST), including: extraction based on frequency (e.g., low pass filtering in the equatorial eastern Pacific), extraction based on spatial pattern (e.g., Empirical Orthogonal Function (EOF)- type analysis), and extraction based on a sophisticated combination of the two (e.g., the Multi-Taper Method - Singular Value Decomposition (MTM-SVD)). All approaches yield a spatial pattern having some similarity to a latitudinally broadened El Nino - Southern Oscillation (ENSO) pattern, but with considerable power in low frequencies (decadal to interdecadal). Equally apparent from the differences between the various results, however, are several basic questions: Are there multiple distinct modes of decadal-scale variability in the North Pacific? Is the low frequency variability a standing or propagating mode? Can the ENSO-like decadal variability be separated from decadal ENSO variability? Is there a physical linkage between the high and low-frequency variability modes that have similar spatial structure?

The current analysis presents several basic combinations of the two more simple techniques, band pass filtering and rotated EOF analysis, guided by the original results from all three approaches (and essentially rebuilding the MTM-SVD technique for a given frequency regime.) This allows a detailed intercomparison of the different approaches and tests robustness of the potential answers to the above questions.

Particular attention is given to the spatial evolution and behavior of the modes in different frequency regimes -- rotated EOF analysis of unfiltered data yields two spatial patterns with signal in the North Pacific and power broadly spread through both (inter)decadal-scale and interannual frequencies, whereas the MTM-SVD analysis yields a single mode with a well-defined spectral peak at inter-decadal scales that evolves between the two patterns.