Tropospheric direct circulations associated with the climatic components of SST variability in the equatorial Pacific

Using an updated version of the Kaplan et al. SST dataset (1870-1998), we have constructed a global complex EOF mode of SST anomalies that reproduces the known canonical aspects of El Niño-Southern Oscillation (ENSO), including phase propagation of SST within and between ocean basins. When the global ENSO mode is subtracted from the data, we are left with a non-ENSO residual dataset for SST anomalies that includes fluctuations from inter-seasonal to multidecadal time scales.

Reconstructed for the NINO3 index region, the global ENSO mode accounts for about 3/4 of the total SST anomaly variability, while the residual, dominated by decadal time scales, accounts for the rest. Interestingly, about 40-50% of the amplitudes of the record-setting 1982-83 and 1997-98 El Niño events is accounted for by the residual variability. Related to this, the ranking of the canonical ENSO events changes significantly with respect to the NINO3 index based on data (e.g., 1972-73 is equal to or stronger than 1982-83 and 1997-98).

Based on indices of the canonical ENSO and residual (non-ENSO) variabilities in the NINO3 region, we construct the associated boreal winter (DJF) global composite maps of the tropospheric direct circulation (NCAR/NCEP reanalysis, 1950-1999): velocity potential and and irrotational flow at 850 & 200 hPa, and vertical velocity at 500 hPa. Questions we address include: (1) Does the tropospheric circulation anomaly associated with decadal-scale warming in the NINO3 region reinforce the corresponding anomaly associated with superimposed ENSO warmings? (2) If the warm/cold associations for the two time scales are significantly different, how so and where, and what are the implications for climate predictions?