A global El Niño‐Southern Oscillation wave in surface temperature and pressure and its interdecadal modulation from 1900 to 1997

Abstract

Zonal wavenumber frequency spectra of sea surface temperature (SST) anomalies along the equator in the Indo‐Pacific basin for the 98 years from 1900 to 1997 and of surface temperature (ST) and sea level pressure (SLP) anomalies extending around the globe along 10°N for the 48 years from 1950 to 1997 display significant peak spectral energy density for standing and eastward propagating waves of 3–7 year periods and 120°–360° zonal wavelengths. The global standing wave is the familiar Southern Oscillation, but the global propagating wave represents a new paradigm for the El Niño‐Southern Oscillation (ENSO). Global distributions of the phase velocities for this global ENSO wave finds covarying SLP and ST anomalies propagating eastward along the mean path of the Intertropical Convergence Zone (ITCZ), with the global zonal wavenumber 1 (2) component taking ∼4 (6) years to cross the tropical Indian, Pacific, and Atlantic Oceans at a zonal average speed of 90° (60°) longitude per year. Along this path the interannual SST and SLP anomalies are directly out of phase. Since thermocline depth anomalies underneath the ITCZ in the Pacific Ocean propagate westward [White et al. 1985], we view the global ENSO wave as a slow coupled SST wave trapped onto the ITCZ. Separating the global ENSO wave from the Southern Oscillation using complex empirical orthogonal function analysis finds the amplitude of the propagating wave to be half that of the standing wave, with the former (latter) accounting for one third (two thirds) of the interannual variability in Niño‐3 SST and SLP indices during the 1980s. The global ENSO wave is shown to be responsible for the eastward propagation of covarying zonal surface wind and thermocline depth anomalies across the equatorial Pacific Ocean and through this mechanism is able to influence both the phasing and intensity of El Niño. Examining the persistence of the global ENSO wave from 1900 to 1997 finds it and the intensity of El Niño in the eastern equatorial Pacific Ocean modulated by interdecadal change. Both were strong (weak or absent) during decades of global tropical cooling (warming).