Interannual Modes of Variability in Atmospheric Angular Momentum

The interannual variability of atmospheric angular momentum over a 26-yr period is studied regionally using monthly analyses of zonal winds derived from the global rawinsonde network. Variations in zonal-mean momentum, filtered to emphasize interannual timescales, exhibit a coherent propagating signal emanating from low latitudes, as identified in other studies using shorter records. Applying extended empirical orthogonal function (EEOF) analyses to zonally varying data, the authors isolate a dominant pair of eigenvectors whose principal component time series and spatial patterns are in quadrature with one another, indicating oscillatory behavior. The oscillation described by the two EEOFs has a period of about 36 months and is linked a posteriori to the time evolution of the El Niño–Southern Oscillation phenomenon. Beginning as an anomaly over the Tropics that extends from the Indian Ocean into the Pacific, the signal is observed to progress eastward and poleward into both hemispheres, leading to a bipolar structure straddling the central tropical Pacific Ocean. A lagged teleconnection analysis between the Pacific centers and remote sectors corroborates the EEOF results. The first pair of eigenvectors contributes substantially to the interannual variance in global angular momentum and to the variability of the zonal-mean momentum field at low latitudes. A second pair of eigenvectors, also in quadrature with one another, describes a biennial oscillation related to zonal momentum variability at higher latitudes.