EOF Analysis of Temperature Anomalies in the Tropics with Radiosonde Data from the SHADOZ Program

A principal component analysis is performed on the vertical temperature profiles of the Southern Hemisphere Additional Ozonesondes (SHADOZ) soundings (1998-2006). Because the soundings are launched at 5-10 day intervals, the analysis resolves variability at time scales that range from approximately two weeks to eight years. For selected SHADOZ sites, which are distributed throughout the tropics close to the equator, the two most dominant empirical orthogonal functions (EOFs) together represent the Quasi-biennial Oscillation (QBO) in the stratosphere. The typical value of combined fractional variance explained by these two leading EOFs is 55 %. The third most dominant EOF is found to be closely associated with the El Nino-Southern Oscillation (ENSO). While the principal component (PC) time series of the third EOF is significantly correlated with the Nino3 index, the time series also shows much shorter time scale fluctuations. Linear regression with global NCEP-NCAR reanalysis wind and temperature data against this PC time series indicates that the lower stratospheric (LS) temperature anomalies are associated with vertically propagating Kelvin waves.

These results indicate (1) that next to the QBO, ENSO has the most significant impact on the tropical LS temperature signal, and (2) that this LS/ENSO relationship is realized through vertically propagating Kelvin waves, which have much shorter time scales than ENSO itself.