Multivariate statistical investigations of the northern hemisphere circulation during the El Niño event 1982/83

Abstract

Using a multivariate statistical testing procedure, the anomalies of the monthly mean northern hemisphere stream function in 1000, 850, 500 and 200 hPa of the period January 1982 to September 1983 are compared to the respective monthly values of the period January 1967 to December 1981, to find out whether or not the El Niño 1982/83 had a statistically significant influence on the northern hemisphere circulation. The multivariate test utilizes the parametric Hotelling T² statistics, whereby the stream functions are projected on a low-order set of global orthogonal functions (spherical harmonics) which reduce the number of spatial parameters to be tested drastically while still retaining most of the spatial variance. To clarify which special wave amplitude or combination of wave amplitudes leads to a significant circulation anomaly, a multivariate test hierarchy is defined. The results show clearly and well-defined significant anomalies at 500 hPa between July 1982 and September 1983, where the dominant anomaly is in wave amplitude Ψml(m= 1, l = 2). In 200 hPa, a clear signal is detected only during the winter season 1983 (January-March). At the two lower levels, a signal is not so clear. Using the anomalous Ψ1/2 amplitude at 500 hPa, three distinct periods for this wave can be distinguished: a build-up phase July 1982 – December 1982, a mature phase January 1983 – March 1983 and a decay period from April 1983 — September 1983.