Extreme Value Analysis of Madden-Julian Oscillation Events

The Madden-Julian Oscilation (MJO) is an eastward equatorially propagating mode with a strong influence on the precipitation in the tropics on sub-seasonal timescales. Although, several studies have widely analysed the MJO, its activation and evolution are not fully understood [1].

The purpose of this study is to analyse the statistical features of the most intense MJO events.

We perform the study using two different indices describing the MJO: The popular Wheeler and Hendon index (1979-2021), based on the first two principal components of a multivariate empirical orthogonal function analysis of a combination of outgoing longwave radiation (OLR) and 200 mb and 850 mb zonal winds, as well as the Oliver and Thompson index (1905-2015) based on surface pressures [2].

In this study an event takes place when the index amplitude exceeds a threshold for a certain number of days. With this, we define the observables of an event; these are, the maximum amplitude, duration and size, which is the sum of the amplitudes along the duration of an event.

We use extreme-value theory to fit the generalized Pareto distribution (GPD) to the different distributions of observables and we compare the results with the fitting of a simple power-law tail and other heavy-tailed distributions. We also compare the performance of several advanced extreme-value-statistics tools to find the threshold over which the GPD holds.

 

1.Kiladis, G. N., Dias, J., Straub, K. H., Wheeler, M. C., Tulich, S. N., Kikuchi, K., ... & Ventrice, M. J. (2014). A comparison of OLR and circulation-based indices for tracking the MJO. Monthly Weather Review, 142(5), 1697-1715.

2.Klotzbach, P. J., and E. C. J. Oliver (2015), Variations in global tropical cyclone activity and the Madden-Julian Oscillation since the midtwentieth century, Geophys. Res. Lett., 42, 4199–4207.