A Rotation Procedure to Improve Seasonally Varying Empirical Orthogonal Function Bases for MJO Indices

Various indices have been defined to characterize the phase and amplitude of Madden-Julian oscillation (MJO). One widely used index is outgoing longwave radiation (OLR) based MJO (OMI), which calculated using spatial pattern 30–96-day eastward-filtered OLR. The empirical orthogonal functions (EOFs) calculate OMI in observations are prone degeneracy exhibit oscillations on order 10–20 days, despite initial filtering We propose a simple modification that involves aligning EOFs between neighboring days while retaining described by EOFs. This rotation method implemented as postprocessing procedure current calculation cleanly removes spurious issues seen standard method. A similar can be calculations other indices.