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