EOFs of the (combined) daily fields of 15°S-15°N averaged OLR, 850 hPa zonal wind, and 200 hPa zonal wind.
Method for obtaining EOF structures
We performed EOF analysis on the combined daily fields of equatorially-averaged
(15°S to 15°N) OLR, 850hPa zonal wind, and 200 hPa zonal wind
for the period of 1979 to 2001 (23 years).
We did this on the covariance matrix with each field normalized by the square-root
of its global mean variance first. This is necessary so that each field contributes
the same amount of variance to the combined field.
Before the EOF analysis, however, we also performed the following:
- Remove the long-term mean and climatological seasonal cycle (3 harmonics) from each field at each grid-point.
- Remove the variability associated with El Nino (that which is linearly related the ENSO SST1 index).
- Remove a 120-day mean of the most recent 120 days at each point..
All of these steps can be performed in real-time and are important for removing
aspects of low frequency variability that can have spatial structures like that of
the MJO.
An ASCII file containing the EOF spatial structures and code for projecting model or
observed data onto them is obtainable from here.
Results and Interpretation
As shown above, the leading EOF structure accounts for 12.8% of the total variance
of the three combined fields. It has a region of negative OLR anomalies, indicating
enhanced atmospheric deep convection, between about 80°E and the date line.
The 850-hPa level winds show westerly anomalies to the west of the main centre of
convection, and easterlies to the east, with the 200-hPa wind anomalies being
close to the opposite. The pattern thus describes a large vertically-oriented
circulation cell along the equator with upward motion being implied in the region
of negative OLR anomalies, and downward motion outside that region.
The second EOF structure, on the other hand, accounts for 12.2% of the total variance,
with it being approximately in quadrature with the first. Together the EOFs form a
degenerate pair, and they can represent the spatially propagating signal of
the MJO. The vertically-oriented circulation cells they describe represent the
same features as were given in one of the original schematics of the oscillation, as
shown below.
