EOFs of the (combined) daily fields of 15°S-15°N averaged OLR, 850 hPa zonal wind, and 200 hPa zonal wind.

EOF 1 spatial structure EOF 2 spatial structure

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: 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.

schematic depiction of MJO and corresponding EOFs