The evolution of planetary‐scale 200 mb divergent flow during the FGGE year

A descriptive study of the large‐scale horizontally divergent wind field during the FGGE year is presented, using empirical orthogonal functions (EOFs) of the 200 mb velocity potential. The mean flow and annual cycle are shown to be realistically related to climatological features. These and large‐scale patterns represented by the EOFs are consistent with very similar patterns of opposite sign seen at 850 mb. The main result of this work is the clear description of an eastward propagating wave in the 200 mb divergences with zonal wavenumber 1 and period 30–50 days. This mode has been deduced in other studies, and related to variability in the Asian summer monsoon. These results show it to exist throughout the FGGE year, and imply that a relationship exists between the planetary‐scale eastward propagating mode and smaller‐scale divergence bands apparently propagating northward over the monsoon region.