The large-scale environment over the eastern Pacific and the Gulf of Mexico during August and September 1998 is the subject of this study. We are motivated by the formation of five tropical cyclones in this region within a short span of 18 days between Aug. 31-Sep. 17 1998. This period was characterized by a dramatic evolution of the large scale environment which is defined here as the 20 day low-pass filtered fields. A large area of convection that first developed over the eastern Pacific eventually progressed eastward and northward into the Gulf of Mexico. At the same time, the prevailing easterlies over the eastern Pacific changed to westerlies. This evolution is consistent with the active phase of the Madden Julian Oscillation (MJO) described in previous studies. The MJO-related convection was accompanied by low level cyclonic vorticity and convergence over a large area - two crucial factors that foster tropical cyclogenesis.

On the synoptic scale, several disturbances that could be tracked back to the west coast of Africa made their way into this region. At least three out of the five storms that formed during this period could be linked to precursor disturbances originating from Africa. An examination of the barotropic exchanges between the disturbances and the environment reveals significant disturbance growth within the Gulf of Mexico and the eastern Pacific during the active MJO period.

Numerical simulations using an idealized barotropic model are also performed to highlight the influence of the MJO on the amplification of waves arriving from upstream. A barotropic model, linearized about the low-pass filtered ECMWF analyses for this period, is used. A Rossby wave generator is specified to simulate upstream waves. The simple numerical model is able to reproduce key features of the observed barotropic energy exchange. The results suggest that the active MJO environment provided conditions favorable for the growth of upstream disturbances and barotropic dynamics played an important part in this process.