The embryo of unseasonal Typhoon Nanmadol (2004) over western North Pacific were tracked back to the Eastern Pacific (120°W) as a counter-clockwise rotating mixed Rossby-gravity wave (MRG) gyre at upper troposphere. The temporal and spatial evolution from an MRG-wave-type disturbance as an equatorial trapped mode to an off-equatorial disturbance is studied in details utilizing NOGAPS (Navy's Operational Global Atmospheric Prediction System) data. After experiencing dramatic reduction in zonal wavelength and amplification (elimination) at the lower (upper) troposphere, the MRG gyre moved away from the equator. The remarked evolution is attributed to the variation of large-scale background such as the mean vertical shear, low-level convergent flow and sea surface temperature (SST).

This case claims that the upper tropospheric MRG waves at the very east of date line are linked to tropical cyclonegenesis over western North Pacific.