Transition from an eastern Pacific upper‐level mixed Rossby‐gravity wave to a western Pacific tropical cyclone

Abstract

The ancestor of unseasonal Typhoon Nanmadol (2004) over the western North Pacific is traced back to the Eastern Pacific (near 120°W) as an upper tropospheric, counter‐clockwise rotating mixed Rossby‐gravity (MRG) wave. The temporal and spatial evolution from an equatorial trapped MRG‐wave‐type disturbance to an off‐equatorial tropical depression is documented by utilizing NOGAPS (Navy's Operational Global Atmospheric Prediction System) analysis data. It is found that the MRG wave moved away from the equator after a dramatic reduction in its zonal dimension and downward development and amplification in the lower troposphere. These dramatic changes in MRG wave properties are attributed to the modulation of the easterly vertical shear, low‐level westerly‐easterly confluence in the large‐scale background flows, and convective coupling, as well as underlying high sea surface temperature (SST). The boundary layer convergence associated with asymmetric pressure field of the MRG wave is likely responsible for the transition from an equatorial MRG to an off‐equatorial tropical disturbance. This case study addresses the possibility that upper tropospheric MRG waves over the far eastern Pacific may provide “seeds” for tropical cyclogenesis over the western North Pacific.