Environmental steering flow analysis for central north Pacific tropical cyclones based on NCEP/NCAR reanalysis data

Abstract

An environmental steering flow analysis for central north Pacific tropical cyclones was made utilizing model wind data from the NCEP/NCAR Reanalysis project, since no aircraft or rawinsonde data are readily available in this region. The results are compared with previous work performed in other hurricane basins. Tropical cyclone best track data were obtained from the Central North Pacific Hurricane Center. Most of the cyclonic activity analyzed in this project was observed at latitudes below 20°N, where most tropical cyclones followed a general west to northwestward track. Environmental steering was defined as a 5°-7° annulus around the cyclone center. On average, tropical cyclones were observed to move faster than the environmental steering flow, moving to the right of the environmental flow at the midlower tropospheric levels between 850 and 600 mb, and to the left at higher levels. These results show agreement with previous work for the north Atlantic basin, and disagree with most previous results for the north-west Pacific (where most cyclones show movement to the left of the environmental steering flow at all levels). Out of 36 candidate steering layers, two were identified as the recommended steering layers for different cyclone intensities: 850-400 mb for tropical storms and depressions, and 850-300 mb for hurricanes. The differences between these steering layers and tropical cyclone motion are smaller south of 20°N, especially southeast of the Hawaiian islands. The possibility of a direct relationship between wind shear and the environmental steering flow was also investigated, but no correlation between these two variables was found.