Internal Variability of the Dynamically Downscaled Tropical Cyclone Activity over the Western North Pacific by the IPRC Regional Atmospheric Model

As synoptic storms, tropical cyclones (TCs) are highly nonlinear systems resulting from multiscale interactions. In particular, the genesis of TCs involves complex nonlinear processes, exhibiting strong internal variability in climate model simulations. This study attempts to examine such internal variability of dynamically downscaled TCs over the western North Pacific Ocean based on four simulations of 20 typhoon seasons (1982-2001) initialized on 4 successive days using the International Pacific Research Center (IPRC) Regional Atmospheric Model (iRAM). The results show that on both seasonal and interannual time scales, the initial conditions significantly affect the downscaled TC activity, with the largest internal variability occurring in August on the seasonal time scale. The spreads between any of the individual simulations and the ensemble mean are comparable to and in some circumstances greater than the interannual variation of the observed TC frequency. The internal variability of the downscaled TC activity is found to be insensitive to the amplitude and the pattern of the initial perturbations. However, day-to-day model solutions are strongly affected by the internal variability. As a result, the development of nonlinear atmospheric instabilities significantly modulates the genesis and development of the TC-like vortices, leading to the large internal variability of the downscaled TC activity. In addition to the traditional initial value problem, criteria (in particular, threshold values) used in the TC detection contribute equally to the internal variability of the downscaled TCs in the simulations. Consistent with earlier studies, the results from this study also show that the ensemble mean provides the better downscaled information on seasonal and interannual frequencies of TC genesis and occurrence.