Forced and internal variability of tropical cyclone track density in 1 the western North Pacific

6 Forced interannual-to-decadal variability of annual tropical cyclone (TC) track density in 7 the western North Pacific between 1979-2008 is studied using TC tracks from observations 8 and simulations by a 25-km-resolution version of the GFDL High-Resolution Atmospheric 9 Model (HiRAM) that is forced by observed sea surface temperatures (SSTs). Two modes 10 dominate the decadal variability: a nearly-basin-wide mode, and a dipole mode between 11 the subtropics and lower latitudes. The former mode links to variations in TC number and 12 is forced by SST variations over the off-equatorial tropical central North Pacific, whereas 13 the latter might be associated with the Atlantic Multidecadal Oscillation. The interannual 14 variability is also controlled by two modes: a basin-wide mode driven by SST anomalies of 15 opposite signs located respectively in the tropical central Pacific and eastern Indian Ocean, 16 and a southeast-northwest dipole mode connected to the conventional eastern Pacific ENSO. 17 The seasonal evolution of the ENSO effect on TC activity is further explored via a joint 18 EOF analysis using TC track density of consecutive seasons, and the analysis reveals that 19 two types of ENSO are at work. 20 Internal variability in TC track density is then examined using ensemble simulations from 21 both HiRAM and a regional atmospheric model. It exhibits prominent spatial and seasonal 22 patterns, and it is particularly strong in the South China Sea and along the coast of East 23 Asia. This makes an accurate prediction and projection of TC landfall extremely challenging 24 in these regions. In contrast, basin-integrated metrics (e.g., total TC counts and TC days) 25 are more predictable. 26