**Long-Term Trends in North Pacific Tropical Cyclone Activity

In recent years, it has been documented that tropical cyclone (TC) activity over the North Atlantic basin undergoes cyclic changes on a multi-decadal time scale (Landsea et al. 1996). For example, TCs were more prevalent during the 1930s to the middle 1960s but were below-normal between the late 1960s to the early 1990s. Since 1995, there have been signs that TC activity over the North Atlantic has again returned to a more active regime.

Here an examination of long-term trends in TC activity over the North Pacific is completed, and the results are compared to those found over the North Atlantic. It is found that TC activity over the Eastern and Central North Pacific (ECNP) basin also undergoes distinct, mult-decadal oscillations which are out-out-phase with activity over the North Atlantic. Two periods can be identified in the TC record. The first period (1966-1981) is characterized by below-normal TC activity with an average of 14.8 TCs per season. Moreover, in the Central North Pacific (CNP) basin only 6 TCs formed over those 16 years. In contrast, the second period (1982-1994) is characterized by enhanced TC activity with an average of 19.5 TCs per season; twenty TCs formed during this period in the CNP.

An analysis is performed to determine whether these changes in TC activity for the respective epochs are statistically significant. The test shows that the first period is significantly less than the second period. Furthermore, since 1995, TC activity has again returned to below-normal levels with an average of 12 TCs per season. These findings mirror those found by Chan and Shi (1996) over the Western North Pacific.

Variations over the North Pacific over the past several decades can be explained in terms of natural climate variability. As is the case in the North Atlantic, these changes are not attributed to a global warming scenario.