Trends in Northwest Pacific Basin Tropical Cyclone Activity (1951-2017)

In a basin where typhoon season is year-round, the northwestern pacific did not spawn its first typhoon in 2016 until early July - ending the longest stretch ever [since mid-December of previous year] without a typhoon in that basin. Understanding inter-annual variability and longer term temporal trends in tropical cyclone (TC) frequencies, their track patterns, and other characteristics is paramount to understanding the role of climate variability and climate change on these systems and hence the impact on damage and insured loss. To that end, Best Track Data from the Japan Meteorological Agency from 1951-2017 was analyzed for temporal changes over the northwest Pacific basin in several important parameters. From a landfall perspective, a 10-year moving average of TC landfalls shows the lowest ratio of landfalls to named storms occurred in the late 1960s to early 1970s. This ratio dramatically increased from 1998 to 2015. From a basin perspective, a 7-year moving average of TC frequency reveals a strong downward trend that is most readily apparent in the second half of the record. By further analyzing this trend in events by intensity, we confirm that not only are there fewer weak typhoon (Category 1 to 3), but also a greater number of strong typhoons (Category 4 to 5). The result is similar to how climate change is expected to alter TC activity later this century. From a track perspective, the overall latitudinal and longitudinal extent of typhoon tracks in a year has changed, such that TCs in later years have not been seen in the outermost regions of the domain, especially over the last ten years. On shorter time scales, it was found that southern regions (e.g., Vietnam and southern China) are more likely to be impacted during La Niña years, while during El Niño years Taiwan, Japan, and South Korea are more likely impacted. While some patterns and trends and their causes have been identified, more work needs to be done to better represent climate variability and climate change within catastrophe models.