Tropical Cyclones

Also known as tropical storms, typhoons, hurricanes, and cyclones, tropical cyclones (TCs) are intense low-pressure systems that form over warm ocean waters. Depending on their strength, TCs can have dramatic effects on human life and property. Impacts due to strong winds, heavy rains, and high seas associated with a landfalling TC can be devastating across multiple sectors, especially for islands like Palau (Merrifield et al., 2019).

Storms are tracked operationally in real time and compiled into post‑season best‑track datasets such as NOAA’s IBTrACS that catalog position and intensity histories (Knapp et al., 2010). Potential changes in the intensity, frequency, and location of TCs is an important consideration with respect to assessing the impacts of climate variability and change. Intensity is often communicated using the Saffir–Simpson Hurricane Wind Scale, which classifies storms into 5 categories based on maximum sustained wind (NOAA NHC).

- Category 1: 64–82 kt (119–153 km/h)
- Category 2: 83–95 kt (154–177 km/h)
- Category 3: 96–112 kt (178–208 km/h)
- Category 4: 113–136 kt (209–251 km/h)
- Category 5: ≥137 kt (≥252 km/h)

Palau sits in a very active cyclone basin, often referred to as a “typhoon alley,” which closely aligns with the area of responsibility of the Regional Specialized Meteorological Centre (RSMC) Tokyo for Tropical Cyclones. TC activity peaks in late boreal summer to autumn (Miles et al., 2020), though TCs can affect Palau at any time of the year (SPC 2022). ENSO affects both the frequency and location of TCs in the western North Pacific near Palau (Wang and Chan, 2002; Camargo and Sobel 2005; Patricola et al., 2018). A key feature is the east-west shift in origin location. During El Niño, the monsoon trough in the western North Pacific tends to move eastward and closer to the equator. As result, TCs form farther east. During La Niña, this pattern is reversed: the monsoon trough remains in the far western Pacific and TCs form farther west, closer to Southeast Asia.

In this section we look at two indicators of change in tropical cyclones. Using observations from the IBTrACS (International Best Track Archive for Climate Stewardship) we will create plots and maps that show changes in frequency, intensity and location of all tropical cyclones and severe tropical cyclones in the area of interest, where severe cyclones are those at fall into category 3 or higher 96–112 kt (178–208 km/h) on the he Saffir-Simpson scale. We will also create a table that summarizes these results.