Skilful multi-week predictions of tropical cyclone frequency in the Northern Hemisphere using ACCESS-S2

Skilful multi-week predictions of tropical cyclone frequency in the Northern Hemisphere using ACCESS-S2

The skill of subseasonal (multiweek) forecasts of tropical-cyclone (TC) occurrence over the Northern Hemisphere is examined in the Australian Bureau of Meteorology's (BoM) multiweek to seasonal prediction system, ACCESS-S2. ACCESS-S2 shows a good representation of the spatial distribution of TCs in the Northern Hemisphere; however, TC track frequency is generally underpredicted in the western North Pacific to the east of the Philippines and in the eastern North Pacific. The reduced activity relative to observations could be due to a significant positive bias in 850–200-hPa wind shear in both of these regions, as well as a significant negative sea-surface temperature (SST) bias in the eastern North Pacific. Despite biases in climatological TC frequency, the observed change in TC track frequency across the Northern Hemisphere with the phase of the Madden–Julian Oscillation (MJO) is well captured by ACCESS-S2. Changes in the large-scale environment (e.g., precipitation, 600-hPa relative humidity, 850-hPa absolute vorticity and 850–200-hPa wind shear) are also well represented, with the location and size of the anomalies comparable to ERA-Interim, apart from SST which shows a different response during some phases. ACCESS-S2 shows skill relative to climatology for multiweek predictions of TC occurrence out to week 5 in the western North Pacific, eastern North Pacific and North Atlantic; and out to week 2 for the North Indian Ocean. Assessment of real-time forecasts for Typhoon Rai (December 2021) showed that ACCESS-S2 provided good guidance of the development and potential landfall of a TC in the Philippines at four weeks lead time.