A Slowdown in Translation Speed of Western North Pacific Tropical Cyclones Undergoing Rapid Intensification

This study examines long-term trends in western North Pacific (WNP) tropical cyclones (TCs) experiencing rapid intensification (RI) from 1971 to 2022. Although there is only a weak slowdown for all intensifying WNP TCs, the average translation speed for RI TCs has significantly decelerated over the RI main development region (7.5 degrees-25 degrees N, 115 degrees-160 degrees E). This slowdown is primarily due to increasing RI probabilities for slower-moving TCs. By contrast, the RI probability of faster-moving TCs remains virtually unchanged. These differences in RI trend probabilities between slow-moving and fast-moving TCs are primarily linked to a deepening of the WNP mixed layer. TC-induced sea surface temperature cooling tends to weaken when the mixed layer is deep. During the intensification stage, the deeper mixed layer is more critical for slower-moving TCs than for faster-moving TCs. Our findings suggest that RI probabilities for slow-moving WNP TCs may continue to increase in a future warming climate. Rapid intensification (RI) poses a considerable challenge for operational tropical cyclone (TC) forecasting. Several studies have found increasing trends in RI frequency, ratio and magnitude over the western North Pacific (WNP) during recent decades. This study focuses on the translation speed for WNP TCs experiencing RI from 1971 to 2022. The average movement of RI TCs has significantly decelerated over the RI main development region, where a majority of RI cases occur climatologically. This slowdown is primarily induced by significantly increasing RI probabilities for slower-moving TCs and virtually unchanged RI probabilities for faster-moving TCs. These differences in RI probabilities between slower-moving and faster-moving TCs cannot be explained by weak changes in the steering flow or favorability of the thermodynamic environment over the WNP. Instead, these results are primarily linked to a deepening of the WNP mixed layer. TC-induced sea surface temperature cooling tends to weaken when the mixed layer is deep. Previous idealized simulations have shown that a deeper mixed layer is more critical to TC intensification for slower-moving TCs than for faster-moving TCs. Our findings suggest that RI probabilities for slow-moving WNP TCs may continue to increase in a future warming climate. There is a significant slowdown in the mean translation speed of western North Pacific tropical cyclones undergoing rapid intensification Slow-moving (fast-moving) tropical cyclones have a significant increasing (virtually unchanged) rapid intensification probability A deepening of the western North Pacific mixed layer is the primary driver of the deceleration of rapid intensification cases