Impact of Subtropical ISO Propagation Routes on Summertime Submonthly Wave Patterns and Tropical Cyclone Activity in the Western North Pacific

Abstract

Abstract This study examined the impact of northward- and westward-propagating summertime intraseasonal oscillations (ISOs) on submonthly wave patterns and tropical cyclones (TCs) in the subtropical western North Pacific. In the ISO westerly phase, submonthly wave patterns associated with the northward-propagating ISO appeared to be more energetic and most of the corresponding TCs maintained their wind speed for a relatively long period. Perturbation kinetic energy exhibited a stronger maximum in the ISO northward mode than in the westward mode. The analysis of barotropic conversion in the ISO northward mode revealed that an increase in barotropic conversion can be attributed to a strong association between the perturbation zonal wind component and the background flow. Therefore, submonthly wave patterns moving in a direction similar to that of the northward-propagating ISO continuously extracted energy from the background flow to the south of the submonthly base region. However, in the westward mode, the ISO propagating in a direction almost perpendicular to the submonthly wave pattern tracks not only altered the direction of the wave pattern but also created a background environment that was detached from submonthly perturbations. Thus, the background flow transferred less energy to submonthly wave patterns, resulting in shorter TC durations in the ISO westward mode than in the northward mode. Significance Statement In this study, we focused on the northward and westward ISO propagation routes in the subtropical western North Pacific to investigate their impact on the submonthly wave pattern and TCs. This is important because the ISO propagating behavior can change the background flow for the submonthly wave pattern. The results showed that the northward ISO tended to enhance the wave pattern through strengthening the background component of the barotropic conversion. TCs associated with submonthly wave patterns tended to maintain their intensity longer in the ISO northward mode. The wave pattern associated with the westward-propagating ISO remained weaker.