A Climatological Perspective on Extratropical Synoptic-Scale Transient Eddy Activity Response to Western Pacific Tropical Cyclones

An observational study focusing on the contribution of tropical cyclones (TCs) that form over the western North Pacific (WNP) to the synoptic-scale transient eddy activity (STEA) over the North Pacific during the boreal autumn and early winter in the period 1979–2019 is presented in this paper. Statistical results show that WNP TCs entering the mid-latitudinal North Pacific provide significant positive effects on the pentad mean strength of STEA, which is primarily concentrated over the Kuroshio/Oyashio Extensions (KOE) and regions from east of Japan to 160°W in the lower and mid-to-upper troposphere, respectively. TC intensity is highly indicative of the subsequent STEA with a correlation coefficient of 0.37/0.33/0.45 at 300 hPa/500 hPa/850 hPa exceeding the 99% confidence level for the period 1979–2019. The strength of STEA in the upper troposphere associated with TCs presents a more significant linear growth with TC intensity than that at the mid-to-lower levels after the cyclones enter the KOE region, suggesting that the impact of TCs on STEA gradually increases with height. Further analyses reveal that the contribution of TCs accounts for 4%–6% of the total STEA change over the KOE region during the late autumn and early winter. In addition, the influence of TCs on STEA experienced an interdecadal decrease from the early 2000s through the early 2010s.