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Along the East Asian coast, the interdecadal variation in tropical cyclone (TC) landfall activities in the early 2010s shows a tripole change pattern characterized by significantly increased (decreased) TC landfalls to the north of 30°N and to the south of 20°N (between 20° and 30°N). The increased TC landfalls in the East Asian subregion north to 30°N mean greater TC damage in the most populous regions, including southeastern China, the Korean Peninsula, and Japan, since the early 2010s. The present work uses an objective clustering technique to analyze the internal dynamical causes of this interdecadal variation. The increased TC landfall activities in the East Asian subregion north of 30°N are attributed to the increase in TCs with a northwestward-moving track (cluster defined as C1), whereas the increased TC landfall activities to the south of 20°N and the decrease between 20° and 30°N are mainly due to the southward shift of TCs with a westward-moving track (cluster defined as C2). This work focuses on different TC track groups (C1 and C2) based on daily data, showing that the interdecadal changes in the synoptic-scale monsoon trough and subtropical high provide important environment for different groups of TCs. The ascending phase of the PDO with significant warming over the central Pacific favors the interdecadal tripole changes in TC landfalls. [ABSTRACT FROM AUTHOR]

The maintenance period (20 days after the onset day) of the South China Sea (SCS) summer monsoon (SCSSM) onset is investigated in this paper, the main focus of which is the roles of synoptic‐scale (2–10‐day period) waves (SSWs) relative to those of 10–20‐day and 30–60‐day intraseasonal oscillations (ISOs). The results reveal that the eddy kinetic energy (EKE)‐based SSW intensity displays a greater amplitude and closer relationship with the SCSSM onset date (correlation coefficient: −0.50) than either ISO during the SCSSM onset maintenance period, indicating that frequent synoptic‐scale processes play an important role in the SCSSM onset. Regression analyses based on the original EKE‐based wave intensity further reveal that SSW‐related horizontal wind anomalies show significant cyclonic anomalies over the SCS, with westerlies occupying the central and southern SCS. Moreover, SSW‐induced low‐pressure systems, including negative outgoing longwave radiation (OLR) anomalies, low‐level convergence and weakening of the 500 hPa height field, are observed over the SCS and maintain the SCSSM onset. A three‐dimensional EKE diagnosis is conducted to investigate the relative contributions of SSWs and both ISOs. The results demonstrate that the SSW‐related horizontal barotropic EKE conversion and EKE conversion from eddy available potential energy cooperate to contribute more than the two ISOs to the dynamic processes during the maintenance stage of the SCSSM onset. [ABSTRACT FROM AUTHOR]

ABSTRACT Using the European Centre for Medium-Range Weather Forecasts Interim Reanalysis data set and the tropical cyclone ( TC) best track data obtained from the International Best Tracks Archive for Climate Stewardship project from June to November for the epoch of 1979-2013, this study reexamines the recent interdecadal shift in TC genesis over the western North Pacific ( WNP) in the late 1990s. An interdecadal decrease in the frequency of the WNP TC genesis was observed during 1998-2013 compared to the period of 1979-1997. The spatial distribution of the interdecadal decrease of the WNP TC genesis running northwest-southeast, consistent with the monsoon trough ( MT) zone. The results imply that the WNP genesis may be closely related to the MT on the interdecadal timescale. After the late 1990s, the intensity of the WNP MT becomes weaker and it extends more westward, thereby providing an unfavourable condition for the TC genesis. Through a diagnosis of the energetics, we suggested that the barotropic energy conversion in the eastern part of the WNP MT tends to be weakened in the late 1990s, hence less energy to support the WNP TC genesis. Both the meridional shear and the convergence of the mean zonal winds over the eastern WNP MT have an important impact on the conversion from mean kinetic energy to eddy kinetic energy, which can be considered as the atmospheric cause of the interdecadal decrease of TC genesis. [ABSTRACT FROM AUTHOR]

ABSTRACT The interdecadal changes in the frequency of tropical cyclone ( TC) genesis in the western North Pacific ( WNP) were examined for the epoch 1979-2013. There was an interdecadal increase in the frequency of TC genesis in the WNP in May during 1999-2013 than during 1979-1998, and such changes differed from those occurring between June and December. This interdecadal change is attributed to the early onset of the South China Sea summer monsoon ( SCSSM) during 1999-2013, when the southwesterly winds spread eastwards and the monsoon trough ( MT) moved into the WNP earlier. The MT favoured enhanced convection and anomalous low-level cyclonic vorticity, facilitating more TC genesis. During 1979-1998, in contrast, the onset of the SCSSM was delayed and the MT was confined to the west of the South China Sea. As a result, weakened convection and anomalous anticyclonic low-level vorticity are observed over the WNP, facilitating less TC genesis. [ABSTRACT FROM AUTHOR]