The present paper uses the satellite era data from 1979 to 2015 to examine the relationship between El Niño-Southern Oscillation (ENSO) and tropical cyclones (TCs) in the western North Pacific (WNP) during the boreal summer from June to August. It is found that WNP TC variability is characterized by two major feature changes: (1) a significant reduction of the TC number since 1998 and (2) a stronger interannual relationship between ENSO and TCs since 1998. Results suggested that such changes are largely due to the synergy effects of a shifting ENSO and the Pacific climate regime shift. Since 1998 with a cool Pacific decadal oscillation phase switching from a warm phase, more La Niña and central Pacific (CP) El Niño events occur. The decreased low-level relative vorticity and increased vertical wind shear during 1998-2015 compared to 1979-1997 are responsible for the TC reduction. The stronger interannual relationship between ENSO and TCs since 1998 is closely associated the change of CP sea surface temperature. It enhances the associations of environmental factors including vertical wind shear and mid-level relative humidity with TCs and thus increases the interannual relationship between ENSO and TCs. These two feature changes also manifest in the mean TC genesis location, with a northwestward shift of the TC genesis location during 1998-2015 and an increased relation to El Niño Modoki index since 1998. This study has an important implication for TC outlooks in the WNP based on climate predictions and projections. Keywords Tropical cyclone variability and climate • ENSO • Pacific decadal oscillation • Interdecadal change of interannual relationship * Chunzai Wang

The study of the impact of global warming on tropical cyclone (TC) intensity is subject to uncertainty in historical datasets, especially in the western North Pacific (WNP) basin, where conflicting results have been found with the TC datasets archived in different organizations. In this study the basinwide TC intensity in the WNP basin is derived dynamically with a TC intensity model, based on the track data from the Joint Typhoon Warning Center (JTWC), the Regional Specialized Meteorological Center (RSMC) of Tokyo, and the Shanghai Typhoon Institute (STI) of the China Meteorological Administration. The dynamically derived TC intensity is compared to the three datasets and used to investigate trends in TC intensity. The associated contributions of changes in SST, vertical wind shear, and prevailing tracks are also examined. The evolution of the basinwide TC intensity in the JTWC best-track dataset can be generally reproduced over the period 1975–2007. Dynamically derived data based on the JTWC, RSMC, and STI track datasets all show an increasing trend in the peak intensity and frequency of intense typhoons, mainly because of the combined effect of changes in SST and vertical wind shear. This study suggests that the increasing intensity trend in the JTWC dataset is real, but that it may be overestimated. In contrast, the TC intensity trends in the RSMC and STI intensity datasets are dynamically inconsistent. Numerical simulations also suggest that the frequency of intense typhoons is more sensitive to changes in SST and vertical wind shear than the peak and average intensities defined in previous studies.

The modulation of tropical cyclone (TC) genesis over the western North Pacific (WNP) by the intraseasonal variability (ISV) is investigated in this study. The two leading ISV modes, i.e., the 40-day Madden-Julian oscillation (MJO) and the 16-day quasi-biweekly oscillation, are found to exert significant impacts on TC genesis over the WNP. A majority of TC geneses over the WNP is found to occur during the period when both the modes are active, suggesting a joint influence of the two modes on TC genesis over the WNP.The modulation of TC genesis over the WNP by the two leading ISV modes can be well depicted by the genesis potential index (GPI). Contributions of the four terms to the total GPI anomalies are further analyzed to determine the key factors involved in modulations of TC genesis by both of the ISV modes. Results indicate that while, in general, the low-level absolute vorticity and the mid-level relative humidity are the two most important factors affecting WNP TC genesis, relative roles of the four GPI factors also tend to be dependent on the ISV phases. This study provides further understanding of the ISV modulation of WNP TC genesis, which could benefit the intraseasonal prediction of the TC activity.

A significant increase in the proportion of tropical cyclones undergoing rapid intensification at least once during their lifetime (RITCs) over the western North Pacific (WNP) is observed since 1998 when an abrupt climate regime shift occurred. Changes of large-scale atmospheric and oceanic conditions affecting TC activity are compared between two subperiods: one before and one since 1998. Results suggest that both a significant decrease in the number of TCs and a nearly unchanged number of RITCs since 1998 caused a significant increase in the frequency of RITCs. The decrease in TC numbers is likely driven by considerably increased vertical wind shear and decreased low-level vorticity. In contrast, the unchanged RITC counts and thus increased ratio of RITCs during the recent decades are largely attributed to the dominance of a more conducive ocean environment with increased TC heat potential and warmer sea surface temperature anomalies. These associated decadal changes are closely associated with the recent climate regime shift. During the recent decades with a mega–La Niña–like pattern, stronger easterly trade winds have caused increased vertical wind shear and a weakened monsoon trough, thus hampering TC formation ability over the WNP. In addition, a steeper thermocline slope that hampered the eastward migration of warm water along the equatorial Pacific has generated a more favorable thermodynamic environment supporting TC rapid intensification over the WNP.

34 35 The intra-seasonal variability of tropical cyclogenesis in the western North 36 Pacific (WNP) basin is explored in this study. The relation of cyclogenesis in each of 37 the five large-scale patterns identified by Yoshida and Ishikawa (2013) is associated 38 with the Madden-Julian Oscillation (MJO). Confirming previous results, more events 39 of cyclogenesis are found during the active MJO phase in the WNP. Furthermore, 40 results indicate that most of the tropical cyclogenesis is associated with the monsoon 41 shear line (SL) large-scale pattern during the active phase. 42 The genesis potential index (GPI) and its individual components are used to 43 evaluate the environmental factors that most contribute towards cyclogenesis under 44 the different phases of the MJO. GPI exhibits a large positive anomaly during the 45 active phase of the MJO and such anomaly is spatially correlated with the events of 46 cyclogenesis. The analysis of each factor indicates that low-level relative vorticity and 47 mid-level relative humidity are the two dominant contributors to the MJO-composited 48 GPI anomalies. The positive GPI anomalies during the active phase are partially offset 49 by the negative contributions from vertical wind shear and potential intensity. This is 50 valid for all five large-scale patterns. 51 It is noteworthy that the easterly wave (EW) large-scale pattern, while exhibiting 52 the same influence of relative vorticity and mid-level humidity contributing towards 53 positive GPI anomalies, presents slightly more cyclogenesis events under the inactive 54 phase of the MJO. This unexpected result suggests that other factors not included in 55 the definition of the GPI and/or changes of environmental flows on other time scales 56 contribute to the tropical cyclogenesis associated with the EW large-scale pattern. 57 58

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The WNP tropical cyclone (TC) activity peaks in July-August-September, accounting for about 60% of the annual TC count (Chan, 2005;Lander, 1994). TCs in this study are defined as those in the data sets with maximum wind speed exceeding the tropical storm intensity (17.2 m s −1 or 34 knots). Climatologically, about four TCs form in July and it usually takes a northward-recurved track and exerts great impacts on subtropical East Asia. However, no TC formation in July 2020 over the WNP was recorded in July 2020 (Figure 1a), which is the first time in the available historical records since 1945. The unprecedented absence of TC formation suggests its possible unique driving factors.It is well known that El Niño-Southern Oscillation (ENSO) is an important factor modulating TC activity over the WNP (Chan, 1985;Lander, 1994;Wang & Chan, 2002). During the El Niño decaying summer, an anomalous anticyclonic circulation is a recurrent pattern in the WNP, which acts to suppress TC genesis over the WNP (

The individual contributions of changes in sea surface temperature (SST), vertical wind shear and tropical cyclone (TC) tracks to the interannual TC intensity change in the western North Pacific (WNP) basin are examined based on the selected 7 warm years and 7 cold years during the period 1970-2007. The selected warm and cold years are defined by the Nino-3.4 SST anomalies index, and correspond to El Niño and La Niña events, respectively. The intensity model used in this study can simulate the spatial distribution and differences of TC intensity when the model is integrated along the observed TC tracks in the warm and cold years. It is found that the change of TC tracks plays a dominant role in the observed TC intensity difference between warm and cold years. During the warm years, TC formation is enhanced in the southeast quadrant, and more TCs take a northwestward track during the warm years than during the cold years because of the interannual change in the large-scale steering flows. As a result, TCs have a longer time for intensification and develop into intense TCs during the warm years when compared to the cold years.