Changes in ENSO Modulation of the Distribution of Rapidly Intensifying Tropical Cyclones over the Western North Pacific in Boreal Autumn

Ye Tian aLow-Carbon and Climate Impact Research Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong, China
bKey Laboratory of Polar Atmosphere-Ocean-Ice System for Weather and Climate, Ministry of Education and Department of Atmospheric and Oceanic Sciences and Institute of Atmospheric Sciences, Fudan University, Shanghai, China

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Wen Zhou aLow-Carbon and Climate Impact Research Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong, China

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Lin Zhang aLow-Carbon and Climate Impact Research Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong, China

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Yue Zhang cCMA-FDU Joint Laboratory of Marine Meteorology and Department of Atmospheric and Oceanic Sciences and Institute of Atmospheric Sciences, Fudan University, Shanghai, China

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Ruhua Zhang bKey Laboratory of Polar Atmosphere-Ocean-Ice System for Weather and Climate, Ministry of Education and Department of Atmospheric and Oceanic Sciences and Institute of Atmospheric Sciences, Fudan University, Shanghai, China

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Abstract

This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Niño–Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence position of WNP RI events experiences a significant westward shift of 5.5° in El Niño years and a significant northward shift of 4.5° in La Niña years. The strengthening of thermodynamic conditions west of 160°N plays a dominant role in the westward shift of RI events in El Niño years, and the northward shift in La Niña years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20°N, and weakening of dynamic conditions within 10°–20°N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140°E in El Niño years, whereas it is much stronger in La Niña years. In addition, the Hadley circulation over the WNP shows little change during El Niño, but the ascending branch around 10°N expands to 20°N during La Niña. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, during the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Niño (La Niña) years, which may also contribute to strengthening the thermodynamic conditions over the WNP.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

This article is included in the U.S. CLIVAR - Hurricanes and Climate Special Collection.

Corresponding author: Wen Zhou, wen_zhou@fudan.edu.cn

Abstract

This work explores the modulation of the Pacific decadal oscillation (PDO) on the relationship between the occurrence position of rapid intensification (RI) events of tropical cyclones (TCs) over the western North Pacific (WNP) in boreal autumn and El Niño–Southern Oscillation (ENSO). From the warm to cold phase of the PDO, the occurrence position of WNP RI events experiences a significant westward shift of 5.5° in El Niño years and a significant northward shift of 4.5° in La Niña years. The strengthening of thermodynamic conditions west of 160°N plays a dominant role in the westward shift of RI events in El Niño years, and the northward shift in La Niña years is associated with the expansion of areas with warm sea surface temperature, high tropical cyclone heat potential and midlevel relative humidity, strengthening of relative vorticity north of 20°N, and weakening of dynamic conditions within 10°–20°N. During the PDO cold phase, the descending branch of the Walker circulation over the western Pacific is weak and shifts west of 140°E in El Niño years, whereas it is much stronger in La Niña years. In addition, the Hadley circulation over the WNP shows little change during El Niño, but the ascending branch around 10°N expands to 20°N during La Niña. These trends reflect the changing responses of the WNP environment to ENSO variation and are consistent with the changing distribution of WNP RI events. Moreover, during the PDO cold phases, SST over the north Indian Ocean is much warmer, and anomalous anticyclonic circulation occurs in the WNP in boreal spring (summer and autumn) during the developing phase of El Niño (La Niña) years, which may also contribute to strengthening the thermodynamic conditions over the WNP.

© 2023 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

This article is included in the U.S. CLIVAR - Hurricanes and Climate Special Collection.

Corresponding author: Wen Zhou, wen_zhou@fudan.edu.cn

1. Introduction

Long-term variation of tropical cyclones (TCs) is a critical problem under global climate change. Recent studies (e.g., Sharmila and Walsh 2018; Wang and Toumi 2021; Lakshani and Zhou 2022) have shown that TCs are approaching closer to coastal regions. Moreover, the intensity and intensification rate of TCs also show a strengthening trend (e.g., Bhatia et al. 2019, 2022; Wang et al. 2020; Y. Li et al. 2022; Wang and Toumi 2022). All the above evidence indicates the growing threat of TCs and the increasing possibility of destructive disasters.

As the most active region for TCs, the western North Pacific (WNP) has also experienced a similar trend. In recent decades, the occurrence position of TC maximum intensity has shifted closer to the coastlines of East Asia (Park et al. 2014; Li et al. 2017; Basconcillo and Moon 2022), leading to an increase in landfall intensity and more landfalling TCs with extreme intensity (e.g., Park et al. 2011; Li et al. 2017; Liu and Chan 2020; Liu et al. 2020, 2021; Chen et al. 2022; G. Li et al. 2022). A northward migration of landfall position has also been found recently, increasing the possibility of damage in East China, the Korean Peninsula, and Japan (e.g., Liu et al. 2020; Xiao 2021; Chen et al. 2022; Yu et al. 2022).

Recently, variation in the rapid intensification (RI) of WNP TCs has received great attention (e.g., H. Zhao et al. 2018; Kang and Elsner 2019; Song et al. 2020). RI refers to a significant increase in TC intensity in a short time, generally referring to an increase in TC intensity of 30 kt within 24 h (Kaplan and DeMaria 2003; 1 kt ≈ 0.51 m s−1), and almost 80% of intense TCs undergo at least one RI process during their lifetime (Kaplan and DeMaria 2003; Lee et al. 2016). In recent decades, RI events over the WNP have also shown a coastward migration. Liu and Chan (2022) pointed out that RITCs (defined as TCs that experience at least one RI process during their lifetime) over the WNP have increased significantly, along with a northwestward shift in their occurrence position.

El Niño–Southern Oscillation (ENSO) is an important factor in annual and interannual variation and has proven essential in controlling the variation of WNP RI events. Guo and Tan (2018) found that short-duration El Niño follows a westward shift in the upper ocean heat content (OHC), causing weaker vertical wind shear (VWS) as well as higher midlevel humidity and tropical cyclone heat potential (TCHP), which results in a westward migration of RI occurrence position. Their subsequent work (Guo and Tan 2021) further indicated that WNP RI events in the peak TC season shift northward in La Niña years, while RI events in the late TC season shift westward in eastern Pacific (EP) El Niño and La Niña years. Shi et al. (2020) also reported that WNP RITCs during EP and central Pacific (CP) El Niño years vary in monthly distribution, occurrence position, and RITC ratio.

The above work shows that the occurrence position of WNP RI events varies annually and is associated with ENSO variation. Although the relationship between ENSO and WNP RI events has been well discussed, the temporal change in this relationship under global climate change remains challenging. Previous studies have shown that the Pacific decadal oscillation (PDO) plays an essential role in the long-term variation of WNP TCs. For example, the abrupt decrease and northwestward migration in WNP TCs in the late 1990s are highly associated with the shift of the PDO from its warm to cold phase (e.g., Hsu et al. 2014; He et al. 2015; Li and Zhou 2018; J. Zhao et al. 2018; Cao et al. 2020; Shan and Yu 2020; Tian et al. 2022). Moreover, the PDO has also proven to be important in modulating the relationship between ENSO and the genesis, landfall, and track of WNP TCs (Zhao and Wang 2019; Li et al. 2021; Huang et al. 2022). Wang and Liu (2016) have also examined how the PDO modulates the effects of ENSO on the frequency of WNP RITCs. However, whether the PDO modulates the response of RI occurrence position to ENSO variation has not been thoroughly investigated. This work aims to further explore the changing relationship between ENSO and WNP RI events.

Generally, May–November is the active season for WNP TCs. However, TCs show different characteristics in boreal summer and autumn. For example, Hsu et al. (2014) have pointed out that the abrupt decrease in WNP TC genesis in the late 1990s occurred mainly in the late TC season. Recent work (e.g., Wang et al. 2015; Ge et al. 2018; Shi et al. 2020) has also shown that WNP RI events are more active in boreal autumn [September to November (SON)] since both the number of RITCs and the proportion of RITCs (PRITC) during this season are much higher than in other seasons, indicating that TCs in boreal autumn are more likely to experience at least one RI event. The high PRITC during autumn also introduces more difficulties for TC forecasting. Therefore, analyzing autumn RI events is worthwhile, and our work focuses on the characteristics of WNP RI events during boreal autumn.

The rest of this paper is organized as follows. The data and methods used are introduced in section 2. Section 3 gives the features of WNP RI events in different PDO and ENSO phases. The differences in the WNP environmental conditions related to RI events in different PDO and ENSO phases are presented in section 4. Section 5 further analyzes possible mechanisms for the WNP environmental variation. A summary and discussion are provided in section 6.

2. Data and methods

TC best-track data during 1979–2020 are provided by the Joint Typhoon Warning Center (JTWC), extracted from the International Best Track Archive for Climate Stewardship (IBTrACS) dataset (Knapp and Kruk 2010; Knapp et al. 2010). TCs before 1979 are not considered because of uncertainty from the presatellite era. In this work, TC intensity is defined by the 1-min maximum sustained wind speed, and only TCs with the lifetime maximum intensity of tropical storms (≥34 kt) are considered. The definition of RI events in this study follows previous work (Wang and Zhou 2008; Shi et al. 2020): the increase in intensity should satisfy the following criteria: 1) 5 kt in TC intensity in the first 6 h, 2) 10 kt in TC intensity in the first 12 h, and 3) 30 kt in TC intensity in 24 h. Correspondingly, RITCs are defined as TCs that experience at least one RI event during their lifetime, and PRITC refers to the proportion of total TCs that are RITCs.

Monthly data, including relative humidity (RH), relative vorticity (RV), and wind, are obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) dataset (Hersbach et al. 2020), which has a horizontal resolution of 0.25° × 0.25°. The 850–200-hPa VWS is calculated as the magnitude of wind vector differences between 850 and 200 hPa. Monthly SST data from the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) dataset (Rayner et al. 2003), with a horizontal resolution of 1° × 1°, are also used for further analysis of environmental conditions.

Potential intensity (PI) is the function that reflects SST, temperature, and moisture. High PI is favorable for TC intensification (Wang et al. 2021). In this work, the monthly temperature (T), mean sea level pressure (SLP), and specific humidity (SH) from the ERA5 dataset and monthly SST data from HadISST are used to calculate PI following the methods of Bister and Emanuel (2002):
PI=TsToCkCD(CAPECAPEb),
where Ts is SST, To is the mean outflow temperature, Ck is the exchange coefficient for enthalpy, Ck is the drag coefficient, CAPE is the convective available potential energy (CAPE) of air lifted from saturation at sea level in reference to the environmental sounding, and CAPEb is the CAPE of the boundary layer air.
Monthly subsurface oceanic temperature is obtained from the EN4 dataset of the Met Office Hadley Centre observation datasets (Good et al. 2013), which are used to calculate tropical cyclone heat potential (TCHP). TCHP, defined as the ocean heat content in water warmer than 26°C (Leipper and Volgenau 1972), has been proven essential for RI events. This work calculates TCHP by
TCHP=cpρD260[T(z)26]dz,
where cp is the heat capacity of seawater at a constant pressure, which is taken as 4178 J kg−1 °C−1; ρ is the density of the 26°C isotherm; T(z) is the in situ temperature. The monthly subsurface oceanic temperature data are obtained from the EN4 dataset with a horizontal resolution of 0.5° × 0.5° and 47 vertical layers. The SST for the TCHP calculation is obtained from the HadISST dataset.

The Oceanic Niño Index (ONI) is defined as the 3-month running mean of SST anomalies in the Niño-3.4 region (5°N–5°S, 120°–170°W). The monthly ONI during 1950–2021 is obtained from the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration (NOAA) to represent the status of ENSO. Based on the SON mean ONI, the research period is separated into El Niño (ONI ≥ 0.5), neutral (−0.5 < ONI < 0.5), and La Niña (ONI ≤ −0.5) years. The PDO index is obtained from the Joint Institute for the Study of the Atmosphere and Ocean, University of Washington (UW/JISAO), and is defined as the leading principal component of North Pacific monthly SST variability. A 9-yr low-pass filter is used to determine the PDO phase, and one PDO warm phase (1979–97, hereafter P1) and one cold phase (1998–2020, hereafter P2) are separated. The classification of years during the ENSO and PDO phases is shown in Table 1.

Table 1.

Classification of years in different PDO and ENSO phases.

Table 1.

In this work, the anomalies of variables are calculated based on the climatological mean during the period of 1981–2010. A two-tailed Student’s t test is performed to analyze the statistical significance of composite differences and correlation analysis.

3. Different shift direction of WNP autumn RI events in El Niño and La Niña years from the PDO warm phase to the cold phase

We first analyze the long-term variation of the boreal autumn WNP RI occurrence position. Figure 1 displays the time series of the annual mean RI occurrence longitude and latitude, along with the state of ENSO. From P1 to P2, significant increases in latitude and decreases in longitude are shown in La Niña years and El Niño years, respectively. The mean RI occurrence position quantitatively experiences a 5.5° westward shift in El Niño years and a 4.5° northward shift in La Niña years, as Table 2 indicates. The change in RI occurrence position is associated with the position of TC genesis. As listed in Table 2, the genesis position of RITCs shows a change similar to that of the RI occurrence position.

Fig. 1.
Fig. 1.

Time series of frequency of mean RI occurrence (a) longitude and (b) latitude. The red and blue circles refer to El Niño and La Niña years, respectively. Gray shading refers to P1, and white refers to P2.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Table 2.

RI characteristics during different PDO and ENSO phases. The differences (P2 minus P1) in boldface are significant at the 90% confidence level, and an asterisk (*) indicates significance at the 95% confidence level.

Table 2.

Figure 2 gives a consistent result for the distribution of RI events. Principally, after 1998, both the mean occurrence position and the kernel density estimation (KDE) of RI events experience a significant westward shift in El Niño years and a northward shift in La Niña years. In comparison, the RI occurrence position in neutral years shows no significant change from the PDO warm phase to the cold phase (shown in Table 2). In addition, as RI events of La Niña years are located mainly in the western WNP, the northward shift of RI events in La Niña years results in a concentration of RI events in coastal regions like the South China Sea (SCS) and the Philippine Sea (PS), as shown in Fig. 2d. Figures 2c and 2f give the differences in RI distribution in El Niño and La Niña years from P1 to P2. In El Niño years RI events become more active in the west of 140°E and north of 10°N, whereas in La Niña years, RI frequency increases in the north of 20°N and decreases south of 20°N. In short, from P1 to P2, WNP RI events experience a significant westward shift in El Niño years and a northward shift in La Niña years, with little position change in neutral years.

Fig. 2.
Fig. 2.

The occurrence position of RI events during (a),(d) P1 and (b),(e) P2. The dots refer to the RI occurrence position, and shading refers to the kernel density estimation (KDE) of the RI occurrence position. (c),(f) The differences in RI frequency between P1 and P2 (P2 minus P1) in each 5° × 5° grid. Also shown are the differences in RI frequency between El Niño years and La Niña years (El Niño minus La Niña) during (g) P1 and (h) P2. (a)–(c) El Niño years and (d)–(f) the La Niña years.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Previous results have pointed out the significant increase in PRITC from P1 to P2, which is mainly due to the frequency of WNP TCs suddenly decreasing and the frequency of WNP RITCs remaining steady (H. Zhao et al. 2018). Table 2 further compares this change in PRITC in different ENSO phases, showing that the change in PRITC during SON occurs mainly in El Niño years, since PRITC in El Niño years increases from 0.49 to 0.68, while the change in La Niña and neutral years is somewhat weaker (from 0.47 to 0.50 and from 0.49 to 0.57, respectively).

Moreover, the RI occurrence latitude in El Niño and La Niña years shows differences in the PDO warm and cold phases. During P1, the mean RI occurrence position in El Niño years is to the south of that in La Niña years. This difference is the opposite during P2. Figures 2g and 2h further show that the differences occur mainly in the area 10°–25°N, 125°–145°E. RI events in El Niño years in this area are inactive in P1 but active in P2, which causes the opposite difference in RI occurrence latitude in El Niño and La Niña years. Quantitatively, in P1, RI events are 2.1° more southerly in El Niño years than in La Niña years, whereas in P2, RI events in El Niño years are 2.7° more northerly (as listed in Table 3). During both P1 and P2, TC genesis and RI occurrence position show significant westward migrations of about 10° in La Niña years compared to El Niño years, which is consistent with previous findings (e.g., Guo and Tan 2021).

Table 3.

Difference in RI characteristics between El Niño and La Niña years (El Niño minus La Niña) during P1 and P2. The differences in boldface are significant at the 90% confidence level, and an asterisk (*) indicates significance at the 95% confidence level.

Table 3.

4. The change in WNP environmental conditions for RI events during different PDO and ENSO phases

The variation of five large-scale environmental factors is investigated, including SST, TCHP, 600-hPa RH, 850-hPa RV, and 850–200-hPa VWS. As reported in previous work (e.g., Wang and Zhou 2008; Shu et al. 2012; Kieu et al. 2014; Wang et al. 2017; Fudeyasu et al. 2018; Guo and Tan 2021; Shimada 2022), these factors are essential for the development of RI events. Warm SST and upper ocean heat content can provide sufficient heat energy to fuel TC intensification. Moisture from a high-RH environment is necessary for the development of TCs. Strong low-level RV is usually associated with strong low-level inflow, which is essential for a steady TC structure and TC development. Weak VWS has also proven to be related to the maintenance of TCs. In this section, PI is also selected to further verify the contribution of thermodynamic conditions. As shown in section 3, the changes in RI occurrence latitude and longitude vary in El Niño and La Niña years. The correlation between different variables and the annual mean occurrence position of RI events during the PDO warm phase and cold phase is first investigated in Figs. 3 and 4.

Fig. 3.
Fig. 3.

Correlation maps between the time series of mean autumn RI occurrence latitude and (a),(b) SST, (c),(d) TCHP, (e),(f) 600-hPa RH, (g),(h) PI, (i),(j) 850-hPa RV, and (k),(l) 850–200-hPa VWS. Columns show (first column),(third column) correlation maps during P1 and (second column),(fourth column) those during P2. The black solid and dashed lines indicate correlation coefficients of 0.4 and −0.4. The dots refer to values that are statistically significant at the 95% confidence level.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 4.
Fig. 4.

As in Fig. 3, but for RI occurrence longitude.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Figure 3 gives the correlation of RI occurrence latitude. For all variables, the distribution of the correlation maps of latitude is the opposite in P1 and P2. The correlation change is stronger for SST, 600-hPa RH, and PI. In P1, RI occurrence latitude has a positive correlation with SST over the southeastern WNP (0°–20°N, 160°E–160°W) and a strong negative correlation with SST over the northeastern WNP (20°–35°N, 160°E–160°W). But this is reversed in P2, as a significant negative correlation occurs in the southeastern WNP and a significant positive correlation in the northeastern WNP. The change in the correlation between occurrence latitude and RH occurs mainly over the latitudinal band of 10°–30°N, from a strong negative correlation to a strong positive correlation over a wider area. PI shows a distribution of correlation maps similar to that of SST. The correlation between latitude and PI is not significant in P1 but is significantly positive over the PS and northeastern WNP. According to Fig. 2, the region 10°–20°N, 120°–160°E is the main region (MR) for RI events. In this region, the correlation between occurrence latitude and thermodynamic factors (SST, TCHP, RH, and PI) is negative in P1 but positive in P2. This difference indicates that sufficient heat energy in the MR is accompanied by a more southerly distribution of RI events in P1, and the opposite in P2. In addition, the correlation between occurrence latitude and VWS and RV is less significant in both phases, which might mean that the latitude change in La Niña years and the difference in occurrence latitude between El Niño and La Niña years are more related to thermodynamic conditions than dynamic conditions.

The correlation maps between factors and RI occurrence longitude are shown in Fig. 4. The correlation maps show similar patterns in P1 and P2, but an expansion of the area with significant and strong correlations for all variables occurs in P2. The occurrence longitude has a significant negative correlation with SST, TCHP, RH, and PI in the southwestern WNP (0°–10°N 100°–160°E) in P1. In P2, the area with a strong negative correlation expands northward. Moreover, their positive correlation over the southeastern WNP also becomes stronger from P1 to P2. The correlations in the MR show that sufficient heat energy is accompanied by the westward shift of RI events. The correlation between occurrence longitude and dynamic factors (VWS and RV) is also strengthened from P1 to P2 over the whole WNP. The above changes in the correlation maps indicate that RI occurrence longitude has a closer relationship with WNP environmental conditions in P2 than in P1.

To explore the change in WNP environments and how the environmental change affects RI distribution, Figs. 510 show the composite anomalies of six variables in different ENSO and PDO phases. In El Niño years during P1, the anomalies of SST, TCHP, and RH are small in the MR, and PI shows basinwide significant negative anomalies. The insufficient thermodynamic conditions suppress RI events, resulting in a low PRITC in El Niño years during P1. In La Niña years during P1, although SST, RH, and PI anomalies are negative north of 15°N, the positive TCHP anomalies south of 15°N and the positive RH anomalies over 10°–20°N still provide sufficient heat energy and moisture for RI development. Moreover, the favorable dynamic conditions, including weak VWS and strong RV, also enhance RI events in the south of 15°N. TCHP, RH, VWS, and RV result in more RITCs in La Niña years than in El Niño years during P1. The distribution of these variables provides an explanation for the more southerly distribution of RI events in La Niña years and the negative correlation between thermodynamic conditions and RI occurrence latitude during P1 in Fig. 3.

Fig. 5.
Fig. 5.

Composite anomalies of SST (unit: °C) during (top) P1 and (bottom) P2 for(left) El Niño and (right) La Niña years. The dots refer to values that are statistically significant at the 95% confidence level.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 6.
Fig. 6.

As in Fig. 5, but for TCHP (unit: 10−7 J m−2).

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 7.
Fig. 7.

As in Fig. 5, but for 600-hPa RH (unit: %).

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 8.
Fig. 8.

As in Fig. 5, but for PI (unit: m s−1).

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 9.
Fig. 9.

As in Fig. 5, but for 850–200-hPa VWS (unit: m s−1)

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 10.
Fig. 10.

As in Fig. 5, but for 850-hPa RV (unit: s−1)

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

From P1 to P2, the strengthening of thermodynamic conditions occurs in both El Niño and La Niña years. In El Niño years, SST and PI show significant positive anomalies throughout the whole WNP. TCHP and RH are also strengthened in most regions over the WNP. The strengthening of thermodynamic conditions enhances the development of RI events and leads to an increase in the frequency of RITCs. In addition, the changes in thermodynamic conditions are more noticeable west of 160°E and cause a concentration of RI events in the west of 160°E and the westward shift of RI events in El Niño years from P1 to P2. VWS and RV also show variation, but their anomalies are small in the MR and may have little impact on RI events. In La Niña years during P2, thermodynamic conditions are similar to those in El Niño years, including a change from negative to positive SST and RH anomalies in the north of 20°N, the expansion of positive TCHP anomalies to the south of 20°N, and basinwide positive anomalies of PI. That strengthening is more noticeable in the north of 10°N and results in the northward shift of RI events in La Niña years. Moreover, significant positive RV anomalies occur north of 20°N, which also enhances RI in the north. Strong VWS and weak RV are found within 10°–20°N, which suppress RI events and cause a decrease in RI frequency in the MR. Overall, the strengthening of thermodynamic conditions and RV in the north and the weakening of dynamic conditions in the south result in an increase in RI events north of 20°N and a decrease in RI events in the MR, which is shown as a northward shift of RI occurrence position.

From P1 to P2, environmental conditions show similar variation in El Niño and La Niña years, but the RI occurrence latitude shows differences, which may lead to the opposite pattern of the correlation maps between RI occurrence latitude and environmental variables in Fig. 3. Moreover, the change in longitude and the composite anomalies of WNP environment fields are also consistent with the results in Fig. 4; the strong thermodynamic conditions west of 160°E are related to a westerly distribution of RI events. The strengthening of SST, TCHP, RH, and PI in P2 also shows consistency with the changes in correlation maps from P1 to P2.

5. Possible mechanisms for the variation in the WNP environment

The above results indicate that the WNP environment varies in different PDO and ENSO phases, which influences the distribution of RI events. To explore the possible mechanisms of change in the WNP environment, we first investigate the associated large-scale circulations. From P1 to P2, both the Walker circulation and the Hadley circulation show consistent change with the change in the distribution of WNP RI events and TC genesis (Figs. 11 and 12). In El Niño years, the Walker circulation around the equator (5°S–5°N) is stronger during P1 than during P2. In P1, strong ascending motion is found at the date line and the descending branch is in the western tropical Pacific. Furthermore, the strong descending branch shifts west of 140°E from P1 to P2, leading to a westward shift of regions with deep convection over the WNP, which results in the westward shift of RI events in El Niño years during P2. In contrast, in La Niña years, the Walker circulation shows a similar pattern in P1 and P2, including an ascending branch to the west and a descending branch to the east of the date line. This similarity leads to little change in the mean position of TC genesis and RI occurrence. Additionally, the Walker circulation is strengthened during P2, making the RI occurrence distribution more concentrated (as shown in Fig. 2d).

Fig. 11.
Fig. 11.

Anomalies of the regional Walker circulation (vectors) averaged over 5°S–5°N, and pressure vertical vorticity (shading; unit: 10−2 P s−1) during (top) P1 and (bottom) P2 for (a),(c) El Niño years and (b),(d) La Niña years. The red and blue curves in the line plots refer to the KDE of RI occurrence and RITC genesis longitude, respectively. The thick black vectors refer to winds that are statistically significant at the 95% confidence level. The contour lines indicate vertical vorticity that is statistically significant at the 95% confidence level.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 12.
Fig. 12.

As in Fig. 11, but for the regional Hadley circulation averaged over 100°E–180° and latitude of RI occurrence and RITC genesis.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Similarly, the change in the Hadley circulation within 100°E–180° indicates a shift in RI occurrence latitude. Figure 12a shows little change in the Hadley circulation in El Niño years. However, the Hadley circulation in La Niña years experiences a significant change. During P1, an ascending branch exists around 10°N, and a descending branch exists around 20°N. Then in P2, the descending branch around 20°N shifts to an ascending branch and results in an abnormal upwelling motion, which is associated with the increase in midlevel RH at 20°N and enhances TC genesis and RI in the midlatitudes. This change in the Hadley circulation provides a further explanation for the northward shift of RI events in La Niña years. Composite anomalies of 200-hPa velocity potential and divergent wind in Fig. 13 give a similar result that confirms the variations in the Walker circulation and Hadley circulation from P1 to P2.

Fig. 13.
Fig. 13.

Composite anomalies of 200-hPa velocity potential (shading; unit: m2 s−1) and divergent wind (vectors; unit: m s−1) during (top) P1 and (bottom) P2 for (a),(c) El Niño years and (b),(d) La Niña years. The thick black vectors refer to winds that are statistically significant at the 95% confidence level. The contour lines indicate vertical vorticity that is statistically significant at the 95% confidence level.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Figures 14 and 15 present the composite fields of SST and 850-hPa wind anomalies from boreal winter (DJF) to boreal autumn (SON) during the developing phase of El Niño and La Niña events. In El Niño years, during P1, a negative SSTA pattern occurs in the Indian Ocean in DJF. In the following seasons, positive SST occurs in the south Indian Ocean (SIO) and expands to the north Indian Ocean (NIO). However, in P2, a basinwide positive SSTA appears over the Indian Ocean and then develops into a significant warm SSTA in the NIO in SON, which may trigger warming of the WNP via a warm Kelvin wave. Additionally, in P1, an anomalous anticyclonic circulation is located in northeast Asia in MAM and JJA, which causes abnormal northwesterly wind from the continent to bring cold and dry air to the WNP. In P2, the anomalous anticyclonic circulation near Japan in MAM triggers warming over the WNP and causes SST anomalies to remain positive in the following JJA and SON.

Fig. 14.
Fig. 14.

Composite fields of SST anomalies (shading; unit: °C) and 850-hPa wind anomalies (vectors; unit: m s−1) from D(0)JF(1) to SON(1) during El Niño developing years for (left) P1 and (right) P2. The thick black vectors refer to winds that are statistically significant at the 95% confidence level. The contour lines indicate vertical vorticity that is statistically significant at the 95% confidence level.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Fig. 15.
Fig. 15.

As in Fig. 14, but for La Niña years.

Citation: Journal of Climate 36, 22; 10.1175/JCLI-D-23-0084.1

Similar to the SSTA evolution in El Niño years, during the developing phase of La Niña in the PDO warm phase, only a weak positive SSTA appears in the SIO. Conversely, during P2, the SSTA over the Indian Ocean is warm at the basin scale, and a significant warm SSTA occurs in the NIO. In addition, an anomalous cyclonic circulation occurs near Japan in SON during P1, causing a cooling of SST, which suppresses RI. The northeasterly wind in the North Pacific in JJA and northerly wind near Japan in SON also transport cold seawater from high latitudes to the WNP and cause a decrease in heat energy. In contrast, during P2, an anomalous anticyclonic circulation exists in the North Pacific in MAM; it then moves southwestward and reaches the WNP in SON. The anomalous anticyclonic circulation provides an accumulation of heat energy over the WNP that enhances RI. It also causes an anomalous easterly wind around 20°N, which favors TC genesis and intensification. Figure 12 shows that a sinking motion shifted to an upward motion within 40°–50°N from P1 to P2, which is consistent with the shift from anomalously cyclonic in P1 to anticyclonic in P2 in SON. Moreover, an anomalous westerly wind is located near the equator, which suppresses the TC activities over the low-latitude area and is linked to the northward shift of RI events.

6. Summary and discussion

Previous work has pointed out that WNP TC activity experienced a significant decadal variation when the PDO shifted from the warm phase to the cold phase around 1998; for example, an abrupt decrease and northwestward shift of genesis position (e.g., Hsu et al. 2014; He et al. 2015; Li and Zhou 2018; J. Zhao et al. 2018; Tian et al. 2022). In addition, associated with this PDO phase shift, the relationship between ENSO and WNP TC activities, such as movement and landfall, also changed (Zhao and Wang 2019; Li et al. 2021; Huang et al. 2022). Following previous studies, this work further examines the relationship between WNP RI occurrence position and ENSO during boreal autumn in different PDO phases (P1: 1979–97; P2: 1998–2020). The migration of RI occurrence position shows a difference in El Niño and La Niña years (Fig. 2). In brief, from P1 to P2, a westward shift of 5.5° occurs in El Niño years, and a northward shift of 4.5° occurs in La Niña years, whereas the change in ENSO neutral years is less noticeable. The genesis position of RITCs also shows northward and westward shifts, similar to the changes in RI events. In different PDO phases, RI occurrence also varies in El Niño and La Niña years. Compared with El Niño years, RI events in La Niña years are more southerly in P1 but more northerly in P2. In both P1 and P2, RI events in La Niña years are more westerly than in El Niño years.

The correlation between WNP environmental factors and RI occurrence position also varies in P1 and P2. The correlation maps of RI occurrence longitude indicate a stronger relationship between the WNP environment and RI longitude in P2 than in P1 since the area with significant correlation expands. Conversely, correlation maps of RI occurrence latitude show opposite patterns in P1 and P2. Composite anomalies of environmental conditions show that the shifts in RI position are triggered mainly by changes in thermodynamic conditions. In El Niño years, the increase in SST, TCHP, and midlevel RH are more significant west of 160°E, which enhances RI and causes the westward migration from P1 to P2. The expansion of warm SST is accompanied by a change in the correlation maps of RI occurrence longitude. In La Niña years, warm areas with high SST, RH, and TCHP expand northward, concentrating heat energy over the northwestern WNP. Noticeable strong RV occurs in the north of 20°N, leading to the strengthening of dynamic conditions. Conversely, strong VWS and weak RV occur within 10°–20°N, which suppresses RI events. The above changes result in a northward shift of RI events in La Niña years from P1 to P2. This migration also results in the changing difference in RI occurrence latitude between El Niño and La Niña during P1 and P2, which shows consistency with the opposite correlation maps of RI occurrence latitude in P1 and P2.

From P1 to P2, the descending branch of the Walker circulation over the western Pacific shifts westward in El Niño years, and the ascending branch of the Hadley circulation in La Niña years expands to 20°–30°N. The above changes are consistent with the shift in the WNP RI occurrence position. Additionally, in P2, a warming trend of SSTA over the NIO occurs in the developing phases of both El Niño and La Niña years, which may trigger the strengthening of thermodynamic conditions over the WNP via a Kelvin wave. The PDO cold phase also triggers an anomalous anticyclonic circulation near Japan in MAM (JJA and SON) in El Niño (La Niña) years, which contributes to the accumulation of heat energy over the WNP and enhances RI.

To minimize the influence of data quality in the presatellite era (before 1979), the research period of this work is 1979–2020, which includes only one PDO warm phase (1979–97) and one PDO cold phase (1998–2020). Therefore, it is difficult to distinguish whether the position shifts of RI events in El Niño and La Niña years are controlled by the PDO phase change, global warming, or their combined effects. Our work shows that thermodynamic conditions are essential for the shift in RI occurrence position. Previous works (e.g., Hu et al. 2018) have discussed the different distribution of OHC over the WNP during different PDO phases. The increasing trend of WNP OHC is also reported to be consistent with the trend of global warming in some research (e.g., Moon and Song 2013; Song et al. 2020). Therefore, we hypothesize that both the PDO phase shift and global warming play important roles in the changing response of WNP RI occurrence position to ENSO. More reliable observational data and numerical simulations are required to further determine their influence in detail.

Recent studies have indicated that the activities of WNP TCs are modulated by other oceans like the Indian, Atlantic, and Antarctic Oceans (e.g., Li and Zhou 2014; Gao et al. 2018; Wang and Chen 2018; Zhang et al. 2018; Zhan et al. 2019; Gao et al. 2020; Wu et al. 2020; Zhao et al. 2020; Song et al. 2022). Results in this study also show that the warming trend of SST over the NIO and North Atlantic is consistent with the position shift of WNP RI events during the PDO cold phase (Figs. 14 and 15). These changes in SST may influence WNP environmental conditions through large-scale oceanic and atmospheric circulations and further modulate the activities of WNP RI events. The processes and inner mechanisms of how other oceans affect WNP RI events are of great importance and will be addressed in our future work.

Acknowledgments.

Research funding for this project was provided by the National Natural Science Foundation of China (Grants 42192563, 42288101, and 42120104001) and the Hong Kong RGC General Research Fund (11300920).

Data availability statement.

We are grateful to the institutions that provided data for this study. TC best-track data were provided by IBTrACS data from https://www.ncei.noaa.gov/products/international-best-track-archive. Monthly atmospheric data were provided by ERA5 (https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5). Monthly sea surface temperature and subsurface oceanic temperature were obtained from the HadISST dataset and the EN4 dataset from the Met Office Hadley Centre (https://www.metoffice.gov.uk/hadobs/en4/download-en4-2-2.html). The monthly ONI index was provided by NOAA CPC (https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php). The monthly PDO index was from UW/JISAO (http://research.jisao.washington.edu/pdo/PDO.latest).

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