Role of the intraseasonal IPCO in the absence of typhoons in July 2020

The in�uence of the intraseasonal Indo-western Paci�c convection oscillation (IPCO) on the absence of typhoons in July 2020 over the western North Paci�c (WNP) was explored. Our observations analysis revealed an unprecedented absence of typhoons over the WNP in July 2020 even though the necessary conditions such as sea surface temperature (SST) and vertical wind shear met the basic requirements of typhoon formation since it began occurring from 1949. Additionally, signi�cant differences were found in the frequency of typhoons between the different phases of the intraseasonal IPCO; the frequency in the positive phase of the intraseasonal IPCO was signi�cantly higher than that in the negative phase of the intraseasonal IPCO. In July 2020, the intraseasonal IPCO was in a strong negative phase, with the third lowest index in history and had the strongest inhibition effect on convection over the WNP on record, leading to large-scale circulation anomalies. The strongest descending movement on record inhibited the upward transport of water vapor and the development of cumulus convection, thereby reducing the release of latent heat of condensation and making it di�cult to form a typhoon warm-core structure. In addition, the geopotential height increased over the WNP, and the western Paci�c subtropical high moved southerly, which inhibited typhoon formation. Simultaneously, the South China Sea monsoon trough weakened signi�cantly, with increased negative vorticity anomaly in the response scale, which hindered disturbance generation. The lowest genesis potential index con�rmed that the large-scale circulation anomaly caused by the intraseasonal IPCO had an unprecedented restraining effect on typhoon generation, leading to the absence of typhoons over the WNP in July 2020.


Introduction
Tropical cyclones (TCs) are cyclonic eddies with warm central structures that occur in tropical and subtropical oceans (Zhu et al. 2007a). TCs that occur over the western North Paci c (WNP) and have a maximum wind speed greater than 17.2 m/s in the center are called typhoons, and these cause serious societal and economic impacts (Li et al. 2016a). TCs are accompanied by strong winds and torrential rain, bringing serious disasters to the affected areas. According to the statistics, typhoons kill approximately 453 people in the coastal areas of China annually, resulting in a direct economic loss of more than 26 billion yuan (Niu et al. 2011). However, TCs are also a valuable potential resource that bring abundant fresh water to mankind. In South and Southeast Asia, the precipitation brought by typhoons accounts for most or even all of the local annual rainfall (Elsberry and Tsai 2016). In addition, TCs also play an important role in driving ocean thermohaline circulation, affecting regional and global climate change (Emanuel 2001;Done et al. 2009). Therefore, TC is one of the focuses of weather and climate research, and both too many and too few typhoons have important research signi cance.
The WNP is one of the main sources of TCs, producing approximately 36% of TCs (Li et al. 2016b), and July, the peak season for TC generation, has approximately four TCs over the WNP every year. However, no typhoon was generated in July 2020 over the WNP for the rst time since 1949 as per records. The four well-known necessary conditions for the formation of typhoons are: warm and broad ocean surface (sea surface temperature above 26.5°C), initial disturbance at low level, su cient Coriolis force, and weak vertical wind shear. The results in section 3 of this study show that these four necessary conditions meet the requirements of typhoon generation, suggesting that there may be other complex and special drivers behind the absence of typhoons in July 2020. Wang et al. (2021) used the dynamic genesis potential index to show the impacts of large-scale circulation conditions on typhoon genesis, and their numerical experiments showed that warming of the Indian Ocean could trigger abnormal anticyclone over the WNP, leading to the absence of typhoons.
Convection also plays an important role in TC formation. Previous studies on the convective characteristics over the tropical Indo-western Paci c using outgoing longwave radiation (OLR) data showed an out-of-phase relationship in convection variation between the Indian Ocean and the WNP, and the location and phase of convection activities varied in different seasons (Lau andChan 1985, 1986; Zhu and Wang 1993; Lee et al. 2012).  found that this convective dipole phenomenon also has important signals on interannual timescales, but the centers of convective actions remain largely the same and de ne the out-of-phase convection anomalies over the Indo-western Paci c as the Indo-western Paci c convection oscillation (IPCO). Zhang et al. (2015) found that the IPCO has signi cant intraseasonal variations, and the average locations of the convection centers of the intraseasonal IPCO change slightly and lie over the eastern equatorial Indian Ocean (EEIO) (5°S-10°N, 70°-100°E) and the WNP (5°-20°N, 110°-160°E), respectively. Wang et al. (2018Wang et al. ( , 2019 explored the modulation effect of the intraseasonal IPCO on TC genesis location, frequency, and path over the Indo-western Paci c. They found that there tend to be more TCs over the WNP when the intraseasonal IPCO was in its positive phase, whereas the TC genesis frequency is lower in its negative phase. In addition, Wang et al. (2018) explored the possible physical mechanism by which the intraseasonal IPCO affects large-scale circulation and its impact on typhoon generation. In July 2020, the IPCO turns to its strong negative phase. Does the intraseasonal IPCO play an important role in the absence of typhoon in July 2020? What is the physical mechanism of the intraseasonal IPCO inhibiting typhoon formation? These questions are the motivation for the present study.
The remainder of this paper is organized as follows: Section 2 brie y reviews the data and methodologies used in the analyses. Section 3 explores conditions necessary for typhoon genesis over the WNP in July 2020. Sections 4 and 5 show the in uence of the intraseasonal IPCO on typhoon genesis and circulation and the absence of typhoons in July 2020, respectively. Section 6 presents a summary of the key results and discussion.

Data
Daily datasets analyzed from 1979 to 2020, consisted of reanalysis data (Kalnay et al. 1996) and interpolated OLR data (Liebmann and Smith 1996) from the National Centers for Environment Prediction (NCEP)-National Centers for Atmospheric Research (NCAR), including geopotential height, relative humidity, vertical velocity, sea surface temperature, wind eld, and interpolated OLR, with a horizontal spatial resolution of 2.5°.
The best-track data for typhoon activity over the WNP during 1979-2020 were obtained from the Regional Specialized Meteorological Center (RSMC) of the Japan Meteorological Agency (JMA). Only TCs that reached tropical storm intensity (maximum sustained 10 m wind speed ≥ 17.2 m s − 1 ) were used in our analysis.

Statistical methods
To study the in uence of the intraseasonal IPCO, anomalies of the aforementioned meteorological elements, except for typhoon data and sea surface temperature, were applied to a 30-60-day Lanczos bandpass lter using 305 weights, and the response function is shown in Fig. 1. It can be seen that the signals beyond 30-60 days are basically removed.
Composite and correlation analyses are performed to explore the possible physical mechanisms. The statistical signi cance of the correlation between two autocorrelated time series is assessed via the two- where N is the sample size, and ρ XX (j) andρ YY (j) are the autocorrelations of the two sampled time series Xand Y at time lagj, respectively.

Genesis potential index
To further con rm our results, 30-60-day Lanczos bandpass-ltered genesis potential index (GPI) anomalies are employed in this study. Developed by Emanuel  where V shear is the magnitude of the vertical wind shear between 850 and 200 hPa (m s − 1 ), rhum is the relative humidity (%) at 600 hPa, PI is the potential intensity (m s − 1 ) and ζ a is the absolute vorticity (s − 1 ) at 850 hPa. The de nition of PI is de ned by Bister and Emanuel (1998), based on Emanuel (1995): where C k and C D denote the surface enthalpy and momentum exchange coe cients, respectively, T s and T o are the sea surface temperature and out ow temperature, respectively, h * o is the saturation moist static energy of the sea surface, and h * is the saturated moist static energy of the free atmosphere.

Necessary Conditions For Typhoon Genesis Over The Wnp In July 2020
Before investigating the possible impact of the intraseasonal IPCO on typhoons, necessary conditions of typhoon genesis are explored to con rm whether the absence of typhoon genesis is caused by the failure of meeting the basic requirement of these conditions. The sea surface temperature (SST) is the fundamental thermal condition and should be greater than 26.5°C for typhoon generation. The warm sea surface is the basis of the formation of a warm-core structure, since it contains a large amount of heat, and strong evaporation. Through the turbulence transport between the air and sea, a large amount of warm and wet air is obtained from the lower atmosphere where the disturbance is located, making the atmospheric strati cation conditionally unstable. Figures 2a and 3a show the spatial patterns of SST anomalies over the WNP in July 2020 and the time series of the area-averaged SST from 1979 to 2020, respectively. It is seen that the arealaveraged SST over the WNP in July 2020 is abnormally high exceeding 30°C, the highest since 1979. In addition, Fig. 2c shows the generation location of typhoons and tropical disturbances in July, which are basically generated between 5°N and 25°N. It is clear that the SST over the WNP in July 2020 is abnormally warm, providing abundant water vapor and heat supply, conducive to the formation and development of typhoons.
In addition to warm SST, weak vertical wind shear is also crucial for the formation of typhoon. When the vertical wind shear is small, the condensation latent heat produced by the cumulus is concentrated in a ( ) ( ) ( ) | | ( ) limited space, favoring formation of the warm-core structure of the typhoon. In contrast, the latent heat of condensation will be rapidly transported from the initial disturbance if the vertical wind shear is extremely large, and a large area becomes slightly warm with low pressure, which is unfavorable for the formation of typhoon. Figures 2b and 3b show the spatial patterns of the vertical wind shear anomaly over the WNP in July 2020 and the time series of the areal-averaged vertical wind shear from 1979 to 2020, respectively.
In terms of spatial distribution (Fig. 2b), the vertical wind shear south of 15°N is signi cantly smaller than the climatological normal, and the wind shear north of 15°N is slightly larger. The areal-averaged results ( Fig. 3b) show that the vertical wind shear in July 2020 is generally lower than the climatological normal, which means that the condensation latent heat is easier to maintain in a small area and is thus favorable to the formation of the warm-core structure of typhoons.
In addition, tropical disturbances can release the unstable energy of the unstable atmosphere and transform it into the kinetic energy of TC development. The number of tropical disturbances is listed in Table 1  The frequency of typhoon genesis over the WNP changed during the different phases of the intraseasonal IPCO in July. Table 2 shows the composite number of typhoons over the WNP in July during the positive and negative intraseasonal IPCO phases from 1979 to 2020 and their differences.
There are signi cant differences in typhoon generation frequency in the different intraseasonal IPCO phases, and tends to be more typhoons over the WNP during the positive phase of the intraseasonal IPCO (about 1.7 times more than in the negative phases), consistent with the results reported by Wang et al.
(2018) on boreal extended summer. The IPCOI could measure the difference in convective intensity between the EEIO and WNP characterizing the phase and strength of the intraseasonal IPCO. In the positive phase of the intraseasonal IPCO, convection over the WNP is strengthened and that over the EEIO is inhibited, while in the negative phase of the intraseasonal IPCO, convection over the WNP is weakened and that over the EEIO is strengthened. Figure 4shows the time series of IPCOI and area-averaged OLR over the WNP (5-20°N, 110-160°E) from 1979 to 2020. As can be seen, the intraseasonal IPCO in July 2020 is the one of the strongest negative phases in history, and further indicated by the area-averaged OLR over the WNP was also experiencing the weakest convective intensity on record, which may be closely related to the absence of typhoon.   (5°N to 25°N), while 20°N to 25°N is basically in the transition region between the northern and southern parts of the dipoles. In the positive phase of the intraseasonal IPCO, the upper-level divergence, low-level convergence, and middle-level ascending motion are enhanced in the range of 5°N-20°N when the large-scale convection over the WNP is strengthened. Such large-scale circulation anomalies are favorable to the lifting and development of low-level disturbances, and can also enhance the upward movement of water vapor and the development of local cumulus convection. The above atmospheric processes are opposite in the negative phase of the intraseasonal IPCO. Figure 5d illustrates the composite difference between the positive and negative phases of the relative humidity anomalies at the 600 hPa level. To some extent, relative humidity represents the available latent heat of condensation, further representing the intensity of cumulus convection (Wang et al. 2018). Owing to the strengthening of convection and upward water vapor transport, in the intraseasonal IPCO-positive phase, a larger amount of condensation latent heat will be released by the cumulus convection, contributing to the development of the warm-core structure of the typhoon, than in the negative phase (Fig. 5d).
In addition, in the positive phase of the intraseasonal IPCO, owing to the strengthening of convection (south of 25°N), troposphere heating is dominated by condensation latent heat release. According to the potential tendency equation, as nonadiabatic heating increases with height, the geopotential height decreases (Zhu et al. 2007a). As shown in Fig. 5e, the low-value center of the geopotential height is located in the South China Sea, whereas the high-value center is in the Sea of Japan during the positive phase of the intraseasonal IPCO, which may be favorable to the northward uplift of the western Paci c subtropical high (WPSH) and the development of tropical disturbance over the WNP. In addition, Fig. 5f shows that the distribution of the vertical wind shear anomaly dipole is 5 latitudes south of the other atmospheric circulation anomalies as a whole, with the high-value center located in the south of the South China Sea. In the positive phase of the intraseasonal IPCO, the vertical wind shear is abnormally high south of 15°N, which is not favorable for the maintenance of condensation latent heat and the formation of a typhoon warm heart structure, whereas it is the opposite north of 15°N.
Moreover, tropical disturbance is the embryonic state of TC, which can convert the unstable energy of an unstable atmosphere into kinetic energy of TC development. The intertropical convergence zone is the most concentrated area of heat and water vapor in the tropics and also the main source of tropical disturbances. The monsoon trough is a type of intertropical convergence zone, and more than 80% of tropical disturbances generate from the intertropical convergence zone in the western Paci c and South China Sea. . The GPI is mainly in uenced by four factors: low-level vorticity, middle-level relative humidity, vertical wind shear, and potential intensity. Table 3 shows the correlation coe cients between IPCOI and GPI and its four elements, and IPCOI is highly correlated with both, indicating that GPI can be used to characterize the in uence of environmental conditions caused by the IPCO on typhoon generation, in accordance with a previous study (Wang et al. 2018). Figure 7 shows the composite difference between the positive and negative IPCO phases in the GPI in July. In the key area of the WNP part of the intraseasonal IPCO, positive values indicate that more typhoons tend to be generated in the intraseasonal IPCO-positive phase than in the negative phase.
In general, the intraseasonal IPCO could affect large-scale circulation anomalies over the WNP and further modulate the generation and development of tropical disturbances and typhoons. Except for vertical wind shear, circulation anomalies have a similar effect on typhoon formation. Circulation anomalies in the positive phase of the intraseasonal IPCO are favorable for typhoon formation, but not in the negative phase. Compared with the positive phase, the circulation anomalies in the negative phase of the intraseasonal IPCO were unfavorable for the generation of TCs over the WNP. In addition, the intraseasonal IPCO in July 2020 is a strong negative phase, and the inhibition on convection over its WNP is unprecedentedly strong.
To explain the role of the intraseasonal IPCO in the absence of typhoons over the WNP, three possible in uencing pathways are proposed: the in uence on cumulus convection, WPSH, and SCSMT. Figure 8 shows the anomalies of the aforementioned large-scale atmospheric circulation in July 2020. Figure 8a shows an anomalous anticyclone in the lower troposphere over the WNP, with its center over the South China Sea and the eastern part of the Philippines, and an abnormal cyclone located along the southeast coast of China. The low-level divergence eld corresponds well to the position of the wind eld, the divergence is strengthened over the South China Sea and the eastern part of the Philippines, and the convergence is strengthened along the southeast coast of China, presenting a north-south dipole distribution. The position of the strongest subsidence movement is consistent with the low-level divergence center and the center of the anomalous anticyclone, whereas the center of the strengthening upward movement is located east of the convergence center (Fig. 8b). Such circulations are not conducive to the lifting and development of low-level disturbances, as the air mass must undergo a considerable amount of forced uplift before reaching the height of free convection and then obtain unstable energy from the atmosphere to continue development. In contrast, the strengthening of the sinking movement inhibits the upward transport of water vapor, leading to a signi cant negative anomaly in tropospheric relative humidity (Fig. 8c), indicating suppression of cumulus convection and the reduction in the release of latent heat of condensation. Simultaneously, the release of condensation latent heat decreases, which is not conducive to the formation of the warm-core structure of the TC. By :

Cumulus convection anomalies
where f is the Coriolis parameter, v the tangential velocity (v > 0 represents the cyclonic ow), r is radial distance to the center of TC, p is pressure, R d is the gas constant for dry air, and T v is virtual temperature.
Eq. (5) shows that the TC warm-core structure favors weakening of the TC cyclonic circulation with height, which implies that the warm-core structure plays a vital role in the development of TC. As shown in Fig. 8c, however, the relative humidity in the middle atmosphere is negative anomaly over the WNP, which covers most typhoon-generating areas. Additionally, due to cumulus convection, the latent heat of condensation decreases with weakening of warm-core structure, and the cyclonic circulation associated with the increase in height. These are not conducive for TC formation. Figure 9 shows the time series of the areal-average 600 hPa relative humidity, 500 hPa vertical velocity and geopotential height, vertical wind shear, and 850 hPa absolute vorticity over the WNP in July. It is seen that the vertical velocity and relative humidity in 2020 are the lowest since 1979, indicating that the subsidence movement in July 2020 was the strongest, and the cumulus convection was the weakest, which hindered the development ( ) of the typhoon structure. An approximate half of the vertical wind shear between 5°N and 25°N has a negative anomaly owing to the weakening of cumulus convection releasing latent heat and reducing condensation, resulting in weak vertical wind shear with di culty in typhoon formation (Fig. 8d).

WPSH anomalies
Owing to the decrease in cumulus convective intensity and condensation latent heat release, tropospheric heating over the WNP is dominated by surface long-wave radiation heating, which means that nonadiabatic heating increases with decreasing height. According to the potential tendency equation, as nonadiabatic heating decreases with height, the geopotential height increases (Zhu et al. 2007a). As shown in Fig. 8c, the geopotential height over the WNP in July 2020 was abnormally high and conducive to the strengthening of the WPSH. Compared with the climatological normal, the 5880 gpm isoline moved westerly and southerly in July 2007, indicating that the WPSH in July 2020 was southward and stronger. The centers of the above-mentioned circulation anomalies other than the geopotential height are basically located in the South China Sea and the eastern part of the Philippines, explaining the low number or absence of typhoons south of 20°N. A north-south dipole distribution of the circulation anomalies and the location of 20-25°N between the two parts of the dipoles, where the circulation anomalies are small, hinder the formation of typhoons that are mainly in uenced by the enhanced WPSH at this latitude. Figure 8f illustrates the horizontal wind eld at the 850 hPa level in July 2020, indicating that the intensity of the SCSMT is very weak and the trough line is located west of 120°E, which is unfavorable for the convergence and uplift of air ow. The low-level vorticity over the WNP is a negative anomaly, which increases the scale of the response and reduces the conversion of latent heat release to rotational motion, thus making it di cult for the formation of the initial disturbance. In addition, the southerly and strong WPSH hindered the uplift and development of the disturbances.

SCSMT anomalies
In general, the negative IPCO in July 2020 affected large-scale circulation anomalies over the WNP, which reduced the latent heat of condensation released by cumulus convection in the region, strengthened the WPSH, and weakened the SCSMT, hindering the generation of tropical disturbances and typhoons. Figure 10a shows the GPI anomaly over the WNP in July 2020. It reveals that the GPI over the WNP is mostly negative, and the area of the negative GPI anomaly covers most of the generation positions of typhoons over the years, consistent with previous large-scale circulation. As a comprehensive index, the GPI shows that when the strong negative phase of the intraseasonal IPCO occurs in July 2020, the anomalies of large-scale atmospheric circulation have an inhibitory effect on typhoon genesis.

GPI perspective
We further areal-averaged GPI over the WNP. Figure 10b shows that the GPI anomalies over the WNP in July 2020 were the lowest on record, indicating that the intraseasonal IPCO negative phase had the strongest inhibitory effect on typhoon genesis over the WNP by affecting large-scale atmospheric circulation. Previous studies have also shown that relative humidity and vorticity play a leading role in TC genesis compared with the other two factors (Camargo et al. 2009;Wang et al. 2018). Figure 9 shows that the relative humidity and vorticity over the WNP were negative anomalies in July 2020, and the relative humidity was the historical minimum, indicating that the intraseasonal IPCO may inhibit the generation of typhoons by affecting the relative humidity.

Conclusion And Discussion
In this paper, the inhibitory effect of the intraseasonal IPCO on typhoon genesis in the "absence of typhoons" over the WNP in July 2020 is discussed. The high SST and small vertical wind shear over the WNP in July 2020 are conducive to typhoon genesis, indicating that it is di cult to explain the absence of typhoons from the conditions necessary for typhoon genesis. In contrast, the intraseasonal IPCO plays an important role in modulating the generation of typhoons during the boreal extended summer over the WNP (Wang et al. 2018). This unprecedented absence of typhoon is consistent with IPCO's unique suppression of convection and the resulting large-scale circulation over the WNP. The in uence of atmospheric circulation anomalies on typhoons, represented by GPI, was the lowest ever recorded. In July 2020, the intraseasonal IPCO turned negative, associated with a strong inhibition on convection over its WNP, leading to large-scale circulation differences and di culty in generating typhoons.
Three possible ways that affect IPCO typhoon genesis are also presented. Figure 11 shows that the descending movement of the troposphere and low-level divergence were strengthened over the WNP in July 2020, weakening the upward lifting of low-level tropical disturbances and transport of water vapor and inhibiting the development of cumulus convection. Thus, the decrease in the release of latent heat of tropospheric condensation was not conducive to the formation of a typhoon warm-core structure and the development of a cyclone circulation with the increase in height. Due to the inhibition of cumulus convection, tropospheric heating increases by surface long-wave radiation and nonadiabatic heating along with a decrease in altitude, which makes the subtropical high southerly and stronger. This system inhibits the formation and development of typhoons. In addition, when the intraseasonal IPCO was in its negative phase, the following happened, namely: a) signi cant weakening of SCSMT intensity; b) increase in Rossby deformation radius by increase in the negative vorticity anomaly; c) increase in the response scale; and d) reduction in the e ciency of latent heat release into rotational motion. These are not conducive to the generation and development of tropical disturbances. On further employing GPI to examine the in uence of large-scale circulation anomalies on typhoon genesis, the lowest on record was for July 2020, indicating the strongest suppression of typhoons in history. In general, the negative phase of the intraseasonal IPCO may be one of the main reasons for the extreme large-scale circulation anomalies, leading to the absence of typhoons over the WNP in July 2020.
This study provides an understanding of the absence of typhoons in July 2020 and the bene ts of operational climate predictions of typhoon activity. Although this study implies that intraseasonal IPCO had an unprecedented inhibitory effect on typhoon in July 2020, various other factors affect its formation. Wang et al. (2021) found that the extremely warm Indian Ocean SST plays an important role in the generation of abnormal anticyclones over the WNP and further leads to circulation anomalies and the absence of typhoons. The additional factors that are responsible for the absence of typhoons in July 2020 and the in uence of other factors, such as ENSO and IOD, remain unknown and require further study.

Declarations Acknowledgments
We thank all the data providers. This work is supported by the National Natural Science Foundation of  Figure 1 The response function of the 30-60-day Lanczos bandpass lter. The lter has 305 weights and cutoff periods of 30 and 60 days.   Time series of areal-averaged 600 hPa relative humidity, 500 hPa vertical velocity and geopotential height, vertical wind shear, and 850 hPa absolute vorticity over the WNP (5°N-20°N, 110°E-160°E) in July. The opposite signs of geopotential height and vertical velocity are used here. Dots indicate that the variables in those years had the minimum value over the study period   The schematic diagram showing the inhibition of the intraseasonal IPCO on typhoon generation in July 2020