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 significant 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.
Table 2
The composite number of typhoons over the WNP in July during the positive and negative intraseasonal IPCO phases (1979−2020) and their difference
|
IPCO (+)
|
IPCO (−)
|
Difference (IPCO (+) − IPCO (−))
|
Typhoons
|
4.8
|
2.8
|
2.0 *
|
Note: The asterisk * indicates significance at the 95% confidence level using the Student’s t-test |
Previous studies have found that the IPCO could affect large-scale circulation in local and remote areas in a number of ways and modulate the weather and climate of the region (Li et al. 2016b; Zheng et al. 2017; Wang et al. 2019; Zhao et al. 2019). Wang et al. (2018) demonstrated that the anomalies of large-scale circulation are significantly different between the different phases of the intraseasonal IPCO in boreal extended summer (May–October) and proposed possible physical mechanisms of large-scale circulation influencing typhoon generation, including the thermodynamic effect of relative humidity in the middle atmosphere and circulation conditions. In July 2020, the intraseasonal IPCO had an unprecedented strong inhibition on convection over the WNP, which may have inhibited the generation and development of typhoons by influencing large-scale circulation anomalies. This was followed by further investigation on the circulation anomalies in July during the different phases of the intraseasonal IPCO.
The anomalies of large-scale circulation affected by the intraseasonal IPCO could be characterized by the 30–60-day Lanczos bandpass-filtered data. Figure 5 shows the composite difference of the 200 hPa and 850 hPa divergence, 500 hPa vertical velocity and geopotential height, 600 hPa relative humidity anomalies, and vertical wind shear in July between the positive and negative IPCO phases. Significant differences are observed in large-scale circulation under different the intraseasonal IPCO phases, and the large-scale circulation anomalies present in the north–south dipole distribution. Except for the vertical wind shear, other atmospheric factors showing significant difference between the different phases of intraseasonal IPCO are located in the southern part of the dipole 5°N–20°N, corresponding to the key area of the WNP part of the intraseasonal IPCO. Moreover, the significantly different area also covers the main region of typhoon generation (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 Pacific 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 Pacific and South China Sea.
Figure 6 shows the composite map of the horizontal wind vorticity anomalies at the 850 hPa level. The position and intensity of the South China Sea monsoon trough (SCSMT) are significantly different in the different phases of the intraseasonal IPCO. In the positive phase of the intraseasonal IPCO, the SCSMT is much stronger than that in the negative phase, and the trough line can reach 150°E. In addition, the north–south range of the SCSMT covers the main area of typhoon generation, and water vapor and heat abound in the SCSMT. The airflow easily converges and then rises in this area, which is conducive to the generation and uplifting of tropical disturbances. The positive anomaly of the vorticity at the trough line leads to a decrease in the Rossby deformation radius, which reduces the scale of response and increases the conversion of latent heat release to rotational motion, thus facilitating the generation of tropical disturbances (Hack and Schubert 1986). In the year of the negative phase of the intraseasonal IPCO, the intensity of the SCSMT is obviously weaker, and the vorticity is a significantly negative anomaly, unfavorable for the generation and development of tropical disturbances.
Previous research has shown that the GPI replicates the interannual variations of TC genesis in several different basins on intraseasonal timescales and reflects the influence of large-scale circulation conditions on TC genesis (Camargo et al. 2007, 2009; Jiang et al. 2012; Zhao et al. 2015a, b). The GPI is mainly influenced by four factors: low-level vorticity, middle-level relative humidity, vertical wind shear, and potential intensity. Table 3 shows the correlation coefficients between IPCOI and GPI and its four elements, and IPCOI is highly correlated with both, indicating that GPI can be used to characterize the influence 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.
Table 3
Correlation coefficient between IPCOI and areal-average vertical wind shear (Vshear), 600 hPa relative humidity (rhum), potential intensity (PI), 850 hPa absolute vorticity, and GPI over the WNP in July
|
Vshear
|
rhum
|
PI
|
Vorticity
|
GPI
|
Correlation coefficient
|
0.72*
|
0.77*
|
-0.42*
|
0.78*
|
0.58*
|
Note: The asterisk * indicates significance at the 99% confidence level using the Student’s t-test |