3.1 The wave train
Although intraseasonal velocity potential anomalies at 200 hPa showed the concordant positive anomalies over the whole WNP (Fig. 1), monthly mean wind and OLR anomalies showed a distinct distribution in August of 1996 and 2014. In August 1996, an obvious wave train extended from the South China Sea and southwestern WNP to the middle latitude of the North Pacific (Fig. 3a). Over the South China Sea and southwestern WNP, a strong anticyclonic circulation and inactive convection was located along with the occurrence of four TC cases. In the northeastern WNP, a strong cyclonic circulation and enhanced convection was observed along with the occurrence of five TC cases (Fig. 3a). In contrast, in August 2014, there was no obvious wave train structure of 850-hPa wind anomalies (Fig. 3f). The strong anticyclonic circulation was observed over most of the WNP and a cyclonic circulation was located in the East China Sea (Fig. 3f). This indicates that the anomalous cyclone associated with the wave train was one of the important factors of the active TC genesis in the northeastern WNP during August 1996. Based on climatological average, only one TC occurs in August in the northeastern WNP. However, total five TCs were generated herein in August 1996, which is the most since 1979 (Fig. 2b). Based on the wind fields with TC signals removed, monthly mean anomalies of 850-hPa winds showed the consistent features in August of 1996 and 2014 (figures not shown).
The averaged OLR anomalies in August 1996 showed active convection around the Maritime Continent and eastern Indian Ocean. From the Maritime Continent to the middle-latitude North Pacific, there was a southwest-northeast -oriented train of negative-positive-negative-positive OLR anomalies, which was consistent with cyclone-anticyclone-cyclone-anticyclone wave train structure of 850-hPa wind anomalies (Fig. 3a). This wave train is associated with the active convection around the Maritime Continent. However, in August 2014, no active convection anomalies were seen over the Maritime Continent, and no corresponding cyclonic circulation was induced over the WNP (Fig. 3f). Positive OLR anomalies and anticyclonic circulation dominated most of the WNP.
The cyclonic circulation created some favorable conditions for the TC genesis in the northeastern WNP. Figures 3b-e showed that there were opposite anomalies of precipitation, mid-level vertical velocity, mid-level specific humidity and lower-level stream function in the northeastern and southwestern WNP in August 1996. In particular, obvious positive precipitation, ascending motion, positive humidity and positive relative vorticity anomalies were located in the northeastern WNP, favorable for the TC genesis herein. In the contrast, negative precipitation, descending motion, weak humidity and negative relative vorticity anomalies occurred over most of the WNP in August 2014, except for limited regions in the East China and Japan (Figs. 3g-j). Those unfavorable conditions resulted in the absence of TC genesis in whole August of 2014.
It is noteworthy that although monthly anomalies of precipitation, vertical motion, specific humidity and relative vorticity were unfavorable, four TCs were still generated in the southwestern WNP during August 1996. It indicates that the TC genesis in the southwestern WNP could not be explained by monthly anomalies of large scale factors. Recent studies have illustrated that the composite based on the local and instantaneous perspective can better reflect the actual environments for the TC genesis (Cao et al., 2018, 2019, Cao and Wu, 2020). Thus, we examine the daily evolution of large-scale conditions to understand the factors responsible for the TC genesis over the WNP in the following section.
3.2 Time evolution of daily anomalies
The four TCs in the southwestern WNP were generated in 4, 12, 16, 21 August of 1996. Figure 4 showed total anomalies (the climatological daily mean removed), intraseasonal anomalies and synoptic anomalies of lower-level winds and relative vorticity at the time of these four TC geneses with the TC signals removed. The total anomalies showed obvious cyclone and positive vorticity around the location of TC genesis on 4 and 21 August of 1996 (Fig. 4a and Fig. 4d). On 12 August of 1996, salient southerly and northerly winds converged along with the positive vorticity anomalies around the location of TC genesis (Fig. 4b). Differently, the TC on 16 August of 1996 was generated in the easterly winds (Fig. 4c). It indicates that this TC genesis was associated with easterly wave. Previous studies have showed that easterly wave is one of the synoptic-scale perturbations that may induce TC genesis (Fu et al., 2007, Li, 2012). Fu et al. (2007) found that 20% TC geneses during the 2000 and 2001 TC seasons were induced by easterly waves.
A clear southeast-northwest-oriented wave train pattern was observed three days prior to the TC genesis on 16 August 1996 (Fig. 5a). An obvious synoptic-scale cyclone and vorticity anomalies were located around the tropical depression location. On 15 August 1996, the wave train began to weaken (Fig. 5c). On the day of the TC genesis, the synoptic-scale wave train became much weaker (Fig. 5d and Fig. 4k). In the following two days, the synoptic scale wave train started to reinforce (Fig. 5e and Fig. 5f). Particularly, on 18 August 1996, the tropical disturbances had intensified into the tropical storm with the maximum wind of 35 kts (Fig. 5f). It indicates that the intensity of TC experienced two oscillations, which is a common feature during the early stage of TC genesis (Li et al. 2006, Ge et al. 2013, Cao et al. 2016). In the end, the TC gradually intensified into a severe typhoon with the maximum surface wind of 95 kts on 21 August 1996.
Although the monthly mean anomalies of dynamic and thermodynamic variables were favorable for the TC genesis in the northeastern WNP (Fig. 3), it is necessary to investigate the lower-level circulations from the local and instantaneous perspective. As such, Figure 6 showed the total, intraseasonal and synoptic anomalies of lower-level winds and relative vorticity at the time of five TC geneses in the northeastern WNP with the TC signals removed. The common feature of all these five TC cases is that obvious cyclonic circulation and positive relative vorticity occupied the location of TC genesis (Figs. 6a-e). It indicates that both monthly and daily mean anomalies supplied a favorable background for TC genesis in the northeastern WNP.
When the intraseasonal winds and synoptic winds were compared with the total anomalies, it is found that the intraseasonal variation had a larger contribution to TC genesis than synoptic variation in each case on the day of TC genesis (Fig. 4 and Fig. 6). The result of case studies is consistent with the composite analysis in Cao et al. (2018) who showed that the contribution of lower-level vorticity to the TC genesis is mainly due to intraseasonal and synoptic variations with the former slightly larger than the latter.
Based on Fig. 3 and Fig. 4, the daily time evolution of lower-level winds and vorticity showed a completely different distribution from monthly mean anomalies. Although monthly mean anomalies were unfavorable for TC genesis in the southwestern WNP, the daily anomalies at the time of TC genesis provided a favorable environment for the TC genesis. It indicates that monthly mean anomalies of large-scale conditions did not match well the atmospheric states around the location of TC genesis and daily anomalies could reflect well the real background for the TC genesis.
Previous studies about the reasons of the absence of TC genesis in August 2014 were mainly focused on monthly mean anomalies (Hong et al., 2015, Yang et al. 2015). For example, Hong et al. (2015) indicated that the combination of the dry ISO phase and downward motion triggered by the warm sea surface temperature led to the extremely dry and warm conditions, which were unfavorable for TC genesis over the WNP. However, from the analysis about TC genesis in the southwestern WNP in August 1996, the unfavorable monthly mean anomalies cannot guarantee an inactive TC activity. It is necessary to examine the daily evolution of atmospheric conditions. Figure 7 showed the intraseasonal anomalies of 850-hPa winds and relative vorticity at three-day interval in August 2014. On 4 August 2014, there was an obvious cyclone around 28°N, 148°E (Fig. 7a). However, the cyclonic disturbance was destroyed by the nearby typhoon in the west in the following three days during the western propagation (Fig. 7b). Meantime, another cyclonic disturbance began to develop around the dateline (Fig. 7b). However, the cyclonic disturbance gradually expanded with a larger size and a weaker intensity until disappearance in the following week (Figs. 7d-f). In late August 2014, no obvious potential cyclonic disturbance was observed over the tropical WNP except for easterly winds (Figs. 7g-i). As such, the cyclonic disturbances were suppressed during whole August 2014. It indicates that the suppressed disturbances combined with the inactive ISO phase were responsible for the absence of TC genesis over the WNP in August 2014.