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Total numbers of tropical cyclone genesis events during Northern Hemisphere summer (MJJASO) of 1979-2015. The box indicates the selected 108 3 108 locations of tropical cyclone genesis for stepwise regression analysis.
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An intraseasonal genesis potential index (ISGPI) for Northern Hemisphere (NH) summer is proposed to quantify the anomalous tropical cyclone genesis (TCG) frequency induced by boreal summer intraseasonal oscillation (BSISO). The most important factor controlling NH summer TCG is found as 500-hPa vertical motion (ω500) caused by the prominent northwa...
Contexts in source publication
Context 1
... BSISO phase is separated into eight categories and each phase composite is achieved when the amplitude of BSISO index is greater than 1.0 [i.e., (PC1 2 1 PC2 2 ) 1/2 $ 1.0] following Wheeler and Hendon (2004). Comparison of the results derived from the three indices is presented in section 7. Figure 1 presents the eight-phase evolution of major convection (OLR) anomalies (shading) and TCGF anomalies (contour) by selected BSISO (OLR 08-208N ) index. The negative enhanced OLR anomalies corre- spond with increased TCGF anomalies through phases 1 to 8. ...
Context 2
... derive an ISGPI, we first enlarged the grid size from 2.58 3 2.58 to 108 3 108 grid and used TCGF anomaly at eight composite BSISO phases in order to include as many TCG samples as possible. Figure 2 displays the locations of the TCG during NH summer. The main TC occurrence is found in the latitude belt of 58-258N. ...
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Citations
... Following the GPI04, subsequent research has introduced a series of enhanced empirical indices that incorporate various environmental variables and their functional forms (Emanuel 2010;Murakami and Wang 2010;Tippett et al. 2011;McGauley and Nolan 2011;Wang and Murakami 2020). Additionally, other investigations have focused on fitting these indices to specific time scales and basins (Bruyère et al. 2012;Wang and Moon 2017;Moon et al. 2018). Both basin-dependent and global genesis indices have their own positive aspects. ...
Tropical cyclone (TC) genesis is initiated by convective precursors or “seeds” and influenced by environmental conditions along the seed-to-TC trajectories. Genesis potential indices (GPIs) provide a simple way to evaluate TC genesis likelihood from environmental conditions but have two limitations that may introduce bias. First, the globally fixed GPIs fail to represent interbasin differences in the relationship between environments and genesis. Second, existing GPIs are only functions of local environmental conditions, whereas nonlocal factors may have a significant impact. We address the first limitation by constructing basin- and time-scale-specific GPIs (local-GPIs) over the eastern North Pacific (ENP) and North Atlantic (NA) using Poisson regression. A sequential feature selection (SFS) algorithm identifies vertical wind shear and a heating condition as leading factors controlling TC genesis in the ENP and the NA, respectively. However, only a slight improvement in performance is achieved, motivating us to tackle the second limitation with a novel trajectory-based GPI (traj-GPI). We merge adjacent nonlocal environments into each grid point based on observed seed trajectory densities. The seed activity, driven mainly by upward motion, and the transition to TCs, controlled primarily by vertical wind shear or heating conditions, are captured simultaneously in the traj-GPI, yielding a better performance than the original GPIs. This study illustrates the importance of seed activity in modeling TC genesis and identifies key environmental factors that influence the process of TC genesis at different stages.
Significance Statement
The genesis potential index (GPI) is an effective tool for modeling the likelihood of tropical cyclone (TC) genesis for a given time and location. This study reveals that existing GPIs are primarily biased by a lack of information about nonlocal TC seed activity, since they are based only on local large-scale environmental variables. According to our study, upward motion and vertical wind shear are the most influential environmental factors in seed genesis and the transition from seed to TC, respectively. Based on the observed seed trajectories, we build trajectory-based GPIs that include the information from seed activity. Spatiotemporal performances of TC genesis are significantly improved over the original GPIs.
... Besides the weakening eastward propagation from the Indian Ocean to the western Pacific, the BSISO also features salient northward/ northeastward propagation over the Indian and western North Pacific monsoon regions, with its large variability extending much farther from the equator (Lau and Chan 1986;Pillai and Sahai 2016). Although the BSISO and its effects are not as widely studied as wintertime MJO, the BSISO behavior has also been reported to exert considerable impacts on some weather and climate systems, such as the tropical cyclones genesis (Moon et al. 2018), monsoon onsets and breaks (Chen et al. 2000), and rainfall variability and flooding in the monsoon regions (Li et al. 2015;Hsu et al. 2016;Olaguera et al. 2022). Therefore, to improve the extended-range forecasting capability during boreal summer or austral winter, more attention should be paid to the evolution of BSISO. ...
... As suggested by previous studies, to some extent, we can expect an interaction between the wintertime MJO and northern stratospheric polar vortex. The poleward propagating wave trains triggered by certain MJO phases, such as MJO phases 6/7 or 3/4, constructively or destructively interfere with climatological fields, strengthening or weakening the upward propagation of planetary waves into the polar stratosphere and then modulating the intensity of the stratospheric polar vortex through the wave-mean flow interaction (Garfinkel et al. 2012(Garfinkel et al. , 2014Schwartz and Garfinkel 2017;Wang et al. 2018;Yang et al. 2019;Ma et al. 2020). Given the MJO's bold fingerprint on the variability in NH stratospheric polar vortex, these specific MJO phases can be treated as the precursory signal for the prediction of stratospheric polar vortex Rao et al. 2019;Wang and Ye 2021). ...
... The interference between the MJO-triggered teleconnection pattern and the climatological planetary wavefield, especially over the North Pacific, is strongly associated with the intensity of upward propagating planetary waves in the NH extratropics, which has been widely used to explain the relationship between MJO and NH SSW event (e.g., Garfinkel et al. 2014;Wang et al. 2018;Rao et al. 2019Rao et al. , 2021b. Likewise, the geopotential height anomalies at 500 hPa triggered by BSISO P5 are examined in Fig. 6. ...
The boreal summer intraseasonal oscillation (BSISO) features more distinctive and complex propagation characteristics than its wintertime counterpart, the Madden–Julian oscillation (MJO). While the relationship between the MJO and the Arctic stratosphere during boreal winter has been widely documented, the linkage between the BSISO and the Antarctic stratosphere during austral winter has not been extensively discussed. Here, after identifying the Southern Hemisphere (SH) stratospheric polar vortex warming (SPVW) events, we reveal the bidirectional connection between BSISO and SPVW. Before onset of the SPVW events, the occurrence frequency and amplitude of BSISO phase 5 (P5) shows a significant increase. The most significant responses of the SH polar stratospheric temperature to the BSISO are found about 10 days after BSISO P5. Thus, to some extent, BSISO P5 can be regarded as a precursor to the SH SPVW event, which is attributed to the enhanced upward propagation and dissipation of planetary waves in the SH stratosphere induced by the BSISO P5. After onset of the SPVW events, a significant increase in the occurrence and amplitude of BSISO P6 is observed, corresponding to the enhanced convection over the South China Sea and southern Philippine Sea. Tropical upwelling associated with the strengthened Brewer–Dobson circulation (BDC) induced by the SPVW tends to result in unstable circumstances in the tropical upper troposphere. Then the high correlation between static stability at 150 hPa and outgoing longwave radiation (OLR) anomalies over the South China Sea and southern Philippine Sea provides robust evidence that the intensity of convective activity in the tropics can indeed be modulated by the variability in SH stratospheric polar vortex.
Significance Statement
The relationship between the boreal summer intraseasonal oscillation (BSISO) and the Southern Hemisphere (SH) polar stratosphere remains unclear. After selecting the stratospheric polar vortex warming (SPVW) events, we reveal the two-way connection between BSISO and SPVW. Before the SPVW, the significant increase in occurrence and amplitude of BSISO phase 5 (P5) suggests that BSISO P5 can be regarded as a precursor to the weakened polar vortex. After the SPVW, the significant increase in occurrence and amplitude of BSISO phase 6 (P6) establishes that environmental alteration in tropical upper troposphere induced by the SPVW provides a favorable condition for the growth of ensuing convective activity. The results here help us better understand the potential link between stratospheric polar vortex and tropical intraseasonal oscillation.
... Gray (1979) first proposed a genesis potential index (GPI) by constructing a function of dynamical and thermodynamical large-scale environmental parameters that are known to be important to TC formation. Several variants of GPI were subsequently developed to improve the quantitative linkage between TC genesis and large-scale environmental conditions from different perspectives (Emanuel and Nolan 2004;Emanuel 2010;Murakami and Wang 2010;Tippett et al. 2011;Menkes et al. 2011;Holland and Bruyère 2013;Zhang et al. 2016;Wang and Moon 2017;Moon et al. 2018;Wang and Murakami 2020). Among them, the GPI proposed by Emanuel and Nolan (2004;ENGPI) is one of the most popular (Camargo et al. 2007). ...
The genesis potential index (GPI) has been used widely to estimate the influence of large-scale conditions on tropical cyclone (TC) genesis. Here we find that two GPIs, the Emanuel–Nolan GPI (ENGPI) and the dynamic GPI (DGPI), show opposite skills in quantifying decadal variability of TC genesis in the western North Pacific (WNP). During 1979–2020, ENGPI shows a reverse decadal variation to the WNP TC genesis with a significant negative correlation of −0.61, while DGPI can reasonably reproduce the decadal variation of the WNP TC genesis with a significant correlation of 0.66. The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis induced by a WNP anomalous cyclonic circulation that controls the decadal variation of TC genesis. On the one hand, the cyclonic circulation leads to favorable dynamical conditions including ascending motion, cyclonic vorticity, and weakened vertical shear, and thus tends to increase the DGPI. On the other hand, the cyclonic circulation leads to unfavorable thermodynamical conditions (decreased maximum potential intensity and midlevel humidity) that tends to decrease the ENGPI. As a result, the DGPI and ENGPI are reversely evolved and eventually lead to their opposite correlation between TC genesis. The significant positive correlation between DGPI and TC genesis suggests a critical role in the large-scale dynamical control of the decadal variability of the WNP TC genesis.
Significance Statement
Tropical cyclones (TCs) account for one-third of the deaths and economic losses from weather-, climate-, and water-related disasters. Understanding variations in TC activity from the perspective of large-scale conditions is of great importance to seasonal forecasting and disaster mitigation. Here we find that two genesis potential indexes (GPIs), the Emanuel–Nolan GPI (ENGPI) and dynamic GPI (DGPI), show opposite skill in quantifying decadal variability of TC genesis in the western North Pacific (WNP). The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis. The result suggests a critical role of large-scale dynamic control in the decadal variability of TC genesis in the WNP.
... The boreal summer intraseasonal oscillation (BSISO), one of the most prominent variabilities in the tropical atmosphere, is usually characterized by a northeastward propagation from the Indian Ocean (IO) to the western North Pacific (WNP) (Hsu et al. 2004;Jiang et al. 2004;Kikuchi 2020). As the cornerstone of the subseasonal forecast, the BSISO has profound influences on the global weather and climate systems (Zhang 2013;Liu et al. 2022), like the monsoon onset, tropical cyclones, heat waves, extreme rainfall, surface air temperature anomalies, Pacific-Japan-like teleconnection (Naumann and Vargas 2010;Shao et al. 2014;Hsu et al. 2016Hsu et al. , 2020Moon et al. 2018;Shao et al. 2018;Wang et al. 2018a;Li et al. 2020a, b;Wang et al. 2022). Thus, enhancing the understanding of the BSISO activity is crucial for improving the subseasonal prediction and simulation capabilities of the BSISO in general circulation models (GCMs) (Lin et al. 2006;Ding et al. 2011;Kim et al. 2011;Jiang et al. 2018). ...
Recent studies suggest that La Niña events can be classified into two categories: mega La Niña and equatorial La Niña. The understanding of the variations in boreal summer intraseasonal oscillation (BSISO) behaviors between such two conditions remains uncertain. Results in this work show during equatorial La Niña summers, in conjunction with the more adequate intraseasonal column-integrated moisture anomalies, the weaker intraseasonal outgoing longwave radiation anomalies are observed over the western North Pacific (WNP) at 3 pentads lag of the peak phase for the Maritime Continent (MC) BSISO events than during mega conditions. Such changes are closely linked with the different propagation features, specifically northwestward and northeastward propagations under mega and equatorial conditions respectively. The distinct propagations under these two conditions could be partly explained by the background column-integrated moisture anomalies. Under equatorial conditions, the less sufficient background moisture anomalies over the tropical western Pacific (WP), in comparison to mega conditions, suppress the activities of the BSISO and its northwestward propagation here. Meanwhile, the enhanced moisture anomalies over the northwestern MC and its surrounding area (NWMC) facilitate the northeastward propagation. Under mega conditions, the background moisture anomalies over the tropical WP are not significant. The southward moisture anomaly gradient over the NWMC hinders the meridional northward propagation and makes some BSISO activities move to the tropical WP region, performing the zonal westward propagation as a whole. The moisture budget and multi-scale interaction diagnoses also emphasize the significant role of the propagation change in the moisture tendency difference averaged over the WNP. Moreover, the extratropical circulation anomalies associated with the MC BSISO events are also discussed. These findings provide new insights into BSISO activity and offer potential improvements for subseasonal forecast.
... We found that the TC days are 11% less in Phases 1-4 (44%) than those in Phases 5-8 (55%) (not shown). This clearly indicates that TC activity is suppressed during the suppressed phases of the BSISO (Yoshida et al., 2014;Moon et al., 2018;Olaguera et al., 2022). Yoshida et al. (2014) found that the large-scale conditions over the WNP are unfavourable for TC genesis in the region in these phases. ...
This study investigates the climatological characteristics of the monsoon breaks during the southwest monsoon season (June–September) of the Philippines from 1979 to 2020. The monsoon break is defined as a period when the average rainfall across the eight western coast stations of the Philippines decreases below 5 mm·day⁻¹ for at least three consecutive days. About 208 monsoon break events were identified, with 131 short‐ (3–4 days; 63% of the total monsoon break events), 71 medium‐ (5–9 days; 34%) and 6 long‐duration (≥10 days; 3%) monsoon break events. The short‐ and medium‐duration monsoon breaks were found to be more frequent during June and less frequent during August. Lag composite analyses of the time series of rainfall, low level zonal wind (UWND850hPa), vertically integrated moisture flux convergence (VIMFC) and mid‐tropospheric geopotential height (HGT500hPa) relative to the starting date of the monsoon break events show that the monsoon break is characterized by decrease in rainfall, weakening of westerly winds, increase in HGT500hPa and suppression of VIMFC over Luzon Island. Further analyses of the large‐scale circulation features show the appearance of an anticyclonic circulation over Luzon Island that suppresses the rainfall in the region and westward extension of the western North Pacific Subtropical High (WNPSH). The occurrence of the monsoon break events appears to be modulated by the 30–60‐day mode of the boreal summer intraseasonal oscillation (BSISO). About 39%, 55% and 82% of the short‐, medium‐ and long‐duration monsoon break events occur between Phases 1 and 4 during significant suppressed phases of the BSISO (i.e., amplitude >1), respectively. Convective activities and tropical cyclone activities are generally suppressed in these phases. The suppression of convective activities near the Philippines may facilitate the westward expansion of WNPSH, which further suppresses the rainfall and weaken the southwest monsoon.
... The likelihood of TCG in a given ocean basin during its active season is usually represented by genesis potential indices, which are empirically determined functions of large-scale environmental variables which influence TCG. Moon et al. (2018) developed an intra-seasonal genesis potential index (IGPI) for the northern hemisphere to assess the anomalous TCG frequency induced by boreal summer intra-seasonal oscillation (BSISO). It is found to perform poorly over the North Atlantic Ocean (NAO) basin. ...
Understanding the role of tropical cloud clusters (TCC) in the development of tropical cyclones involves various complexities and, thus, necessitates precise research. The study on TC development from the TCCs is still minimal. The present research is carried out to investigate the predictability of tropical cyclogenesis (TCG) by examining the Rossby radius ratio (RRR) and daily genesis potential (DGP) of different cloud clusters over the four ocean basins in the Northern Hemisphere, viz., North Indian Ocean (NIO), North Atlantic Ocean (NAO), West Pacifc Ocean (WPO), and East Pacifc Ocean (EPO). The analysis of the TCC data, taken for the period 1996–2005, shows that both the predictors are skilled at identifying the developed and non-developed TCCs. The method of cumulative distribution is implemented to identify the threshold ranges of RRR and DGP. In
addition, the forecast skill scores are estimated for the selected predictors. The rough set theory based on different condition-decision support is implemented to estimate the certainty in the TCG prediction scheme with each predictor individually and in combination. The result shows that higher certainty in TCG prediction is observed when RRR ≤ 24 and DGP ≥ 1.21× 10−5 for the NIO basin. However, it is to be noted that the combination of both RRR and DGP provides better confidence in the predictability of TCG over the NAO basin (RRR≤38 and DGP≥0.71× 10−5) and EPO basin (RRR ≤ 28.7 and DGP ≥ 0.47× 10−5). Furthermore, RRR (threshold value≤28.2) individually gives better predictability for TCG over the WPO basin. The forecasts of TCG with RRR and DGP are validated with the observations from 2006 to 2009.
... Upon its northward movement, TC "Choi-wan" (2021) acted as an immediate trigger for the SCSSM onset in the low levels. Both TC events seemed to be triggered by the MJO event (Wang and Moon, 2017;Zhao and Li, 2019;Ye et al., 2020), which is supported by the analysis of the empirical genesis potential index (Camargo et al., 2009;Moon et al., 2018) in the supporting information (Text S3, Figs. S4, S5 in the ESM). ...
The South China Sea Summer Monsoon (SCSSM) onset is characterized by an apparent seasonal conversion of circulation and convection. Accordingly, various indices have been introduced to identify the SCSSM onset date. However, the onset dates by various indices sometimes are very inconsistent. It not only limits the determination of onset dates, but also misleads the assessment of prediction skills. In 2021, the onset time identified by the circulation criteria was May 20, which is 12 days earlier than that by considering convection in addition. The present study mainly ascribed such circulation-convection inconsistency to the activities of tropical cyclones (TCs) modulated by the Madden-Julian Oscillation (MJO). The convection of TC “Yaas” released adiabatic heating in the north of the SCS in the upper level, facilitating the circulation transition. Afterward, TC “Choi-wan” over the western Pacific assisted the persistence of westerlies in the lower level, but suppressed the convection over the SCS. Accurate predictions using the ECMWF S2S forecast system were obtained only after the MJO formation. The skillful prediction of MJO during late spring may provide an opportunity to accurately predict the SCSSM establishment several weeks in advance.
... The TC track is primarily controlled by steering flow that can be approximately represented by the lower-tropospheric circulation near the U.S. coast, given the high correlation between the 850 hPa and 500 hPa geopotential height anomalies (Supplementary Fig. 1). The factors determining the subseasonal modulation of TC genesis by MJO and Boreal Summer Intraseasonal Oscillation (BSISO) 32,33 are to some extent different from the factors controlling the seasonal TC genesis [34][35][36] and are also basindependent 36 . The most prominent controlling factor is the 500 hPa vertical motion for most basins, except the western North Atlantic, where the relative vorticity in the lowertroposphere plays the dominant role in determining the subseasonal variability of TC genesis 36 . ...
... The TC track is primarily controlled by steering flow that can be approximately represented by the lower-tropospheric circulation near the U.S. coast, given the high correlation between the 850 hPa and 500 hPa geopotential height anomalies (Supplementary Fig. 1). The factors determining the subseasonal modulation of TC genesis by MJO and Boreal Summer Intraseasonal Oscillation (BSISO) 32,33 are to some extent different from the factors controlling the seasonal TC genesis [34][35][36] and are also basindependent 36 . The most prominent controlling factor is the 500 hPa vertical motion for most basins, except the western North Atlantic, where the relative vorticity in the lowertroposphere plays the dominant role in determining the subseasonal variability of TC genesis 36 . ...
... The factors determining the subseasonal modulation of TC genesis by MJO and Boreal Summer Intraseasonal Oscillation (BSISO) 32,33 are to some extent different from the factors controlling the seasonal TC genesis [34][35][36] and are also basindependent 36 . The most prominent controlling factor is the 500 hPa vertical motion for most basins, except the western North Atlantic, where the relative vorticity in the lowertroposphere plays the dominant role in determining the subseasonal variability of TC genesis 36 . Motivated by this, we focus on understanding the major atmospheric circulation modes in the lower troposphere that may influence both TC track and genesis. ...
Landfalling tropical cyclones (LTCs) are the most devastating disaster to affect the U.S., while the demonstration of skillful subseasonal (between 10 days and one season) prediction of LTCs is less promising. Understanding the mechanisms governing the subseasonal variation of TC activity is fundamental to improving its forecast, which is of critical interest to decision-makers and the insurance industry. This work reveals three localized atmospheric circulation modes with significant 10–30 days subseasonal variations: Piedmont Oscillation (PO), Great America Dipole (GAD), and the Subtropical High ridge (SHR) modes. These modes strongly modulate precipitation, TC genesis, intensity, track, and landfall near the U.S. coast. Compared to their strong negative phases, the U.S. East Coast has 19 times more LTCs during the strong positive phases of PO, and the Gulf Coast experiences 4–12 times more frequent LTCs during the positive phases of GAD and SHR. Results from the GFDL SPEAR model show a skillful prediction of 13, 9, and 22 days for these three modes, respectively. Our findings are expected to benefit the prediction of LTCs on weather timescale and also suggest opportunities exist for subseasonal predictions of LTCs and their associated heavy rainfalls.
... Consistent with the enhanced convective activity, anomalous moistening formed in the middle troposphere, along with northwestward moisture flux (Fig. 6b) and enhanced upward motion (Fig. 6c); these changes created favorable conditions for TC genesis and development, thus explaining the simulated enhancement of TC activity in both cases in the southwestern WNP. These findings are consistent with previous conclusions based on long-term ISO analyses (Moon et al. 2018;Fowler and Pritchard Fig. 6 Composite of the ISO active phase (deviations from period-mean values) (a) OLR and 850-hPa horizontal wind vectors, (b) 700-hPa relative humidity and vertically integrated moisture flux, and (c) 500-hPa vertical velocity and mass-weighted 300−1,000-hPa integral of zonal wind vectors in CTL (left panels) and MWNPCLM (center panels). The difference (CTL−MWNPCLM) during the ISO active phase is also plotted (right panels). ...
In this study, we explored the impacts of midlatitude western North Pacific (WNP) sea surface temperature (SST) on tropical cyclone (TC) activity at intraseasonal to seasonal time scales during the 2018 boreal summer. During this period, a positive SST anomaly occurred in the midlatitude WNP and subtropical central Pacific; TC activity was abnormally high under the influence of the strong Asian summer monsoon. We performed sensitivity experiments using a global cloud system-resolving model for global SST (control, CTL) and SST that were regionally restored according to midlatitude WNP climatology (MWNPCLM). TC track density in the eastern WNP was higher in CTL than in MWNPCLM, in association with large-scale atmospheric responses; enhanced monsoon westerlies in the subtropical WNP, moist rising (dry subsiding) tendencies, and reduced vertical wind shear in the eastern (western) WNP. Enhanced TC activity in the eastern WNP was more distinct for intense TCs and during the active phase of intraseasonal oscillation (ISO). These results suggest that the impacts of midlatitude SST anomalies can reach lower latitudes to affect TC activity via large-scale atmospheric responses and ISO, which are usually overwhelmed by the impacts of SST anomalies in the tropics and subtropics.
... In the subseasonal timescales, some studies have demonstrated the role of the boreal summer intraseasonal oscillation (BSISO) in modulating the rainfall variability including the rainfall extremes during the summer monsoon season over the Asian monsoon region (e.g., Kikuchi et al., 2012;Li et al., 2015;Hsu et al., 2016;Moon et al., 2018;Ren et al., 2018). Hsu et al. (2016) noted that the convective and circulation anomalies associated with the BSISO may induce changes in the local rainfall variation by altering the background conditions. ...
... Unlike the Madden-Julian Oscillation (MJO; Madden and Julian, 1971), which propagates eastward along the equator in boreal winter, the BSISO shows a more complex nature and with a dominant northward/northwestward propagation over the Indian and western North Pacific monsoon regions (e.g., Kikuchi et al., 2012;Moon et al., 2018). Kikuchi et al. (2012) developed a bimodal index that captures the eastward propagation of MJO and northward/ northwestward propagation of BSISO based on the extended empirical orthogonal function (EEOF) analysis of the bandpass filtered (25-90 days) outgoing longwave radiation (OLR). ...
... The total TC-induced extreme rainfalls days over the Visayas and Mindanao regions are both less than 24% of the total BSISO days, indicating less TC influence in these regions. Moon et al. (2018) showed that the TC genesis frequency over the western North Pacific is enhanced in these phases (see their fig. 1). ...
This study investigates the impact of the northward/northwestward propagating 30–60‐day mode of the boreal summer intraseasonal oscillation (BSISO) on the extreme rainfall events in the Philippines during the June–September (JJAS) season from 1979 to 2018. The Philippines domain is divided into the three latitudinal regions: Luzon region (13°–22°N), Visayas region (10°–13°N), and Mindanao region (5°–10°N) to account for the regional differences in the timing of extreme rainfall events. The probability density functions of JJAS rainfall are skewed towards higher values relative to the non‐BSISO days in BSISO Phases 6–8, Phases 5–7, and Phases 4–6 over the Luzon, Visayas, and Mindanao regions, respectively, during which the probability of extreme rainfall events at the 95th percentile increases by as much as 80% in some stations in these regions. Further analyses of the large‐scale circulation features show that the increase (decrease) in the probability of extreme rainfall events is associated with enhanced moisture convergence (divergence) induced by the cyclonic (anticyclonic) circulation anomalies of the BSISO and appearance of multiple tropical cyclones. About 36% of the total extreme rainfall events over the Luzon region are associated with TCs during Phases 7–8. On the other hand, TCs contribute by no more than 24% in all phases over the Visayas and Mindanao regions, indicating less TC influence in these regions. This study is the first attempt to clarify the impact of the BSISO on the extreme rainfall events in the Philippines.
