Tropical Cyclogenesis in the Western North Pacific As Revealed by the 2008–09 YOTC Data

1038 WEATHER AND FORECASTING VOLUME 28

Tropical Cyclogenesis in the Western North Paciﬁc as Revealed by the 2008–09 YOTC Data*

YAMEI XU Department of Geosciences, Zhejiang University, Hangzhou, China, and International Paciﬁc Research Center, and Department of Meteorology, University of Hawaii at Manoa, Honolulu, Hawaii

TIM LI International Paciﬁc Research Center, and Department of Meteorology, University of Hawaii at Manoa, Honolulu, Hawaii, and Key Laboratory of Meteorological Disaster, and College of Atmospheric Science, Nanjing University of Information Science and Technology, Nanjing, China

MELINDA PENG Naval Research Laboratory, Monterey, California

(Manuscript received 1 October 2012, in ﬁnal form 15 March 2013)

ABSTRACT

The Year of Tropical Convection (YOTC) high-resolution global reanalysis dataset was analyzed to reveal precursor synoptic-scale disturbances related to tropical cyclone (TC) genesis in the western North Pacific (WNP) during the 2008–09 typhoon seasons. A time filtering is applied to the data to isolate synoptic (3–10 day), quasi- biweekly (10–20 day), and intraseasonal (20–90 day) time-scale components. The results show that four types of precursor synoptic disturbances associated with TC genesis can be identified in the YOTC data. They are 1) Rossby wave trains associated with preexisting TC energy dispersion (TCED) (24%), 2) synoptic wave trains (SWTs) unrelated to TCED (32%), 3) easterly waves (EWs)(16%),and4)acombinationofeitherTCED-EWor SWT-EW (24%). The percentage of identifiable genesis events is higher than has been found in previous analyses. Most of the genesis events occurred when atmospheric quasi-biweekly and intraseasonal oscillations are in an active phase, suggesting a large-scale control of low-frequency oscillations on TC formation in the WNP. For genesis events associated with SWT and EW, maximum vorticity was confined in the lower troposphere. During the formation of Jangmi (2008), maximum Rossby wave energy dispersion appeared in the middle troposphere. This differs from other TCED cases in which energy dispersion is strongest at low level. As a result, the midlevel vortex from Rossby wave energy dispersion grew faster during the initial development stage of Jangmi.

1. Introduction environmental conditions are necessary, the timing of TC genesis depends on the occurrence of synoptic-scale wave Tropical cyclone (TC) genesis is a process through disturbances that trigger individual cyclogenesis events. which random convective systems are organized into Different from the tropical Atlantic, where the typical a mesoscale vortex under favorable large-scale conditions perturbation type is African easterly waves (Burpee 1972, (Gray 1968, 1979; Montgomery and Enagonio 1998; also Landsea 1993), three major types of low-level precursor see Li 2012 for a review). Whereas the favorable disturbances associated with tropical cyclogenesis in theWNPare1)Rossbywavetrainsinducedbyenergy dispersion of a preexisting TC (TCED), 2) northwest– * School of Ocean and Earth Science and Technology Contri- bution Number 8926 and International Pacific Research Center southeast-oriented synoptic wave trains (SWTs, some- Contribution Number 979. times referred to as tropical depression type (TD type) disturbances) unrelated to TC energy dispersion, and Corresponding author address: Tim Li, IPRC, and Dept. of 3) Pacific easterly waves (EWs; Fu et al. 2007). Meteorology, University of Hawaii at Manoa, Honolulu, HI 96822. The first type of precursor synoptic-scale disturbances E-mail: [email protected] for cyclogenesis in the WNP is associated with TCED.

DOI: 10.1175/WAF-D-12-00104.1

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A Rossby wave train with alternating anticyclonic and (QuikSCAT) wind data, Fu et al. (2007) found that 21% cyclonic vorticity perturbations formed in the wake of of cyclogenesis events in the WNP were associated with a preexisting TC due to its Rossby wave energy disper- EWs during the summers of 2000 and 2001. This is in sion (Frank 1982; Flier 1984; Holland 1995; McDonald contrast to Chen et al. (2008), who suggested that 80% 1998; Li et al. 2003; Krouse et al. 2008; Krouse and Sobel of TC formations in the WNP are due to the influence of 2010). Although TCED is essentially of a barotropic EWs either directly or indirectly. Therefore, what per- vorticity dynamics nature (Carr and Elsberry 1995), centage of TC formation in the WNP is induced by EWs a Rossby wave train associated with a realistic 3D TC remains unclear. structure has a baroclinic vertical structure (Ge et al. In addition to the aforementioned three types of 2008, 2010), and vorticity asymmetry between upper- precursor disturbances, other pathways to cyclogenesis and lower-tropospheric wave trains depends strongly on in the WNP include the influence of a tropical upper- the sign of the vertical shear of the zonal mean flow (Ge tropospheric trough (TUTT; Sadler 1976, 1978; Briegel et al. 2007). A new TC may form in the cyclonic vorticity and Frank 1997), monsoon gyres (Lander 1994; Holland region of the wave train under favorable environmental 1995; Ritchie and Holland 1999; Lee et al. 2008), equa- conditions (Li and Fu 2006; Li et al. 2006). torial waves (Frank and Roundy 2006; Schreck et al. The second type of precursor disturbance associated 2011, 2012), and cross-equatorial flows (Love 1985; Xu with TC genesis in the WNP is a northwest–southeast- 2011; Beattie and Elsberry 2012). Some of the upper- oriented wave train that does not involve a preexisting and lower-level precursor signals may occur simulta- TC. Lau and Lau (1990) showed that this TD type of neously. For example, Briegel and Frank (1997) suggested perturbation is a dominant synoptic-scale mode in the that 49% of TC genesis events have both an upper- and summertime WNP. Chang et al. (1996) examined the a lower-level feature. wave trains in the Navy Operational Global Atmo- Fu et al. (2007) noticed that during the 2000–01 WNP spheric Prediction System (NOGAPS) analysis data summer seasons 70% of cyclogenesis events are associ- and found that the wave trains are well presented re- ated with the aforementioned three types of precursor gardless of whether a TC bogus was used in the analysis. disturbances, whereas the remaining were unclassified Dickinson and Molinari (2002) attributed the genera- due to uncertainty in the relatively coarse (2.5832.58) tion of SWTs to the development of equatorial mixed resolution of the National Centers for Environmental Rossby–gravity (MRG) waves located initially near the Prediction–National Center for Atmospheric Research equator. A theoretical study by Li (2006) suggested that (NCEP–NCAR) reanalysis data. Recently, the Year of the SWT resulted from the instability of the summer Tropical Convection (YOTC) project has provided un- mean flow in the WNP regardless of the initial pertur- precedented high-resolution global analysis data with bation pattern. Recently, Wang et al. (2012) applied a so- a horizontal resolution of 0.2258 and 26 vertical levels called pouch theory (Dunkerton et al. 2009; Wang et al. expanding from the surface to 1 hPa. The unique high- 2010) to the northwestward-propagating SWT system. resolution YOTC data make it possible to identify the The third type of precursor synoptic-scale disturbance finer 3D structures and patterns of evolution of pre- is Pacific EW (Ritchie and Holland 1999). The non- cursor synoptic-scale disturbances associated with TC divergent barotropic model experiments by Kuo et al. genesis events. (2001) suggested that the scale contraction of EWs could The lower-frequency atmospheric oscillations may lead to the accumulation of kinetic energy at a critical significantly modulate TC genesis in the WNP. It has longitude where monsoon westerlies meet trade east- been suggested that the atmospheric intraseasonal os- erlies. This energy accumulation mechanism may lead to cillation [ISO, 20–90-day mode; Yamazaki and Murakami the successive development of TCs at the critical longi- (1989); Hartmann et al. (1992); Liebmann et al. (1994); tude. The origin of the Pacific EWs is from the south- Sobel and Maloney (2000); Maloney and Hartmann ward propagation of Rossby wave energy from the (2000, 2001, 2002); Maloney and Dickinson (2003); upper-tropospheric jet in the North Pacific (Tam and Li Hogsett and Zhang (2010)] may exert a large-scale 2006). Based on the pouch theory (Dunkerton et al. control on TC formation through enhancing or sup- 2009; Wang et al. 2010), EWs provide a favorable en- pressing the TD-type disturbances in the lower-level vironment for TC development. Frank (1988) reported troposphere (Zhou and Li 2010). The enhancement of that only a small percentage of EWs in the WNP the TD-type disturbances is attributed to either Rossby (;10%) developed into TCs. Using 8 yr of data from an wave accumulation caused by large-scale confluent flows Australia Bureau of Meteorology tropical analysis field, (Kuo et al. 2001) or barotropic energy conversion that Ritchie and Holland (1999) attributed 18% of WNP involves both rotational and divergent winds (Hsu et al. cyclogenesis events to EWs. Using Quick Scatterometer 2011). The quasi-biweekly oscillation (QBW) with

Unauthenticated | Downloaded 09/28/21 03:37 AM UTC 1040 WEATHER AND FORECASTING VOLUME 28 a period of 10–20 days is one of the dominant modes significant spectrum of SWTs revealed by Lau and Lau over the WNP during boreal summer (Li and Wang (1990) appears in this bandwidth. In the 10–20- and 20– 2005; Chen and Sui 2010). The perturbation associated 90-day bandwidths, the strongest response appears with the QBW has a northeast-tilted structure and around 14 and 43 days, respectively. By isolating these propagates northwestward in the WNP (Gao and Li two time-scale motions, we examine how the quasi- 2011). The QBW has been suggested to be closely as- biweekly and intraseasonal oscillations modulate cy- sociated with convectively coupled equatorial Rossby clogenesis activity. waves and affect TC genesis in the WNP (Li and Wang Different types of synoptic-scale perturbations as TC 2005; Kikuchi and Wang 2009; Gao and Li 2011). precursors are identified based on the following ap- The rest of the paper is organized as follows. In section proach. By examining the daily synoptic-scale maps of 2, the data and analysis methods are described. Pre- low-level winds prior to TC genesis, we group cyclo- cursor synoptic-scale disturbances during the YOTC genesis events into four categories. If a TC formed in the (2008–09) are analyzed in section 3. In section 4, the cyclonic circulation embedded within a wave train pro- impact of the atmospheric QBW and ISO on TC genesis duced by a preexisting TC, this genesis event belongs to during the 2-yr period is further examined. A summary the TCED category. If a TC formed within a synoptic is given in section 5. wave train that does not involve a preexisting TC, it belongs to the SWT group. If a TC formed in association with the pronounced westward propagation of pertur- 2. Data and analysis methods bation kinetic energy, vorticity, and total column water The primary dataset used in this study is the high- vapor, it belongs to the EW group. If two or more of the resolution global analysis from the Year of Tropical above genesis scenarios were involved, it belongs to Convection (YOTC) program, a coordinated research a combined cyclogenesis group. program jointly conducted by the World Weather Re- To examine to what extent TCs may affect 3–10-day search Program [WWRP/The Observing System Re- filtered wind fields, we compared the results with and search and Predictability Experiment (THORPEX)] without the removal of TCs. The TC removal technique and the World Climate Research Program (WCRP) follows Schreck et al. (2011), in which a Gaussian including observing, modeling, and forecasting of orga- function with a radius of 500 km is applied. The results nized tropical convection. Based on decadal substantial show that the influence of TCs on the filtered synoptic- investments in the earth science infrastructure, in- scale wave train pattern is small. This implies that the cluding satellite observing systems and operational buoy TC impact on the wind is not as significant as the rainfall arrays in each of the tropical oceans, the European field shown in Schreck et al. (2011). Centre for Medium-Range Weather Forecasts (ECMWF) generated a global analysis dataset with a horizontal 3. Characteristics of four cyclogenesis groups resolution of 0.2258 at 26 standard pressure levels for a period of 2 yr from 1 May 2008 to 30 April 2010 Twenty-five WNP TC genesis events during the boreal (Waliser et al. 2011). A more detailed description of the summer seasons of 2008 and 2009 were investigated. The YOTC dataset can be found online (www.ucar.edu/ selection of these 25 events was based on the following YOTC). The other data used in this work include TC criteria: 1) the genesis location appears south of 308N best-track data from the Joint Typhoon Warning Center and 2) the maximum surface wind speed during its life 2 (JTWC) and observed daily outgoing longwave radia- cycle exceeds 35 kt (kt 5 0.51 m s 1). Table 1 lists the tion (OLR) data from the National Oceanic and At- names of the 25 TC cases selected, along with the mospheric Administration (NOAA). The OLR field has characteristics of their identified pregenesis disturbance a horizontal resolution of 2.5832.58 and will be used to types. Among the 25 TCs, 24 (96%) are associated with identify the atmospheric low-frequency oscillations in- the aforementioned four genesis groups, with 6 cases cluding the QBW and ISO. being associated with TCED, 8 associated with SWT, Bandpass filtering (Murakami 1979) is applied to 4 cases are associated with EW, and 6 cases are associ- separate the synoptic-scale (3–10 day), quasi-biweekly ated with the combined genesis scenario. Among the six (10–20 day), and intraseasonal (20–90 day) components combined genesis cases, two belong to TCED-EW and from the original dataset. In the 3–10-day bandwidth, four belong to SWT-EW. If one counts a combined the strongest response is around 5 days, with a maximum genesis event as one-half of an event for each of the response of 1.0. By isolating the synoptic-scale (3–10 involved groups, the percentages for cyclogenesis asso- day) motion, one may examine precursor synoptic per- ciated with TCED, SWT, and EW during the 2 yr are turbation signals prior to TC genesis. In fact, the most 28%, 44%, and 24%, respectively. The relative contribution

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TABLE 1. Classiﬁcation of the 25 TCs between 2008 and 2009 (month the TC occurred is shown in parentheses).

Genesis type 2008 2009 TCED Kammuri (Aug), Jangmi (Sep), Mekkhala (Sep) Etau (Aug), Parma (Sep), Melor (Sep) SWT Fengshen (Jun), Kalmaegi (Jul) Linfa (Jun), Molave (Jul), Goni (Aug), Krovanh (Aug), Koppu (Sep), Dujuan (Sep) EW Fung-wong (Jul), Vongfong (Aug), Hagupit (Sep) Morakot (Aug) Combined TCED-EW Higos (Sep) Nangka (Jun) SWT-EW Nuri (Aug), Sinlaku (Sep) Ketsana (Sep), Choi_wan (Sep) Unclassified Vamco (Aug) of the three genesis scenarios is quite similar to that train in their wakes, and among them 8 resulted in analyzed by Fu et al. (2007), although the absolute value a subsequent TC genesis event. of the genesis percentage for each group is greater. By examining the vertical structure of these Rossby In the following we will examine each of the four wave trains, we noted that the maximum vorticity as- genesis scenarios. sociated with the wave trains mostly formed in the lower troposphere. This is consistent with 3D TC energy dis- a. Cyclogenesis associated with TCED persion characteristics (Ge et al. 2008). Therefore, a The analysis of 3–10-day filtered YOTC wind fields general feature for this category is that TCED-induced shows that 16 out of the 25 TCs induced a Rossby wave Rossby wave trains are confined at lower levels throughout

21 25 21 FIG. 1. A 1-day sequence of the 3–10-day ﬁltered wind (vector, m s ) and relative vorticity (shaded, 10 s )at 850 hPa for (a) 0000 UTC 21 Sep, (b) 0000 UTC 22 Sep, (c) 0000 UTC 23 Sep, and (d) 0000 UTC 24 Sep 2008. A red typhoon mark denotes the genesis location of TC Jangmi, which formed at 0000 UTC 24 Sep 2008 in the Rossby wave train of preexisting TC Hagupit (denoted by a black typhoon mark). The centers of the anticyclone and cyclone within the wave train are labeled as A and C, respectively.

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25 21 FIG. 2. Vertical cross section of the 3–10-day filtered vorticity (shaded, 10 s ) and horizontal crosswind velocity 2 (contour, m s 1) along the wave train axis shown in Fig. 1 for (a) 0000 UTC 21 Sep, (b) 0000 UTC 22 Sep, (c) 0000 UTC 23 Sep, and (d) 0000 UTC 24 Sep 2008. the development. However, energy dispersion associated formation of Jangmi at 0000 UTC 24 September 2008 with Typhoon Hagupit (2008) appears to be a special case. (Fig. 1d). Jangmi then developed into a supertyphoon on The evolution of the Rossby wave train associated 27 September 2008, when the observed maximum surface with Typhoon Hagupit (2008) and subsequent genesis of wind exceeded 140 kt. TC Jangmi (2008) is shown in Fig. 1. Jangmi’s first The vertical cross section of the TCED-induced wave warning was issued by JTWC at 1800 UTC 23 September train and its evolution are shown in Fig. 2. Here, each 2008. The precursor synoptic signal prior to the Jangmi vertical cross section is plotted along the wave train axis formation can be traced back to 21 September 2008, (i.e., black line in each panel of Fig. 1). In Fig. 2 we show about 3 days prior to the JTWC warning (Fig. 1a). At both the 3–10-day filtered vorticity and the wind com- that time a northwest–southeast-oriented Rossby wave ponent normal to the wave train axis. The latter is de- train was clearly seen, in association with a previous fined to be positive (negative) if air flows from southwest typhoon (Hagupit, during 2008, whose center is denoted (northeast) to northeast (southwest). It is interesting by a black typhoon mark in Fig. 1). The Rossby wave to note that during the early development stage, the train had a typical wavelength of 2500 km and was maximum vorticity and circulation of the wave train composed of alternate anticyclonic and cyclonic cir- are confined in the middle troposphere (around 700– culation patterns. During the initial stage of Hagupit, 400 hPa; Fig. 2a). This feature appears in both the anticyclonic circulation in its wake was weak (Fig. 1a). As cyclonic and cyclonic circulations of the wave train. the storm intensified, the wave train strengthened due With the continuing strengthening of Hagupit, the wave to the TC energy dispersion. The weak cyclonic circula- train further developed and stretched in the vertical, tion in the wake seen in Figs. 1a and 1b developed into extending from the surface to about 200 hPa (Figs. 2c a strong cyclone with a closed circulation at 0000 UTC and 2d). Both the anticyclonic and cyclonic circulations 23 September 2008 (Fig. 1c, with its center labeled as C). of the wave train during this stage show a quasi-barotropic The strengthening of the wave train eventually led to the structure below 200 hPa.

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The stronger midlevel vorticity during the initial developing stage implies that Jangmi underwent a typical ‘‘top down’’ developing process in which a precursor perturbation signal first appeared in the middle level (Fu et al. 2007; Li 2012) and then developed from a weak and shallow system (Fig. 2a) into a strong and deep one (Fig. 2d) prior to Jangmi’s formation. An E-vector approach (Hoskins et al. 1983; Trenberth 1986; Sobel and Bretherton 1999; Li and Fu 2006) may be used to measure the Rossby wave energy dispersion. The E vectors are defined as [(2u0u0 1 y0y0)/2, 2 u0y0], where the overbar denotes time averaging, and u0 and y0 denote zonal and meridional wind components associated with synoptic-scale disturbances, respectively. The E vectors indicate energy propagation during a specified period. Figure 3a shows the horizontal map of the calculated E vector at 500 hPa during the wave train development stage (from 0000 UTC 20 September to 0000 UTC 24 September 2008). The results show that as the previous TC (denoted by a black typhoon mark each day in Fig. 3) moved northwestward, it emitted energy southeastward, leading to the strengthening of systems behind it, including not only the wake cyclone (C in Figs. 1c and 2c), in which TC Jangmi (a red typhoon mark in Fig. 3) formed later, but also the anticyclone (A in Figs. 1 and 2) before the cyclone (C). By comparing the E vectors at each vertical level (from the surface to 300 hPa), we note that the greatest energy propagation occurs at 500 hPa (Fig. 3b). This seems to explain why the anomalous circulation in the wake developed first in the middle troposphere. The result above suggests a new midlevel vortex genesis scenario, in which TC Rossby wave energy dispersion plays an essential role. This differs from the classical scenario proposed by Simpson et al. (1997), in which evaporative cooling of raindrops under stratiform clouds set up a midlevel vortex. A calculation of E vectors for other TCED cases shows that maximum energy dispersion indeed occurred in the lower troposphere. The energy dispersion feature is consistent with the vertical structure of corresponding Rossby wave trains. A composite E vector calculated based on a 4-day period prior to the genesis of each TC in the TCED groupincludedinTable1ispresented in Fig. 3c. Figure 3c 2 22 indicates that an averaged new TC appeared to the east- FIG. 3. (a) E vectors (m s ) at 500 hPa calculated based on southeast of the previous existing composite TC. Krouse a 4-day period prior to the genesis of Jangmi. (b) The vertical cross et al. (2008) suggested that nonlinearity could alter the section of the E-vector component along the red line in (a). The structure of the wave trains, making them more zonally genesis location of Jangmi is denoted by a red typhoon mark, and four black typhoon marks denote the locations of the preexisting oriented, as opposed to northwest–southeast oriented. TC during the 4-day period. (c) Composite E vector at 850 hPa b. TC genesis associated with SWT calculated based on a 4-day period prior to the genesis for each of the TCs in the TCED group, as shown in Table 1. The location of During the 2008 and 2009 typhoon seasons, 8 out of the composite previous TC is denoted by a black typhoon mark and the 25 TC genesis cases were related to SWTs. The the composite new TC by a red typhoon mark.

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FIG. 4. As in Fig. 1, but for a time sequence of synoptic disturbances (at a 36-h interval) for the genesis of TC Molave (denoted by a red typhoon mark in Fig. 4d) at 1800 UTC 16 Jul 2009 and the evolution of a precursor SWT from (a) 0600 UTC 12 Jul to (c) 0600 UTC 15 Jul 2009. The centers of the anticyclones and cyclones in the SWT are labeled as A and C, respectively.

formation of TC Molave (2009) is an example (Fig. 4). wave train and was composed of well-defined cyclonic The first warning by JTWC for TC Molave was issued at and anticyclonic circulation patterns with a wavelength 0600 UTC 15 July 2009. At 1800 UTC 16 July 2009, 36 h of about 2500 km, covering a region between 08–258N later, it became a tropical storm with maximum surface and 1208–1508E (Fig. 4c). As the SWT continues inten- winds of 35 kt. It later developed into a typhoon (TY) sifying and moving northwestward, TC Molave formed and made landfall in Hong Kong. The 3–10-day filtered at 1800 UTC 16 July in the cyclonic vorticity region of 850-hPa wind field can clearly identify the synoptic wave the wave train (Fig. 4d). train pattern as early as 1800 UTC 11 June 2009, 5 days Figure 5 illustrates the vertical cross section of the prior to the TC genesis. Figure 4 shows the time sequence synoptic-scale vorticity along the wave train axis. Dif- maps (every 36 h) of the SWTs from 0600 UTC 12 July to ferent from the TCED case shown in Fig. 2, the strongest 1800 UTC 16 July. During the early stage, cyclonic (la- perturbation vorticity was initially confined in the lowest beled as C) and anticyclonic (labeled as A) centers were level (Fig. 5a). It gradually extended upward, and by not aligned along a straight line and the systems appeared 1800 UTC 13 July, the positive vorticity has penetrated weak and shallow (Figs. 4a and 5a). The cyclonic and into the upper troposphere. The maximum vorticity, anticyclonic centers gradually aligned into a northwest– however, is still kept in the lower troposphere (Figs. 5c southeast-oriented line, as they intensify and deepen and 5d). It appears that this cyclogenesis event resembles (Figs. 4b and 5b). Note that there is no preexisting a typical ‘‘bottom up’’ development process. The time tropical cyclone associated with these waves. On 15 July, evolution of area-averaged vorticity, divergence, verti- the SWT became a typical northwest–southeast-oriented cal motion, and relative humidity fields averaged over

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FIG. 5. As in Fig. 2, but at (a) 0600 UTC 12 Jul, (b) 1800 UTC 13 Jul, (c) 0600 UTC 15 Jul, and (d) 1800 UTC 16 Jul 2009 along the SWT axis shown in Fig. 4. a 1.8831.88 domain centered at the maximum 850-hPa or underestimating the influence of the EW, in addition vorticity are shown in Fig. 6. At day 25, positive vorticity to examining the horizontal maps of synoptic-scale dis- is confined in the lower troposphere (below 700 hPa). It turbances prior to TC formation, we also examined the gradually expands upward as the TC develops (Fig. 6a). time–longitude cross section of the 3–10-day filtered The increase in the synoptic-scale vorticity is closely kinetic energy, vertical vorticity, and total-column water linked to low-level convergence (Fig. 6b). A marked in- vapor fields along the latitudinal band (58 width) where crease in relative humidity throughout the troposphere a TC formed. The results show that 4 out of 25 TC happened prior to genesis time, indicating the important genesis cases were associated with the EW during the role of deep-layer moistening in preconditioning TC 2008–09 typhoon seasons. genesis (Nolan 2007; Ge et al. 2013). These features Figure 7 shows the formation of TC Hagupit (2008) as support the bottom-up development hypothesis. an example. A warning for Hagupit was first issued by The genesis event of TC Molave shown above is JTWC at 1200 UTC 17 September 2008. After 42 h, at representative of all of the genesis events in the SWT 0600 UTC 19 September 2008, it developed into a trop- group. In these cases, maximum vorticity was confined at ical storm with maximum surface winds of 35 kt. It later low levels, suggesting that the bottom-up mechanism became a category 4 typhoon with a maximum surface may play a dominant role in the SWT group as shown by wind speed of 125 kt. Hagupit made its landfall in the developing process for Molave. Guangdong Province, China, and was the first known category 4 typhoon to hit the province. The estimated c. TC genesis induced by EW damage was around $1 billion (U.S. dollars) and at least A subjective but strict method was applied to identify the 67 people were killed (Bell and Montgomery 2010). The EW-induced cyclogenesis events. To avoid overestimating easterly wave signal associated with the formation of

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25 21 FIG. 6. Time–vertical section of the 3–10-day ﬁltered (a) vorticity (greater than 10 s shaded), (b) divergence 2 2 2 (negative values shaded, 10 5 s 1), (c) vertical p velocity (negative values shaded, Pa s 1), and (d) relative humidity (greater than 5% shaded) averaged over a 1.8831.88 domain centered at the maximum cyclonic vorticity region of the SWT where TC Molave formed (day 0 corresponding to 1800 UTC 16 Jul 2009; TC Molave’s genesis time).

Hagupit can be traced back 7 days prior to its genesis. genesis time. In the early stage, the wave intensity ap- The 3–10-day filtered 850-hPa wind field shows a closed pears weak, as it propagates slowly westward. From day cyclonic circulation pattern at low levels 7 days prior to 23 to day 0, the wave perturbation energy and vorticity Hagupit’s genesis (Fig. 7a). During the early 4-day pe- intensify rapidly, while the wave phase speed increases riod (Figs. 7a–c), the disturbance moved westward but slightly compared to the earlier period. It is worth noting its intensity was rather weak. A closed cyclonic circu- that there are zonal phase differences among the wave lation can be seen in Fig. 7a, and it becomes open in Figs. perturbation energy, vorticity, and total-column water 7b and 7c. The disturbance started to intensify rapidly at vapor fields. It appears that the perturbation energy 0600 UTC 17 September, when a closed cyclonic circu- leads the vorticity, while the latter leads the total- lation pattern could be clearly seen (Figs. 7d and 7e) and column water vapor. The exact cause of such a phase the TC formed on 19 September (Fig. 7f). No clear relationship is unknown at the moment. It is speculated northwest–southeast-oriented wave train was observed that the vorticity leading may be attributable to the in the genesis region. The time-sequence maps of the Rossby wave response to the convective heating (Hsu synoptic perturbations clearly indicate that Hagupit’s et al. 2011), whereas the perturbation energy leading formation arose from a precursor EW perturbation. may be attributable to zonal and meridional wave To confirm that Hagupit’s genesis originates from an structures. EW perturbation, we examined the longitude–time The time evolution of the vertical profile of the area- section of the perturbation kinetic energy, vorticity, and averaged synoptic-scale vorticity shows a bottom-up total-column water vapor (Fig. 8). The EW signals from developing process for this EW-induced cyclogenesis these fields can be traced back to 6–7 days prior to the case (figure not shown). The maximum cyclonic vorticity

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FIG. 7. As in Fig. 1, but for a time sequence of synoptic disturbances for the genesis of TC Hagupit (denoted by a red typhoon mark) at 0600 UTC 19 Sep 2008 and the evolution of a precursor EW at (a) 0600 UTC 12 Sep, (b) 0600 UTC 14 Sep, (c) 0600 UTC 16 Sep, (d) 0600 UTC 17 Sep, (e) 0600 UTC 18 Sep, and (f) 0600 UTC 19 Sep 2008. A black dot in (f) denotes the same maximum vorticity location in (a).

was mainly confined in the lower troposphere; positive Various factors may contribute to the EW inten- vorticity penetrated into the upper troposphere as the sification. First, as the wave moves westward to warmer disturbance developed. As in the SWT case (Fig. 6), SST regions, environmental moisture increases; this fa- there was oscillatory development in the area-averaged vors greater latent heat release. Second, as the wave vorticity, divergence, vertical velocity, and relative hu- moves toward a critical longitude where the monsoon midity fields. westerly meets the trade easterly, the wave energy may

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d. Combined genesis type

Among the 25 TC genesis events, 6 were associated with the combination of different genesis scenarios, either combined SWT-EW cases or combined TCED-EW cases. The genesis of TC Sinlaku is an example of the combined SWT-EW scenario. It is worth mentioning that Sinlaku eventually developed into a supertyphoon and became one of the most impacting TCs in the WNP during 2008. The time sequence of the 3–10-day filtered 850-hPa wind field shows clearly the evolution of an EW, from 0600 UTC 5 September to 1800 UTC 6 September 2008 (Figs. 9a–c). The EW was composed of an open anticyclonic circulation (ridge, denoted by an A at 208N, 1458E) and a cyclonic circulation (denoted by a C at 178N, 1608E) at 0600 UTC 5 September 2008 (Fig. 9a). Eighteen hours later, a trough (denoted by a red dashed line) associated with the EW developed (Fig. 9b), as it slowly moves westward. Note that during the same period, a northwest–southeast-oriented SWT with a combined cyclonic and anticyclonic circulation pattern appeared to the west of 1308E (purple line in Fig. 9a). As the SWT intensified, it overlapped with the EW, leading to the formation of a closed cyclonic circulation at 148N, 1308E (denoted by a C at 148N, 1318E in Fig. 9c). After the formation of the cyclonic circulation, the EW seemed to dissipate slightly, while the SWT continued intensifying as it moved northwestward. At 1200 UTC 8 September 2008, a new TC, identified as Sinlaku, formed in the cyclonic vorticity region of the SWT. The rapid development of Sinlaku during its genesis stage may be attributable to the reenforced cyclonic flow from the EW and SWT. It is likely that both the EW and SWT may affect the cyclogenesis through wave energy dispersion and accumulation. To demonstrate this, we calculated E vectors during the period centered at 1800 UTC 6 September. Figure 10 shows that during the period there was clear southeastward energy dispersion associated with the SWT, as well as westward energy dispersion associated with the EW. The convergence of these wave activity fluxes led to energy accumulation in the genesis region. FIG. 8. Time–longitude cross section of the 3–10-day filtered The formation of TC Higos is a genesis example that 2 (a) perturbation kinetic energy (m2 s 2) at 850 hPa, (b) vorticity 25 21 22 was influenced by both the EW and TCED. Figure 11 (10 s ) at 850 hPa, and (c) total-column water vapor (kg m ) illustrates the daily sequence of 3–10-day filtered 850-hPa along the W–E line shown in Fig. 7f (day 0 corresponding to 0600 UTC 19 Sep 2008). wind fields from 22 to 29 September. On 22 September, there was a clear Rossby wave train associated with the energy dispersion of the preexisting TC Hagupit (Fig. accumulate (Kuo et al. 2001). The decrease in zonal 11a). Two days later, TC Jangmi formed at 138N, 1358E wavelength associated with the deceleration of the mean (Fig. 11c). Meanwhile, a weak EW may be identified easterly may also lead to wave activity flux convergence east of 1508E (with the A and C denoting anticyclonic near the critical longitude (Tam and Li 2006). and cyclonic vorticity, respectively). As Jangmi rapidly

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FIG. 9. As in Fig. 1, but for the genesis of TC Sinlaku (denoted by a red typhoon mark) at 1200 UTC 8 Sep 2008 and the evolution of an SWT and an EW at (a) 0600 UTC 5 Sep, (b) 0000 UTC 6 Sep, (c) 1800 UTC 6 Sep, (d) 1200 UTC 7 Sep, (e) 0000 UTC 8 Sep, and (f) 1200 UTC 8 Sep 2008. Purple lines show the axis of either the SWT or the EW. A red dashed line in (b) denotes the trough of the EW.

intensified and moved northwestward, it emitted Rossby over the previous red triangle region (Figs. 11d and 11e). wave energy southeastward, leading to the development Two days later, a new TC named Higos (labeled as a red of a closed anticyclone circulation (labeled as A at 88N, typhoon mark in Fig. 11h) formed in the closed cyclonic 1408E in Fig. 11d). Meanwhile, the EW also strength- circulation region. A calculation of E vectors shows that ened as it moved westward. As a result of the combined indeed there was wave activity flux convergence in the forcing of the TCED and EW, a closed cyclonic circula- cyclone-developing region due to energy dispersion tion (labeled as C at 88N, 1408E in Fig. 11f) developed from Jangmi (figure not shown).

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2007; Gao and Li 2011). As seen from Figs. 13e and 13f, there is only one exception in which the combined 10– 20- and 20–90-day OLR anomaly is positive. It was related to the formation of TC Higos (last TC shown in Fig. 13) in 2008. Higos formed due to the triggering of synoptic perturbations associated with both an EW and a Rossby wave train, as shown in Fig. 11. To more strictly define an active phase of QBW or ISO, we use half of the OLR standard deviation averaged over 58–208N, 1308–1608E during the 2008 and 2009 typhoon seasons as a threshold for an active phase. The results show that 68% and 64%, respectively, of TC genesis cases during the 2 yr occurred in the region where the QBW and ISO are in an active phase, and 80% of the cases occurred in the region where the combined QBW 2 22 FIG. 10. E vectors (m s ) at 850 hPa calculated based on a 3-day and ISO modes are in an active phase. The same meth- period centered on 1800 UTC 6 Sep 2008. Purple lines are the same odology is also applied to the wind field. We note that SWT and EW axes shown in Fig. 9c. 60% of genesis cases occurred in the region where the combined QBW and ISO modes have a westerly anomaly 2 of greater than 2 m s 1. While the TCED-induced wave train effect is quite The composite patterns of the filtered OLR and obvious, as seen from in Fig. 11, the EW effect appears 850-hPa wind fields associated with the QBW and ISO weaker. To demonstrate the EW effect, we plotted the modes during the cyclogenesis time are shown in Fig. 14. time–longitude section of perturbation kinetic energy, The TC genesis is associated with the active phase of vorticity, and total-column water vapor in Fig. 12. There QBW and ISO, with favorable low-level cyclonic cir- was clear westward propagation of wave kinetic energy, culation, ascending vertical motion, and higher low- vorticity, and water vapor. The EW signals from these level and midtropospheric relative humidity. Note that fields can be clearly seen 4 days prior to the formation of the horizontal scale of the QBW mode is less that its ISO Higos. Compared to the pure EW case in Fig. 8, the counterpart, while their intensities are comparable. amplitude of the EW in this combined genesis case is weaker, implying that the TCED process may play 5. Summary and discussion a more important role in this particular TC genesis event. The high-resolution (0.2258) YOTC analysis data were used to analyze precursor synoptic-scale wave features associated with TC genesis in the WNP during 4. Modulation of cyclogenesis by low-frequency the 2008–09 typhoon seasons. Four genesis scenarios waves were identified based on low-level synoptic-scale pre- In this section we examine the impacts of the 10–20- cursor signals. They are associated with energy disper- day (QBW) and 20–90-day (ISO) modes on WNP TC sion of a preexisting TC (TCED), synoptic-scale wave genesis during the 2008–09 summer seasons. The com- train (SWT), Pacific easterly wave (EW), and the combined influence of the two modes is also examined. bination of TCED-EW or SWT-EW. Figure 13 shows the area-averaged low-frequency OLR Among the 25 cyclogenesis cases investigated, 6 and zonal wind anomalies for the 25 TC genesis cases. (24%) are associated with TCED, 8 (32%) are associ- Here, the OLR anomaly was calculated based on a 583 ated with SWT, 4 (16%) are associated with EW, and 6 58 box centered at the TC genesis location, whereas the (24%) are associated with the combined genesis sce- zonal wind anomaly was calculated based on a same- nario. There is one unclassified case in which we could sized box centered at 2.58 south of the TC genesis loca- not identify a significant precursor signal in either the tion. As one can see from Fig. 13, 92% and 80% of TC lower or upper levels. genesis cases during the 2 yr occurred in the region One interesting cyclogenesis case, Typhoon Jangmi, where the QBW and ISO, respectively, have a negative associated with TCED indicates that the strongest OLR anomaly. This suggests that TC activities are, to Rossby wave energy dispersion from the previous Ty- a large extent, coupled to both the QBW and ISO phoon Hagupit does not happen in the lower level; in- modes, consistent with previous studies (e.g., Fu et al. stead, its maximum energy dispersion appeared in the

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FIG. 11. As in Fig. 1, but for the genesis of TC Higos (denoted by a red typhoon mark) at 1200 UTC 29 Sep 2008 and the evolution of a TCED-induced Rossby wave train (with the preexisting TC denoted by a black typhoon mark) and an EW at (a) 1200 UTC 22 Sep, (b) 1200 UTC 23 Sep, (c) 1200 UTC 24 Sep, (d) 1200 UTC 25 Sep, (e) 1200 UTC 26 Sep, (f) 1200 UTC 27 Sep, (g) 1200 UTC 28 Sep, and (h) 1200 UTC 29 Sep 2008. A red triangle in (d) and (e) denotes the region where a cyclone forms due to interaction of the wave train and EW.

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EW scenarios, on the other hand, showed a typical ‘‘bottom up’’ development process, with maximum vorticity always appearing in the lower troposphere. A common feature associated with all of the aforementioned genesis scenarios is that the precursor circulation was usually weak and shallow in the early stages, but developed into a deep-layer cyclonic system with closed circulation, strong vorticity, and a near- saturated column in the later stages prior to genesis. Intensification appeared not only in the cyclonic circulation (where a TC finally formed), but also in the anticyclonic circulation. For example, in the cases of TCED and SWT, not only did the cyclonic vorticity region of the wave train develop, but the anticyclonic region of the wave train developed as well. In the case of EW, both the trough and ridge intensify with time. Another interesting finding based on the YOTC data analysis is that sometimes two or more genesis precursors can be identified and they worked together to trigger cyclogenesis. For instance, Sinlaku was a result of combined SWT and EW forcing; Higos formed in the both TCED and EW scenarios. While identifying the coexistence of two or more precursor disturbances, we do not know the relative importance of these disturbances in causing TC formation. Further sensitivity numerical experiments are needed in order to isolate different triggering processes. The percentage of TC genesis events associated with TCED during the 2-yr (2008–09) period is 32%, which is consistent with the result (approximately 30%) obtained for a much longer period [1948–2005; Krouse and Sobel (2010)]. However, caution is needed when comparing the two results as the methods used to identify the TCED events differ. While a detailed wave train structure was tracked in the current study, in Krouse and Sobel (2010) any TCs that formed within a distance of 5000 km to the east of an existing TC were counted. The YOTC data analysis reveals a close relationship between the atmospheric low-frequency oscillation (including QBW and ISO) and TC genesis. Twenty-four out of 25 genesis cases occurred when the combined QBW and ISO modes have a negative OLR anomaly. It was found that 80% of the genesis events occurred in the FIG. 12. Time cross section of the 3–10-day filtered (a) pertur- region where the combined QBW and ISO modes are in 2 2 2 bation kinetic energy (m2 s 2) at 850 hPa, (b) vorticity (10 5 s 1)at 2 an active phase. 850 hPa, and (c) total-column water vapor (kg m 2) along the black Caution is also needed in interpreting the temporally line shown in Fig. 11h (day 0 corresponding to 1200 UTC 29 Sep 2008). filtered wave train patterns, because the strength of these wave train patterns may be overestimated due to the presence of tropical cyclones in the original re- midtroposphere. As a result, a midlevel vortex de- analysis data. Schreck et al. (2011) proposed a way to veloped initially in the wake of Hagupit, and subsequent remove TC signals with the use of a Gaussian function for cyclogenesis underwent a ‘‘top down’’ development a 500-km radius. By applying this TC removal technique process. Genesis events associated with the SWT and to the YOTC data, we noted that the wave train intensity

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22 21 FIG. 13. The 58358 box-averaged OLR (red bars, W m ) and 850-hPa zonal wind (black bars, m s ) anomalies associated with the (top) QBW, (middle) ISO, and (bottom) combined QBW and ISO centered at the TC genesis location during the genesis time. Horizontal axis denotes the day from 1 Jun to 30 Sep for (left) 2008 and (right) 2009. The 850-hPa zonal wind was calculated 58 south of the genesis center. indeed became weaker, even though the wave train pat- formula was used to illustrate energy dispersion charac- tern remained. An open issue related to this is how to teristics. Caution is needed here as well, as there might be make a clean removal of TC signals without impacting some nontrivial issues in cases where the time-mean ﬂow other scale motions. In this study a barotropic E-vector is complex or the averaging period is too short. It might

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21 22 FIG. 14. Composite patterns of 850-hPa wind (vector, m s ) and OLR (shaded, W m ) associated with the QBW, ISO, and combined QBW and ISO during (left) 2008 and (right) 2009. The vertical (horizontal) axis denotes the relative distance in degrees latitude (longitude) to the genesis location.

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