Importance of Tropical Cyclone Heat Potential for Tropical Cyclone Intensity and Intensification in the Western North Pacific

Journal of Oceanography, Vol. 63, pp. 427 to 447, 2007

AKIYOSHI WADA* and NORIHISA USUI

Meteorological Research Institute, Japan Meteorological Agency, Nagamine, Tsukuba, Ibaraki 305-0052, Japan

(Received 2 October 2006; in revised form 5 December 2006; accepted 11 January 2007)

Which is more important for tropical cyclone (TC) intensity and intensification, sea Keywords: surface temperature (SST) or tropical cyclone heat potential (TCHP)? Investigations ⋅ Tropical cyclone using best-track TC central pressures, TRMM/TMI three-day mean SST data, and heat potential, ⋅ an estimated TCHP based on oceanic reanalysis data from 1998 to 2004, show that tropical cyclone the central pressure is more closely related to TCHP accumulated from TC formation intensity, ⋅ sea surface to its mature stages than to the accumulated SST and its duration. From an oceanic temperature, environmental viewpoint, a rapid deepening of TC central pressure occurs when TCHP ⋅ rapid intensifica- is relatively high on a basin scale, while composite distributions of TCHP, vertical tion. wind shear, lower tropospheric relative humidity, and wind speed occurring in cases of rapid intensification are different for each TC season. In order to explore the influence of TCHP on TC intensity and intensification, analyses using both oceanic reanalysis data and the results of numerical simulations based on an ocean general circulation model are performed for the cases of Typhoons Chaba (2004) and Songda (2004), which took similar tracks. The decrease in TCHP due to the passage of Chaba led to the suppression of Songda’s intensity at the mature stage, while Songda maintained its intensity for a relatively long time because induced near-inertial currents due to the passage of Chaba reproduced anticyclonic warm eddies appearing on the leftside of Chaba’s track before Songda passed by. This type of intensity-sustenance process caused by the passage of a preceding TC is often found in El Niño years. These results suggest that TCHP, but not SST, plays an important role in TC intensity and its intensification.

1. Introduction sification, SST or tropical cyclone heat potential (TCHP)? One of the decisive factors influencing the tropical This issue continues to be controversial. According to the cyclone (TC) intensity and its intensification is ocean ther- reply of Scharroo (2006) to the comment of Sun et al. mal energy in the upper ocean. The relationship between (2006) on the importance of dynamic topography in the TC potential intensity and sea surface temperature (SST) intensification of Hurricane Katrina (2005), high SST is has been discussed for the past decade (e.g. DeMaria and a necessary but insufficient condition for hurricane in- Kaplan, 1994a). The intensity-SST relationship also plays tensification (Scharroo et al., 2005). an important role in statistical intensity prediction The ocean thermal energy is defined as TCHP (Gray, schemes such as the National Hurricane Center Statisti- 1979) and is calculated by summing the heat content in a cal Hurricane Intensity Prediction Scheme (SHIPS) column where the sea temperature is above 26°C (Leipper (DeMaria and Kaplan, 1994b, 1999; DeMaria et al., 2005). and Volgenau, 1972). A conventional methodology for However, recent studies focus on the relationship between estimating TCHP using the European Remote Sensing the intensity and the upper ocean heat content. Which is Satellite-2 (ERS-2) and TOPEX/Poseidon satellite more important for the potential intensity and the inten- altimetry observations has recently been developed (e.g. Shay et al., 2000). We can find the results of real-time monitoring of TCHP (Goni and Trinanes, 2003) via the * Corresponding author. E-mail: [email protected] Web page (http://www.aoml.noaa.gov/phod/cyclone/data/ Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer [cited 30 September 2006]).

427 (a) Formation (b) Development 40 40 0

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4 Latitude Latitude 2 20 4 20 6

10 6 10

20 100 110 120 130 140 150 160 170 180 100 110 120 130 140 150 160 170 180 Longitude Longitude

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100 110 120 130 140 150 160 170 180 100 110 120 130 140 150 160 170 180 Longitude Longitude

Fig. 1. Frequency and locations of tropical cyclones at the (a) formation, (b) development, (c) mature, and (d) decay stages from 1998 to 2004 using RSMC tropical cyclone best-track data. The distribution expands into the western North Pacific during the tropical cyclone formation stage, while it is concentrated around Okinawa Island as the stage shifts from development to decay.

This conventional methodology for TCHP estima- the case of Typhoon Rex (1998) that the evolution of a tion has also been applied to case studies of hurricanes in simulated TCHP was well correlated with that of the TC the North Atlantic (Shay et al., 2000; Goni and Trinanes, central pressure. Another interest is how and to what ex- 2003). These studies suggested that warm core eddies or tent warm core eddies in the western North Pacific with Loop Current in the Gulf of Mexico were important in high TCHP vary following TC passage, influencing sub- decreasing the impact of the negative feedback on the sequent TC intensity. According to Bender and Ginis intensification of hurricanes. In general, a strong wind (2000), the intensity prediction for Hurricane Fran (1996) accompanied by TCs stirs and cools the underlying sea was significantly improved when the coupled model was water. This leads to the reduction of sensible and latent run with the well-represented cold wake of Hurricane heat fluxes from the ocean to the atmosphere (e.g. Ginis, Edouard (1996). 1995). In contrast, a deep warm layer with a positive sea This paper is organized as follows. Section 2 de- surface height anomaly (SSHA) is hardly cooled by a scribes the data and the specification of the ocean gen- strong wind. This helps TCs maintain their intensity or eral circulation model used in the present study. Section intensify rapidly in places where the SSHA is relatively 3 describes the statistical relationship between minimum high. This effect has been found not only in hurricanes in central pressures (MCPs) and TCHPs around the TC the North Atlantic (Bao et al., 2000; Hong et al., 2000; center. In Section 4 we investigate the relationship be- Shay et al., 2000) but also in typhoons in the western tween locations of rapid intensification and atmospheric North Pacific (Holliday and Thompson, 1979; Lin et al., or oceanic environmental factors such as vertical wind 2005). This process is one of the environmental factors shear, lower-tropospheric relative humidity and wind ve- that control the intensity and intensification of TCs locity, and TCHP. Section 5 describes the relationship (Emanuel et al., 2004; DeMaria et al., 2005). between TCHP and intensity or intensification in the cases An interesting question regarding these TC-ocean of Typhoons Chaba (2004) and Songda (2004). The im- interactions is whether we can discuss the predictability pact of TCHP on the intensification of Chaba and Songda of TC potential intensity statistically using the above is investigated in Section 6. Section 7 explores the mentioned TCHP, instead of using SST as in the SHIPS baroclinic ocean response to a moving TC, related to the approach of DeMaria et al. (2005). The predictability is maintenance of Songda’s peak intensity, from the view- expected to apply not only to intensification but also to point of TCHP variation. In Section 8 further discusses the overall life stages of TCs. Wada (2002) reported in the relationship between MCP and TCHP accumulated

428 A. Wada and N. Usui Fig. 2. Relationship between three-day mean SST and tropical cyclone best-track central pressure from 1998 to 2004. Cross marks with gray shaded squares show the SST at the formation stage. Circles show the SST at the development stage. Filled squares show the SST at the mature stage. Triangles show the SST at the decay stage. Three empirical functions (DeMaria and Kaplan, 1994a; Whitney and Hobgood, 1997; Baik and Paek, 1998) are drawn in the same panel. The maximum potential intensity derived from the three empirical functions is usually overestimated compared to the observations.

from formation to the mature stages. A formation mecha- Figure 1 illustrates the horizontal distribution of TC nism for warm core eddies in the western North Pacific locations for each life stages. TCs are frequently formed and its impact on Songda’s intensity is also discussed in around 120°E, 20°N in the South China Sea and around Section 8. Section 9 is devoted to an overall summary. 130° to 140°E, 18°N (Fig. 1(a)). In Fig. 1(b), TCs develop west of the Philippines in the South China Sea or 2. Methods around Okinawa Island. Around Okinawa Island, TCs frequently sustain their intensity as well (Fig. 1(c)). In fact, 2.1 Tropical cyclone best-track data the horizontal distribution of TC locations in the mature We employed Regional Specialized Meteorological stage is similar to that of the development stage. How- Center (RSMC) TC best-track data obtained from 1998 ever, the TC locations in the mature stage tend to shift to 2004. The data include the TC center positions, its cen- northward compared with those in the development stage. tral pressure, and its 10-minute-averaged maximum sus- A peak in the number of TC locations in the decay stage tained wind measured four times a day, at 0600, 1200, is found along the Japan coast, partly because many ob- 1800, and 2400 UTC and so on. The data include these servations are made around Japan (Fig. 1(d)). Neverthe- elements taken eight times a day when a TC is located less, the appearance of a peak with an eastward exten- within 300 km of the coast of Japan, or every hour when sion at high latitudes is one of the characteristics of the a TC makes landfall in Japan. distribution of TC locations in the decay stage. Here, each TC central position over the ocean from 1998 to 2004 is counted in 5 × 5° bins for each life stage: 2.2 Satellite sea surface temperature TC formation, development, mature, and decay stages. The Tropical Rainfall Measuring Mission (TRMM)/ TC formation is defined as occurring when a TC reaches TRMM Microwave Imager (TMI) three-day mean SST wind speeds of at least 17 m/s. The development stage is covering January 1998 to December 2004 was used to a time when the central pressure decreases continuously, reconfirm the relationships between SST and the TC maxi- including a temporary period when the central pressure mum potential intensity (MPI) derived from empirical does not change. The post stages, defined as the mature functions of the monthly SST. The TRMM/TMI three- and decay stages, occur after the central pressure of the day mean SST covers a global region extending from 40°S TC reaches MCP. The mature stage is defined as the pe- to 40°N with a horizontal resolution of 0.25°. One of the riod from the end of the development stage to the begin- important features of TRMM/TMI microwave retrievals ning of the decay stage. The decay stage is a period dur- is that SST can be measured through clouds. ing which the central pressure rises monotonically. We also used a daily Microwave Optimally Interpo-

Importance of TCHP for TC Intensity and Intensification 429 lated (OI) SST (MW OI SST), for 2004 only. The MW OI when the SST is above 29°C. SST covers the global ocean with a horizontal resolution The quantitative difference between the results of the of 0.25°. MW OI SSTs include data from TRMM/TMI present study and those of previous studies may be caused and from Aqua/Advanced Microwave Scanning Radiom- by the representative depth of each SST. According to eter for Earth observing system (AMSR-E) satellite radi- Donlon et al. (2002), the TRMM/TMI SST (TSST) is rep- ometers. Because AMSR-E was initiated to perform ob- resentative of the SST at the bottom of the skin SST, at servations beginning in May 2002, the SST data were here depths within a thin layer (nearly 500 µm). The repre- only applied to case studies in 2004. sentative depth of the TSST or SST on a low-frequency The relationship between the three-day mean SST (6 to 10 GHz) microwave radiometer is shallower than 1 and the RSMC best-track central pressure is denoted in mm. In contrast, the representative depth of bucket SST Fig. 2 with three MPI empirical functions of the monthly (BSST) is a few meters. The difference between BSST SST (DeMaria and Kaplan, 1994a; Whitney and Hobgood, and TSST is usually above 1°C during daytime. 1997; Baik and Paek, 1998). Since some of the MPI empirical formulas (DeMaria and Kaplan, 1994a; Whitney 2.3 Atmospheric reanalysis data and Hobgood, 1997) are functions of SST and maximum In order to investigate the impact of the atmospheric wind velocity, the MPI expressed by maximum wind ve- environment on rapid intensification of TCs from 1998 locity is replaced with that expressed by central pressure, to 2004, the National Centre for Environmental Predic- using an empirical formula representing the modified ver- tion (NCEP)—Department of Energy Atmospheric Model sion of Atkinson and Holliday’s wind-pressure relation- Intercomparison Project reanalysis (abbreviated NCEP R- ship (Atkinson and Holliday, 1977) proposed by Knaff 2) (Kanamitsu et al., 2002) is used in the present study. and Zehr (2006). Since the difference in sea-level pres- NCEP R-2 is also used to provide the atmospheric forc- sure between the environment and the center is derived ing to perform numerical simulations of the ocean re- from this formula, we assume that the environmental pres- sponse to Typhoons Chaba (2004) and Songda (2004). sure is 1010 hPa. The horizontal resolution is 2.5° square in a latitude-lon- Baik and Paek (1998) determined empirical functions gitude coordinate system, and the time interval is six that cap storm intensity over the western North Pacific, hours. The horizontal resolution of the daily atmospheric using only SST. They applied least-squares fitting to ex- forcing data for the numerical simulation is 1.875° by lin- ponential and polynomial functions of order up to 5 us- ear interpolation. ing observed maxima and 99th intensity percentile data. In order to realistically reflect the cyclonic storm In the present study, the regression curve for the 99th in- circulations of Chaba and Songda in the atmospheric forc- tensity percentile is used for comparison. Note that the ing, each Rankin vortex, determined from the central po- above mentioned MPI empirical functions utilize only sition, maximum sustained wind velocity, and a distance SST, with attempts made to estimate an upper bound on of the radius of 50 knots wind velocity, is merged into TC intensity for given atmospheric and oceanic condi- the JMA/Global ANALysis (GANAL) data as a TC bo- tions (see a review of Wang and Wu, 2004). gus (Wada, 2002). The horizontal resolution of JMA/ Figure 2 reveals that TCs are formed when the SST GANAL is 1.25° square in a latitude-longitude coordi- is above 26.5°C, which is consistent with the previous nate system and the time interval is six hours. report from Palmén (1948). In contrast, SST variations are not always dependent on TC intensity, especially dur- 2.4 Oceanic reanalysis data and tropical cyclone heat ing the development, mature and decay stages. The in- potential tensity rarely reaches its MPI because a TC may be af- Oceanic reanalysis data from the North Pacific ver- fected by many other possible negative influences (Evans, sion of the Meteorological Research Institute multivariate 1993). Nevertheless, the empirical function of Baik and ocean variational estimation (MOVE) system is used Paek (1998) is closest to the relationship between the (Usui et al., 2006) to estimate daily TCHP from 1998 to TRMM/TMI three-day SST and the lower bound of the 2004. The MOVE system includes an ocean general cir- best-track central pressure. The empirical function is more culation model (OGCM: Ishikawa et al., 2005) and a vari- reasonable, especially during the decay stage when the ational analysis scheme to synthesize observations such SST is always below 27°C. It is worth noting that the as sea temperature, salinity, and sea-surface height in their value of the lower bound of the best-track central pres- own time and space. The horizontal resolution of MOVE sure tends to be high compared to the value determined is 0.5° square in a latitude-longitude coordinate system. from empirical functions in which SST is above 29°C. The model domain of MOVE covers from 15°S to 65°N This implies that the lower bound of the best-track cen- and 100°E to 75°W. tral pressure may not always be reliable enough to use as Using this reanalysis data, the TCHP, based on that verification of numerically predicted central pressures of Leipper and Volgenau (1972), is calculated using the

430 A. Wada and N. Usui gitude coordinate system. The model domain covers 10° to 50°N and 120° to 160°E. MRI.COM has a total of 54 layers. The maximum bottom depth is set to 5625 m. The thickness at the bottom is 250 m. There are 17 layers above a 200-m depth at levels of 0.5 m, 1.5 m, 4.0 m, 7.0 m, 12.0 m, 18.0 m, 26.0 m, 38.0 m, 50.0 m, 66.0 m, 82.0 m, 100.0 m, 118.0 m, 138.0 m, 158.0 m, 178.0 m, and 200.0 m. The turbulent closure scheme of Noh and Kim (1999) is used. Tuning parameters α and m in Noh and Kim (1999) are 5 and 100 in the present study. The empirical bulk formula (Kondo, 1975) is employed to calculate sensible and latent heat fluxes. The water flux is corrected by restoring sea surface salinity to the climatology with a Fig. 3. Relationship between TRMM/TMI three-day mean SST restoring time of 24 hours. and TCHP derived from the MOVE system. The regression curve, its formula, and the regression coefficient are de- 3. Relationship between Tropical Cyclone Heat Po- noted in the panel. The regression curve shows the expo- tential and the Central Pressure of Tropical Cy- nential increase of TCHP as the SST linearly increases. clones As discussed in the previous section, the SST-derived MPI is usually deeper than the best-track central pressure. This indicates that SST alone is an inadequate pre- following formula: dictor of TC intensity (Evans, 1993). This is because TC intensity is controlled not only by SST but also by the H thermodynamic environment, the vertical shear, and dis- =−ρ ()∆ () QCTZTCHP∑ p 26 1 turbances in the upper troposphere (Emanuel et al., 2004). h=0 In addition, the SST-derived MPI is not influenced by ocean thermal energy or the upper oceanic structure un- ρ where is the density of the sea water at each layer, Cp is derneath the storms. The concern is how accurately the the specific heat at constant pressure, T is the sea tem- TCHP-derived MPI can predict the best-track central pres- perature, and ∆Z is the thickness at each layer. When T is sure when we use TCHP as a substitute for SST. below 26°C, it is reset to 26°C. The ocean heat content Parameters used for the above mentioned investiga- QTCHP represents TCHP in the upper ocean where the sea tion are as follows. temperature is above 26°C. In the present study, the unit • Attainable TC intensity is defined as sea-level is transformed from kcal cm–2 to kJ cm–2. Figure 3 pressure when a TC reaches its minimum central pres- presents the relationship between the TRMM/TMI three- sure (MCP), corresponding to the best-track MCP. day mean SST and the TCHP. The TCHP increases • Duration of a TC is defined as the period from exponentially while the SST increases linearly. However, the formation stage to the mature stage, when the central the relationship between the SST and the TCHP is not pressure has just reached the MCP. always unique. The second power of the correlation co- • Accumulated TCHP (ATCHP) and Accumulated efficient between them is approximately 0.45. TCHP is SST (ASST) are defined as the accumulation and integra- sometimes high (low) during low (high) SST when the tion of the average of THCP or SST within a 1.5° square SST is above 26°C. This means that TCHP is not only around the TC center during the six-hour duration. It is dependent on the SST but also on the depth of the 26°C noted that attainable TC intensity, duration, ATCHP, and isotherm. ASST, all are determined uniquely for each TCs. Figure 4 presents the relationships between the at- 2.5 Ocean general circulation model tainable TC intensity and the ASST, between the attain- The OGCM is a version of the community ocean able TC intensity and the duration, and between that in- model developed by the Meteorological Research Insti- tensity and the ATCHP from 1998 to 2004. The intensity- tute (MRI) (Ishikawa et al., 2005). Runs of the MOVE ASST correlation coefficient denoted in Fig. 4(a) is the system and the MRI Community Ocean Model smallest of the three relationships. The intensity-duration (MRI.COM) are usually forced by daily wind stress, heat relationship (Fig. 4(b)) is considered to be closely tied to flux, and fresh water flux data provided from NCEP R-2. the intensity-ASST relationship because the SST is rela- In the present numerical simulations, the horizontal tively high before a TC reaches the attainable TC inten- resolution of MRI.COM is 0.25° square in a latitude-lon- sity (Fig. 2) and its subsequent variation is relatively

Importance of TCHP for TC Intensity and Intensification 431 Fig. 5. Predictability of tropical cyclone intensity using the average accumulated SST (ASST: cross), duration (gray circles), and average accumulated TCHP (ATCHP: filled circles). Horizontal axis represents the tropical cyclone best- track central pressure; vertical axis is the predicted central pressure. Tropical cyclone intensity predicted using ATCHP is closer to the best-track intensity than that using SST or duration.

small. The longer the duration is, or the longer the TC experiences high SST during the duration, the stronger the TC becomes. The intensity-ATCHP relationship is more reasonable for the attainable TC intensity (Fig. 4(c)). The TC is much stronger when it undergoes high TCHP continuously over its duration. The predictability of potential intensity derived from the ATCHP, the duration, and the ASST is examined using the relationship between the attainable TC intensity and the ATCHP, the duration, and the ASST (Fig. 5). As- suming that the distribution of TCHP (SST) does not change from TC formation to its mature stage, the ATCHP (ASST) can be calculated along the TC best track every six hours using the TCHP (SST) distribution at the time of TC formation. The correlation coefficients between the best-track MCP and ATCHP, duration, and ASST are approximately 0.47, 0.33, and 0.40. The correlation coefficients and gradients of the regression functions shown in Fig. 5 indicate that the predicted potential intensity derived from ATCHP is superior to that based on duration Fig. 4. Relationships between a tropical cyclone’s central pres- alone or derived from ASST. However, it is worth noting sure at the mature stage and (a) the average accumulated that the problem of TC intensity prediction still remains, SST (ASST), (b) the duration of the tropical cyclone, and even though the potential intensity derived from ATCHP (c) the average accumulated TCHP (ATCHP). The regression line, the formula, and the coefficient are denoted in agrees with the best-track MCP better than the intensity each panel. This shows the analytical value of ATCHP for derived from ASST and the duration. In fact, the poten- its relation to tropical cyclone intensity. tial intensity predicted by the ATCHP tends to underesti- mate strong TCs and overestimate weak TCs. These er-

432 A. Wada and N. Usui Table 1. Rapid intensification cases and tropical cyclone for- south of the equator in the Pacific under conditions of mation in the western North Pacific from 1998 to 2004. rapid TC intensification (Fig. 6(a)). The high-TCHP area expanded northwestward, probably due to easterly winds Year The number of rapid The number of TC formation south of a subtropical high. Therefore, from a intensification cases in the western North Pacific climatological viewpoint, rapid intensification occurs in 1998 3 16 the western North Pacific where the TCHP is relatively 1999 0 22 high. In fact, TCs intensify rapidly when the TCHP is 2000 5 23 above 120 kJ cm–2. 2001 1 26 The horizontal distribution of TCHP may be influ- 2002 4 26 enced by intraseasonal or intra-annual climate phenom- 2003 4 21 ena such as the Madden-Julian Oscillation or the El Niño 2004 9 29 Southern Oscillation (ENSO) occurring from 1998 to 2004. In other words, atmospheric environments associ- Total 26 163 ated with the rapid intensification of TCs are not always uniquely determined climatologically (see the next section). In the 2000 season (Fig. 6(b)) there were four cases rors may be attributed to atmospheric or oceanic envi- of rapid TC intensification. During that season, the TCHP ronmental conditions. was not particularly high south of the equator in the Pa- cific, in contrast to the distribution of TCHP in Fig. 6(a). 4. Relationship between the Atmospheric and Oce- The TCHP was high south of the equator in the central anic Environments and Tropical Cyclone Loca- Pacific during the 2004 season (Fig. 6(c)). In turn, the tions of Rapid Intensification TCHP east of the Philippines was below 60 kJ cm–2. It is Besides its potential intensity, the rapid intensifica- worth noting that the locations of rapid intensification tion of a TC is an important aspect of accurately predict- were different in the 2000 and 2004 seasons. In particu- ing TC intensity. In fact, it is so difficult that rapid TC lar, the TCHP east of the Philippines during the 2000 sea- intensification is numerically predicted using sophisti- son was higher than the TCHP in 2004, although there cated numerical models with a relatively coarse horizon- was less rapid intensification in the 2000 season than in tal resolution. According to a statistical analysis by Kaplan the 2004 one. This suggests that the frequency and loca- and DeMaria (2003), rapid intensification occurs where tions of rapid intensification are influenced not only by the SST is relatively warm, the lower-tropospheric rela- high TCHP but also by the atmospheric environment and tive humidity is relatively high, and the vertical shear is the TC track, although high TCHP is a necessary condi- relatively weak. In the present study, rapid intensification. In other words, the frequency of rapid intensification is defined as occurring when the best-track central tion does not increase with higher local TCHPs. In addi- pressure falls by more than 10 hPa in six hours. Under tion, the location is not affected by higher local TCHPs, this definition, there were a total of 26 rapid intensifica- at least in the western North Pacific. This result is simi- tion cases from 1998 to 2004 (Table 1). As seen in Table lar to the conclusion reached by Chan and Liu (2004). 1, the number of rapid intensification cases is not related This conclusion does not involve the ideas of Camp and to the amount of TC formation. Interestingly, half of the Montgomery (2001), referring to Shay et al. (1992). The rapid intensifications occurred from 2003 to 2004. use of local TCHP does not always aid in producing an The interesting point is whether or not rapid TC in- MPI formulation that is more representative of the tensification is related to atmospheric and/or oceanic en- intensities that storms actually achieve. This is because vironments, especially TCHP. In the following section we TC rapid intensification requires favorable atmospheric describe the relationship between rapid TC intensifica- conditions. tion and atmospheric and/or oceanic environmental factors such as TCHP, lower tropospheric relative humidity 4.2 Lower tropospheric relative humidity and wind at a level of 850 hPa, as well as vertical shear In the case of rapid TC intensification, the lower between levels of 200 hPa and 850 hPa. tropospheric environment was investigated by a composite analysis of the NCEP R-2 winds and relative humidity 4.1 Tropical cyclone heat potential at a level of 850 hPa for both the 2000 (Fig. 7(a)) and Figure 6 presents composite maps of TCHP in cases 2004 (Fig. 7(b)) seasons. The composite analysis was of rapid TC intensification from 1998 to 2004. The com- made using the six-hour NCEP R-2 reanalysis of the pe- posite analysis was made using the daily TCHP observed riod when rapid TC intensification occurs. Rapid intensi- when rapid TC intensification occurs. The TCHP com- fication occurred in both years when the southwesterly posite analysis showed that the composite TCHP was high monsoon from the South China Sea and the easterly wind

Importance of TCHP for TC Intensity and Intensification 433 (a)

(b)

(c)

Fig. 6. Tropical cyclone heat potential (kJ cm–2) in the North Pacific from a composite analysis of cases of rapid intensification (central pressure falling more than 10 hPa per six-hour) (a) from 1998 to 2004, (b) in 2000, and (c) in 2004. The composite analysis was made using the daily TCHP at the time of rapid intensification. “Typhoon” symbols are shown at locations of rapid intensification.

on the southern edge of a subtropical high were conflu- that high TCHP is not always correlated with high lower- ent. However, moisture conditions were different between tropospheric relative humidity. the two years. In the 2000 season, rapid intensification In contrast, the monsoonal trough shifted eastward occurred under relatively dry conditions, while intensifi- in the 2004 season. Under the prevailing atmospheric cation occurred under relatively moist conditions in the environmental conditions, the rapid intensification area 2004 season. Since a southwesterly monsoon was trapped was covered with moist air, although the TCHP east of west of the Philippines in the South China Sea during the the Philippines was relatively low. Easterly winds on the 2000 season, the center of a monsoonal trough was lo- southern edge of the subtropical high meandered east of cated in that place. In addition, easterly winds brought the area where rapid intensification occurred. The typi- dry air into the area where the rapid intensification oc- cal wave-like perturbations in the easterly flow over the curred. Therefore, the area was relatively dry, although central Pacific, with a wavelength of 2000 km, meridi- the TCHP around the area was relatively high. This means onal wave number n = 3, and a period of 5 to 10 days

434 A. Wada and N. Usui (a)

(b)

Fig. 7. Winds, relative humidity at a height of 850 hPa, and isobaric height of 850 hPa, all from a composite analysis of cases of rapid intensification using NCEP-DOE AMIPII R-2 data (a) in 2000 and (b) in 2004. “Typhoon” symbols are shown at locations of rapid intensification. The composite analysis was made using a six-hour NCEP R-2 reanalysis at the time of rapid intensification.

(Reed and Recker, 1971) slowed down upon approaching Therefore, the lower-tropospheric relative humidity may the monsoonal westerly flow. At the same time, not be a necessary condition, or the reanalysis system with perturbations moving eastward in the monsoonal wester- its coarse horizontal resolution may not express the hu- lies tended to become trapped in the confluent zone midified condition around a TC. According to Chan and (Holland, 1995). This meandering led to the enhancement Liu (2004), the interannual variation in monsoonal trough of tropical cyclogenesis around the rapid intensification strength is mostly contributed by the ENSO event rather zone under high TCHP, although further external forcing than by the local SST. This implies that a higher local was not required for the intensification beyond its own TCHP alone may not significantly impact lower-tropo- interaction with the ocean (Briegel and Frank, 1997). spheric relative humidity.

Importance of TCHP for TC Intensity and Intensification 435 (a)

(b)

Fig. 8. As Fig. 7 except for winds at the height of 200 hPa and vertical shear between the levels of 200 hPa and 850 hPa.

4.3 Vertical shear between levels of 200 hPa and 850 falls in Japan. hPa Weak vertical wind shear is one of the crucial condi- Figure 8 presents composite maps showing the winds tions for TC formation (Gray, 1979). It seems that weak at a height of 200 hPa and the second power of the verti- vertical wind shear is a favorable condition for TC inten- cal wind shear between 200 hPa and 850 hPa in the 2000 sification, too. In other words, strong vertical shear may (Fig. 8(a)) and 2004 (Fig. 8(b)) seasons. Figure 8 reveals possibly have inhibited intensification, even when TCs that TC rapid intensification occurs under weak vertical passed over warm sea water. Rapid TC intensification wind shear in a divergent field at a level of 200 hPa. The invariably occurred under weak vertical wind shear with- weak vertical wind shear spread more widely in the 2000 out being inhibited by the unfavorable environment. season than in the 2004 season. In the 2004 season the amplitude of vertical wind shear was relatively high in 5. Impact of Tropical Cyclone Heat Potential on In- the mid-latitudes, along the southern edge of the subtropi- tensification of Typhoons Chaba (2004) and cal high, and in the South China Sea. Therefore, the area Songda (2004) with weak vertical wind shear in the 2004 season was This is a case study investigating the impact of TCHP much smaller than in the 2000 season, leading to a nar- on TC intensification. Chaba and Songda became tropi- rower divergent field at a level of 200 hPa in the 2004 cal storms over the sea around the Marshall Islands. They season. The horizontal distribution of the vertical wind moved along a similar track, making landfall in Japan shear and the divergent field may influence not only the while passing through a similar course (Fig. 9). Their locations of rapid TC intensification but also the TC tracks durations, about 11 days for each typhoon, were relatively from 2004, especially the extraordinary number of land- long compared to the mean duration of typhoons, which

436 A. Wada and N. Usui was about six days in 2004 (Wada, 2005a). Chaba reached its peak strength with a central pressure of 910 hPa over the sea west of the Mariana Islands from 23 to 25 August (Fig. 10(a)). In contrast, Songda reached its peak strength with a central pressure of 925 hPa over the ocean north- west of Saipan from 31 August to 2 September, and again southwest of Okinawa from 4 to 5 September (Fig. 10(b)). The tendencies of intensification for the two typhoons were different. During the development and mature stages of Chaba, the SST had been maintained above approximately 29°C. In contrast, the TCHP derived from the reanalysis by MOVE gradually decreased from approximately 120 kJ cm–2 to 50 kJ cm–2 during the period. The potential intensities of Chaba and Songda derived from the empirical relation shown in Fig. 4(c) are 948.7 hPa and 953.13 hPa. These PIs are weaker than the MCP in the Fig. 9. Tracks of typhoons Chaba (2004) and Songda (2004). best-track data. There might also be errors in the best- “Typhoon” symbols indicate the central position of Chaba track data. However, TCHP is not solely responsible for (2004). Filled typhoon symbols indicate the central position of Songda (2004). the rapid intensification of the two typhoons. The difference between the potential intensities of Chaba and

Fig. 10. Time series of daily SST, three-day mean SST, TCHP, and central pressure in the cases of Typhoons (a) Chaba (2004) and (b) Songda (2004). Upper panel indicates time series of the SST and TCHP, while lower panel indicates that of central pressure.

Importance of TCHP for TC Intensity and Intensification 437 Fig. 11. (a) Relationship between SST and central pressure during the decay stage of typhoon Chaba (2004). (b) As (a) except of the relationship between TCHP and central pressure. (c) As (a) except for typhoon Songda (2004). (d) Relationship between TCHP and central pressure during the decay stage of typhoon Songda (2004).

Songda is only about 5 hPa, while that between their pressure in Fig. 11(c) (about 0.96) is higher than that be- analyzed MCPs is 15 hPa. This implies that the TC inten- tween SST and central pressure in Fig. 11(d) (about 0.76). sity of Songda was subject to influence by air-sea inter- This reveals that TCHP is more strongly correlated with action. central pressure than SST during the decay stage. During the decay stage of Chaba, the central pres- In the case of Songda, SST had been maintained sure rose sharply when the typhoon passed through a low- above 28°C during the development and mature stages. TCHP area, even though the SST had been maintained In contrast, the TCHP varied with high amplitudes. In above 29°C. This suggests that the tendency of Chaba’s particular, during the rapid intensification stage of central pressure during its decay stage was not correlated Songda, the TCHP around the area was above 120 with the SST but rather was related to TCHP. Figure 11 kJ cm–2. A peak in TCHP was also found during the ma- portrays the relationship between SST and central pres- ture stage of Songda. In contrast, the variations in TCHP sure and between TCHP and central pressure during the were not always correlated with the underlying SST where decay stages of Chaba and Songda. No correlation be- Songda passed by. The intensity of Songda or its intensi- tween SST and central pressure was found (Fig. 11(a)), fication was influenced by the variation of TCHP. There- while the correlation coefficient between TCHP and cen- fore, local TCHP is better correlated with the intensity tral pressure was about 0.47, which is relatively high com- and intensification than the local SST. pared to that between SST and central pressure (Fig. 11(b)). Both of Songda’s correlation coefficients are 6. Relationship between Warm Core Eddies and the higher than those for Chaba. This is consistent with the Intensification of Typhoons Chaba (2004) and suggestion that the TC intensity of Songda is subject to Songda (2004) influence by air-sea interaction. In the case of Songda, Oceanic reanalysis data taken by MOVE for the du- the correlation coefficient between TCHP and central ration of Chaba and Songda was used to explore the rela-

438 A. Wada and N. Usui tionship between local TCHP variations and the tendencies of central pressure in the cases of Chaba and Songda. The five-day mean TCHP field for 21 to 25 August reveals that Chaba developed over the ocean with high TCHP, above 80 kJ cm–2 (Fig. 12(a)). When Chaba reached its peak strength, the typhoon moved northwestward where the TCHP became relatively low. During its sustenance of MCP at 910 hPa, Chaba passed over two warm core eddies (WCEs). From 26 to 31 Au- gust, Chaba weakened in intensity (Fig. 12(b)) as it moved northwestward toward Japan. The typhoon made landfall with a central pressure of 950 hPa although the TCHP was low where the typhoon passed by. The strong typhoon brought on a disaster caused by torrential rains, destruc- tive winds, and storm surges. Songda had moved west-northwestward from 26 to 31 August after the passage of Chaba (Fig. 12(b)). The TCHP became low where Chaba passed by. This decrease in TCHP was probably caused by multiple effects of upwelling and turbulent mixing. These effects are the same as those of sea surface cooling (SSC) (Wada, 2005b). Since the TCHP also became low when Songda passed by, it reached its peak strength of MCP of 925 hPa, 15 hPa weaker than that of Chaba. Nevertheless, Songda sustained a central pressure of around 925 to 930 hPa from 1 to 5 September (Fig. 12(c)). During this period, Songda passed over two WCEs. One of the WCEs was located in around 135°E, 21°N (hereafter W1). W1 had been already dominant before the arrival of Chaba. During the passage of Chaba, the TCHP of W1 became lower than before the passage of the typhoon. During the passage of Songda, however, the TCHP increased. The other WCE was located around 128°E, 27°N (hereafter W2) along the Kuroshio stream. This WCE appeared only from 1 to 5 September when Chaba passed by to the right of WCE W2. Upstream from W2, where there was a source of the Kuroshio stream, another WCE was found southeast of Taiwan (hereafter W3). This WCE had been continuously salient, even during the passage of Chaba and Songda. WCE W3 may help WCE W2 develop or maintain its amplitude through the Kuroshio stream. Fig. 12. Distribution of mean TCHP in the western North Pa- Unlike warm eddies in the Gulf of Mexico, those in cific by MOVE reanalysis data. (a) 21 to 25 August. (b) 26 the western North Pacific can be separated or propagated to 31 August. (c) 1 to 5 September 2004. These panels in- far from the place of formation. Thus, each evolution of clude the typhoon names and their central pressures. W1, WCEs W1, W2, and W3 is examined from 1998 to 2004 W2 and W3 indicate warm eddies. The domain of W1 cov- using the reanalysis data taken by MOVE. The areas of ers from 130° to 135°E and 20° to 22.5°N, that of W2 from these WCEs are defined as follows: W1 is located be- 127.5° to 130°E and 26° to 29.5°N, and that of W3 from tween 130°E and 135°E and between 20°N and 22.5°N, 121° to 125°E and 19° to 22.5°N. Open “typhoon” symbols W2 is located between 127.5°E and 130°E and between denote the locations of Chaba (2004). Filled ones denote 26°N and 29.5°N, and W3 is located between 121°E and those of Songda (2004). 125°E and between 19°N and 22.5°N. Figure 13 depicts the evolution of TCHP around the WCEs, accompanied by seasonal variations. The evolu-

Importance of TCHP for TC Intensity and Intensification 439 (a)

Songda

Chaba

(b)

Fig. 13. Time series of the TCHP around (a) W1, (b) W2, and Fig. 14. (a) SST and ocean current after 300-hour integration, (c) W3 from 1998 to 2004. The domains of W1, W2, and at 1200 UTC 5 September 2004. (b) TMI and AMSRE fu- W3 are as in Fig. 12. sion daily SST on 5 September 2004. “Typhoon” symbols with lines indicate the central positions and tracks of Chaba (2004) and Songda (2004). tion of TCHP has high frequencies, especially in W1 and W3. Each TCHP value for W1 (Fig. 13(a)) and W2 (Fig. 13(b)) in the 2003 season is the highest, while the value extraordinarily high compared to the value for 1998 to for W3 (Fig. 13(c)) in the 2004 season is the highest in 2003. The formation mechanism of the extraordinary any year. The TCHP value for W2 in the 2004 season is WCE W3 in the 2004 season is beyond the scope of this relatively low, partly because moderately strong typhoons study. However, it may be related to the extraordinary frequently passed WCE W2 during the 2004 season. In number of typhoon landfalls in Japan with only a small contrast, the TCHP value for W3 in the 2004 season was increase in central pressure from the peak.

440 A. Wada and N. Usui Fig. 15. Vertical section of sea temperature in JMAR experiment, differences between JMAR and NCEP R-2, and difference in ocean current along 22°N between them. “C” indicates cold water and “W” indicates warm water in the JMAR experiment. Chaba (2004) passed by near 137°E, 22°N where upwelling is dominant.

7. Effect of the Baroclinic Ocean Response to Ty- phoon Chaba (2004) on Warm Core Eddies and Fig. 16. Time series of differences in tropical cyclone heat on the Intensity of Typhoon Songda (2004) potential (kJ cm–2) and depth of 26°C isotherm (m) around After the passage of Chaba, the TCHPs of W1 and the center position of (a) Chaba (2004) and (b) Songda W2 (Fig. 12(c)) evidently increased. Each increase in (2004) between JMAR and NCEP R-2. Solid line represents TCHP probably influenced the sustenance of TC inten- tropical cyclone heat potential, and broken line represents depth of the 26°C isotherm. sity of the subsequent typhoon Songda. However, the process responsible for the TCHP increase has never been examined. Our interest was to explore the process of TCHP increase in the WCEs. In order to investigate the of each numerical simulation is 2400 UTC 23 August effect of the baroclinic ocean response to Chaba on the 2004. Two numerical integrations were performed be- WCEs, especially W1 and W2, numerical simulations tween the initial time and 2400 UTC 4 September 2004. were performed by MRI.COM. Figure 14(a) illustrates the horizontal distribution of Two specifications for atmospheric forcing were pre- SST simulated by MRI.COM for 1200 UTC 5 September pared for the numerical simulations in the present study. 2004 in JMAR. The simulation enables one to reproduce One was determined from the NCEP R-2 wind stress and SSC after the passages of Chaba and Songda. Around wind velocity by linear interpolation; the other replaced 137°E, 22°N, the SSC caused by the passage of Songda the above mentioned wind stress and wind velocity with overlapped with that caused by the passage of Chaba. values obtained from JMA/GANAL by linear interpola- Songda sustained its peak intensity where the typhoon tion with an artificial Rankin vortex. The Rankin vortex passed over the relatively low SST compared to the SST was determined from the best-track central position, a before the passage of Chaba. Figure 14(b) illustrates the distance of the radius of 50 knots wind velocity, and maxi- horizontal distribution of TMI and AMSER fusion daily mum sustained wind velocity. Hereafter, this experiment SST on 5 September 2004. SSC was produced around the will be referred to as JMAR and the other as NCEP. Pro- track of Chaba, especially to the right of the running di- duction of JMAR atmospheric forcing from the JMA/ rection. This represents the ocean response to a fast mov- GANAL and best-track data is described in detail in Wada ing storm (Wada, 2005b). SSC was dominant around (2002). The time interval of atmospheric forcing in JMAR 137°E, 22°N. These results are consistent with results of or NCEP is 10 minutes, corresponding to the time step of JMAR seen in Fig. 14(a). the numerical simulation from MRI.COM. The initial time Figure 15 presents a vertical section of sea tempera-

Importance of TCHP for TC Intensity and Intensification 441 current induced by the passage of Chaba produced a convergence near the surface outside the sea temperature cooling region around the typhoon center. Around the convergence region, the sea temperature in JMAR was higher than that in NCEP around a depth of 80 m. En- trainment induced by wind forcing on the left side of the track may have contributed to the increase in near-surface sea temperature through downward transportation of warm near-surface water into the ocean interior, since vertical turbulent mixing on the left side of the track was too small to entrain cold water by upwelling behind the typhoon. Figures 14 and 15 provide no evidence of the TCHP increase around each typhoon track because the increase in SST is negligibly small compared to the decrease of SST due to the passage of the typhoons. In order to clarify the TCHP increase, TCHP was estimated by the formula of Leipper and Volgenau (1972), using the results of numerical simulations in JMAR and NCEP. Figure 16 depicts a time series of differences in TCHP and the depth of the 26°C isotherm (Z26) between JMAR and NCEP, around the center of Chaba and Songda. For Chaba, the TCHP in JMAR is smaller than that in NCEP, although Z26 is deeper in JMAR than it is in NCEP (Fig. 16(a)). In contrast, the TCHP in JMAR is larger than that in NCEP in the case of Songda, with a relatively deep Z26 (Fig. 16(b)). It is worth noting that the difference between NCEP and JMAR without the Rankin vortex is negligible (not shown). This suggests that the Rankin vortex plays an important role in producing TCHP variations around the tracks. Figure 17 indicates each evolution of TCHP and Z26 around the WCEs: W1 (Fig. 17(a)), W2 (Fig. 17(b)), and W3 (Fig. 17(c)). The TCHP and Z26 in JMAR was high around W1 (Fig. 17(a)) and W3 (Fig. 17(c)), partly because Typhoon Aere (2004), which was generated between Chaba and Songda, had passed by near the area. In particular, a near-inertial variation is found in Fig. 17(a). Fig. 17. Time series of TCHP and depth of the 26°C isotherm In fact, the Rankin vortex of Aere is artificially included around (a) W1, (b) W2, and (c) W3 from 24 August to 5 in the JMAR atmospheric forcing. In contrast, TCHP and September in 2004. “NCEP” and “JMAR” represent kinds Z26 in NCEP were high around WCE W2 (Fig. 17(b)) of atmospheric forcing. Domains of W1, W2, and W3 are until 29 August. After that, both TCHP and Z26 increased as in Fig. 12. gradually in JMAR, accompanied by inertial oscillations. The inertial oscillations found in W2 were probably caused by near-inertial currents induced by the passage of Songda. ture, the difference in sea temperature between JMAR and NCEP, and the temperature of the ocean current between 8. Discussion them along the 22°N line at 2400 UTC 31 August 2004. Around 137°E, SSC by upwelling is dominant. The peak 8.1 Relationship between accumulated tropical cyclone of sea temperature cooling from the conditions just be- heat potential and tropical cyclone intensity fore the passage of Chaba was greater than 6°C at a depth We have demonstrated in the previous sections that around 40 m, accompanied by cyclonic circulation. In the potential intensity derived from ATCHP was more contrast, the northwestward or westward near-inertial reliable than that derived from ASST or the duration of a

442 A. Wada and N. Usui TC. This was also reported in Wada (2006), using a longer. As for the duration of TCs, the eastward shift of nonhydrostatic model coupled with a slab ocean model. TC-genesis locations leads to an elongation of the TC The SSC caused by the passage of a TC, accompanied by track over warm SST regions in El Niño years (Camargo mixed layer deepening by entrainment, leads to a decrease and Sobel, 2005). The study of Camargo and Sobel (2005) in TCHP around the TC center. This is because the TCHP shows that ENSO indices are positively correlated with variations are accompanied by variations in mixed-layer accumulated cyclone energy (ACE) in the western North thickness as well as in the sea temperature above 26°C. Pacific. In contrast, since 1995, the ACE indices for all In contrast, differences in central pressure between the but two Atlantic hurricane seasons have been above nor- coupled and non-coupled experiments reported in Wada mal; the exceptions are the El Niño years of 1997 and (2006) are correlated not with TCHP but with ATCHP. 2002 (Trenberth, 2005). The concept of ACE is similar to This suggests that increases in central pressure simulated that of ATCHP from the standpoint of the accumulation by the atmosphere-ocean coupled model are closely re- of energy (kinetic or thermal). Therefore, the concept of lated not to the SSC around the TC center but to the re- accumulation may be more important in discussing TC duction of ATCHP where TCs have been experienced. intensity and intensification than the concept of localiza- In an observational study, Cione and Uhlhorn (2003) tion. reported that the energy available to the TC was an order In order to qualitatively evaluate the effect of ATCHP of magnitude greater than the energy extracted by the TC. increases due to the passage of TCs on their central pres- A numerical study by Chan et al. (2001) found that the sures, we estimated the ATCHP increase in 2004 around original peak intensity of a TC did not weaken after leav- WCEs W1 and W2. The difference in ATCHP between ing a warm core eddy, even though the TC attained its NCEP and JMAR was 0.6 MJ day–1 for W1, and 0.22 peak at the center of the warm core eddy. These results, MJ day–1 for W2, corresponding to intensifications of ap- along with those of the present study, reveal that a TC proximately 12 hPa day–1 and 4.5 hPa day–1 determined carries a kind of reservoir of heat from the ocean which in the case of JMAR using the relation shown in Fig. 4(b). is closely related to its intensity. This reservoir is not re- In fact, other atmospheric environmental conditions, such lated to the TCHP at a particular place but rather to ATCHP as vertical shear or an upper trough, may affect the inten- during the overall duration. Local SSC caused by the pas- sification of TCs, as suggested in the previous section. In sage of a TC plays a role in inhibiting any ATCHP in- any case, the ATCHP contributes to sustaining TC inten- crease. sity. ATCHP can influence TC intensity through transpor- A comparison between 2000 and 2004 reveals that tation of water-vapor flux (or latent heat flux) from the rapid intensification areas are confined to places where ocean to the atmosphere by eddy diffusion in the plan- the divergence in the upper troposphere is relatively small, etary boundary layer. The transportation of water-vapor although the TCHP was relatively high on average around flux is generally estimated under the assumption that the the basin in both years. This shows that rapid TC intensi- atmosphere at the sea surface is saturated, in equilibrium fication is controlled by the atmospheric environment or with the SST. The exchange coefficients of the momen- a related TC track rather than the average TCHP distribu- tum and enthalpy fluxes are calculated in the surface- tion. Moreover, the pattern in the upper troposphere may boundary scheme of the atmospheric model. According be related to the tracks typically making landfall in Ja- to Emanuel (1995) and Schade and Emanuel (1999), these pan, while the average TCHP distribution may be related coefficients, especially for moisture flux, play an essen- to TC intensity through the accumulation of TCHP along- tial role in determining TC intensity. side TC tracks. In order for the moisture flux to affect TC intensity, the moisture supplied to the atmospheric boundary layer 8.2 Formation mechanism of warm eddies in the west- needs to be transported to the warm core of a TC through ern North Pacific secondary circulation around the eyewall. During trans- The ATCHP increase caused by the ocean response portation to the warm inner core by neutral moist ascent, to a TC is explored here. Lin et al. (2005) discussed the the moisture flux continues to be transported from the impact of warm core eddies on the intensity of Typhoon ocean to the atmosphere, even when the TC becomes Maemi (2003). However, they did not mention the im- weak. Therefore, a TC can sustain its intensity by the pact of TCs on variations in WCEs, nor the impact of continuous supply of moisture flux. The above-mentioned those WCEs on subsequent TCs passing through the area process occurs in a cumulative way so that the ATCHP after the passage of a previous TC. Bender and Ginis but not the TCHP is important to TC intensity. The proc- (2000) reported that the intensity of a subsequent hurri- ess is also linked to the effects of deep-layer inflow on cane did not intensify but rather was suppressed due to intensification described by Ooyama (1982). the passage over the SSC area of a previous hurricane. The ATCHP becomes higher when the duration is According to the observational research of Pudov et al.

Importance of TCHP for TC Intensity and Intensification 443 (1978) and Fedorov et al. (1979), the sea temperature around the typhoon center was reduced by upwelling, while areas outside the reduction area were warming, es- Ma-on pecially around a depth of 50 m. A schematic diagram Tokage from Ginis (1995) indicates that a warming region for- Nock-ten ward and outside of the radius of deformation of a TC was brought about not only by entrainment but also by a surrounding convergence caused by divergent flow induced by cyclonic wind stress near the TC center. This causes downward transportation of warm sea water from the sea surface to the ocean interior. On the left side of Ma-on the TC track, an anticyclonical circulation is formed or Tokage enhanced by near-inertial oscillation. The anticyclonical Nock-ten circulation is favorable for sustaining the amplitude of WCEs. It is worth noting that the sea temperature warming is not remarkable at the sea surface but is significant around a depth of 50 m, corresponding to the mixed layer base. This is one reason why TCHP increases significantly in areas surrounding the TC. Fig. 18. Lines depicting the tracks of typhoons Ma-on (2004), Unlike the TCHP around WCE W1, the TCHP around Tokage (2004), and Nock-ten (2004). “Typhoon” symbols WCE W2 increases in NCEP. The following processes indicate central position of each typhoon. Size of the ty- are believed to influence TCHP variations around WCE phoon symbol represents intensity of each typhoon. Cen- W2: the baroclinic response to Typhoon Aere (2004), the tral positions with central pressures above 960 hPa are development of WCE W3, and advection from WCE W3 shown grey, while those with central pressures below 960 toward WCE W2. Initially, SSC caused by the passage of hPa are shown black. “Cloud” region represents a suste- Aere led to a decrease in TCHP around WCE W2 in nance region for typhoon intensity. JMAR. Subsequently, Typhoon Songda (2004) ap- proached WCE W2. Near-inertial currents caused by the passage of Songda led to an increase in TCHP ahead of the direction of TC translation. In addition, warm water the TCHP can increase south (decrease north) of the track is transported from WCE W3 through the Kuroshio. The due to transportation of warm water by near-inertial cur- increase in TCHP has become significant since 1 Sep- rents (by upwelling and vertical mixing). Previous stud- tember 2004. ies of the ocean response to storms have focused on the Songda passed by to the left of the track of Typhoon SST cooling rate, which was dominant in a slow transla- Chaba (2004). This is similar to another case involving tion (Price, 1981; Wada, 2002). In the case of Chaba, typhoons in October and November 2004: Typhoons however, the typhoon re-developed during a slow trans- Ma-on (2004), Tokage (2004), and Nock-ten (2004), seen lation when the typhoon passed over warm eddy W1. Af- in Fig. 18. Ma-on experienced rapid intensification around ter the passage of Chaba, SSC occurred alongside the 130°E, 23°N. After the passage of Ma-on, Tokage passed track. The pattern of SSC was similar to previous by on the left side of Ma-on’s track. Tokage was weaker modeling studies (e.g. Bender and Ginis, 2000). It is worth than Ma-on. However, Tokage sustained its intensity while noting that it is difficult to define a threshold between moving westward. Moreover, Nock-ten passed by to the slow and fast translation in the present study. This diffi- left of Tokage’s track, sustaining its peak intensity. This culty is because the best-track TC position has been re- track pattern was notably found around 120 to 135°E, 16 corded every six hours, except around Japan, where rapid to 23°N, denoted by a cloud symbol in Fig. 18. SSC was TC intensification rarely occurs. Moreover, the calcula- frequently found here in 2004 due to the frequent pas- tion-period of ATCHP does not involve the mature stage sage of typhoons (Wada, 2005a). when the SST cooling rate is the greatest with a slow trans- According to Riehl (1972), TCs are not at their peak lation. If we were to separate all the cases into two cat- intensity when they begin to recurve with a relatively slow egories; for example, fast-moving TCs and slow-moving speed of TC translation. Around the recurvature area, sig- ones, the ATCHP-TC intensity relation would be modi- nificant SSC results from the slow translation. The SSC fied. However, the other question arises in the aforemen- caused by the moving TC appears along the track as a tioned discussion: whether or not a TC can continue to narrow band of cold water in the western North Pacific. intensify with a slow translation? This is beyond the scope In the particular case of a TC moving northwestward, of this study.

444 A. Wada and N. Usui Table 2. List of typhoons having minimum central pressure within a week. The second typhoon moved to the right side of the first typhoon, then later moved to the left side.

Year Month El First typhoon MCP of the first typhoon Second typhoon MCP of the second typhoon (hPa) (hPa) 2004 Aug.−Sep. Chaba 910 Songda 925 October Tokage 940 Nockten 945 2002 Jun.−Jul. E Chataan 930 Halong 945 Aug.−Sep. E Rusa 950 Sinlaku 950 1998 October Zeb 900 Babs 940 1993 December E Lola 955 Manny 955 1992 November E Gay 900 Hunt 940 1987 August E Betty 890 Cary 960 1981 November Hazen 955 Irma 905

The combination of WCE and SSC leads to a sharp or El Niño-like seasons. The baroclinic ocean response gradient SST alongside the track. According to the nu- to TC intensity may not always have a positive impact. merical study of Chan et al. (2001), the intensification Indeed, the baroclinic ocean response in the case of a process over a sharp SST gradient is similar to that over “cross” track pattern is one of the factors responsible for a warm core eddy. The present study demonstrates that intensification or sustenance of peak TC intensity in El the formation of a sharp SST gradient is equivalent to the Niño and El Niño-like seasons. formation of anticyclonical circulation on the left side of the TC track, which is consistent with the result of Chan 9. Conclusion et al. (2001). We employed tropical cyclone heat potential (TCHP) However, local SSC is not responsible for the subse- in the western North Pacific from 1998 to 2004 as esti- quent movement of TCs toward warm regions in the low mated from oceanic reanalysis data to explore the rela- latitudes. Previous studies have reported that the behavior tionship between TCHP and tropical cyclone (TC) inten- of TCs is hardly affected by SSC in general; so many sity, as well as that between the TRMM/TMI three-day factors could influence the behavior that it would be dif- mean sea surface temperature (SST) and TC intensity. A ficult to isolate the effect of SST. From the standpoint of regressive empirical function derived from the TCHP was the effect of SST distribution of TC translation, Chang more reliable for determining the maximum potential in- and Madala (1980) reported in their idealized numerical tensity of TCs than empirical functions derived from SST. experiments that the behavior of TCs was influenced by The ATCHP, an accumulation of TCHP around a TC center SST distribution. In their result, TCs tended to move into every six hours from the formation to the mature stage, is regions of warmer SST, translating the mean flow over better correlated with the intensity in the mature stage the ocean downwind, with SST gradients perpendicular than is the ASST, an accumulation of SST around the to the mean ambient flow vector. In the present study, the center every six hours, along with the duration. SST gradient had been produced after the passage of a Rapid intensification of a TC was found to occur previous TC (Chaba or Ma-on in this case). Futhermore, when TCHP was relatively high in the western North Pa- the subsequent TC (Songda or Tokage in this case) moved cific from 1998 to 2004. Rapid TC intensification was into a high-TCHP region (not shown). Even though the frequently observed in 2000 and 2004. Atmospheric and report of Chang and Madala (1980) is for a case that is oceanic environments, including the relative humidity and not terribly realistic, it may be necessary to reexamine wind in the lower troposphere, and the vertical shear be- the relationship between subsequent TC translation and tween levels of 200 hPa and 850 hPa, occurring between SST or TCHP distribution produced as a result of the pas- 2000 and 2004, all are different. Rapid intensification is sage of a previous TC. confined to areas where the divergence in the upper tropo- These TC track patterns have not been typical in re- sphere is relatively small. The pattern in the upper tropo- cent decades. Interestingly, the above mentioned TC track sphere may be related to the tracks typically making land- pattern has occurred only nine times in the western North fall in Japan. Rapid intensification is controlled both by Pacific since 1981, as seen in Table 2. It is worth noting TCHP on a basin scale and atmospheric environment. that this track pattern has increased in recent years. Table In the cases of Typhoons Chaba (2004) and Songda 2 also indicates that this pattern often appears in El Niño (2004), the intensity and rapid intensification were re-

Importance of TCHP for TC Intensity and Intensification 445 lated not to SST variations but to variations in TCHP. References However, the potential intensity derived from ATCHP in Atkinson, G. D. and C. R. Holliday (1977): Tropical cyclone the cases of Chaba and Songda was weaker than the best- minimum sea level pressure/maximum sustained wind re- track TC intensity. At SSTs above 29°C, the tendencies lationship for the western North Pacific. Mon. Wea. Rev., of TC intensity agree well with those of TCHP. The TCHP 105, 421–427. Baik, J.-J. and J.-S. Paek (1998): A climatology of sea surface variations were brought about by the baroclinic ocean temperature and the maximum intensity of western North response to Chaba. This implies that the atmospheric en- Pacific tropical cyclones. J. Meteor. Soc. Japan, 76, 129– vironment may also contribute to rapid intensification ir- 137. respective of TCHP. The SSC due to the passage of Chaba Bao, J.-W., J. M. Wilczak, J.-K. Choi and L. H. 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