**3. Structural and intensity change of concentric eyewall**

In order to study the structural and intensity changes of CE TCs, we excluded the case when the TC's outer eyewall was within 200 km from land in the period of 24 h before and after CE formation, or where the satellite temporal resolution was greater than 12 h in the WNP (ATL) basin. There were 83 and 34 CE cases analyzed in the WNP and ATL basins, respectively. Three different structural change processes were defined after CE formation. The eyewall replacement cycle (ERC) cases were classified based upon the dissipation of the inner eyewall in less than 20 h after CE formation. The cases in which part of the outer eyewall dissipates within 20 h are classified as no replacement cycle (NRC) cases. The cases where the CE structure is maintained for more than 20 h are classified as concentric eyewall maintained (CEM) cases. The similar inner core size requirement was used to avoid assigning a CE TC with multiple ERC processes into one single CEM case.

The ERC classification had 42 of 83 cases and 19 of 34 cases (51 and 56%) in the WNP and ATL basins, respectively. The CEM classification had 19 and 5 cases (23 and 15%) in the WNP and ATL basins, respectively. The NRC classification had 22 and 10 cases (27 and 29%). Examples of the three classifications for the CE processes are shown in **Figure 2**. The NRC cases resemble "the shear stop ERC mode" and the CEM cases "the large radius outer eyewall and CE structure maintained for a time cases" as discussed in the study of Hawkins and Helveston [16].

**Figure 3a** and **b** shows the composite time series of intensities for the ERC, CEM, NRC cases as well as the average of the total CE sample. In the WNP basin, the average intensity of CEM cases is stronger than that of the ERC and NRC cases before and after CE formation. In particular, the CEM storms intensified continuously for 18 h after CE formation and maintained the intensity for another 24 h. The composite intensity of ERC and NRC cases is similar to that before CE formation. In the ATL basin, although the composite intensity of ERC is stronger than that of CEM continuously for 18 h before CE formation and 36 h after CE formation, the CEM cases maintained similar intensity before CE formation. Furthermore, the intensity of NRC decreases quickly after CE formation in both basins. **Figure 3a** and **b** indicates that a key feature of CE formation appears to be the maintenance of a relatively high intensity for a longer duration rather than a rapid intensification process to a high intensity. The stronger core intensity may play a pivotal role in the axisymmetrization dynamics of asymmetric convection outside the core to produce the CE structure [8, 9]. This asymmetric convection is also shown in **Figure 2**.

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tween the typhoon center to the point where *T*<sup>B</sup> = 0.5 × *σ*inner + *T*Binner. The moat width was determined by the distance between the points of *T*<sup>B</sup> ≧ 0.5 × *σ*outer + *T*Bouter and *T*<sup>B</sup> ≧ 0.5 × *σ*inner + *T*Binner. Finally, the outer eyewall width was determined by the distance of the region that satisfies *T*<sup>B</sup> < 0.5 × *σ*outer + *T*Bouter in the outer eyewall region. The inner eyewall radius, the moat, and the outer eyewall width were calculated by averaging the radial profiles of

In order to study the structural and intensity changes of CE TCs, we excluded the case when the TC's outer eyewall was within 200 km from land in the period of 24 h before and after CE formation, or where the satellite temporal resolution was greater than 12 h in the WNP (ATL) basin. There were 83 and 34 CE cases analyzed in the WNP and ATL basins, respectively. Three different structural change processes were defined after CE formation. The eyewall replacement cycle (ERC) cases were classified based upon the dissipation of the inner eyewall in less than 20 h after CE formation. The cases in which part of the outer eyewall dissipates within 20 h are classified as no replacement cycle (NRC) cases. The cases where the CE structure is maintained for more than 20 h are classified as concentric eyewall maintained (CEM) cases. The similar inner core size requirement was used to avoid assigning a CE TC with multiple

The ERC classification had 42 of 83 cases and 19 of 34 cases (51 and 56%) in the WNP and ATL basins, respectively. The CEM classification had 19 and 5 cases (23 and 15%) in the WNP and ATL basins, respectively. The NRC classification had 22 and 10 cases (27 and 29%). Examples of the three classifications for the CE processes are shown in **Figure 2**. The NRC cases resemble "the shear stop ERC mode" and the CEM cases "the large radius outer eyewall and CE structure maintained for a time cases" as discussed in the study of Hawkins and Helveston [16].

**Figure 3a** and **b** shows the composite time series of intensities for the ERC, CEM, NRC cases as well as the average of the total CE sample. In the WNP basin, the average intensity of CEM cases is stronger than that of the ERC and NRC cases before and after CE formation. In particular, the CEM storms intensified continuously for 18 h after CE formation and maintained the intensity for another 24 h. The composite intensity of ERC and NRC cases is similar to that before CE formation. In the ATL basin, although the composite intensity of ERC is stronger than that of CEM continuously for 18 h before CE formation and 36 h after CE formation, the CEM cases maintained similar intensity before CE formation. Furthermore, the intensity of NRC decreases quickly after CE formation in both basins. **Figure 3a** and **b** indicates that a key feature of CE formation appears to be the maintenance of a relatively high intensity for a longer duration rather than a rapid intensification process to a high intensity. The stronger core intensity may play a pivotal role in the axisymmetrization dynamics of asymmetric convection outside the core to produce the CE structure [8, 9]. This asymmetric convection is

**3. Structural and intensity change of concentric eyewall**

the eight sections as shown in **Figure 1**.

176 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

ERC processes into one single CEM case.

also shown in **Figure 2**.

**Figure 2.** The imagery sequences and averaged TB radial profile for (a) Typhoon Saomai 2000-ERC, (b) Haitang 2005- NRC, (c) Ewiniar 2006-NRC, (d) Winnie 1997-CEM, (e) Dianmu 2004-CEM, and (f) Chaba 2004-CEM. From Yang et al. [26], courtesy of American Meteorological Society.

Maclay et al. [13] use aircraft data to construct the *K-V*max diagram for the intensity and structural changes. However, there is few active aircraft reconnaissance program for the WNP basin. Therefore, the structural and intensity variability is illustrated here using the *T-V*max diagram (where *T* is the *T*B and *V*max is the best track estimated intensity). The convective activity (CA) is indicated by the areal averaged *T*<sup>B</sup> contrast to the background *T*<sup>B</sup> in the 400 km square area of satellite imagery centered at the eye (CA ≡ − TB1 <sup>−</sup> TB0). The background *T*B0 is calculated as the highest 5% of *T*B in the 400 km square area. The 400 km square box in general is sufficient to cover the structure of CE TCs.1 Yang et al. [30] also used *T-V*max diagram to analyze Typhoon Soulik (2013), which had two long-lived CE episodes. **Figure 3c** and **d** shows the *T-V*max diagrams for average values of intensity and CA for the no-CE TCs with intensity category 4 or above and far from land (NCE), and CE TCs. The CE cases have stronger averaged CA, in particular, the CEM cases indicates significant CA increase 24 h after CE formation in the WNP basin. The maintenance or a slight increase of the CA for three types and a slower decrease than that of NCE cases in both basins are in general agreement with the notion that the CE TCs can lead to storm growth [13]. The decrease of areal averaged *T*B and the increase of kinetic energy both occurred after the ERC process.

**Figure 3e** and **f** indicates that the outer eyewall width is larger with a larger moat width (*R*<sup>2</sup> = 0.5) in both basins. All the CEM cases have moat widths greater than 30 km in both basins. In particular, the CEM cases on average have slightly higher intensities, larger moat widths, and larger outer eyewall widths than those of ERC and NRC cases. The CEM cases in the ATL basin also have similar characteristics except the average intensity slightly lower than that of ERC. The ATL basin is smaller than WNP basin, only five CEM cases are classified in the ATL basin. If we choose 15 h (10 h) for CEM criteria, eight (18) cases are classified into CEM cases in the ATL basin. These CEM cases on average have higher intensity, larger moat widths, and larger outer eyewall widths. In general, the moat size and outer eyewall width are approximately 20– 50% (15–25%) larger in the CEM cases than that in the ERC and NRC cases. The very large moat and outer eyewall width in the CEM cases may have some implications for the long duration of CE structure. Willoughby [31] presented a scale analysis on the validity of the balance model and the transverse circulation equation in the TC. Rozoff et al. [32] used the balanced model transverse circulation equation to study the ERC dynamics. In this manner, the balance dynamics of the CE is scale free, namely, the dynamics may occur in different scales where the balance equation assumption is valid. Thus, it is possible that the larger CE storms simply end up taking much longer time to contract due to their larger scale. Rozoff et al. [32] showed that the decay of the inner eyewall may be related to the fact that the upper warm core has a larger stabilization effect on the convection in the inner eyewall than it does on the convection in the outer eyewall. The stabilization effect of upper warm core argument cannot explain why the inner eyewall is maintained for such a long time in the CEM cases. We also note that the CE variabilities of intensity and structural changes in the WNP basin are larger than that in the ATL basin as shown in **Figure 3**.

<sup>1</sup> Typhoons Winnie (1997) and Amber (1997) were very large, and these quantities are calculated using a 600 km square box.

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Maclay et al. [13] use aircraft data to construct the *K-V*max diagram for the intensity and structural changes. However, there is few active aircraft reconnaissance program for the WNP basin. Therefore, the structural and intensity variability is illustrated here using the *T-V*max diagram (where *T* is the *T*B and *V*max is the best track estimated intensity). The convective activity (CA) is indicated by the areal averaged *T*<sup>B</sup> contrast to the background *T*<sup>B</sup> in the 400 km square area of satellite imagery centered at the eye (CA ≡ − TB1 <sup>−</sup> TB0). The background *T*B0 is calculated as the highest 5% of *T*B in the 400 km square area. The 400 km square box in general

analyze Typhoon Soulik (2013), which had two long-lived CE episodes. **Figure 3c** and **d** shows the *T-V*max diagrams for average values of intensity and CA for the no-CE TCs with intensity category 4 or above and far from land (NCE), and CE TCs. The CE cases have stronger averaged CA, in particular, the CEM cases indicates significant CA increase 24 h after CE formation in the WNP basin. The maintenance or a slight increase of the CA for three types and a slower decrease than that of NCE cases in both basins are in general agreement with the notion that the CE TCs can lead to storm growth [13]. The decrease of areal averaged *T*B and the increase

**Figure 3e** and **f** indicates that the outer eyewall width is larger with a larger moat width (*R*<sup>2</sup>

0.5) in both basins. All the CEM cases have moat widths greater than 30 km in both basins. In particular, the CEM cases on average have slightly higher intensities, larger moat widths, and larger outer eyewall widths than those of ERC and NRC cases. The CEM cases in the ATL basin also have similar characteristics except the average intensity slightly lower than that of ERC. The ATL basin is smaller than WNP basin, only five CEM cases are classified in the ATL basin. If we choose 15 h (10 h) for CEM criteria, eight (18) cases are classified into CEM cases in the ATL basin. These CEM cases on average have higher intensity, larger moat widths, and larger outer eyewall widths. In general, the moat size and outer eyewall width are approximately 20– 50% (15–25%) larger in the CEM cases than that in the ERC and NRC cases. The very large moat and outer eyewall width in the CEM cases may have some implications for the long duration of CE structure. Willoughby [31] presented a scale analysis on the validity of the balance model and the transverse circulation equation in the TC. Rozoff et al. [32] used the balanced model transverse circulation equation to study the ERC dynamics. In this manner, the balance dynamics of the CE is scale free, namely, the dynamics may occur in different scales where the balance equation assumption is valid. Thus, it is possible that the larger CE storms simply end up taking much longer time to contract due to their larger scale. Rozoff et al. [32] showed that the decay of the inner eyewall may be related to the fact that the upper warm core has a larger stabilization effect on the convection in the inner eyewall than it does on the convection in the outer eyewall. The stabilization effect of upper warm core argument cannot explain why the inner eyewall is maintained for such a long time in the CEM cases. We also note that the CE variabilities of intensity and structural changes in the WNP basin are larger

1 Typhoons Winnie (1997) and Amber (1997) were very large, and these quantities are calculated using a 600 km square

Yang et al. [30] also used *T-V*max diagram to

=

is sufficient to cover the structure of CE TCs.1

178 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

of kinetic energy both occurred after the ERC process.

than that in the ATL basin as shown in **Figure 3**.

box.

**Figure 3.** (a and b) Composite time series of intensity for the ERC, CEM, NRC cases, and the all CE cases in WNP and ATL basins. (c and d) The averaged *T*B and intensity changes in 48 h before and after the peak intensity/CE formation for the NCE, ERC, CEM, and NRC cases in WNP and ATL basins. Case numbers are given. The dots are CE formation time for CE cases and *V*max time for NCE cases. (e and f) Scatter diagrams of the moat width versus outer eyewall width in the WNP and ATL basins. The linear fitting line and formula are also shown. The table indicates the average intensity, moat width, and outer eyewall width of CEM, ERC, and NRC cases, respectively.

On the other hand, the occurrence of barotropic instability will invalidate the axisymmetric balance assumption. Kossin et al. [33] identified two types of barotropic instabilities in the vorticity field with CE structure: the instabilities across the outer eyewall (Type I) and across the moat (Type II) due to the sign reversal of the radial vorticity gradient. These instabilities may work against the maintenance of the CE structure. The large moat size in the CEM cases has two dynamic implications. It reduces the growth rate of the Type II instability across the moat which is favorable for the CE structure maintenance; and it also lessens the stabilization of the core vortex on the Type I instability across the outer eyewall which is not favorable for the CE maintenance. As demonstrated by Kossin et al. [33], the thicker outer eyewall is more stable for the type I instability, which is favorable for maintaining the outer eyewall structure. These observations of the large outer eyewall and moat widths are in general agreement with the concept that barotropic dynamics may play a significant role in maintaining the CE structure for CEM cases.

Finally, we note that the large moat size in the CEM cases may have an impact on the convection and subsidence in both eyewalls. The interference between the convection/subsidence couplet of the inner and outer eyewalls may be reduced when the moat size is very large. The large moat size may assist the inner core by suppressing potentially competing convection while the subsidence concentrated radial outward may make it less likely to penetrate to the eyewall. Zhou and Wang [34], in the modeling study, revealed that the demise of the inner eyewall is primarily due to the interception of the BL inflow supply of entropy by the outer eyewall. The interception process becomes inefficient when the moat size is large. **Figure 3e** and **f** suggests that the internal structure of CE TCs, such as the general high intensity with the large widths of the moat and outer eyewall, may be important for the maintenance of the CE structure in the CEM cases.

**Figure 4.** (a) Tracks within 48 h centered at CE formation in the WNP. The ERC, NRC, and CEM cases are represented by blue, green, and red colors, respectively. The circles with (without) a dot are the location of the secondary eyewall formation with intensity greater than or equal to (less than) category 4 on the Saffir-Simpson scale. The triangle symbols represent the composite location of CE formation and 24 h after CE formation. The average translation speed of zonal and meridional between CE formation and 24 h after CE formation is shown. (b) As in (a) but the cases in the ATL basin.

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moat which is favorable for the CE structure maintenance; and it also lessens the stabilization of the core vortex on the Type I instability across the outer eyewall which is not favorable for the CE maintenance. As demonstrated by Kossin et al. [33], the thicker outer eyewall is more stable for the type I instability, which is favorable for maintaining the outer eyewall structure. These observations of the large outer eyewall and moat widths are in general agreement with the concept that barotropic dynamics may play a significant role in maintaining the CE

180 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

Finally, we note that the large moat size in the CEM cases may have an impact on the convection and subsidence in both eyewalls. The interference between the convection/subsidence couplet of the inner and outer eyewalls may be reduced when the moat size is very large. The large moat size may assist the inner core by suppressing potentially competing convection while the subsidence concentrated radial outward may make it less likely to penetrate to the eyewall. Zhou and Wang [34], in the modeling study, revealed that the demise of the inner eyewall is primarily due to the interception of the BL inflow supply of entropy by the outer eyewall. The interception process becomes inefficient when the moat size is large. **Figure 3e** and **f** suggests that the internal structure of CE TCs, such as the general high intensity with the large widths of the moat and outer eyewall, may be important for the maintenance of the CE structure in

**Figure 4.** (a) Tracks within 48 h centered at CE formation in the WNP. The ERC, NRC, and CEM cases are represented by blue, green, and red colors, respectively. The circles with (without) a dot are the location of the secondary eyewall formation with intensity greater than or equal to (less than) category 4 on the Saffir-Simpson scale. The triangle symbols represent the composite location of CE formation and 24 h after CE formation. The average translation speed of zonal and meridional between CE formation and 24 h after CE formation is shown. (b) As in (a) but the cases in the

structure for CEM cases.

the CEM cases.

ATL basin.

**Figure 5.** The time series of SST and 850–200 mb vertical shear (VWS) in the WNP and ATL basins. The solid lines mean average value. The dash lines mean average value ± 1 standard deviation.

**Figure 4a** shows the locations of CE formation on the tracks 24 h before and after the formation. Most CEM cases are located west of 140°E with the smallest northward translation speed of 2.2 m s−1 in the WNP basin. This suggests TCs tend to be more intense in the western part of Pacific after a long journey over ocean. In the ATL basin, the average location of CEM cases is farther west than that of ERC and NRC cases with the smallest northward translation speed of 1.4 m s−1. Compared with ERC and NRC cases, CEM cases encounter warmer SST after CE formation. On the other hand, the NRC cases have a larger northward translation speed (3.8 m s−1) in the WNP basin, and the average location of CE formation is farther north (25°N) than that of ERC and CEM cases in the ATL basin. The composite time series with respect to CE formation time for SST, relative humidity (RH), ocean heat content (OHC), and maximum potential intensity (MPI) decrease and vertical shear increases with time because TCs in general move toward the northwest direction by the Statistical Typhoon/Hurricane Intensity Prediction Scheme model data (STIPS/SHIPS; [35, 36]). The large northern translation speed of the NRC cases causes them to experience colder SST, larger vertical wind shear, smaller RH, smaller OHC, and smaller MPI 24 h after CE formation (not shown; the result of WNP cases is presented in **Figure 10** of reference [26]). These phenomena are consistent with the sharp decrease of CA and intensity in NRC cases as shown in **Figure 3**. The dissipation of the outer eyewall in the NRC cases presumably may also be related to the strong vertical shear in the high latitudes [16]. Moreover, the CEM cases were under small vertical wind shear, high SST, OHC and MPI, and high low- to mid-level RH throughout the period of CE formation. These favorable environment factors may help CEM cases maintain their intensity and eyewall structures. The environmental conditions play a role in the structural and intensity changes of CEM and NRC TCs. **Figure 5** shows that the variabilities of SST and 850–200 mb vertical wind shear during CE formation and after CE formation are larger in the WNP basin than that in the ATL basin. This results in the larger CE variabilities of intensity and structural changes in the WNP basin (**Figure 3**).

#### **4. The relationship between CE TC and ENSO**

The environmental factors and TC activities are deeply affected by ENSO and have been examined by many previous studies. We have discussed the importance of environmental factors for CE TCs in Section 3. In this section, we followed Yang et al. [38] but included CE cases in the ATL basin. Furthermore, we examined the CE TCs in different phases of ENSO. There are 46 months for five warm episodes, 62 months for four cold episodes, and 103 months for eight neutral episodes during 1997–2014 period according to NCEP data. There are 38 (18), 16 (10), and 59 (22) CE TCs occurred in the warm, cold, and neutral episodes in the WNP (ATL) basin.

**Figure 6.** Number of (a) category 1–5 TCs, (c) category 4–5 TCs, (e) CE cases (multiple CE formations are included), and (f) CE TCs by years (histograms and left ordinate) and the mean ONI by year (line and right ordinate) in the WNP basin. The correlation between number and mean ONI is shown. (b, d, f, and h) As in (a, c, e, and h) but in the ATL basin.

In the WNP basin, the correlation between annual CE TCs number (CE cases) and ONI by year is 0.77 (0.70) as shown in **Figure 6**. **Figure 6** also shows that the correlation between ONI and annual strong TCs of Category 4 or stronger is 0.75, which means more intense TCs occur in the El Niño phase than that in the La Niña phase. This is in general agreement with previous studies [19, 22]. All the CE-related correlations are higher than the correlation of TC number and ONI by year (0.55). The better correlation of CE TCs and ONI may be due to the fact that the CE structure is likely to occur in strong TCs [3]. In the ATL basin, the negative correlation of annual TCs (strong TCs) and ONI is −0.46 (−0.51), which is consistent with previous studies [20, 23, 24, 37] that suggest unfavorable environment for TC formation in the El Niño phase. Moreover, the worse negative correlation of CE TCs and ONI may be because of only 33 CE TCs in the ATL basin. In addition, the better correlation with CE TCs than with CE cases may be due to the fact that multiple CE formation may be controlled by both internal and environmental factors in both basins.

**4. The relationship between CE TC and ENSO**

182 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

basin.

basin.

The environmental factors and TC activities are deeply affected by ENSO and have been examined by many previous studies. We have discussed the importance of environmental factors for CE TCs in Section 3. In this section, we followed Yang et al. [38] but included CE cases in the ATL basin. Furthermore, we examined the CE TCs in different phases of ENSO. There are 46 months for five warm episodes, 62 months for four cold episodes, and 103 months for eight neutral episodes during 1997–2014 period according to NCEP data. There are 38 (18), 16 (10), and 59 (22) CE TCs occurred in the warm, cold, and neutral episodes in the WNP (ATL)

**Figure 6.** Number of (a) category 1–5 TCs, (c) category 4–5 TCs, (e) CE cases (multiple CE formations are included), and (f) CE TCs by years (histograms and left ordinate) and the mean ONI by year (line and right ordinate) in the WNP basin. The correlation between number and mean ONI is shown. (b, d, f, and h) As in (a, c, e, and h) but in the ATL

In the WNP basin, the correlation between annual CE TCs number (CE cases) and ONI by year is 0.77 (0.70) as shown in **Figure 6**. **Figure 6** also shows that the correlation between ONI and annual strong TCs of Category 4 or stronger is 0.75, which means more intense TCs occur in the El Niño phase than that in the La Niña phase. This is in general agreement with previous

**Figure 7.** Histogram of the no-CE TCs and CE TCs in three different episodes. The numbers of total TC and CE TCs in each episode are indicated. The letters W and A mean WNP and ATL basin, respectively.

**Figure 7** indicates that 42% (35%) of TCs possessed CE structures in their lifetimes during warm (neutral) episodes in the WNP basin. In contrast, only 25% of TCs formed CE structure in the cold episodes. In the ATL basin, 36% (approximately 24%) of TCs possessed CE structures in their lifetimes during warm (neutral and cold) episodes. Moreover, **Table 2** shows that there are 1.5 (0.5), 1.3 (0.7), and 1.0 (0.5) TC formations per month in the WNP (ATL) basin during warm, neutral, and cold episodes, respectively. The monthly CE formation frequencies are 0.8 (0.4), 0.6 (0.2), and 0.3 (0.2) in the WNP (ATL) basin during warm, neutral, and cold episodes, respectively. **Figures 6** and **7** and **Table 2** suggest that ENSO may create a better environment for CE formation in the WNP basin. In the ATL basin, the slightly higher monthly CE formation frequency during warm episode may be due to the farther south CE location as shown in **Figure 8**. For the WNP basin, **Figure 8a** suggests that CE cases during cold (warm) episodes tend to occur farther west (east). The eastward shift of the genesis region may be due to the warm sea water and moist air extending farther east (west) over the WNP during warm (cold) episodes, and the weak vertical wind shear in the southeast part of WNP during warm episodes. This result is consistent with the eastward shift of the TC genesis region in the warm episode [19, 20]. For the ATL basin, **Figure 8b** shows that the average location of CE formation during warm and cold episodes is similar. After 24 h of CE formation, however, the 850–200mb vertical wind shear in the cold episode is 2–6 m s−1 weaker than that in other episodes (not shown) and may help in CE maintenance. The farther south CE location during warm episode may lead to the slight CE formation frequency.


**Table 2.** The number of TCs, CE cases, and multiple CE cases per month during warm, cold, and neutral episodes.

**Figure 8.** Same as **Figure 4** but during warm (red), cold (blue), and neutral (black) episodes. The triangle symbols represent the composite location of CE formation. The standard deviations of zonal and meridional locations are shown.

**Figure 9** suggests that there are only three CEM cases during cold episode in the WNP basin. In addition, there are 12% (3%) of CE cases with more than 30 h long duration in the WNP (ATL) basin. The CE storms in the cold episode have a more rapid intensification rate than that of the warm episode before the CE formation (**Figure 10**). Specifically, there are 13 out of 16 storms in cold episode with intensity change which meet the rapidly intensifying criteria of ∆*V*max ≥ 19.5 m s−1 in 24 h [39]. The storm intensity change after the CE formation during warm (cold) episodes often decreases slowly (quickly). The quick decline of intensity during cold episodes may be due to the encountering of unfavorable environmental factors such as the colder SST. **Figure 10b** indicates the similar trends of the average intensity before CE formation in the ATL basin. However, a rapid decreasing trend 24 h after CE formation is in the warm episode. In summary, the CE formation in the ATL basin may not be affected by ENSO because of the average location of CE formation during warm episode farther south over the ATL. After CE formation, the unfavorable environment which is created by ENSO may reduce the TC intensity quickly during warm episode.

during warm and cold episodes is similar. After 24 h of CE formation, however, the 850–200mb vertical wind shear in the cold episode is 2–6 m s−1 weaker than that in other episodes (not shown) and may help in CE maintenance. The farther south CE location during warm episode

WNP TC number/month 69/46 = 1.5 65/62 = 1.0 130/103 = 1.3 WNP CE cases/month 38/46 = 0.8 16/62 = 0.3 59/103 = 0.6 ATL TC number/month 22/46 = 0.5 34/62 = 0.5 71/103 = 0.7 ATL CE cases/month 18/46 = 0.4 10/62 = 0.2 22/103 = 0.2

**Table 2.** The number of TCs, CE cases, and multiple CE cases per month during warm, cold, and neutral episodes.

**Figure 8.** Same as **Figure 4** but during warm (red), cold (blue), and neutral (black) episodes. The triangle symbols represent the composite location of CE formation. The standard deviations of zonal and meridional locations are shown.

**Figure 9** suggests that there are only three CEM cases during cold episode in the WNP basin. In addition, there are 12% (3%) of CE cases with more than 30 h long duration in the WNP (ATL) basin. The CE storms in the cold episode have a more rapid intensification rate than that of the warm episode before the CE formation (**Figure 10**). Specifically, there are 13 out of 16

**Warm Cold Neutral**

may lead to the slight CE formation frequency.

184 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

**Figure 9.** The number of CE cases in different episodes as a function of duration time. The letters W and A mean the WNP and ATL basins, respectively.

**Figure 10.** Same as **Figure 3** but during warm, cold, and neutral episodes and all CE cases.
