The North Equatorial Current and rapid intensification of super typhoons

Domain: earth_science
Published: 2024-03-07
Editorial outcome: major_revision

Abstract
Super Typhoon Mangkhut, which traversed the North Equatorial Current (NEC; 8–17°N) in the western North Pacific in 2018, was the most intense Category-5 tropical cyclone (TC) with the longest duration in history—3.5 days. Here we show that the combination of two factors—high ocean heat content (OHC) and increased stratification—makes the NEC region the most favored area for a rapid intensification (RI) of super typhoons, instead of the Eddy Rich Zone (17–25°N), which was considered the most relevant for RI occurrence. The high OHC results from a northward deepening thermocline in geostrophic balance with the westward-flowing NEC. The stratification is derived from precipitation associated with the Inter-Tropical Convergence Zone in the summer peak typhoon season. These factors, which are increasingly significant over the past four decades, impede the TC-induced sea surface cooling, thus enhancing RI of TCs and simultaneously maintaining super typhoons over the NEC region.

Introduction
Tropical cyclones (TCs) obtain their energy from the ocean in the form of enthalpy flux1–3 which is driven in large part by the sea surface temperature (SST) underneath TCs4,5. The intensification of TCs has been observed near regions of high SST6 and statistical and idealized theoretical models have used pre-storm SSTs as a predictor for TC intensity change7,8.

An important factor controlling the degree of SST cooling is strong vertical mixing that occurs during the TC-ocean coupling processes9. During TC’s intensification, wind mixing entrains and pumps colder subsurface water into the surface layer2,3,5,10 which reduces the SST. This cooling effect is a well-known negative feedback to restrain TC’s own intensification; the smaller the cooling effect, the more the air-sea heat flux for TC’s intensification10. As such, ocean heat content (OHC), defined as the upper-ocean heat content relative to the 26°C isotherm depth (D26)11 (Methods), maybe a better diagnostic tool to predict TC-induced cooling than using along-track SST alone10,12,13. Over high OHC (deep D26 regions), the TC-induced ocean cooling effect tends to be much smaller, thus more air-sea enthalpy fluxes are available for TC’s intensification.

Mesoscale eddies, a transient rotational circulation feature of order 100-200 km in diameter with alternating cyclonic and anticyclonic flows (see Fig. 1a), are known to affect significantly the TC intensity during its lifecycle in the western North Pacific14–19. Warm eddies, characterized by an anticyclonic circulation and positive temperature/sea level anomalies, cause a deepening of the warm upper layer20 and thus an increase in OHC, and vice versa for cold eddies. Due to their much deeper D26 and high OHC, warm eddies can effectively restrain the TC-induced ocean cooling effect6,18,19. Since the cooling effect controls the air-sea enthalpy fluxes, when TC encounters a warm eddy, it is favorable to develop rapid intensification (RI define as wind intensification rate > 15.4 ms−1 day−1)14–17. Assuming other atmospheric factors (e.g., vertical wind shear, TC translation speed)21 and sea state22 are also favorable, encountering warm eddies increases the probability of RI, as the negative feedback effect can be effectively restrained. As a consequence, super typhoons, or Saffir-Simpson scale category-4 (CAT4) or higher (maximum sustained wind greater than 58 ms−1) have been frequently observed in the eddy-rich zone (ERZ) between 17°–25°N18,19, where mesoscale eddy activity is prevalent, with the frequent occurrence of both cold and warm eddies (Fig. 1a). Notable examples include Super Typhoon Maemi18 which devastated South Korea in 2003, and ‘Killer Cyclone’ Nargis23 which devastated Myanmar in 2008.

In contrast, the area south of the ERZ exhibits little eddy activity and is dominated by a persistent large-scale circulation, characterized by the westward-flowing strong North Equatorial Current (NEC) between 8°−17°N (Fig. 1a). However, analysis of all major TCs (≥ CAT3; maximum wind speed greater than 50 ms−1) during 1984–2021, using the Joint Typhoon Warning Center (JTWC) best track database (see Data), reveals a significant number of super typhoons occurring within the NEC region compared to the ERZ (Supplementary Fig. 1a), although mesoscale eddy activity is lacking in the region south of 17°N (Fig. 1a). During the main typhoon season (July–October), eddy activity reaches a maximum in the ERZ23. In contrast, during the off-peak typhoon season (December–April), mesoscale eddy activity is at a minimum within the ERZ23, thus super typhoon activity, while uncommon, is typically observed only within the NEC region (Supplementary Fig. 1b).

For both regions, RI of major TCs most often occurs within the NEC region (52%), compared to 34% within the ERZ (Fig. 1b). The regional contrast is even greater when only CAT5 (maximum wind speed greater than 70 ms−1) TCs are considered (NEC~66%; ERZ~21%; see Fig. 1c). The northern NEC region centered between 14°–15°N and west of 150°E appear to be the most favored region for TC RI for both major TCs and CAT5 TCs (Fig. 1b, c). As indicated similarly in Fig. 1b, TCs moving through the NEC have a much higher probability (mean 27.6%) to experience RI than those (mean 6.1%) moving through other regions (Supplementary Fig. 1c). This difference in probabilities was statistically significant based on the Student’s t-test (p < 0.001). Here, we show the key role of the NEC in the RI of super typhoons and their extended persistence in this region by using Mangkhut as a case study.

Results
Mangkhut was not only the strongest TC in the western North Pacific for 2018, it was the strongest TC globally for that particular year. It moved westward along 14°–17°N over the northern NEC, a region where the least eddy activity occurs, regardless of seasons23. On 10 September, Mangkhut’s maximum sustained wind increased from 46 to 72 ms−1 over 24 h i.e., its RI phase. Mangkhut traveled westward from ~140°E to 122°E (Figs. 1a and 2b) and achieved CAT5 at 06:00 UTC on September 11 and maintained this intensity through 18:00 UTC September 14, making it the longest duration (3.5 days) CAT5 on record.

The cooling inhibition (CI) index (Methods)24,25 measures the potential energy input required to cool the ocean surface through TC-induced vertical mixing, hence accounts for the pre-storm upper-ocean stratification which resists the heat loss at the surface. The RI region of Mangkhut (150°–140°E) and the area over which Mangkhut maintains CAT5 intensity (140°–122°E) are associated with a large value of CI > 35 (J m−2) 1/3 (Fig. 2d), suggesting that the enhanced upper-ocean stratification (Fig. 2c) by warm/fresher water along the NEC (Fig. 2b, e) exerts a strong inhibition of vertical mixing-driven cooling. Enhanced OHC also prevailed in the NEC region with a maximum centered at its northern boundary (17°N) (Fig. 2a), resulting from the westward-flowing strong NEC as well as eddy activity near the southern boundary of ERZ, which will be further discussed in climatology. The combination of the NEC-driven high OHC and low salinity-driven upper-layer stratification makes the whole NEC an enhanced region of maximum dynamic potential intensity (DPI, Methods) (Fig. 2d) and explains the persistence of the observed Super Typhoon Mangkhut (Fig. 2f).

Fresher (or low-salinity) water with sea surface salinity lower than 34 psu (practical salinity unit) originated from the rainy Inter-Tropical Convergence Zone (ITCZ) (Fig. 3b). With the seasonal shift of ITCZ’s precipitation (Supplementary Fig. 4), fresher water moves northward into the NEC region, coinciding with the peak typhoon season. Dedicated numerical experiments based on the 3D Price-Weller-Pinkel model26 conducted along the Mangkhut’s track show that the fresh water can contribute an additional 19% suppression of SST cooling, a factor which favors TC intensification (Supplementary Fig. 5d, e).

Fresher water (Fig. 2b) combined with elevated pre-storm tropical SST of order 29.0–29.5°C (Fig. 2e) induces enhanced near-surface stratification (Fig. 2c), resulting in a stronger oceanic inhibition of vertical mixing, which is a very favorable condition for the intensification of TCs (Fig. 2d), assuming other necessary atmospheric conditions also remain favorable. This view is in line with the barrier layer effect for TC intensification27, as the increased stratification and stability in the salinity-induced barrier layer reduce the TC-induced vertical mixing and SST cooling. Consequently, only limited TC-induced SST cooling between 0.5–0.8°C, with a peak of order 1°C, was estimated during Mangkhut (Fig. 2e).

Potential intensity (PI) (Methods) is a theoretical upper bound of TC intensity under given environmental conditions of the atmosphere and the pre-storm SST28,29. In contrast, dynamic PI (DPI) (Methods) takes into account the TC-induced SST cooling, significantly improving the PI estimates30. The temporal evolution of DPI estimates is in good agreement with the observed JTWC maximum sustained winds of Mangkhut during its long-lasting CAT5, while PI overestimates the observations by about 10 ms−1 (Fig. 2f).

While a number of previous work15,17–19,21 have emphasized the tight link between warm eddies and RI of super typhoons, there were no warm eddies present along Mangkhut’s track between 11–13 September (Fig. 1a), thus the existence of the persistent CAT5 can be attributed largely to the OHC maximum centered at 17°N for the Mangkhut case (Fig. 2a) compared to 14°N in climatology (Fig. 3f). The longest persistence of Super Typhoon Mangkhut owes to its mostly westward translation, which consistently maintained CAT5 within the northern NEC band. In contrast, the climatological OHC in the ERZ decreases sharply northwards (Fig. 3f), making it more difficult for super typhoons to sustain their intensity for a long time due to the alternating positive/negative OHC anomalies associated with the spatially-varying polarity of transient mesoscale eddy field cut by the TC track. Climatology of best track data shows that no previous CAT5 TCs have been observed to persist for more than 1 day in the ERZ, as was the case with Megi (2010), which maintained a CAT5 for a half day only at 18–19°N10. In addition, Maemi (2003) persisted as a CAT5 for less than a day at around ~24°N18.

It is well known that the vertical wind shear (Methods) greater than a critical value of 10 ms−1 is unfavorable TC intensification31–35. The pre-storm-mean vertical wind shear during the RI phase of Mangkhut was moderate at 7.12 ms−1, whereas during its westward journey as a CAT5 was 7.24 ms−1 (Supplementary Fig. 2). The TC translation speed can also exert a significant control on TC intensity by modulating the TC-induced SST cooling, with faster-moving storms tending to lessen the negative SST feedback36. The TC translation speed of Mangkhut varied between 5–11 ms−1 (Supplementary Fig. 3), considerably more than the 4 ms−1 generally accepted as the minimum speed to maintain CAT536. Taken together, this information suggests that the large-scale atmospheric environment was also favorable for the intensification and persistence of CAT5 Mangkhut.

The NEC is the largest westward-flowing, wind-driven surface current in the tropical western North Pacific between 8°–17°N and is bounded on both sides by two eastward currents: North Equatorial Counter Current in the south and the Subtropical Countercurrent in the north37–39 (Fig. 3c). The 30-year (1991–2020) climatology of zonal-mean (126°–160°E) meridional sections of water properties (Fig. 3a–c) shows that the upper-layer thermocline is shallowest in the ITCZ at 8°N, which deepens gradually both to the south and to the north until encountering a local maximum at 14°N before shoaling again to the north in the ERZ. The meridional gradient of upper-layer isopycnals (23.0–26.5 kg m−3) appears in geostrophic balance with the above-mentioned system of three currents (Fig. 3c): northward deepening of isopycnals by the strong westward NEC between 8–17°N, in contrast to their northward shoaling by both the strong eastward North Equatorial Counter Current south of 8°N and the weak eastward Subtropical Countercurrent in the ERZ north of 17°N.

The northern flank of the NEC between 123° and 160°E exhibits the deepest thermocline, with the 26°C isotherm (used for the computation of OHC) deepening from a minimum of 90 m at 7°N to a maximum of 130 m at 14°N (Fig. 3a) due to the westward-flowing NEC. More specifically, deepening of thermocline results in OHC values > 100 kJ cm−2 over the NEC latitude range (8°–17°N), with a peak of 120 kJ cm−2 at 14°N (Fig. 3f). For the ERZ north of 17°N, however, the climatological OHC drops sharply below 100 kJ cm−2, reach a value as low as 40 kJ cm−2 at 25°N. Consequently, in the ERZ the presence of warm eddies is indispensable for compensating the climatologically low OHC, and creates a critical value of in-situ OHC for the accommodation of super typhoons. This process is not required for the NEC region.

In addition, rain-induced freshwater from the ITCZ enhances the stratification in the upper 50 m in the NEC region (Fig. 3b, e), suggesting the presence of a barrier layer27. The ITCZ undergoes seasonal migration from the equatorial region in winter northwards into the NEC region in summer, with the peak summer precipitation dominating over winter precipitation (Fig. 4a–c). This feature results in the salinity-driven squared buoyancy N2 (indicative of stratification) in summer overwhelming winter N2 in the NEC region (Fig. 4d, e). The ITCZ freshening forms a strong barrier layer (Supplementary Fig. 4d, f), contributing positively to the CI index by 7.5–12.5% in the NEC region compared to that without the freshening effect (Supplementary Fig. 6), implying that it is a significant secondary factor in the NEC for the RI of TCs. Theoretical cooling by CAT5 super typhoons translating at 6–7 ms−1 is estimated to be less than 0.8–0.9°C in the NEC region (Supplementary Fig. 7a; Methods), in agreement with the observed cooling during the mature phase of Mangkhut (Fig. 2e). In addition, summer freshening occurs just on time to coincide with the peak typhoon season, leading to RI of TCs, with heat flux over 800 W m−2 (Methods) similar or higher than the reported magnitudes2,15 during RI.

Consequently, the combined effect of the ITCZ freshening-induced enhanced stratification in the upper layer (though a secondary factor) together with the deepest thermocline due to the westward-flowing strong NEC (as a primary factor) make the whole NEC region a hotspot for accommodating the strongest TCs. This finding is consistent with the predominant occurrence of RI events for super typhoons in the northern NEC region (Fig. 1b, c).

We have shown that the upper-ocean stratification in the NEC region naturally features the highest OHC due to the northward thermocline deepening in geostrophic balance with the strong westward-flowing NEC. The highest OHC combined with enhanced subsurface stability due to the fresher water supply by the summer northward migration of the ITCZ makes the northern NEC region between 14°–15°N west of 150°E the most favorable corridor for the intensification and persistence of super typhoons (Fig. 1b, c).

To understand the key role that the NEC region plays in RI of super typhoons in a global perspective, Fig. 5 compares the climatological PI and DPI in the North Pacific south of 35°N. The area with PI values over 70 ms−1 (a lower bound of CAT5 TC) extends up to 25°N, encapsulating most of the ERZ (Fig. 5a), while the DPI zone contracts into the NEC region (Fig. 5b). The difference between these two indices indicates the contribution from the TC-induced negative SST feedback, which accounts for less than 10 ms−1 in the NEC region, compared to a sharp increase of up to 30 ms−1 in the ERZ. The associated SST cooling is minimal and less than 0.8°C in the NEC region (Fig. 5c), similar to the climatology in the literature25 and our climatology (Supplementary Fig. 7a), as well as to the Mangkhut observations (Fig. 2e).

These climatological results are consistent with the Mangkhut case (Fig. 2), which confirms that the westward NEC-associated high OHC and the ITCZ-related haline stratification are optimally conducive for inhibiting TC-induced SST cooling, thus systematically favoring the intensification and persistence of super typhoons in the NEC region. However, Mangkhut is not an isolated case, as we have also observed in May 2023 another long-lasting (2.7 days) super typhoon in the NEC region, even during the pre-typhoon season (Supplementary Fig. 8). In effect, Mawar intensified from CAT3 to CAT5 on 25 May soon after it changed its translation direction westward from NW to WNW at 14°N, 144°E. It then maintained its CAT5 intensity for 1.5 days (25–26 May) while it moved westward in the northern part of the NEC region (14°–16°N), before turning northward and gradually weakening to become CAT2 in the ERZ north of 17°N after 28 May. Upper-ocean stratification conditions during the CAT5 phase of Mawar on 26 May were very favorable (OHC~130 kJ cm−2 and SST cooling ~0.5°C), although the low salinity inflow from the ITCZ was not significant (Supplementary Fig. 8). These upper-ocean conditions during the CAT5 phase of Mawar are contrasted with significantly less favorable conditions during its weakening phase on 27 May (OHC < 70 kJ cm−2 and SST cooling > 1.5°C). Note also that the atmospheric conditions were also favorable during the CAT5 phase of Mawar, with VWS less than 5 ms−1 and the translation speed greater than 5 ms−1 (Supplementary Fig. 9). Therefore, Mawar supports our findings from Mangkhut that super typhoons are likely to both rapidly intensify and persist whenever they travel westward within the NEC region under favorable atmospheric conditions.

We have emphasized the utmost importance of the NEC as a preferred region for rapid intensification and long-lasting persistence of maintaining super typhoon strength in the western North Pacific, which has hitherto received little attention. First identified from the analysis of Mangkhut, the unexpected contribution of the NEC to the RI of super typhoons is found to be a quasi-permanent climatic feature. Of particular importance in this regard is the increase in OHC over the NEC at a rate of ~9 kJ cm−2 decade−1 over the last 4 decades (Fig. 6a), which far exceeds the global (1.32 kJ cm−2 decade−1) and all-Tropics (2.47 kJ cm−2 decade−1) trends (Supplementary Fig. 10). Furthermore, the mean intensification rate of TCs in the NEC region has increased by approximately ~1.5 kt day−1 decade−1, which equates to a probability of RI of 3.1% per decade (Fig. 6b).

Previous work40–42 as well as Fig. 6c show that the overall increase in OHC and RI in the NEC region arise from warming (upward trend in sea level) of the western North Pacific. The El Nino-Southern Oscillation (ENSO) may also play a role in the interannual variability of OHC and RI in the NEC region43, which needs further analysis, although removing the ENSO signal is unlikely to affect the long-term trend of OHC (Fig. 6a).

Methods
Environmental conditions from satellite and reanalysis data
The daily absolute dynamic topography, produced by the satellite altimeter data with a spatial resolution of 0.25°×0.25°, was used for the subsurface thermal condition and the existence of eddies44. It was obtained from the Copernicus Marine Environment Monitoring Service (http://marine.copernicus.eu). For the estimation of atmospheric conditions for Typhoon Mangkhut, we used the 6-hourly fifth generation European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5) with a spatial resolution of 0.25°×0.25° and 37 vertical levels45. To analyze ocean states, we used vertical profiles of water temperature and density from the Global Ocean Reanalysis and Simulation (see GLORYS in Methods)46. The daily GLORYS12V1 was used with a standard regular grid with 1/12° (approximately 8 km) interval and 50 standard levels. The data are available at https://data.marine.copernicus.eu/products. To investigate the impact of climatological atmospheric conditions and ocean states on the TC intensification, the monthly ERA5 and Ocean Reanalysis System 5 (see ORAS5 in Methods)47 with 75 vertical levels were utilized with a horizontal grid of 1°.

JTWC best track data
For tracking Mangkhut, we used the best track data from the Joint Typhoon Warning Center (JTWC).

Satellite-based surface ocean environmental data
Satellite-based data were used for examining the surface environmental conditions associated with the spatio-temporal evolution of Super Typhoon Mangkhut (2018). Optimally-interpolated microwave and infrared SST data were used48. Daily absolute dynamic topography and sea level anomaly data with a spatial resolution of 0.25°×0.25° were utilized for describing the surface circulation and mesoscale eddies44. To identify the effect of low salinity water from the ITCZ, daily sea surface salinity data interpolated into a 0.25° grid with an accuracy of 0.2 psu were used49.

GLORYS12V1 model data
GLORYS12V1 is the CMEMS global ocean eddy-resolving (1/12° horizontal resolution and 50 vertical levels) reanalysis covering the altimetry era (1993–2018)46. It is largely based on the current real-time global forecasting CMEMS system (Marine.Copernicus.EU). The ocean model component is the NEMO50 driven at the surface by the ECMWF ERA-Interim reanalysis with a horizontal resolution of ~80 km51. The global ocean data were displayed on a standard regular grid with 1/12° (approximately 8 km) intervals and 50 standard levels.

ORAS5 reanalysis data
ORAS5 is global ocean and sea-ice reanalysis data produced by the ECMWF OCEAN5 analysis-reanalysis system with a spatial resolution of 0.25°×0.25° and 75 vertical levels47. The ORAS5 system comprises 5 ensemble members and is forced by the ECMWF atmospheric reanalysis and observation data from 1979 onwards. This system uses the 3D-var FGAT assimilation technique. The ORAS5 data is constrained by global atmospheric reanalysis and observation data (SST, salinity, sea-ice concentration, sea-level trend, and climatological variation of the ocean mass).

ERA5 reanalysis data
ERA5 provides hourly and monthly estimates of the state of the atmosphere on pressure and single levels with the regular lat-long grid of 0.25°45. The data are produced by the ECMWF combining model data with observations and available from 1940 onwards. A 10-member ensemble at 3-h intervals is used to produce an uncertainty estimate.

Ocean heat content (OHC)
According to Leipper, D.F. & Volgenau11, the pioneer of this parameter, the sea surface loses heat to the atmosphere in a TC, leading to cooling by turbulent convection through the mixed layer. Considering the air temperature of 26°C, which is a dew-point air temperature in the tropics, the sea-to-atmosphere heat transfer would continue until the mixed layer temperature drops to 26°C. This is the maximum heat utilized by a TC, initially referred to as ‘TC heat potential’, equivalent to OHC, which is computed based on the following formula from ref. 11.

OHC = \int_{H26}^{0} Q(z)dz, with Q(z) = \rho c_p (T(z) - 26) (1)

where H26 is the depth of the 26°C isotherm, \rho is water density (~1025 kg m−3), c_p is the specific heat at constant pressure (~3850 J (kg °C)−1), and T(z) is water temperature in °C at a depth z. OHC timeseries figures (Fig. 6a and Supplementary Fig. 10) are plotted using the original timeseries of ORAS5 (Methods).

Cooling inhibition index (CI)
Vincent et al.25 suggested an idea to assess oceanic control on the amplitude of sea surface cooling induced by TCs. Given a pre-storm upper-ocean density profile, one can calculate the potential energy increase \Delta E_p(\Delta T) necessary to produce a given \Delta T cooling assuming heat conservation and a perfectly homogeneous mixed layer after the mixing by the typhoon. The larger \Delta E_p, the more energy has to be injected into the ocean to produce TC-induced cooling. This is to characterize the propensity of the pre-storm ocean state to yield a weak or strong surface cooling in response to a surface kinetic energy input.

\Delta E_p(\Delta T) = \int_{hm}^{0} g(\rho_i - \rho_f)z dz (2)

where \rho_i is the initial unperturbed profile of the density, \rho_f is the homogeneous final density profile, g is the acceleration due to gravity, and z is ocean depth. Following Vincent et al.25, we adopted the Cooling Inhibition index (CI) as the cube root of the necessary potential energy to induce a 2°C SST cooling:

CI = [\Delta E_p(\Delta T = 2)]^1/3 (3)

The GLORYS12V1 output was used for the density profile needed to calculate the CI.

Potential intensity (PI)
Emmanuel29 demonstrated that potential intensity (PI) is a useful statistical analysis of TC intensity because it provides the upper limit TC can achieve under the thermodynamic state of the atmosphere and sea surface. PI27 is expressed as

V_max = \alpha \sqrt{C_k/C_D (T_s/T_o) (CAPE^* - CAPE)} / RMW, (4)

where \alpha is the factor to reduce gradient wind by friction for 10-m wind as 0.8 by default, C_k = C_D is the ratio of the exchange coefficient for enthalpy (C_k) to the drag coefficient (C_D) as 0.9 by default, replaced by 1.0 in this study, T_s is the SST, T_o is the outflow temperature, CAPE^* is the convective available potential energy of saturated air lifted from sea level to the outflow level, and CAPE is that of the environment near the radius of maximum wind (RMW). The 6-hourly ERA5 and daily GLORYS data 2 days before the TC arrival were used to calculate the 6-hourly PI. For the climatological PI, monthly ERA5 and ORAS5 were used.

Dynamic potential intensity (DPI)
Balaguru et al.30 suggested the dynamic PI (DPI) reflect both ocean stratification and TC state by replacing SST with the vertically averaged seawater temperature T_dy over the mixing length L as

T_dy = 1/L \int_{0}^{L} T(z)dz, (5)

The mixing length forced by the TC circulation is calculated as

L = h + (2\rho_o \kappa g u_*^3 t^3)^1/3, (6)

where h is the initial mixed layer depth defined as the depth where a potential temperature decreases by 0.2°C relative to its value at 10 m depth, \rho_o is the seawater density, u_* is the friction velocity in the mixed layer calculated from the maximum sustained wind of TC obtained using a drag coefficient for high winds52,53, air density of 1.2 kg m−3 30, \kappa is the von Kármán constant of 0.4, g is the gravitational acceleration, and \alpha is the rate of increase of potential density with depth below the mixed layer. t is the mixing time interpreted as the duration of the TC passage across the mean radius of maximum wind. The 6-hourly ERA5 and daily GLORYS data were used to estimate environmental states associated with Typhoon Mangkhut. To calculate the spatial distribution of DPI on 9 September 2018, 2 days before Mangkhut reached its CAT5 intensity, all of the grids were forced by Mangkhut with intensity of 75.45 ms−1, translation speed of 6.54 ms−1, and radius of maximum wind of 34.4 km which are the mean values over the period of sustained CAT5 (0600 UTC 11–1800 UTC 14 September 2018; Supplementary Fig. 3). The temporal evolution of DPI over the typhoon center during the period of 1200 UTC 08–1800 UTC 14 September 2018 was calculated from the JTWC best track data for Mangkhut, with a fixed radius of maximum wind of 35 km.

Vertical wind shear (VWS)
The magnitude of vertical wind shear (VWS) is defined as below.

\sqrt{(U_200hPa - U_850hPa)^2 + (V_200hPa - V_850hPa)^2}, (7)

where U_200hPa and V_200hPa (U_850hPa and V_850hPa) are horizontal winds at 200 (850) hPa. The VWS along the track of Mangkhut was calculated by averaging within a radius of 200 km relative to the TC center 2 days before the TC arrival, although the results are not sensitive to lags 0–2 days (see Supplementary Fig. 2c).

TC-induced Sea Surface cooling (dT)
CAT5 super typhoon with a translation speed of 6–7 ms−1 corresponds to wind power index WPi = 4.1. (Fig. 4b of ref. 25). Following Vincent et al.25, dT is depend on WPi and cooling inhibition index CI from a least square fit. For the fixed WPi = 4.1, TC-induced cooling dT is a function of CI only (Fig. 7b of ref. 25). dT is estimated from a 2nd order polynomial fitting between dT and CI (Fig. 7b of ref. 25) as:

dT = 5.5578 - 0.2233 \times CI + 0.0025 \times CI^2 (8)

Latent Heat Flux during Rapid Intensification in the NEC.
Following previous work2, the latent heat flux bulk formula is defined as:

LHF = \rho_a L_e C_e U_10 (Q_s - Q_a), (9)

where \rho_a = 1.12 kg m-3 is the air density, L_e = 2.5 \times 10−6 is the latent heat of vaporization, C_e = 1.4 \times 10−3 (HWRF, 2017) is turbulent exchange coefficient, U_10 is 10-m wind speed, Q_s is saturation specific humidity, and Q_a is specific humidity.

The latent heat flux estimated from (9) using representative parameters for RI in the NEC region is usually around 838-915 W m−2, which is similar to or higher than the reported enthalpy flux (600–900) W m−2 for RI2,15. This order of magnitude of latent heat flux confirms that RI can easily occur in the NEC region due to its favorable upper-ocean stratification described in the text.

Sea Level Rise Trend in the NEC
Sea level data from satellite altimetry is available from NOAA (https://www.star.nesdis.noaa.gov/sod/lsa/SeaLevelRise/LSA_SLR_timeseries.php). The NEC region is one of the steepest sea level rising areas in the world’s oceans (Fig. 6c). Sea level change in the western North Pacific is due mostly to the upper-ocean thermosteric component40, implying that it is a good proxy of OHC change. Note that the sea level rising trend in the NEC region during the past three decades is much stronger than that in the ERZ by a factor of 2.

(ARGO I, II) data
ARGO I data are available by the international Argo Program, measured by a traditional Argo float54 with one day interval and ARGO II data are measured by a similar type of float with three-hour interval data, during May 25 to 29, 2023. They were used for plotting Supplementary Fig. 8.

Data availability
These satellite-based sea surface temperature and salinity data were provided by the Remote Sensing Systems in Santa Rosa, California (http://www.remss.com). The satellite altimeter data were obtained from the Copernicus Marine Environment Monitoring Service (https://cds.climate.copernicus.eu/cdsapp#!/dataset/satellite-sea-level-global?tab=form). Tropical cyclone information is obtained from the Joint Typhoon Warning Center (JTWC), available at http://www.metoc.navy.mil/jtwc/jtwc.html. GLORYS12V1 model data are available at. https://data.marine.copernicus.eu/product/GLOBAL_MULTIYEAR_PHY_001_030/description. ORAS5 reanalysis data are available at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-oras5?tab=overview. ERA5 reanalysis data are available at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels?tab=form. The ARGO I data are available at https://doi.org/10.17882/42182#5906387 and ARGO II data are available upon request.

Code availability
Codes used to produce results and figures were obtained using Matlab and Grads software package. All codes used in this study can be obtained from the corresponding author upon request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Comments on “The North Equatorial Current and Rapid Intensificafion of Super Typhoons” by Kang et al.\nThis study explores the cause of typhoon Mangkhut in 2018. The authors especially emphasize the role\nof North Equatorial current. They also analyzed the role of ITCZ induced freshening. Overall, this work is\ninteresfing. I think most of the analysis focuses on the happening of the long-lasfing super typhoon\nMangkhut. So, it is more like a case study. But the most intrinsic reason for this is sfill not clear based on\nthe current evidence. The main reason I think is too much analysis is about the climatology. I will show\nsome examples below. Thus, I think the authors need further invesfigafion into this parficular case, that\nis the typhoon Mangkhut. Please see specific comments below.\nSpecific comments:\n1. The TC intensificafion is closely related to the upper ocean heat supply of course, especially for the\ntyphoon Mangkhut, which moves along the NEC region. So, it will be not surprise that the OHC in NEC\nregion plays crifically important role in sustaining the strong intensity of Mangkhut. The authors indeed\nshow several largescale condifions that favorable for the intensificafion, like the large OHC, moderate\nverfical wind shear and freshwater induced by ITCZ shift. But only Figure 2 is about this parficular case.\nFigures 3 and 4, which is also important for TC intensificafion, are both on climatological perspecfive.\nAlthough the authors have analyzed in many details, it is sfill unclear how these climatological variables\nimpact the parficular case Mangkhut. For example, in L39-41, the ITCZ northward migrafion is seasonal\nmarch, but why only Mangkhut sustained its high intensity, while other TCs doesn’t. This issue exists in\nthe analysis of precipitafion, salinity, buoyancy et al. I suggest the authors to carried out these analysis\nfor Mangkhut as in Figure 2.\n2. The atmosphere changes much faster than the ocean. Thus, I concern that in the analysis of verfical\nwind shear around L149-158, whether it is sfill proper to analyze the pre-storm VWS as that of the OHC.\nThe VWS is likely to change with fime during Mangkhut. So, how does VWS change during all life cycle of\nMangkhut should be invesfigated.\n3. Figure 6. The authors show the long-term trend of the OHC and TC intensificafion rate over the NEC\nregion. Actually, many study show that the overall OHC in the WNP has increased not limited to NEC\nregion. So, the figure 6a only shows a part of the overall warming paftern of the WNP. This should be\nacknowledged in the manuscript and also should be discussed on the potenfial relafionship between\nNEC warming and the overall warming paftern. Another issue is that ENSO may plays a role in the long-\nterm trend of OHC. Thus, it’s signal should be removed.\n4. The following study is quite relevant to this study, which found that a type of fast decaying El Nino\nevents leads to westward migrafion of RI events by affecfing the NEC. The authors should not miss it in\nreviewing the related background:\nGuo Y. P., and Z. M. Tan", "reviewer_2018": "Westward migration of tropical\ncyclone rapid-intensification over the Northwestern Pacific during short\nduration El Nino, Nature Communications, 9, 1507", "reviewer_doi": "n10.1038/s41467-018-03945-y.\nWe have now cited this article and discussed the results within this context.\n5. Figure 6c: The significance. The authors say shading means significant.\nDoes it mean the trends of sea level over the whole Pacific is statistically\nSignificant?\nThe areas of significant trends are indicated by black dots in Fig. 6c. The figure caption is also\nreformulated.\nReviewer: Sam Hardy\nOverview\nThe authors present an interesting and informative analysis of the role of\nthe North Equatorial Current region on the rapid intensification of super\ntyphoons in the western North Pacific. Data are presented impressively both\nfor the case study of Typhoon Mangkhut (2018) and over the longer period of\nstudy used for climatological analysis. The interesting findings represent a\npositive contribution to the field, particularly the analysis on the salinity\nand ocean stratification, and I congratulate the authors on putting together\nthis piece of work. However, there are several aspects of the discussion that\nneed improving before the paper is suitable for publication, which I have\noutlined below. My recommendation is Major Revisions.\nThank you for your positive comments and recognition of the contribution of this article to the\nfield of tropical cyclone research.\nGeneral comments\n- Reduce the number of acronyms throughout the paper. It’s difficult for the\nreader to keep track of them after a certain point.\nWe have reduced the number of acronyms by omitting 8 acronyms.\n- Given the central importance of mesoscale eddies to your analysis, you need\nto introduce the topic in more detail at the start of the paper. It currently\nfeels like you’re assuming that the reader already knows what all these\nconcepts mean.\nThank you very much for pointing this out. We have added the introduction to this topic in\nmore details. L54-56\n- Linked to the previous point, how does mesoscale eddy activity contribute\nto the rapid intensification of tropical cyclones? You need to better\nintroduce the established literature on this topic to give the reader a more\nsolid understanding by the time you get to your specific analysis.\nYes, as above, we have added the related text and references. L56-69\nthe reader in more detail early in the paper and discuss their relevance to\ntropical cyclone intensification. Much of the analysis that comes later would\nbe easier to follow if the introduction of these terms was more robust.\nYes, very good point and sorry that it was not clear in the earlier version. We have revised this\naccordingly as well. In addition to the above response for the ERZ, we added a sentence\nintroducing the NEC. L70-72.\n- In the abstract you mention the intensification of super typhoons in a\nwarming climate. However, you do not discuss this point any further in the\nmanuscript, only touching on the topic again right at the end of the paper\n(L248-250). Either expand this part of your analysis or modify the abstract\nto avoid confusing the reader.\nThis is a very good point and we decided to modify the abstract to avoid unnecessary emphasis\non a warming climate.\n- You need more robust discussion of the relationship between upper-ocean\nstratification and tropical cyclone intensification. Making this change would\nreally add depth to your analysis.\nWe strengthened the analysis of the Mangkhut case in Fig.2. L99-111\n- Create a stronger link between your analysis of Typhoon Mangkhut and your\noverall conclusions, for example in the Summary and Discussion section. It\nalmost feels like the paper contains two separate bits of analysis, which are\nnot properly linked together. Along the same lines, the paper should end with\na more confident and conclusive statement on your findings.\nAs stated above, we added in Fig. 2 supplementary analysis of the upper-ocean stratification for the\nMangkhut case and discussed the latter in relation to the climatology. In the Summary and Discussion\nsection, we also emphasized the link between the analysis of Mangkhut and the overall conclusions. To\nstrengthen the Mangkhut case, we also added the analysis of Mawar (2023), another long-sustained super\ntyphoon in the NEC region recently observed in May 2023. L220-235 and Supplementary Figs. 8 and 9.\n- L31-45. Reduce the number of acronyms in the abstract.\nYes, we reduced the number of acronyms, and the following 8 acronyms were omitted:\nSTY (super typhoon), ADT (absolute dynamic topography), SSS (sea surface salinity), 3DPWP\n(3D Price-Weller-Pinkel), NWP (western North Pacific), NECC (North Equatorial Current),\nSTCC (Subtropical Countercurrent), SLA (sea level anomaly).\nL33. Avoid using \"this\" at the start of the sentence without telling the\nreader what you’re referring to (also L128, L180, L194, L218, L237).\nWe have replaced ”this” by a more suitable word or phrase in each case.\n- L35. Define \"freshening\" here (and again in the main text).\nL34. ”freshening” is replaced by ”freshening in salinity”\nL144. ”fresher water” is replaced by ”fresher (or low-salinity) water”\n- L31-45. Discuss how the occurrence of Typhoon Mangkhut relates\nto your results in a bit more detail. You only mention Mangkhut\nonce (in the first sentence).\nAt the end of the sentence (L42), we added a phrase: ”as clearly demonstrated by\nMangkhut.”\n- L53. Spell acronyms out fully when they occur at the start of a\nsentence (e.g. \"OHC\").\nYes, done.\n- L56. What do you mean by \"warm-core eddies\"? You introduce this\nterm in conjunction with a single reference but don’t go into any\nmore detail.\n- L57. Be more specific about \"other atmospheric factors\".\nWe specified these factors within the parentheses: ”(e.g., vertical wind shear, TC\ntranslation speed)”. L62-63\n- L77. Spell out \"NEC\" fully; you have introduced the term in the\nabstract and the caption of Figure 1, but not in the main text.\nYes, done. L71-72\n- Figure 1. What is your data source for the track of Typhoon\nMangkhut? You haven’t defined this clearly in the figure\ncaption.\nWe reformulated the figure caption, indicating clearly the JTWC data source. L596-597\n- Figure 1. It’s not obvious to me how you’ve defined rapid\nintensification (RI) in your analysis of Figures 1a and 1b. If RI\noccurred more than once for a given storm in the Joint Typhoon\nWarning Center dataset, did you choose the 24-hour period with the\nbiggest increase in maximum sustained wind speed (one period of RI per\nstorm) or include all periods of RI? You need to add these details\ninto the text.\nFor the statistics in (b) and (c), we included all periods (or events) of RI, which is added\nin the figure caption. L602\n- Figure 2. Why have you chosen to plot the dynamic potential\nintensity and cooling inhibition index on 7th September 2018, 2\ndays before Mangkhut reached category 1 intensity? Surely 8th or\neven 9th September would still be classed as \"pre-storm\" along most\n7\nof Mangkhut’s track? For the later part of Mangkhut’s track between\n12th to 15th September (furthest west), 7th September becomes\nless and less representative of the pre-storm environment.\nWe agree with your suggestion. We replotted Fig. 2 by changing the pre-storm period\nfrom 7th to 9th September and recalculating the corresponding CI index and DPI. L611\n- L93. Does \"NWP\" refer to western North Pacific (you haven’t spelled\nout the acronym fully anywhere)? Given how infrequently you use this\nterm, I would remove the acronym elsewhere in the paper too (see\ngeneral comment about acronyms).\nYes, it refers to western North Pacific. We removed the acronym of NWP.\n- L103. Be more precise when describing Mangkhut’s evolution (\"the\narea over which it persists\" should be replaced with something like\n\"the area over which Mangkhut maintains Category-5 intensity\").\nWe changed the phrase as suggested. Thank you. L102\n- L118. Spell out \"psu\" fully (first time you’ve introduced it).\nYes, done. L608, L112-113\n- L129. What is the barrier layer effect for tropical\ncyclone intensification? A bit more detail here would be useful\nfor the reader.\nThe corresponding sentence is reformulated by adding at its end the following sentence:\n”, as the increased stratification and stability in the salinity-induced barrier layer reduce\nthe TC-induced vertical mixing and SST cooling.” L124-125\n8\n- L138-140. The implication in this sentence is that a tropical\ncyclone can only maintain category-5 intensity if passing over an\nocean heat content maximum or if warm eddies are present. If this\nstatement is correct, you need to support it more strongly with\nreferences from the literature. You do address this point somewhat\nby exploring some other possible factors later in the section (L149-\n158), but I think that a stronger link to the literature on the\nrelationship between tropical cyclone intensity and ocean heat\ncontent/warm eddies is required.\nAt the beginning of the sentence, we added a phrase concerning the tight link between\nwarm eddies and RI of TCs by citing a number of references. L133-134.\n- L141-145. I think I know what you’re trying to say, but it’s\ndifficult for me to visualise fully what you mean by the\n\"positive/negative polarity of transient mesoscale eddies\nencountered during the passage of TCs\". Could you edit the description\nslightly, or perhaps refer to a figure in another paper?\nWe reformulated the corresponding sentence. L139-142\n- L170. Define isopycnals.\nYes, we added the range of isopycnals (23.0 - 26.5 kg m-3). L166\n- L174. Be more precise with your use of longitude and replace\n\"west of 160oE\" with \"between 123o and 160oE\", to match the\ndescription in Figure 3.\nYes, done. L171\n- Figure 3. Are the cross-section plots in (a) to (e) valid at a\nsingle time, or time-averaged?\nIn the figure caption we added the period (1991-2020) over which time-averaging was\nperformed. L622\n9\n- L176-178. Following on from the previous point, you state that\nthe deepening of the thermocline results in the changing ocean\nheat content values, which suggests that temperature (Fig. 3a) is\ntime-averaged for a single month, in the same way as ocean heat\ncontent (Fig. 3f).\nFigs 3a-e stand for the climatological annual-mean (not monthly mean) sections, while\nonly Fig. 3f shows climatological monthly-mean values for different months. This\ninformation is added in the figure caption. L624-625\n- Figure 4 (and related discussion). Be careful about defining\nAugust-September as \"summer\", when it typically refers to June-July-\nAugust (same with winter, which usually includes December). At least\nmake the reader aware that you know the difference.\nWe removed ”winter/summer” in the figure caption.\n- L200. Does this cooling (0.8 - 0.9oC) correspond to either an\narea or a length of time?\nThe cited cooling corresponds to the NEC region from the climatology. The\ncorresponding sentence is reformulated. L188-191\n- L229. Small typo, add \"the\" before \"NEC region\".\nYes, done.\n- L236. Replace \"reference\" with the paper you’re referencing.\nWe reformulated the corresponding sentence. L215\n10\n- Figure 5. You need to discuss how you calculated the shaded field in\nFigure 5(c). In the figure caption you have referenced Mangkhut, but in\nthe main text you haven’t done this at all. This misalignment makes it\ndifficult for the reader to follow your discussion of the figure.\nThe caption is rewritten, indicating also the method of cooling computation for Fig. 5c.\nIn the main text we linked the climatological analysis with Mangkhut analysis. L638-\n640, L214- 215\n- L442. Small typo, \"comprises of\" should be \"comprises\".\nYes, corrected. Thank you.\nL465-466. What do you mean by the \"common air temperature of\nhurricanes of 26oC\"?\nThe corresponding sentence is reformulated. L301-302\n- L465. Replace all instances of \"hurricanes\" with \"tropical\ncyclones\", especially since your analysis doesn’t focus on the\nNorth Atlantic.\nYes, done.\n- L491. Replace \"TC\" with \"TCs\".\nYes, done\n- L516. Replace \"typhoon Mangkhut\" with \"Typhoon Mangkhut\" (also\nL614).\nYes, done.\n11\n- L528. What’s the justification for the decision to calculate the\nvertical wind shear 5 days prior to the TC arrival? Have previous\nstudies used this value?\nTo be consistent with the pre-storm upper ocean parameters, we changed the pre-storm\nVWS period from 5 days to 2 days prior to the TC arrival. L367-368. See also the above\nresponse to Reviewer 1.\n- L582. You talk about strong vertical shear \"ventilating\"\nthe\ntropical cyclone warm core. Could you be more specific? Is this\nventilation the most important effect of vertical shear on\ntropical cyclone structure?\nThe corresponding sentence is removed.\n- L600. How robust is the difference between the salinity during\nAugust-September and January-February (Extended Data Fig. 4a-c)? Do these\ndifferences (0.5 to 1.0 psu) represent pronounced changes in the ocean\nsalinity?\nThe quoted differences (0.5 to 1.0 psu) are significant and non-negligible in Oceanography\nbecause the internationally-required measurement accuracy of salinity is 0.003 psu. In our\ncase, it is the seasonal difference in salinity-induced stratification N2 (Supplementary Fig.\n4f) which matters best for the barrier layer effect, as the latter effect controls the TC-\ninduced vertical mixing in the upper layer. We found 10% in difference in CI (cooling\ninhibition) index due to the seasonal difference in salinity\nL610. Could you be more precise than using \"probably\" here?\nThe corresponding sentence is reformulated. Supplementary Information L38-44\n12", "reviewer_2": "Reviewer: Sam Hardy\nOverview\nThe authors present an interesfing and informafive analysis of the role of the North Equatorial Current\nregion on the rapid intensificafion of super typhoons in the western North Pacific. Data are presented\nimpressively both for the case study of Typhoon Mangkhut (2018) and over the longer period of study\nused for climatological analysis. The interesfing findings represent a posifive contribufion to the field,\nparficularly the analysis on the salinity and ocean strafificafion, and I congratulate the authors on pufting\ntogether this piece of work. However, there are several aspects of the discussion that need improving\nbefore the paper is suitable for publicafion, which I have outlined below. My recommendafion is Major\nRevisions.\nGeneral comments\n• Reduce the number of acronyms throughout the paper. It’s difficult for the reader to keep track of\nthem after a certain point.\n• Given the central importance of mesoscale eddies to your analysis, you need to introduce the topic in\nmore detail at the start of the paper. It currently feels like you’re assuming that the reader already knows\nwhat all these concepts mean.\n• Linked to the previous point, how does mesoscale eddy acfivity contribute to the rapid intensificafion\nof tropical cyclones? You need to befter introduce the established literature on this topic to give the\nreader a more solid understanding by the fime you get to your specific analysis.\n• Introduce the “eddy-rich zone” and the “North Equatorial Current” region to the reader in more detail\nearly in the paper and discuss their relevance to tropical cyclone intensificafion. Much of the analysis\nthat comes later would be easier to follow if the introducfion of these terms was more robust.\n• In the abstract you menfion the intensificafion of super typhoons in a warming climate. However, you\ndo not discuss this point any further in the manuscript, only touching on the topic again right at the end\nof the paper (L248-250). Either expand this part of your analysis or modify the abstract to avoid\nconfusing the reader.\n• You need more robust discussion of the relafionship between upper-ocean strafificafion and tropical\ncyclone intensificafion. Making this change would really add depth to your analysis.\n• Create a stronger link between your analysis of Typhoon Mangkhut and your overall conclusions, for\nexample in the Summary and Discussion secfion. It almost feels like the paper contains two separate bits of\nanalysis, which are not properly linked together. Along the same lines, the paper should end with a\nmore confident and conclusive statement on your findings.\nSpecific comments\n• L31-45. Reduce the number of acronyms in the abstract.\n• L33. Avoid using “this” at the start of the sentence without telling the reader what you’re referring to\n(also L128, L180, L194, L218, L237).\n• L35. Define “freshening” here (and again in the main text).\n• L31-45. Discuss how the occurrence of Typhoon Mangkhut relates to your results in a bit more detail.\nYou only menfion Mangkhut once (in the first sentence).\n• L53. Spell acronyms out fully when they occur at the start of a sentence (e.g. “OHC”).\n• L56. What do you mean by “warm-core eddies”? You introduce this term in conjuncfion with a single\nreference but don’t go into any more detail.\n• L57. Be more specific about “other atmospheric factors”.\n• L77. Spell out “NEC” fully; you have introduced the term in the abstract and the capfion of Figure 1,\nbut not in the main text.\n• Figure 1. What is your data source for the track of Typhoon Mangkhut? You haven’t defined this clearly\nin the figure capfion.\n• Figure 1. It’s not obvious to me how you’ve defined rapid intensificafion (RI) in your analysis of Figures\n1a and 1b. If RI occurred more than once for a given storm in the Joint Typhoon Warning Center dataset,\ndid you choose the 24-hour period with the biggest increase in maximum sustained wind speed (one\nperiod of RI per storm) or include all periods of RI? You need to add these details into the text.\n• Figure 2. Why have you chosen to plot the dynamic potenfial intensity and cooling inhibifion index on\n7th September 2018, 2 days before Mangkhut reached category 1 intensity? Surely 8th or even 9th\nSeptember would sfill be classed as “pre-storm” along most of Mangkhut’s track? For the later part of\nMangkhut’s track between 12th to 15th September (furthest west), 7th September becomes less and\nless representafive of the pre-storm environment.\n• L93. Does “NWP” refer to western North Pacific (you haven’t spelled out the acronym fully anywhere)?\nGiven how infrequently you use this term, I would remove the acronym elsewhere in the paper too (see\ngeneral comment about acronyms).\n• L103. Be more precise when describing Mangkhut’s evolufion (“the area over which it persists” should\nbe replaced with something like “the area over which Mangkhut maintains Category-5 intensity”).\n• L118. Spell out “psu” fully (first fime you’ve introduced it).\n• L129. What is the barrier layer effect for tropical cyclone intensificafion? A bit more detail here would be\nuseful for the reader.\n• L138-140. The implicafion in this sentence is that a tropical cyclone can only maintain category-5\nintensity if passing over an ocean heat content maximum or if warm eddies are present. If this statement\nis correct, you need to support it more strongly with references from the literature. You do address this\npoint somewhat by exploring some other possible factors later in the secfion (L149-158), but I think that\na stronger link to the literature on the relafionship between tropical cyclone intensity and ocean heat\ncontent/warm eddies is required.\n• L141-145. I think I know what you’re trying to say, but it’s difficult for me to visualise fully what you\nmean by the “posifive/negafive polarity of transient mesoscale eddies encountered during the passage\nof TCs”. Could you edit the descripfion slightly, or perhaps refer to a figure in another paper?\n• L170. Define isopycnals.\n• L174. Be more precise with your use of longitude and replace “west of 160ºE” with “between 123º and\n160ºE”, to match the descripfion in Figure 3.\n• Figure 3. Are the cross-secfion plots in (a) to (e) valid at a single fime, or fime-averaged?\n• L176-178. Following on from the previous point, you state that the deepening of the thermocline\nresults in the changing ocean heat content values, which suggests that temperature (Fig. 3a) is fime-\naveraged for a single month, in the same way as ocean heat content (Fig. 3f).\n• Figure 4 (and related discussion). Be careful about defining August-September as “summer”, when it\ntypically refers to June-July-August (same with winter, which usually includes December). At least make\nthe reader aware that you know the difference.\n• L200. Does this cooling (0.8 – 0.9ºC) correspond to either an area or a length of fime?\n• L229. Small typo, add “the” before “NEC region”.\n• L236. Replace “reference” with the paper you’re referencing.\n• Figure 5. You need to discuss how you calculated the shaded field in Figure 5(c). In the figure capfion\nyou have referenced Mangkhut, but in the main text you haven’t done this at all. This misalignment\nmakes it difficult for the reader to follow your discussion of the figure.\n• L442. Small typo, “comprises of” should be “comprises”.\n• L465-466. What do you mean by the “common air temperature of hurricanes of 26ºC”?\n• L465. Replace all instances of “hurricanes” with “tropical cyclones”, especially since your analysis\ndoesn’t focus on the North Atlanfic.\n• L491. Replace “TC” with “TCs”.\n• L516. Replace “typhoon Mangkhut” with “Typhoon Mangkhut” (also L614).\n• L528. What’s the jusfificafion for the decision to calculate the verfical wind shear 5 days prior to the TC\narrival? Have previous studies used this value?\n• L582. You talk about strong verfical shear “venfilafing” the tropical cyclone warm core. Could you be\nmore specific? Is this venfilafion the most important effect of verfical shear on tropical cyclone\nstructure?\n• L600. How robust is the difference between the salinity during August-September and January-\nFebruary (Extended Data Fig. 4a-c)? Do these differences (0.5 to 1.0 psu) represent pronounced changes\nin the ocean salinity?\n• L610. Could you be more precise than using “probably” here?", "reviewer_author_response": "Response to Reviewers\nReviewer #1 (Remarks to the Author):\nThis study explores the cause of typhoon Mangkhut in 2018. The authors\nespecially emphasize the role of North Equatorial current. They also analyzed\nthe role of ITCZ induced freshening. Overall, this work is interesting. I\nthink most of the analysis focuses on the happening of the long-lasting super\ntyphoon Mangkhut. So, it is more like a case study. But the most intrinsic\nreason for this is still not clear based on the current evidence. The main\nreason I think is too much analysis is about the climatology. I will show\nsome examples below. Thus, I think the authors need further investigation\ninto this particular case, that is the typhoon Mangkhut. Please see specific\ncomments below.\nThank you for your comments and your interest in this work. We are aware that the main focus of this\narticle is the super typhoon Mangkhut, and in some ways it is a case study of this TC, but Mangkhut was\nextraordinary in its formation, and we would suggest that this maybe a precursor for future tropical\ncyclones. So, from this perspective, Mangkhut is an important milestone in the evolution of TC’s.\nBased on these insightful comments, we have strengthened the analysis of Mangkhut and further related\nthe results to the climatology (see below for details). We respond to each of your specific comments\nbelow.\nSpecific comments:\n1. The TC intensification is closely related to the upper ocean heat supply\nof course, especially for the typhoon Mangkhut, which moves along the NEC\nregion. So, it will be not surprise that the OHC in NEC region plays\ncritically important role in sustaining the strong intensity of Mangkhut. The\nauthors indeed show several largescale conditions that favorable for the\nintensification, like the large OHC, moderate vertical wind shear and\nfreshwater induced by ITCZ shift. But only Figure 2 is about this particular\ncase. Figures 3 and 4, which is also important for TC intensification, are\nboth on climatological perspective. Although the authors have analyzed in\nmany details, it is still unclear how these climatological variables\nimpact the particular case Mangkhut. For example, in L39-41, the ITCZ northward\nmigration is seasonal march, but why only Mangkhut sustained its high\nintensity, while other TCs doesn’t. This issue exists in the analysis of\n1\nprecipitation, salinity, buoyancy et al. I suggest the authors to carried out\nthese analyses for Mangkhut as in Figure 2.\nWe have added three supplementary analyses for Mangkhut in Figure 2 showing the spatial\ndistribution of (a) OHC (b) salinity and (c) salinity-induced N 2 for 9th September 2018, 1 day\nprior to its rapid intensification. The corresponding figure caption (L607-616) and text (L99-111\nand L119-126) are updated in the revised draft.\nWe have also added in the discussion section further analysis on persistent Super Typhoon Mawar\nobserved in May 2023 in the NEC region (see L220-235 and Supplementary Fig. 8 and 9).\n2. The atmosphere changes much faster than the ocean. Thus, I concern that in\nthe analysis of vertical wind shear around L149-158, whether it is still\nproper to analyze the pre-storm VWS as that of the OHC. The VWS is likely to\nchange with time during Mangkhut. So, how does VWS change during all life\ncycle of Mangkhut should be investigated.\nWe have tested different lags in the computation of VWS at each moving center of TC. The results\nare not very sensitive to the choice of lags between 0 and 2 days for RI and CAT5 phases of\nMangkhut (see Supplementary Fig. 2c). Therefore, we have chosen the pre-storm atmospheric\nconditions 2 days prior to the TC arrival, consistent with the case of the pre-storm upper-ocean\nparameters. L367-368\n3. Figure 6. The authors show the long-term trend of the OHC and TC\nintensification rate over the NEC region. Actually, many study show that the\noverall OHC in the WNP has increased not limited to NEC region. So, the\nfigure 6a only shows a part of the overall warming pattern of the WNP. This\nshould be acknowledged in the manuscript and also should be discussed on the\npotential relationship between NEC warming and the overall warming pattern.\nAnother issue is that ENSO may plays a role in the long-term trend of OHC.\nThus, it’s signal should be removed.\nAs responding to editor’s comments, the increase in OHC at a rate of 9 kJ cm−2 decade−1 over the last\n4 decades (Fig. 6a) is compared with global increasing rate of OHC. It far exceeds the global (1.32 kJ\ncm−2 decade−1) and all-Tropics (2.47 kJ cm−2 decade−1) trends. (L240-242, Supplementary Fig.10)\nENSO signals have been removed by subtracting the ENSO-regressed time series from the original time\nseries in the NEC region. The corresponding time series is added in Fig. 6a (solid black line). Note that\nthe long-term trend of OHC does not change with or without ENSO signals. L246-249, L648-651, and\nFig.6a\n4. The following study is quite relevant to this study, which found that a\ntype of fast decaying El Nino events leads to westward migration of RI events\n2\nby affecting the NEC. The authors should not miss it in reviewing the related\nbackground: Guo Y. P., and Z. M. Tan"}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed most of my comments. However, the reply to my first major comments is sfill not quite adequate. Thus, I sfill recommend a major revision this round. Please see my specific comments below.\nIn the last review, I commented that why the favorable climatological environmental condifion only\nmakes one TC case, that is Mangkhut, to experience RI. The authors just found another case, the Mawar.\nI think it sfill lacks stafisfical meaning. One good way to makes the arguments that NEC is important to TC\nRI stronger is to show whether the TCs moving through NEC have much higher probability to experience\nRI than those that are not. And this difference should also be stafisfically significant. Only in this way\ncould make the argument more universal.", "reviewer_2": "You have addressed all my comments thoroughly and professionally and produced a much-improved\nanalysis, which I believe is ready for publicafion. I only have a couple of very minor comments, which I\nhave listed below. Well done on producing such an interesfing and thought-provoking study!\nComments\n• L24. I would introduce as ‘Super Typhoon Mangkhut’ rather than just ‘Mangkhut’.\n• L78. Does ERZ23 mean the region at 23ºN?\n• L235. I would be slightly less strong with your wording here. Maybe something like “…supports our\nnew findings from Mangkhut that super typhoons are likely to both rapidly intensify and persist…”."}, "author_response": "Response to Reviewers\nReviewer #1 (Remarks to the Author):\nThe authors have addressed most of my comments. However, the reply to my\nfirst major comments is still not quite adequate. Thus, I still recommend\na major revision this round. Please see my specific comments below.\nIn the last review, I commented that why the favorable climatological\nenvironmental condition only makes one TC case, that is Mangkhut, to\nexperience RI. The authors just found another case, the Mawar. I think it\nstill lacks statistical meaning. One good way to makes the arguments that\nNEC is important to TC RI stronger is to show whether the TCs moving\nthrough NEC have much higher probability to experience RI than those that\nare not. And this difference should also be statistically significant.\nOnly in this way could make the argument more universal.\nThank you for this comment and we apologize for missing this in our initial revision. This is an\nimportant point for our article, and we agree that the statistical significance of the NEC for the\nRI of TCs is critical for the universality of our argument. In order to address this we calculated\nthe statistics for the NEC and non-NEC regions, based on the percentages presented in Fig 1b\nand 1c. These statistics are presented in supplementary Fig. 1c, and shows that TCs moving\nthrough the NEC have a mean probability of 27.6% for experiencing RI compared to 6.1% for\nthose that move through other regions. This difference in probabilities was statistically significant\nbased on the Student's t-test (p-value < 0.001). We added text to reflect this on L85-89.\nWe hope this addresses your final comment on our manuscript. If however does not satisfy your\nrequirements, please specify how we can address this.\nYou have addressed all my comments thoroughly and professionally\nand produced a much-improved analysis, which I believe is ready\nfor publication. I only have a couple of very minor comments,\nwhich I have listed below. Well done on producing such an\ninteresting and thought-provoking study!\nThank you very much for your valuable comments. We think the comments\ncontribute greatly to produce much improved version of the first draft.\nComments\n• L24. I would introduce as ‘Super Typhoon Mangkhut’ rather than\njust ‘Mangkhut’.\nThanks. Agreed. We have added “Super Typhoon” to Mangkhut. L23\n• L78. Does ERZ23 mean the region at 23ºN?\nThanks. It was corrected as ERZ23. L79\n• L235. I would be slightly less strong with your wording here.\nMaybe something like “…supports our new findings from Mangkhut\nthat super typhoons are likely to both rapidly intensify and\npersist…”.\nThanks. We revised it with less strong wording. L237\nAuthors (Remark to the Editor and Reviewers):\nError correction: We found that error number was added to calculate probability\nof RI event (Fig.6b). The Fig.6b for probability of RI event (red line; %) and\ntheir linear regression (thick lines) in the NEC region was corrected (new\nFig.6b). L247, L649 (Fig.6b).\n2"}]

Decision letter
REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author):
The authors have well addressed my comments. I have no further quesfions. I recommend an
acceptance of this manuscript.