The North Equatorial Current and rapid intensification of super typhoons

The study investigates why rapid intensification (RI) and long-lived Category-5 (CAT5) super typhoons preferentially occur and persist in the North Equatorial Current (NEC) region rather than the traditionally emphasized Eddy Rich Zone (ERZ) in the western North Pacific. Tropical cyclones derive energy from oceanic enthalpy fluxes, largely governed by sea surface temperature (SST). TC-induced vertical mixing typically cools the surface by entraining colder subsurface water, imposing a negative feedback on intensification. Therefore, ocean heat content (OHC)—heat above the 26 °C isotherm depth—is a superior predictor of TC-induced cooling compared to SST alone. Prior emphasis has been on mesoscale warm eddies in the ERZ that locally elevate OHC and suppress cooling. However, analysis of major TCs (1984–2021) shows a higher RI probability within the NEC, prompting the hypothesis that large-scale NEC dynamics (deep thermocline/high OHC) and seasonal freshening from the Inter-Tropical Convergence Zone (ITCZ) enhance upper-ocean stratification, suppress TC-induced cooling, and favor RI and CAT5 persistence. Using Super Typhoon Mangkhut (2018) as a case, the study aims to demonstrate the NEC’s key role in RI and sustained intensity.

Previous studies established links between high pre-storm SST, upper-ocean thermal structure, and TC intensity change, with OHC/TC heat potential being critical to restraining TC-induced cooling. Mesoscale warm eddies (anticyclones) in the ERZ deepen the thermocline and enhance OHC, often associated with RI of super typhoons (e.g., Maemi 2003; Nargis 2008). Potential Intensity (PI) provides the thermodynamic upper bound for TC intensity, while Dynamic Potential Intensity (DPI) incorporates ocean stratification and TC-induced cooling, improving intensity estimates. Barrier layers and salinity-driven stratification can reduce mixing and cooling, favoring intensification. Large-scale atmospheric factors such as vertical wind shear (VWS) and translation speed modulate RI occurrences. Nonetheless, the NEC’s contribution—via its geostrophic structure causing a deep thermocline and its seasonal ITCZ-driven freshening—has been underappreciated relative to the ERZ’s eddy effects.

Data and periods: Major TCs (≥CAT3) from 1984–2021 were analyzed using the JTWC best track dataset to compute RI occurrences and probabilities. RI is consistent with an intensification threshold of ~15.4 m s−1 day−1 (≈30 kt per 24 h). Statistical comparisons of RI probabilities between regions used Student’s t-test. Ocean-atmosphere datasets: Satellite altimetry absolute dynamic topography (0.25°) from CMEMS characterized eddies and sea level anomalies. Atmospheric conditions used ERA5 reanalysis (0.25°, 6-hourly); ocean states used GLORYS12V1 (1/12°, 50 levels) for event-based analyses and ORAS5 (0.25°, 75 levels) for climatology. Satellite SST (microwave/infrared OI) and sea surface salinity (0.25°) were used to track surface conditions and ITCZ freshening. Seasonal precipitation from ERA5 supported ITCZ characterization. Sea level trends (1993–2021) were from altimetry with Mann–Kendall significance testing. ENSO signals in OHC time series were regressed out to isolate trends. Diagnostics: OHC was computed relative to the 26 °C isotherm depth (D26). Cooling Inhibition (CI) index quantified the potential energy needed to achieve 2 °C surface cooling, reflecting pre-storm stratification resistance to mixing. PI followed Emanuel’s formulation using ERA5/GLORYS/ORAS5. DPI replaced SST with vertically averaged temperature over a mixing length that incorporates initial mixed layer depth, friction velocity, stratification, and TC passage time (driven by wind speed, radius of maximum wind, and translation speed). TC-induced SST cooling was estimated from a polynomial fit relating cooling to CI at a fixed wind power index. Latent heat flux during RI was computed via a bulk formula using representative NEC RI conditions. Case studies and climatology: Mangkhut (2018) was examined along-track using ERA5 and GLORYS fields sampled 2 days prior to arrival for PI/DPI/CI and cooling estimates. A 3D Price–Weller–Pinkel (PWP) model along Mangkhut’s track isolated the ITCZ-freshening contribution to cooling suppression. Climatological meridional sections (123–160°E) from ORAS5 (1991–2020) characterized thermocline depth, salinity, density, and N², and monthly climatologies mapped OHC seasonal structure across NEC vs ERZ. A second case, Mawar (May 2023), was analyzed with Argo profiles to corroborate findings. Climatological PI/DPI mapping: June–November climatological fields (ERA5+ORAS5) were used to compute PI/DPI and expected SST cooling across the North Pacific south of 35°N. All grid points were forced by representative CAT5 TC parameters inspired by Mangkhut (max wind ~70 m s−1, RMW ~35 km, translation speed ~5 m s−1) to compare NEC vs ERZ responses.

• Spatial occurrence of RI and CAT5 within regions: For major TCs (≥CAT3), 51.9% of RI events occur in the NEC versus 34.1% in the ERZ. For CAT5 TCs, 65.6% occur in the NEC versus 21.2% in the ERZ. Mean RI probability along tracks is 27.6% within the NEC versus 6.1% elsewhere (t-test p < 0.001).
• Mangkhut (2018) case: Mangkhut intensified rapidly on 10 September (max winds from 46 to 72 m s−1 in 24 h) and maintained CAT5 for 3.5 days (11 Sep 06:00–14 Sep 18:00 UTC) while traversing 14–17°N in the NEC. The RI/CAT5 corridor coincided with CI > 35 (J m−2)1/3, indicating strong inhibition of vertical mixing and limited SST cooling. Observed/estimated TC-induced SST cooling along track was typically 0.5–0.8 °C (peak ~1 °C). DPI closely matched observed JTWC winds during the CAT5 phase, while PI overestimated by ~10 m s−1.
• Ocean structure in NEC vs ERZ: Climatological thermocline is deepest on the northern flank of the NEC (D26 ~130 m at ~14°N vs ~90 m at 7°N), yielding OHC >100 kJ cm−2 across 8–17°N with a peak ~120 kJ cm−2 at ~14°N. In the ERZ (>17°N), OHC drops below 100 kJ cm−2, reaching as low as ~40 kJ cm−2 at 25°N, implying warm eddies are required in the ERZ to meet OHC thresholds for super typhoons, whereas the NEC meets them climatologically without eddies.
• Role of ITCZ freshening and stratification: Northward-migrating summer ITCZ precipitation freshens the upper ocean (SSS <34 psu), enhancing near-surface haline stratification and forming a barrier layer. The freshening increases N² in summer relative to winter and contributes a 7.5–12.5% increase to CI regionally. PWP experiments indicate freshening suppressed SST cooling by an additional ~19% along Mangkhut’s track. Theoretical CAT5 cooling in NEC given typical speeds (6–7 m s−1) is <0.8–0.9 °C, consistent with observations.
• Atmospheric environment for Mangkhut: Pre-storm mean VWS during RI was moderate (7.12 m s−1) and during CAT5 persistence 7.24 m s−1; translation speed varied 5–11 m s−1, above the ~4 m s−1 lower bound typically needed to maintain CAT5, indicating atmospheric conditions were favorable.
• PI vs DPI climatology: PI ≥70 m s−1 extends to ~25°N, covering much of the ERZ, but DPI ≥70 m s−1 contracts into the NEC due to stronger negative SST feedback outside the NEC. The SST cooling penalty on intensity is <10 m s−1 in the NEC but up to ~30 m s−1 in the ERZ; associated SST cooling is minimal (<0.8 °C) in the NEC.
• Additional case support: Typhoon Mawar (May 2023) intensified from CAT3 to CAT5 near 14°N, 144°E and maintained CAT5 for 1.5 days within 14–16°N (NEC) under favorable oceanic conditions (OHC ~130 kJ cm−2; cooling ~0.5 °C) and low VWS (<5 m s−1). It weakened upon entering the ERZ with OHC <70 kJ cm−2 and cooling >1.5 °C.
• Long-term trends: NEC OHC has increased by roughly 9 kJ cm−2 per decade over the last four decades, exceeding global and pan-tropical trends. The mean intensification rate in the NEC increased by ~1.5 kt day−1 per decade, corresponding to a RI probability increase of ~3.1% per decade. Sea level rise trends (a proxy for thermosteric OHC) are strongly positive in the western North Pacific and particularly pronounced in the NEC.

Findings demonstrate that the NEC’s large-scale geostrophic structure deepens the thermocline and elevates OHC, while seasonal ITCZ freshening enhances upper-layer stratification and forms a barrier layer. Together these factors minimize TC-induced SST cooling, thereby sustaining higher enthalpy fluxes and favoring both rapid intensification and prolonged CAT5 persistence in the NEC, provided atmospheric conditions (e.g., moderate/low wind shear and sufficient translation speed) are favorable. DPI successfully captures the ocean feedback and aligns with observed intensity evolution better than PI. In contrast, the ERZ relies on transient warm eddies to locally raise OHC; absent such features, stronger cooling and larger negative feedback reduce RI likelihood and CAT5 persistence. Climatological DPI maps confirm the NEC as a persistent corridor for strong TCs, and observed increases in OHC and RI metrics over recent decades suggest that ongoing regional warming enhances the NEC’s role. These insights refine the understanding of where and when super typhoons are likely to intensify rapidly and maintain peak strength, informing forecasting emphasis on upper-ocean structure and seasonal salinity stratification within the NEC.

The study identifies the NEC as the primary corridor for the RI and sustained CAT5 intensity of super typhoons in the western North Pacific, overturning the ERZ-centric view. High OHC from a NEC-deepened thermocline and seasonal haline stratification from ITCZ precipitation jointly suppress TC-induced cooling, as confirmed by Mangkhut’s record-long CAT5 phase and corroborating climatology and a second case (Mawar). DPI diagnostics demonstrate that accounting for ocean feedback is essential to estimate achievable intensity. Long-term increases in NEC OHC and RI probability imply that super typhoons may more frequently intensify and persist in the NEC under continued regional warming. Future work should quantify ENSO’s modulation of NEC OHC/stratification and RI on interannual timescales, assess predictive skill gains from real-time CI/DPI assimilation, and expand case and regional analyses to other basins with analogous current systems.

• The attribution of long-term trends relies on reanalysis products and altimetry-derived proxies (thermosteric sea level); uncertainties in assimilation and sampling could affect magnitudes.
• ENSO’s role in interannual variability of OHC and RI is acknowledged but not fully resolved; while ENSO-regressed OHC suggests a robust long-term trend, detailed mechanism and predictability impacts require further analysis.
• DPI/CI computations and climatological DPI maps use assumed TC parameters (e.g., RMW, translation speed) and bulk formulations that may vary among storms.
• The barrier layer/haline stratification is identified as a significant secondary factor; quantifying its spatial-temporal variability and interaction with wind-driven mixing across events remains limited by observational coverage.
• Case studies (Mangkhut, Mawar) are illustrative; broader statistical attribution separating oceanic from atmospheric controls was not exhaustively performed beyond regional contrasts and selected metrics.