Unleashing the power of the Sun: the increasing impact of the solar cycle on off-season super typhoons since the 1990s

Typhoons in the western North Pacific usually occur from May to October under favorable wind shear and warm SSTs, but super typhoons can occasionally form during the off-season (November–April), posing severe risks, as exemplified by Typhoon Haiyan (Category 5) in November 2013. From 1945–2018, 402 off-season typhoons formed with genesis concentrated in 4–15°N, 122–180°E. The yearly number of off-season super typhoons co-varies with the 11-year solar sunspot number (SSN), with more events during solar maxima than minima, while no such link appears for total off-season typhoon counts. Lead–lag analysis indicates the strongest correlation when SSN leads super-typhoon counts by 1 year, consistent with seasonality in the solar-induced SST footprint that matures by the following boreal winter–spring. A 21-year running correlation shows the solar modulation intensified since the 1990s. The study addresses: (1) the physical mechanism by which the 11-year solar cycle modulates off-season super typhoons; and (2) why the modulation strengthened after the 1990s.

Observational and reanalysis datasets: (a) Typhoon tracks and intensity from the JTWC best track database (1945–2019). PDI defined as the time integral of the cube of 6-hourly maximum surface wind over storm lifetime; LMI defined as the maximum surface wind over a storm’s lifetime; typhoon duration defined from first exceedance of 34 kt to drop below 34 kt. (b) Sunspot number (SSN) from WDC SILSO, Royal Observatory of Belgium. (c) Monthly total solar irradiance (TSI) from the Community-Consensus TSI Composite (Kopp et al.). (d) SSTs from ERSST v5 (2° grid, 1854–present), with linear trends removed. (e) Atmospheric fields from ERA5 (0.25°, 37 levels, 1950–present), JRA-55 (1.25°, 37 levels, 1957–present), and NCEP/NCAR reanalysis (1.875°, 17 levels, 1948–present), with linear trends removed. Vertical wind shear (VWS) computed as sqrt[(U200−U850)^2+(V200−V850)^2]. Relative vorticity computed from winds. (f) Dynamic Genesis Potential Index (DGPI) per Wang & Murakami (2020): DGPI = (2 + 0.1×VWS) × (5.5 − Uy×1e−5)^2 × (5 − 20×w)^3 × (5.5 + ζ0×1e−5)^2 × e^(−1.0), where VWS is vertical wind shear, Uy is meridional gradient of zonal wind at 500 hPa, w is vertical velocity at 500 hPa, and ζ0 is absolute vorticity at 850 hPa. (g) D26 (depth of 26°C isotherm) and upper-ocean heat content from NOAA GODAS (0.333°×1°, 40 levels, 1980–present), detrended. Climate indices and definitions: PMM index defined as November–April SST anomalies averaged over 10–20°N, 160–120°W. AMO index from NOAA (detrended SST anomalies over the North Atlantic, equator–70°N). PDO index from NOAA (PC of EOF1 of North Pacific SST anomalies). SSNmax (active) defined as SSN ≥ 100 and SSNmin (inactive) as SSN ≤ 20 for contrasting environments. Statistical methods: Lead–lag and running correlations with effective degrees of freedom considering autocorrelation; Student’s t-tests for significance. ENSO influence reduced via 7-year running mean; PDO and AMO signals further removed by linear regression when testing robustness. Model experiments: Idealized CESM1.2.2 (B1850C5CN, f19_g16; CAM5.3 ~2° with 30 levels; POP2 ~1° with 60 levels). Control run with fixed TSI (1361 W/m²); solar-forced run with fivefold-amplified observed 11-year TSI variations for AD850–959 (110 years). Other forcings follow Last Millennium setup; both runs 110 years (AD850–959). Realistic solar forcing: NASA GISS-E2-2-G high-top model (≈2°×2.5°, 102 atmospheric levels; ocean ≈1°, 40 levels) hist-sol simulation for 1850–2014 with time-varying solar forcing only; other external forcings fixed at preindustrial. Analyses include spectra of PMM, regressions of SST, mass streamfunction, SLP and 850-hPa wind, and diagnostics of VWS and vorticity responses to TSI.

- A statistically significant link exists between the 11-year solar cycle and the yearly number of off-season (Nov–Apr) super typhoons in the western North Pacific, strongest during 1985–2018 with SSN leading by 1 year (R = 0.60, P < 0.1). Spectra show a clear 11-year peak in off-season super-typhoon counts. - The correlation remains robust after removing ENSO (via 7-year running mean) and regressing out PDO and AMO signals, indicating independence from these internal climate modes. - Ocean conditions in the OMDR (4–15°N, 122–180°E) during active solar periods tend to be less favorable for cyclones: off-season SST and D26 regress negatively onto SSN, implying cooler surface waters and shallower warm layers. Thus, oceanic state alone cannot explain increased super typhoon frequency during solar maxima. - Atmospheric conditions become more favorable during active phases: regressions (SSN leading by 1 year) show decreased vertical wind shear, increased 850-hPa relative vorticity, and increased DGPI east of 140°E within the OMDR across ERA5, JRA-55, and NCEP/NCAR. - Genesis shifts eastward by about 5° during active periods (~10°N, 144°E vs ~11°N, 139°E), storms turn poleward more often, and remain over water longer. Mean duration increases to 135.7 h during SSNmax vs 103.7 h during SSNmin (≈1.3×; P < 0.01). - Intensity metrics increase: LMI is ~1.3× and PDI is ~1.4× higher during SSNmax than SSNmin (differences significant at 99%). Yearly SSN correlates with season-mean PDI at R = 0.55 (P < 0.1; SSN leads by 1 year). - Mechanism: Active solar phases weaken the Brewer–Dobson circulation, warming the lower stratosphere/upper troposphere, reducing tropical deep convection and weakening the Hadley circulation. This weakens the North Pacific subtropical high and trades, producing a subtropical-to-tropical Pacific warm SST footprint that matures by the following boreal winter–spring, weakens the Walker circulation, shifts convection east, reduces VWS, and increases low-level cyclonic vorticity—creating conditions conducive to off-season super typhoons. - Modeling support: CESM1 solar-forced and GISS-E2-2-G hist-sol simulations exhibit an 11-year spectral peak in the PMM index; regressions onto TSI reveal the observed SST footprint and atmospheric responses (weakened Hadley circulation, weakened Pacific High, reduced VWS, increased low-level vorticity). PMM–TSI 1-year lag correlation increases from R = 0.08 (control) to R = 0.57 (solar-forced; P < 0.01). - Decadal modulation: The SSN–super typhoon running correlation aligns with the AMO phase more than PDO. Correlation between running correlation and AMO index is 0.68 (P < 0.1) versus 0.15 (PDO; P > 0.1). Positive AMO enhances subtropical coupling, magnifying the solar footprint mechanism; modulation intensified since the AMO turned positive in the 1990s. - Seasonal contrast: In-season (May–Oct) super typhoon counts show a weak negative correlation with SSN (R = −0.16, not significant), consistent with prior work that enhanced solar forcing reduces CAPE and suppresses in-season intensification, while off-season benefits from the SST footprint mechanism.

The study addresses how and why the 11-year solar cycle modulates off-season super typhoon activity in the western North Pacific. Observations and model experiments converge on a mechanism whereby solar-induced stratospheric–tropospheric interactions weaken the Hadley circulation and North Pacific subtropical high during active phases, generating a subtropical SST footprint that propagates into the tropical central Pacific by the following winter–spring. This footprint weakens the Walker circulation and creates atmospheric conditions with reduced vertical wind shear and increased low-level cyclonic vorticity, shifting genesis eastward and prolonging storm-ocean interaction. These changes increase storm duration and intensity, raising the likelihood of super typhoon occurrence off-season, despite slightly less favorable local ocean thermal conditions in the OMDR. The modulation intensified since the 1990s due to the AMO’s positive phase enhancing subtropical air–sea coupling efficiency. The mechanism also explains opposite seasonal responses: suppression of in-season activity via reduced CAPE versus enhancement of off-season super typhoons via the SST footprint pathway. Practically, incorporating solar cycle information (e.g., SSN) can improve decadal-scale risk assessment and preparedness for off-season super typhoons.

This work identifies and validates a physical pathway linking the 11-year solar cycle to increased off-season super typhoon activity in the western North Pacific: a stratosphere-amplified solar forcing weakens the Hadley circulation and subtropical high, generating a subtropical SST footprint that intrudes into the tropical central Pacific by the following winter–spring, reducing wind shear and enhancing low-level vorticity. Since the AMO’s transition to a positive phase in the 1990s, this coupling has strengthened, intensifying solar modulation. Key impacts include eastward-shifted genesis, longer storm lifetimes, and higher intensity (LMI and PDI), elevating super typhoon frequency during solar maxima. These insights enable the use of solar-cycle indicators for decadal disaster planning. Future work should replicate experiments with multiple coupled climate models to assess model dependence, expand datasets (e.g., longer UOHC records), and further investigate interactions with low-frequency modes (AMO, PDO) and ENSO.

Findings rely on observational analyses combined with specific coupled climate model experiments (CESM1 and GISS-E2-2-G), so results may be model-dependent; replication across diverse state-of-the-art models is needed. Some ocean measures (e.g., D26/UOHC) lack sufficiently long records to assess earlier periods. While atmospheric favorability and intensity metrics increase during solar maxima, the total number of off-season typhoons does not change substantially, concentrating effects on the super typhoon subset. Statistical significance is sometimes modest (e.g., P < 0.1), and effective sample sizes are limited by decadal timescales.