Lapsed El Niño impact on Atlantic and Northwest Pacific tropical cyclone activity in 2023

The 2023 tropical cyclone (TC) season was unusually active in the North Atlantic (NA) yet strongly suppressed in the western North Pacific (WNP), despite the development of a strong El Niño—conditions that typically suppress NA genesis and produce a northwest–southeast dipole in WNP TCGF. From June to November 2023, the NA recorded 20 TCs (third highest since 1980), while the WNP had only 13 TCs, about nine fewer than the 1980–2023 climatological mean (−2 standard deviations), marking one of the lowest counts on record. This inter-basin contrast and the NA surpassing the WNP by a large margin were unexpected and poorly captured by early seasonal outlooks. The study asks why El Niño’s canonical impacts on NA and WNP TCGF lapsed in 2023 and investigates the roles and relative contributions of other oceanic modes—particularly the Pacific Meridional Mode (PMM), tropical North Atlantic (TNA) SST anomalies—and background global warming (GW).

Prior work shows that strong El Niño events suppress NA TCGF via enhanced vertical wind shear and induce a dipole in WNP TCGF, with reduced genesis in the northwest and increased in the southeast. Studies have also highlighted the PMM’s influence on WNP TCGF, with its negative phase suppressing genesis, and documented the TNA’s role in modulating NA and WNP TC activity through atmospheric circulation changes. While PMM and TNA effects have been examined separately, their combined impacts and relative contributions alongside ENSO and GW across basins remained unclear. Basin-dependent trends in TCGF under GW have been reported—an upward trend in the NA and a downward trend in the WNP—associated with a La Niña-like SST warming pattern and consistent circulation trends. These gaps motivate an integrated, cross-basin assessment of ENSO, PMM, TNA, and GW, particularly given 2023’s complex SST anomaly patterns.

Data and indices: The study uses best-track TC data from iBTRACS (1980–2023), with genesis defined at first occurrence of sustained 35 kt winds. Additional agency datasets (STI/CMA, JMA) are used for WNP counts ranking. Atmospheric reanalyses include ERA5 and NCEP/NCAR; SSTs are from HadISST. TCGF is analyzed for JJASON (June–November). ENSO is defined as JJASON Niño3.4 SST; PMM index from NOAA PSL; TNA is JJASON SST over the equatorial–20°N, 10°–60°W Atlantic; GW index is the normalized global mean JJASON SST. SST reconstruction: JJASON 2023 SST anomalies are reconstructed as a linear combination of regressed SST patterns associated with normalized indices of ENSO, PMM (reversed), TNA, and a spatial GW trend component, with the GW magnitude adjusted by minimizing variance between reconstructed and observed SST anomalies across 45°S–45°N (optimal t=30 years, yielding ~0.37 °C mean bias; <0.2 °C in 15°S–15°N). Independence among indices is verified (ENSO–PMM r≈−0.09; ENSO–TNA r≈0.04). Regression of TCGF: A sample-independent, leave-one-out multiple linear regression model relates gridded TCGF to ENSO, PMM, TNA, and GW indices to cross-validate contributions and reconstruct 2023 spatial anomalies. High-resolution modeling: Sensitivity experiments use GFDL HiRAM (~50 km, 32 levels), 30-member ensembles with perturbed initial conditions. Experiments: (1) CLIM Run (1980–2022 climatological monthly SST), (2) All Run (observed 2023 SST), (3) Reconstruction Run (climatological SST plus reconstructed anomalies of ENSO, PMM, TNA, GW), (4) No_ElNiño (Reconstruction excluding ENSO anomalies), (5) No_GW (Reconstruction excluding GW trend), (6) PMM_TNA (only PMM and TNA anomalies), and single-mode PMM and TNA Runs. To limit cross-basin contamination, PMM-induced SST anomalies are applied over the Pacific and TNA-induced anomalies over the Atlantic. TC-like vortices are objectively detected from 6-hourly outputs using thresholds on surface wind (≥17 m s−1), 850 hPa vorticity (≥5×10−5 s−1), warm-core structure (300–500 hPa), and lifespan (≥3 days). TCGF spatial fields are formed by counting genesis within 20°×10° windows at 1° resolution. Statistical significance is assessed with appropriate degrees of freedom; p<0.1 indicated in maps. Sensitivity to GW trend estimation is tested using trends from 1960–2023 and 1990–2023, confirming robustness of inferred GW effects.

- Observed anomalies in 2023: NA had 20 TCs (third highest since 1980); WNP had 13 TCs (≈9 fewer than climatology; −2 SD), with a basin-wide decrease replacing the usual ENSO dipole. El Niño strength (JJASON) was high (normalized Niño3.4=1.59; fifth strongest since 1980). - Record-breaking modes: PMM index was at a record low (−2.03 SD). TNA warming reached record levels (≈+2.5 SD with linear trend removed; ≈+3.1 SD including trend). NA mean SST warming was also record-breaking (surpassing 2.5–3.1 SD depending on trend treatment). - Statistical relationships: NA TCGF correlates negatively with Niño3.4 (r=−0.38; reversed Niño) and positively with TNA (r=0.66); multi-regression using ENSO, PMM, TNA, and GW explains NA variability with r=0.76. WNP TCGF shows significant links to reversed PMM and reversed TNA (both r=0.52), and to the multi-regression model (r=0.64). ENSO–WNP relationship is subregional (dipole) rather than basin-wide. - SST reconstruction and circulation: Reconstructed 2023 SST anomalies from ENSO, PMM, TNA, and GW closely match observations (mean bias ~0.37 °C, tropics ~0.2 °C) and reproduce key circulation responses: anticyclonic anomalies over the North Pacific and equatorial easterlies over the WNP (suppressing WNP genesis), and cyclonic anomalies with equatorial westerlies over the NA (enhancing NA genesis). - Model validation: HiRAM simulations reproduce 2023 anomalies. Basin totals (ensemble means): CLIM vs All Run—NA 9.8 vs 17 TCs; WNP 24.5 vs 17.7 TCs—consistent with observed increase in NA and decrease in WNP. Reconstruction Run mirrors All Run, confirming that the four-mode SST anomalies account for 2023 behavior. - Quantified contributions (box plots across ensembles): • El Niño: Removing ENSO (No_ElNiño) further increases NA TCGF; El Niño contributes −34.7% to NA TCGF in 2023 (i.e., suppressed the increase). WNP basin total less sensitive due to dipole structure. • Global warming (GW): Contributes about +4.3 TCs (~43.8%) to the NA increase; contributes to WNP decreases by ~3.6 basin-wide and ~3.1 in E_WNP (≈14.7% and 18.5%, respectively). GW offsets El Niño’s suppressive effect on NA TCGF and enhances shear over the WNP, favoring fewer WNP TCs. • PMM and TNA: Limited influence on W_WNP but strong in E_WNP. Together they explain ~42.9% of the WNP decrease and ~92.9% of the NA increase. Locally, TNA anomalies dominate NA genesis enhancement; both negative PMM and positive TNA suppress E_WNP genesis, with TNA relatively more influential. - Environmental drivers: Changes in low-level westerlies, 850 hPa vorticity, and vertical zonal wind shear correlate significantly with TCGF anomalies (NA: r≈0.46 for westerlies/vorticity, r≈−0.55 for shear; E_WNP: r≈0.38–0.43 for westerlies/vorticity, r≈−0.56 for shear), linking the oceanic modes to local genesis conditions. - Implication: The typical strong El Niño impacts on regional TC genesis can be overridden or reshaped by concurrent PMM, TNA, and GW signals, explaining the 2023 lapsed El Niño impact across basins.

The study resolves why El Niño’s canonical modulation of NA and WNP TCGF failed in 2023. Record NA warming and a record negative PMM, together with a La Niña-like GW background, produced circulation anomalies that promoted NA genesis and suppressed WNP genesis, overwhelming El Niño’s expected effects. Regression and high-resolution model experiments consistently attribute most of the NA increase to TNA/Atlantic warming and GW, with El Niño acting to partially offset the increase. In the WNP, the negative PMM and positive TNA jointly drove suppression, amplified by GW-induced shear, leading to a basin-wide decrease rather than the usual ENSO dipole. These results underscore that concurrent climate modes can nonlinearly interact with El Niño’s influence, complicating seasonal prediction. The findings also hint at a possible weakening of El Niño’s historical suppressive effect on NA TCGF in a warming climate, as NA TCGF during prior strong El Niños lay below the trend while 2023 was above. The demonstrated pathway from oceanic modes to atmospheric controls (westerlies, vorticity, shear) clarifies mechanisms and informs forecast models.

This work demonstrates that the atypical 2023 TC genesis patterns—active NA and suppressed WNP despite a strong El Niño—were primarily driven by extraordinary Atlantic warming (TNA), a record negative PMM, and the background La Niña-like global warming pattern. Reconstructed SST anomalies and HiRAM sensitivity experiments show that Atlantic warming dominated the NA TCGF increase and, together with the negative PMM, suppressed WNP TCGF; GW further boosted NA genesis and reduced WNP genesis, effectively diminishing El Niño’s canonical impact. The paper provides an integrated, quantitative attribution framework for inter-basin TC genesis anomalies under concurrent climate modes. Future work should investigate potential long-term weakening of El Niño’s influence on NA TCGF, refine representation of Indian Ocean effects, reduce regional modeling biases (e.g., South China Sea), and enhance seasonal outlooks by jointly accounting for ENSO, PMM, TNA, and GW in a warming climate.

- The analysis does not explicitly include Indian Ocean Dipole influences, despite a strong positive phase in 2023; prior studies suggest moderate effects on WNP TC activity. - Model simulations show a northeastward shift of negative WNP TCGF anomalies relative to observations and under-reproduce suppression over the South China Sea, likely due to internal atmospheric variability and coastal-region model challenges. - Attribution of GW relies on linear trends and an optimized scaling period (1980–2023, t=30); results are robust but still sensitive to chosen trend periods and low-frequency variability (IPO/AMO). - PMM and TNA have some interrelation (r≈−0.4), and geographic constraints were applied in experiments to minimize cross-basin contamination; residual interactions may persist. - WNP basin-wide responses mask subregional dipole behavior typical of ENSO; interpretations emphasize E_WNP where signals are strongest.