An intense narrow equatorial jet in Jupiter's lower stratosphere observed by JWST

The study investigates the dynamics of Jupiter’s equatorial atmosphere at altitudes between the well-tracked cloud tops (≈500–700 mbar) and the stratospheric levels (≈0.1–40 mbar) inferred from thermal infrared retrievals. Prior work shows alternating zonal jets with altitude and latitude, multiyear stratospheric thermal/wind oscillations (Jupiter’s equatorial stratospheric oscillation, JESO), and extensive equatorial hazes near and above the tropopause (~100–200 mbar) that have historically hindered precise wind tracking at those levels. The central research question is whether a distinct jet exists within the equatorial hazes near the tropopause and how it relates to the known stratospheric oscillations and to the vertical wind shear from cloud tops upward. The purpose is to leverage JWST/NIRCam’s high spatial resolution and sensitivity to hazes to resolve small-scale features, measure winds at these altitudes, and clarify couplings between the upper troposphere/lower stratosphere and the time-variable stratospheric oscillation. This is important for understanding momentum transport and wave–mean flow interactions shaping equatorial dynamics on rapidly rotating giant planets, and for comparative planetology with Saturn and Earth’s QBO/SAO.

- Zonal jets on Jupiter and Saturn are broad and prograde at the equator, with deep extensions thousands of kilometers below cloud tops; above clouds, wind speeds generally decay with height to near-zero by ~20 mbar at mid-latitudes. - Saturn uniquely shows a strong narrow equatorial jet (~400 m s−1, ~5° width) at ~50–60 mbar tracked via high-altitude hazes. Both planets exhibit equatorial stratospheric oscillations with alternating eastward/westward jets between ~0.1–40 mbar; periods differ: JESO ~3.9–5.7 years vs Saturn’s SAO ~15 years (seasonally influenced). - Prior UV and visible observations on Jupiter indicated slight increases in equatorial winds near ~500–600 mbar and suggested vertical shear but suffered from low contrast and featureless hazes, limiting wind tracking above cloud tops. - Thermal wind analyses from infrared spectra infer time-variable stratospheric winds and temperatures descending with time, analogs to Earth’s QBO/SAO, likely driven by wave forcing (e.g., gravity waves from convection). Links between tropospheric convection and stratospheric variability have been suggested; upper-tropospheric temperatures show multiyear variability anticorrelated with JESO at near-equatorial latitudes. - Equatorial hazes (≥200 mbar) are variable in opacity/albedo and can brighten strongly in methane bands (e.g., 890 nm), complicating consistent wind sensing across wavelengths; previous attempts at wind retrievals in hazes faced large uncertainties.

Observations: JWST/NIRCam imaged Jupiter on 27 July 2022 in filters centered on strong methane and H2–H2/H2–He absorption bands to avoid detector saturation: F164N (1.644 μm), F212N (2.120 μm), F335M (3.365 μm), F360M (3.621 μm), and F405N (4.055 μm). Most filters (F164N, F212N, F335M, F360M) were acquired twice, separated by ~1 Jovian rotation (~10 hours), enabling wind tracking; F360M and F405N were each used once in the sequence. Subarrays (SUB640) and rapid readouts were used for bright filters to minimize exposure; dithers and mosaics filled detector gaps. Image processing: The JWST NIRCam calibration pipeline produced calibrated, geometrically corrected images. Multiple non-destructive read groups were combined to avoid saturation and rotational smearing while maximizing SNR. Bad pixels were removed with adaptive median filtering. Images were navigated using WinJupos, rotation-corrected and co-added over minutes, limb-darkening corrected, high-pass filtered to emphasize small-scale features, and map-projected (oversampled relative to native resolution). Filter penetration/sensing depths were assessed with a simplified aerosol-free radiative transfer model (gaseous absorption + Rayleigh scattering) to map two-way optical depth (τ≈1–5) versus pressure for each filter; sensing levels depend on local clouds/hazes. Wind tracking: Zonal winds were measured by comparing map-projected image pairs separated by one rotation using the PICV3 correlation software. Regions containing large-scale systems (e.g., GRS) were excluded. Correlation boxes were typically 20° longitude × 1° latitude, adapted for contrast. F164N used subarray pairs across detectors (16 datasets) combined into a single profile. For F212N, visually trackable features were also measured to validate correlation results. Zonal wind profiles were derived across latitudes, with focus on the equatorial zone (EZ, ±10° planetographic), and compared with historical cloud-top winds (HST, Cassini) and UV-derived winds. Context and ancillary data: HST images/maps obtained two rotations later (28 July 2022) provided cloud-top context; ground-based observations confirmed quiescent atmospheric conditions during July 2022. Brightness contrasts across wavelengths and Table 1 summarized approximate sensing levels in the EZ considering clouds and hazes.

- Discovery of a narrow, intense equatorial jet in the lower stratosphere/upper troposphere hazes: peak ~140 m s−1 near 100–200 mbar, confined to ±3° latitude, exceeding cloud-top equatorial winds by ~70 m s−1. - Vertical wind shear: At the equator, wind speeds increase from ~70 m s−1 at cloud tops (~600 mbar) to 105±16 m s−1 (F212N), 115±35 m s−1 (F164N), and 140±20 m s−1 (F335M), indicating a strong positive vertical shear within the haze layers. Estimated mean shear from cloud tops to hazes: ~40–70 m s−1 per scale height (~20 km). Within hazes: more uncertain 10–110 m s−1 per scale height. - Outside the central ±3° jet but within the EZ (±10°), winds decrease with altitude: at ~±7°, zonal winds decrease from cloud tops to hazes by ~45–60 m s−1, implying mean vertical shear of −25 to −60 m s−1 per scale height. - The new jet lies below stratospheric thermal oscillations (0.1–40 mbar) and likely represents a deep component of the JESO, implying potential temporal variability of the jet strength. - Multiwavelength morphology and sensing: EZ brightness enhancements relative to mid-latitudes were ~5× (F164N), ~9× (F212N), ~4× (F335M), and ~2× (F360M). F405N (minimal gas absorption) senses deep tropospheric clouds (500–600 mbar). Table 1 indicates that F164N and F212N primarily sense elevated equatorial hazes (<200–240 mbar), while F335M/F360M sample mixed contributions (50–500 mbar), and F405N senses cloud tops (500–600 mbar). - Feature correspondence: Crisp F212N features often correspond to bright, compact clouds seen in F405N (lower troposphere), suggesting vertical connections or convective activity reaching at least one scale height above the main cloud deck; many features evolve rapidly between rotations, consistent with strong shear. - Outside the EZ, JWST zonal wind profiles match historical cloud-top winds, indicating that away from equatorial hazes, NIRCam filters sample the main cloud deck. - Comparative context: Jupiter’s equatorial circulation now shows a sharply peaked near-tropopause jet similar in structure to Saturn’s high-altitude equatorial jet, though Jupiter’s exhibits stronger increase with altitude (40–70 m s−1 per scale height vs ~8 m s−1 per scale height on Saturn in the Cassini epoch).

The detection of a sharp, intense equatorial jet confined to ±3° near 100–200 mbar indicates that Jupiter’s equatorial stratospheric oscillation (JESO) likely extends downward to just above the cloud tops, unlike Earth’s QBO, which does not penetrate below the tropopause. The vertical structure—winds strengthening with altitude at the equator but weakening outside ±3°—parallels the equatorial wind architecture on Saturn, reinforcing the idea of common dynamical processes across rapidly rotating giant planets (e.g., wave–mean flow interactions, gravity-wave or Kelvin/Rossby-gravity wave momentum deposition). The jet’s location beneath the stratospheric thermal oscillations and the observed multiyear periodicity of JESO imply that the near-tropopause jet could vary in time, potentially reversing or changing intensity as thermal anomalies descend. Extrapolation of mid-2019 thermal maps to 2022 suggests a warm anomaly near ~40 mbar during the JWST epoch, consistent with winds decreasing at greater depths, and predicts a sign change at ~40 mbar in 2023–2024 that could modulate the equatorial jet. A holistic, vertically resolved view emerges by combining visible (cloud-top), near-IR (hazes at 100–200 mbar), thermal IR (1–40 mbar), and millimeter Doppler winds (~−4 mbar), revealing alternating jets aloft and a newly recognized intense jet at the base of the stratosphere. The observations also underscore the potential role of convection (seen in F405N and HST) in generating waves that feed energy and momentum into the stratosphere, thereby coupling troposphere and stratosphere.

JWST/NIRCam resolved fine-scale equatorial haze structures and enabled the first robust wind measurements in Jupiter’s near-tropopause hazes, revealing a narrow, intense equatorial jet (~140 m s−1 at 100–200 mbar) that is faster than cloud-top winds and likely represents a deep manifestation of the JESO. This finding revises Jupiter’s equatorial wind structure to include a strong near-tropopause jet and strengthens comparative links with Saturn’s equatorial circulation. The work demonstrates JWST’s capability to probe intermediate atmospheric levels that bridge cloud-top and stratospheric dynamics, offering crucial insights into vertical momentum transport and oscillation penetration depths. Future research should focus on time-series JWST observations to capture expected multiyear variability of the jet, coordinated multiwavelength campaigns (visible, near-IR, thermal IR, millimeter Doppler) to construct continuous vertical wind profiles, and modeling efforts incorporating wave–mean flow interactions and convective wave forcing to reproduce the observed jet and its variability.

- Temporal coverage: Winds in the hazes were measured at a single epoch (two images separated by one rotation), limiting assessment of temporal variability and phase within the JESO cycle. - Vertical sensing ambiguity: Filters sample broad and location-dependent vertical ranges; aerosol/cloud opacity variations affect effective sensing levels, introducing uncertainty in attributing winds to precise pressures. - Measurement dispersion: Larger dispersions in F164N and F335M arise from smaller subarray coverage (F164N), lower spatial resolution and higher noise (F335M), and sensitivity to wider vertical columns, potentially mixing altitudes in the presence of vertical shear. - Limited filters for wind tracking: F360M and F405N were obtained only once, precluding wind measurements in those filters; high-latitude wind retrievals degrade due to elevated subpolar hazes obscuring tropospheric clouds. - Interpretation relies partly on extrapolated thermal maps (2019→2022) to infer oscillation phase, adding uncertainty to dynamical context.