The IRIS experiment on board Voyager 2 provided key information about the latitudinal thermal structure on these planets. Near the tropopause, the observed temperature is found minimal at mid-latitudes and maximal at the equator and poles on both planets. Surprisingly, despite different radiative forcing (owing to their obliquity and different solar longitude at the date of observation), their thermal structure is relatively similar. By analysing the temperature simulated in our model, two main conclusions can be drawn:

- The simulated meridional thermal structure is inconsistent with the one observed by Voyager 2 at the same solar longitude on Uranus and Neptune and also by ground-based observations. The location of minima and maxima are at odds between our simulations and the observations. For Uranus, the simulated temperature near the tropopause is quite in agreement with the one retrieved from the IRIS experiment at high latitudes. However, the local temperature maximum at the equator was not produced by our model. On Neptune, the temperature at the tropopause is only consistent at the equator. The origin of the observed local maxima and minima could be explained by a meridional circulation which consists in subsidence at the equator and poles (adiabatic warming) and upwelling at mid-latitudes (adiabatic cooling) as proposed by Fletcher et al. (2020) and by thermal wind balance;

- In our simulations, a seasonal variation was seen above the tropopause where the maximum latitudinal contrast was shifted by $L_s = +90°$ following the summer solstice on Uranus and $+50°$ on Neptune. Below the tropopause, very slight variations were simulated on both planets, consistently with long radiative timescales. Contrary to simulations, no significant seasonal change has been observed since the Voyager 2 flyby at the tropopause and lower stratosphere on Uranus (Roman et al. 2020). On Neptune, significant variations in temperature have been observed since the Voyager 2 flyby (Roman et al. 2022). These variations cannot be explained by fully radiative seasonal forcings and suggest other effects with a subseasonal trend. This may indicate that the dynamics control the thermal structure at these levels, or that the assumption of a latitudinally uniform methane abundance and aerosol distribution in the lower stratosphere is not appropriate.

Recent and future observations from James Webb Space Telescope should provide new information on the thermal structure of Uranus' and Neptune's troposphere and stratospheres with the Mid-InfraRed Instrument (MIRI) spectrometer and potentially on their haze physical properties and spatial distribution with the Near-Infrared Spectrograph (NIRSPEC) and the Near-InfraRed Camera (NIRCam; Norwood et al. 2016). First spectroscopic observations performed as part of a Guaranteed Time Observation programme (PI: Leigh Fletcher) took place in January 2023 for Uranus and June 2023 for Neptune in the thermal infrared with MIRI. At that time, Uranus was close to the summer solstice ($L_s \approx 64°$) and Neptune's season corresponds to southern summer ($L_s \approx 310°$). At 0.1 hPa, the thermal contrast predicted from our 1D seasonal model on Uranus is 2.2 K between the equator and the north pole and 7.1 K for Neptune between the equator and the south pole. These new observations will enhance our knowledge of the thermal structure and its variations over time on these planets, and will help to constrain and complete future seasonal radiative-convective models. At the same time, the development of GCMs that take atmospheric dynamics into account appears necessary, given the puzzles that remain regarding the link between their thermal structure and meridional circulation.

*Acknowledgements.* G. Milcareck, S. Guerlet and A. Spiga acknowledge funding from Agence Nationale de la Recherche (ANR) project SOUND, ANR-20-CE49-00009-01. The authors acknowledge also Jan Vatant d'Ollone for his contribution at an early stage of this project. T. Cavalié acknowledges funding from CNES. Fletcher and Roman were supported by a European Research Council Consolidator Grant (under the European Union's Horizon 2020 research and innovation programme, grant agreement no. 723890) at the University of Leicester.

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The IRIS experiment on board Voyager 2 provided key information about the latitudinal thermal structure on these planets. Near the tropopause, the observed temperature is found minimal at mid-latitudes and maximal at the equator and poles on both planets. Surprisingly, despite different radiative forcing (owing to their obliquity and different solar longitude at the date of observation), their thermal structure is relatively similar. By analysing the temperature simulated in our model, two main conclusions can be drawn:

- The simulated meridional thermal structure is inconsistent with the one observed by Voyager 2 at the same solar longitude on Uranus and Neptune and also by ground-based observations. The location of minima and maxima are at odds between our simulations and the observations. For Uranus, the simulated temperature near the tropopause is quite in agreement with the one retrieved from the IRIS experiment at high latitudes. However, the local temperature maximum at the equator was not produced by our model. On Neptune, the temperature at the tropopause is only consistent at the equator. The origin of the observed local maxima and minima could be explained by a meridional circulation which consists in subsidence at the equator and poles (adiabatic warming) and upwelling at mid-latitudes (adiabatic cooling) as proposed by Fletcher et al. (2020) and by thermal wind balance;

- In our simulations, a seasonal variation was seen above the tropopause where the maximum latitudinal contrast was shifted by $L_s = +90°$ following the summer solstice on Uranus and $+50°$ on Neptune. Below the tropopause, very slight variations were simulated on both planets, consistently with long radiative timescales. Contrary to simulations, no significant seasonal change has been observed since the Voyager 2 flyby at the tropopause and lower stratosphere on Uranus (Roman et al. 2020). On Neptune, significant variations in temperature have been observed since the Voyager 2 flyby (Roman et al. 2022). These variations cannot be explained by fully radiative seasonal forcings and suggest other effects with a subseasonal trend. This may indicate that the dynamics control the thermal structure at these levels, or that the assumption of a latitudinally uniform methane abundance and aerosol distribution in the lower stratosphere is not appropriate.

Recent and future observations from James Webb Space Telescope should provide new information on the thermal structure of Uranus' and Neptune's troposphere and stratospheres with the Mid-InfraRed Instrument (MIRI) spectrometer and potentially on their haze physical properties and spatial distribution with the Near-Infrared Spectrograph (NIRSPEC) and the Near-InfraRed Camera (NIRCam; Norwood et al. 2016). First spectroscopic observations performed as part of a Guaranteed Time Observation programme (PI: Leigh Fletcher) took place in January 2023 for Uranus and June 2023 for Neptune in the thermal infrared with MIRI. At that time, Uranus was close to the summer solstice ($L_s \approx 64°$) and Neptune's season corresponds to southern summer ($L_s \approx 310°$). At 0.1 hPa, the thermal contrast predicted from our 1D seasonal model on Uranus is 2.2 K between the equator and the north pole and 7.1 K for Neptune between the equator and the south pole. These new observations will enhance our knowledge of the thermal structure and its variations over time on these planets, and will help to constrain and complete future seasonal radiative-convective models. At the same time, the development of GCMs that take atmospheric dynamics into account appears necessary, given the puzzles that remain regarding the link between their thermal structure and meridional circulation.

*Acknowledgements.* G. Milcareck, S. Guerlet and A. Spiga acknowledge funding from Agence Nationale de la Recherche (ANR) project SOUND, ANR-20-CE49-00009-01. The authors acknowledge also Jan Vatant d'Ollone for his contribution at an early stage of this project. T. Cavalié acknowledges funding from CNES. Fletcher and Roman were supported by a European Research Council Consolidator Grant (under the European Union's Horizon 2020 research and innovation programme, grant agreement no. 723890) at the University of Leicester.

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