Saturn atmospheric dynamics one year after Cassini: Long-lived features and time variations in the drift of the Hexagon

The zonal velocity of the equatorial white spot was 452.4 ± 1.7 m s−1 in 2018.

“and 452 . 4 ± 1 . 7 ms -1 in 2018”

A bright white spot in the Equatorial Zone can be tracked continuously since 2014 with minimal changes in its zonal velocity.

“A bright white spot in the Equatorial Zone that can be tracked continuously since 2014 with minimal changes in its zonal velocity”

The zonal velocity of the equatorial white spot was 444.3 ± 3.1 m s−1 in 2014.

“which was 444 . 3 ± 3 . 1 ms -1 in 2014”

• cites S´ anchez-Lavega et al., 2016Not checked yet

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The subpolar vortices and additional atmospheric features studied follow drift rates consistent with zonal winds obtained by Cassini.

“These vortices and additional atmospheric features here studied follow drift rates consistent with zonal winds obtained by Cassini.”

The drift rate of Saturn's North polar hexagon varied through the epochs 2015-2018, Cassini 2007-2014, HST 1990s, and Voyagers 1980-1981.

“We find variations in the drift rate of the hexagon through these epochs”

The long-term tracking of the White Spot covers 162 rotations over the planet's system III longitude.

“This long-term tracking covers 162 rotations of the bright feature over the planet's system III longitude.”

In 2018, a bright cloud feature in the South Equatorial Zone at approximate latitude (6.7 ± 2.2)° S was observed regularly.

“In 2018 a bright cloud feature in the South Equatorial Zone (SEZ) at approximate latitude (6 . 7 ± 2 . 2) ◦ S was also observed regularly (examples are given in Figures 7a and Figure 6).”

This bright cloud is not visible in later images in July or August.

“This bright cloud is not visible in later images in July or August.”

The morphology of the equatorial feature in 2017 and 2018 is similar in most observations with enough quality to inspect its shape.

“The morphology of this equatorial feature in 2017 and 2018 is similar in most of the observations with enough quality to inspect its shape (figure 7).”

In the best images, there were elongated features that drifted at different speeds.

“Besides the bright spot described above, in the best images there were some elongated features that drifted at different speeds.”

Later ground-based observations in 2017 and 2018 showed a bright equatorial cloud with high contrast.

“Later ground-based observations in 2017 and 2018 showed a bright equatorial cloud with high contrast.”

Over the second half of 2018, other equatorial features were visible at the same latitude as the White Spot.

“Over the second half of 2018 other equatorial features were visible at the same latitude.”

New bright clouds appeared at the same latitude in the South Equatorial Zone in September.

“New bright clouds appeared at the same latitude in the SEZ in September and their tracking is shown in the right panels of figure 10.”

The drift rate of the White Spot changed gradually from 444 ± 3 ms-1 (-36.7 ± 0.2° day-1) in 2014 to 452.4 ± 1.7 ms-1 (-37.32 ± 0.02° day-1) in 2018.

“Figure 8 shows that the drift rate of this White Spot changed gradually from 444 ± 3 ms -1 ( -36 . 7 ± 0 . 2 ◦ day -1 ) in 2014 to 452 . 4 ± 1 . 7 ms -1 ( -37 . 32 ± 0 . 02 ◦ day -1 ) in 2018.”

• AssumptionThe authors state this as an assumption: There is an uncertainty of a full rotation over the planet between the 2014 and 2015 data for the White Spot.

These cloud systems moved with a zonal speed of 72.6 ± 1.0 ms-1 (-6.72 ± 0.04° day-1).

“These cloud systems moved with a zonal speed of 72 . 6 ± 1 . 0 ms -1 ( -6 . 72 ± 0 . 04 ◦ day -1 ).”

Over the last few years, AV maintains a stable size.

“Over the last few years AV maintains a stable size and the reasons for its lower visibility on ground-based images are possibly related to a decrease of the contrast with respect to its environment in visible wavelengths.”

Some images in 2018 showed additional cloud systems at other tropical latitudes at 19, 24 and 36° N.

“Some images in 2018, including those acquired by HST, also showed additional cloud systems at other tropical latitudes at 19, 24 and 36 ◦ N.”

The tracking of positions shows that features A2 and A3 separated from the linear fits around July 2018.

“The tracking of their positions show that these features separated from the linear fits around July 2018, which may have been caused by a close interaction between them.”

Many observations over 2018 showed a subtle distortion in the North boundary of the Equatorial Zone at 15.2 ± 0.9° N.

“Many observations over 2018 showed a subtle distortion in the North boundary of the Equatorial Zone at 15 . 2 ± 0 . 9 ◦ N.”

The vortex changed in latitude, drift rate and size over the course of eight years.

“The vortex changed in latitude, drift rate and size over the course of eight years.”

The tracking of the vortex over 2018 results in a zonal drift of 13.4 ± 0.4 ms-1 (-1.43 ± 0.02° day-1).

“Panels a-c in Figure 15 show the tracking of the vortex over 2018, resulting in a zonal drift of 13 . 4 ± 0 . 4 ms -1 ( -1 . 43 ± 0 . 02 ◦ day -1 ).”

A similar feature is observable in Cassini images in 727 nm in April 2016.

“A similar feature is observable in Cassini images in 727 nm in April 2016 (figure 2).”

Cassini images in 2016 show several cyclones at this latitude as dark patches in MT2 images accompanied by elongated clouds in CB2 images.

“Cassini images in 2016 show several cyclones at this latitude as dark patches in MT2 images accompanied by elongated clouds in the CB2 images (Figure 2).”

In HST images and some exceptional amateur images, a small slightly dark vortex surrounded by a faint ring of brighter material is visible at ∼42° N.

“In HST images and in some exceptional amateur images, a small slightly dark vortex surrounded by a faint ring of brighter material is visible at ∼ 42 ◦ N.”

A one-to-one identification of the 2016 Cassini cyclones to the 2017 features is not possible because of lack of additional data in 2016.

“A one to one identification of the 2016 Cassini cyclones to the 2017 features is not possible because we lack additional data in 2016.”

HST observations in 2018 allowed mapping these features close in time to frequent amateur observations, enabling unambiguous identification of each feature in amateur images.

“HST observations in 2018 (Figure 6) allowed to map these features close in time to the frequent amateur observations. This enabled to identify unambiguously each feature in the amateur images.”

A bright cloud that lasted about 45 days held a drift rate of 391 ± 6.3 ms-1 (-32.2 ± 2.5° day-1).

“A bright cloud that lasted for about 45 days held a drift rate of 391 ± 6 . 3 ms -1 ( -32 . 2 ± 2 . 5 ◦ day -1 ).”

After July 20, A3 was not observed again.

“After July 20 A3 was not observed again.”

Three observations of the cloud at 24° N (feature G1) fit well with a linear drift of 174 ± 3.5 ms-1 (-15.4 ± 0.03° day-1).

“Three observations of the cloud at 24 ◦ N (feature G1 in Figure 6) fit well with a linear drift of 174 ± 3 . 5 ms -1 ( -15 . 4 ± 0 . 03 ◦ day -1 ).”

PlanetCam observations in 2017 and HST observations in 2018 show an undulating pattern at ∼47° N.

“PlanetCam observations in 2017 in the 1.0-1.7 µ m wavelength range and HST observations in 2018 show an undulating pattern at ∼ 47 ◦ N.”

Amateur image measurements of the ACA system from 2016 to 2018 show a large scatter with respect to a linear fit.

“The measurements obtained in amateur images in 2016 to 2018 show a large scatter of data with respect to a linear fit”

In some of the best observations of 2018, a dark small spot at ∼60° N is visible.

“In some of the best observations of 2018 a dark small spot at ∼ 60 ◦ N is visible in the images (Fig. 3).”

Their latitude corresponds to the southern limit of the ribbon wave observed on PlanetCam SWIR images, measured as 47.0 ± 0.2°, and in the HST map.

“Their latitude corresponds to the southern limit of the ribbon wave observed on PlanetCam SWIR images, which is measured as 47 . 0 ± 0 . 2 ◦ , and in the HST map of the planet (see Figs. 5 and 6).”

These bright clouds were very difficult to track because they seem to appear and disappear frequently.

“In spite of being generally bright (an example is given in Fig. 7h), these clouds were very difficult to track, since they seem to appear and dissapear frequently.”

A very limited number of images also showed features at 51.5° N.

“Finally, a very limited number of images also showed features at 51 . 5 ◦ N.”

The new WS ground-based data in 2017-2018 show a sustained fast feature with a continuous acceleration of its zonal speed from 444.3 ± 3.1 ms^-1 in 2014 to 452.4 ± 1.7 ms^-1 in 2018.

“The new WS groundbased data in 2017-2018 are more precise and show a sustained fast feature with a continuous acceleration of its zonal speed from 444 . 3 ± 3 . 1 ms -1 in 2014 to 452 . 4 ± 1 . 7 ms -1 in 2018”

Motions at the two Cassini ISS layers (CB and MT filters) are very different in the equator for the same years.

“Motions at both layers are very different in the equator for the same years”

The long-lived features where (β - u_yy) changes sign include the chain of cyclones at 30°N, the AV vortex in its southern limit, the bright features associated to the ribbon, the feature at 53°N and White Spot 3.

“Those where ( β -u yy ) changes sign are highlighted and include the chain of cyclones at 30 ◦ N, the AV vortex in its southern limit, the bright features associated to the ribbon, the feature at 53 ◦ N and the White Spot 3”

Drift rates for each hexagon vertex in 2015-2018 lie in the range from -3.0×10^-3 to 9.3×10^-3 ° day^-1.

“Drift rates for each vertex in this period lie in the range from -3 . 0 × 10 -3 ◦ day -1 to 9 . 3 × 10 -3 ◦ day -1”

The overall acceleration of the WS zonal speed is 1.9 ms^-1 per year.

“with an overall acceleration of 1.9 ms -1 per year.”

Features that separated from WS in early August 2018 moved at a slower rate over September 2018.

“The features that separated from WS in early August 2018 moved at a slower rate over September 2018.”

A short-lived feature observed in November 2018 moved at the same rate as the WS.

“A short-lived feature observed in November 2018 moved at the same rate as the WS.”

The acceleration of Cassini zonal winds in CB filters from 2004-2009 to 2014 is 10-20 ms^-1.

“This can be compared with an acceleration of the Cassini zonal winds in CB filters of 10-20 ms -1 from 2004-2009 to 2014”

The velocities of the features that separated from WS are compatible with the most recent measurements of cloud motions at the visible cloud layer with Cassini ISS images in 2014.

“Their velocities are compatible with the most recent measurements of cloud motions at the visible cloud layer with Cassini ISS images in 2014.”

Measurements of the WS in different years are very similar.

“Measurements of the WS in different years are very similar”

The NPS observed in the 1980s and 1990s was not observed in any of the thermal infrared or visible images examined.

“The NPS that had been observed in the 1980s and 1990s was not observed in any of these thermal infrared or visible images.”

Equatorial Zone cloud systems are the only ones that present large differences in their velocities with the Cassini zonal winds.

“we remark that these cloud systems are the only ones that present large differences in their velocities with the Cassini zonal winds.”

The mean drift rate of the hexagon vertices in 2015-2018 is (4.6 ± 4.3)×10^-3 ° day^-1 (0.012 ± 0.011 ms^-1).

“with a mean value of (4 . 6 ± 4 . 3) × 10 -3 ◦ day -1 (0 . 012 ± 0 . 011 ms -1 )”

The bright white spot (WS) observed from 2014 to 2018 roughly matches the VIMS zonal winds obtained in 2007 and the Voyager zonal winds at the cloud level.

“The bright white spot (WS) observed from 2014 to 2018 roughly matches the VIMS zonal winds obtained in 2007 and the Voyager zonal winds at the cloud level”

The ACA system is located in a region of nearly null relative vorticity.

“note that the ACA system is located in a region of nearly null relative vorticity”

The NPS drift rates are within the error bars of the hexagon measurements.

“and are within the error bars of the measurements.”

The motions of the polar storms roughly followed the zonal wind profiles.

“The motions of the polar storms roughly followed the zonal wind profiles”

Most bright features close in latitude to the ribbon moved at velocities compatible with the zonal winds from Cassini images.

“Most of these cloud systems moved at velocities compatible with the zonal winds from Cassini images.”

When using longitudinal positions over previous years, the drift rate of subpolar vortices matches the Cassini zonal winds within 2 ms^-1.

“When using longitudinal positions over previous years, their drift rate matches the Cassini zonal winds within 2 ms -1 .”

The chain of details at slightly different latitudes seem to follow the meridional wind shear of the zonal winds.

“The chain of details at slightly different latitudes seem to follow the meridional wind shear of the zonal winds”

One of the additional SEZ features moved with the same wind speed as those measured at cloud level on Cassini ISS images.

“One of these cloud systems moved with the same wind speed as those measured at cloud level on Cassini ISS images.”

The shear-like feature in the Northern edge of the EZ moves ~35 ms^-1 slower than the zonal winds obtained by Cassini at the same latitude.

“the shear-like feature in the Northern edge of the EZ also moves at a different speed than the zonal winds, moving ∼ 35 ms -1 slower than the zonal winds obtained by Cassini at the same latitude.”

AV's drift rate over 2010-2018 is intermediate between the Cassini winds in 2004-2009 and the winds measured in August 2011, right after the end of the convective activity.

“AV's drift rate over 2010-2018 is intermediate between the Cassini winds in 2004-2009 and the winds measured in August 2011, right after the end of the convective activity, and measured over a highly perturbed atmosphere.”

Additional features in the SEZ that appeared over September 2018 moved at slower velocities.

“Additional features in the SEZ that appeared over September 2018 moved at slower velocities.”

The shear-like feature is located at a very particular latitude in the limit of the hazes that cover the EZ.

“This feature is located at a very particular latitude in the limit of the hazes that cover the EZ.”

Subpolar vortices in 2018 like the ACA and the 60°N vortex follow closely the zonal winds.

“Subpolar vortices in 2018 like the ACA and the 60 ◦ N vortex follow closely the zonal winds.”

A statistical analysis of the differences between the zonal winds and the bright features implies a difference of -15 ± 28 ms^-1.

“A statistical analysis of the differences between the zonal winds and the bright features implies a difference of -15 ± 28 ms -1 .”

Tropical features moved with a similar velocity to the zonal winds at these latitudes, with differences on the order of 1-4 ms^-1.

“The tropical features moved with a similar velocity to the zonal winds at these latitudes, with differences on the order of 1-4 ms -1 .”

The white spot moves slightly faster than the deep zonal wind profiles.

“However, the white spot moves slightly faster than the deep zonal wind profiles.”

A peculiar cloud feature in the northern edge of the EZ, observed from 2016 to 2018, moved 35 m s−1 slower than the zonal winds.

“A peculiar cloud feature in the northern edge of the EZ and observed from 2016 to 2018 moved 35 ms -1 slower than the zonal winds.”

A similar bright and fast feature has been found in the South Equatorial Zone at 6.7 ± 2.2° S during most of 2018.

“A similar bright and fast feature has been found in the South Equatorial Zone at 6 . 7 ± 2 . 2 ◦ S during most of 2018.”

Except for its elongated shape, the South equatorial bright spot is very similar in size, brightness, drift rate and closeness to the deep winds obtained by VIMS.

“Except for its elongated shape, this South equatorial bright spot is very similar in size, brightness, drift rate and closeness to the deep winds obtained by VIMS.”

Cassini zonal winds in 2004-2009 and 2014 changed in the two layers sensed by the continuum band and methane band filters sensitive to the lower clouds and upper hazes respectively.

“Cassini zonal winds in 2004-2009 and 2014 changed in the two layers sensed by the continuum band and methane band filters sensitive to the lower clouds and upper hazes respectively.”

The hexagon's drift rate changes in time.

“Still, the hexagon's drift rate changes in time”

Zonal winds obtained in Cassini VIMS images sensitive to the ∼2 bar level are closer to the speed of the bright white spot than winds in the upper layers measured in Cassini ISS images.

“Zonal winds obtained in Cassini VIMS images sensitive to the ∼ 2 bar level (Choi et al., 2009; Studwell et al., 2018) are closer to the speed of the bright white spot than winds in the upper layers measured in Cassini ISS images.”

• cites Choi et al., 2009 sourceNot checked yet

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• cites Studwell et al., 2018 sourceNot checked yet

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Many long-lived features were regularly present at Saturn's tropical latitude at around 30° N.

“Many long-lived features were regularly present at Saturn's tropical latitude at around 30 ◦ N.”

The white spot velocities are closer to the equatorial zonal winds at cloud level at the time of the Voyagers than during the Cassini mission.

“These values are closer to the equatorial zonal winds at cloud level at the time of the Voyagers than during the Cassini mission.”

The long-term tracking of a bright white spot at 6.7 ± 1.6° N discussed by Sánchez-Lavega et al. (2016) is extended from 2014-2015 to 2014-2018.

“The long-term tracking of a bright white spot at 6 . 7 ± 1 . 6 ◦ N discussed by S´ anchez-Lavega et al. (2016) is extended with respect to data in that publication from 2014-2015 to 2014-2018.”

• cites S´ anchez-Lavega et al. (2016)Not checked yet

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Several equatorial clouds tracked in September-October 2018 in the North and South latitudes of the EZ moved at intermediate speeds between those of the bright white spots and those of zonal winds at the cloud level.

“Several equatorial clouds were tracked in September-October 2018 in the North and South latitudes of the EZ moving at intermediate speeds between those of the bright white spots and those of zonal winds at the cloud level.”

The AV's drift rate is compatible with zonal winds slightly larger than those measured in 2009.

“Its drift rate is compatible with zonal winds slightly larger than those measured in 2009.”

The interactions they experienced in 2015 that resulted in a subpolar planetary scale perturbation have not developed again.

“The interactions they experienced in 2015 that resulted in a subpolar planetary scale perturbation (del R´ ıoGaztelurrutia et al., 2018) has not developed again.”

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The large AV that formed after Saturn's GWS in 2011 is still present in the planet but with a reduced contrast with the atmosphere.

“The large AV that formed after Saturn's GWS in 2011 is still present in the planet but with a reduced contrast with the atmosphere.”

The measurements of Saturn's hexagon drift rate are extended in time, showing changes from the measurements obtained by the Voyagers and HST in the 1980s and 1990s compared with a more stable and smaller drift rate during the Cassini mission.

“We extend the measurements of Saturn's hexagon drift rate in time showing changes from the measurements obtained by the Voyagers and HST in the 1980s and 1990s compared with a more stable and smaller drift rate during the Cassini mission.”

• AssumptionThe authors state this as an assumption: The identification of hexagon vertices in Figure 23 is correct.

The zonal velocity of the equatorial white spot in 2018 is remarkably different from the zonal winds at the cloud level at its latitude during the Cassini mission.

“This velocity is remarkably different from the zonal winds at the cloud level at its latitude during the Cassini mission”

A large long-lived Anticyclone Vortex (AV) formed after the Great White Spot of 2010-2011.

“A large long-lived Anticyclone Vortex, here AV, that formed after the Great White Spot of 2010-2011.”

• cites Fletcher et al., 2011 sourceNot checked yet

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• cites S´ anchez-Lavega et al., 2012Not checked yet

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• cites Sayanagi et al., 2013 sourceNot checked yet

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A system of subpolar vortices at latitudes 60-65°N has been present at least since 2011.

“A system of subpolar vortices at latitudes 60-65 ◦ N present at least since 2011.”

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The different drift rates reinforce the role of the North Polar Spot present in the Voyager epoch and in the early 90s in causing a faster drift rate of the hexagon at that time compared with the current slower one.

“Instead, the different drift rates reinforce the role of the North Polar Spot that was present in the Voyager epoch and in the early 90s to cause a faster drift rate of the hexagon at that time compared with the current slower one.”

The variations in the hexagon's drift rate cannot be fit by seasonal changes in the polar area.

“We find variations in the drift rate of the hexagon through these epochs that can not be fit by seasonal changes in the polar area.”

The bright White Spot moved at 450 ms-1.

“The bright cloud moved at 450 ms -1 .”

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The White Spot velocity of 450 ms-1 is significantly faster than zonal winds from cloud tracking on Cassini ISS continuum band filters through 2004-2009 (~380 ms-1).

“This velocity is significantly faster than zonal winds obtained from cloud tracking on Cassini ISS images on the continuum band filters through 2004-2009 ( ∼ 380 ms -1 , see Garc´ ıa-Melendo et al. 2010).”

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The bright White Spot moved at similar velocities to equatorial clouds observed in visible light in 1980-1981 by Voyager 1 and 2.

“Instead, the bright WS moved at similar velocities to those of equatorial clouds observed in visible light in 1980-1981 by Voyager 1 and 2 (S´ anchez-Lavega et al., 2000).”

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AV is currently the second longest-lived vortex observed in Saturn, after the North Polar Spot discovered in Voyager 1 flyby in 1980 and observed until 1995.

“AV is currently the second longest-lived vortex observed in Saturn, after the North Polar Spot discovered in the Voyager 1 flyby in 1980 and observed until 1995 (SanchezLavega et al., 1997).”

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Two of these cloud systems (A1 at 30.1 ± 1.1° N and A2 at 29.7 ± 0.7° N) possibly interacted very closely around January 2018, interchanging their relative positions.

“Results of this extended analysis are shown in figure 14, which suggests that two of these cloud systems (A1 at planetographic latitude 30 . 1 ± 1 . 1 ◦ N and A2 at planetographic latitude 29 . 7 ± 0 . 7 ◦ N) possibly interacted very closely around January 2018 interchanging their relative positions.”

This vortex is the same one that formed after the Great White Spot convective storm in 2010-2011.

“This vortex is the same one that formed after the Great White Spot (GWS) convective storm in 2010-2011 (Fletcher et al., 2011; S´ anchez-Lavega et al., 2012; Sayanagi et al., 2013; Trammell et al., 2016).”

• cites Fletcher et al., 2011 sourceNot checked yet

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• cites S´ anchez-Lavega et al., 2012Not checked yet

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• cites Sayanagi et al., 2013 sourceNot checked yet

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The 2017 and 2018 chain of tropical features are interpreted as possible cyclones.

“However, we interpret the 2017 and 2018 chain of tropical features as possible cyclones.”

The reasons for AV's lower visibility on ground-based images are possibly related to a decrease of contrast with respect to its environment in visible wavelengths.

“Over the last few years AV maintains a stable size and the reasons for its lower visibility on ground-based images are possibly related to a decrease of the contrast with respect to its environment in visible wavelengths.”

The sudden change in AV's drift rate after 2015 was possibly accompanied by a small change in latitude.

“After 2013 changes were much less intense and more difficult to constrain, and after 2015 AV became less frequently observed following a sudden change in its drift rate, possibly accompanied by a small change in latitude.”

The vortex was visible in thermal maps as a cold feature at the time of the Great White Spot.

“It was also visible in thermal maps of the planet as cold feature at the time of the GWS (Fletcher et al., 2011).”

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The nearly round size and longevity of the cloud systems over 2017 and 2018 imply that they are most probably non-convective.

“We highlight the nearly round size of the cloud systems over 2017 and 2018 and their longevity implying that, most probably, they are non-convective.”

This northwards migration made the vortex reach latitudes where zonal winds caused a faster zonal drift.

“This northwards migration made the vortex reach latitudes where zonal winds caused a faster zonal drift.”

The ACA system's track in 2013-2015 exhibited oscillations in longitude with an amplitude of ~10° and period of ~8.2 months with respect to a single linear fit.

“According to del R´ ıo-Gaztelurrutia et al. (2018), the track of the ACA system in the period 2013-2015 exhibited oscillations in longitude with an amplitude of ∼ 10 ◦ and period of ∼ 8 . 2 months with respect to a single linear fit”

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The ACA system at 64° N was first observed in Cassini ISS images in 2011.

“The ACA system at 64 ◦ N was first observed in Cassini ISS images in 2011 (del R´ ıo-Gaztelurrutia et al., 2018).”

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Due to the small number of measurements in 2016-2018 and errors in amateur image measurements, it is not possible to find a single oscillation with a clear period.

“However, due to the small number of measurements in 2016 to 2018 and the errors in measurements of amateur image it is not possible to find a single oscillation with a clear period.”

This drift rate is stable enough to lead a search of this vortex over 2017 and 2016.

“This drift rate is stable enough to lead a search of this vortex over 2017 and 2016.”

The perturbation was the consequence of the interaction of a system of three vortices located at 65° N (ACA) with the dark vortex at 60° N.

“The perturbation was the consequence of the interaction of a system of three vortices located at 65 ◦ N and named ACA after Anticyclone-Cyclone-Anticyclone with the dark vortex at 60 ◦ N (del R´ ıo-Gaztelurrutia et al., 2018).”

• cites del R´ ıo-Gaztelurrutia et al., 2018 sourceNot checked yet

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The differences in the 2016, 2017 and 2018 data can be explained by a small change in the drift rate of this vortex of less than 1.6 ms-1 (0.25° day-1).

“The differences in the 2016, 2017 and 2018 data can be explained by a small change on the drift rate of this vortex of less than 1.6 ms -1 (0.25 ◦ day -1 ).”

It was a stable system of three vortices observed in Cassini ISS images until 2017.

“It was a stable system of three vortices that has been observed in Cassini ISS images until 2017 (features A2, C1 and A3 in figure 1 in Ingersoll et al. 2018).”

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The large scatter in ACA system amateur measurements suggests that similar oscillations continued years after the 2015 perturbation.

“that suggests that similar oscilations continued years after the perturbation.”

Most probably, the low visibility of the ACA in 2018 was due to the polar storms developing at 67° N, which masked the area of ACA and made it completely invisible when the polar storms grew into a full longitudinal disturbance after June 2018.

“Most probably, the low visibility of the ACA in 2018 was due to the polar storms developing at 67 ◦ N, which masked the area of ACA and made it completely invisible when the polar storms grew into a full longitudinal disturbance after June 2018.”

This vortex was one of the vortices implied in a polar perturbation that developed in Saturn in May-July 2015.

“This vortex was one of the vortices implied in a polar perturbation that developed in Saturn in May-July 2015.”

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In Voyager and early HST images, the north polar spot (NPS) drifted in longitude at a similar rate as the hexagon.

“In Voyager and early HST images, a large anticyclone outside the hexagon, the so-called north polar spot (NPS), drifted in longitude at a similar rate as the hexagon”

• cites Allison et al., 1990 sourceNot checked yet

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• cites Godfrey, 1990 sourceNot checked yet

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• cites Sanchez-Lavega et al., 1997 sourceNot checked yet

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The narrow and intense jet in the upper hazes and its temporal variations could be related to the equatorial thermal oscillations.

“the narrow and intense jet in the upper hazes and its temporal variations could be related to the equatorial thermal oscillations (Fouchet et al., 2008; Orton et al., 2008; Guerlet et al., 2018).”

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• cites Orton et al., 2008 sourceNot checked yet

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• cites Guerlet et al., 2018 sourceNot checked yet

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The hexagon was later observed in Cassini ISS images in visible light since 2012 until the end of the mission.

“The hexagon was later observed in Cassini ISS images in visible light since 2012 until the end of the mission”

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• cites Antu˜ nano et al., 2015Not checked yet

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• cites Sayanagi et al., 2018Not checked yet

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The positions of the hexagon's vertices are almost steady with respect to Saturn's system III longitudes.

“Although the hexagon lies at a fast Eastward jet, the positions of the vertices that define its shape are almost steady with respect to Saturn's system III longitudes.”

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The Equatorial Zone has quasi-periodic thermal oscillations in the stratosphere that occur both in altitude and in time.

“as well as quasi-periodic thermal oscillations in the stratosphere that occur both in altitude and in time (Fouchet et al., 2008; Orton et al., 2008; Guerlet et al., 2018).”

• cites Fouchet et al., 2008 sourceNot checked yet

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• cites Orton et al., 2008 sourceNot checked yet

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• cites Guerlet et al., 2018 sourceNot checked yet

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The hexagon is almost stationary with respect to system III longitudes.

“that makes the hexagon almost stationary with respect to system III longitudes.”

The quasi-periodic oscillations of the stratosphere were disrupted with the development of the 2010-2011 Great White Spot.

“The quasi-periodic oscillations of the stratosphere where disrupted with the development of the 2010-2011 Great White Spot (Fletcher et al., 2017)”

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The hexagon manifested in the stratosphere in 2014.

“and eventually also manifested in the stratosphere in 2014 (Fletcher et al., 2018).”

• cites Fletcher et al., 2018 sourceNot checked yet

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Differences between the drift rate of WS1 and the zonal winds are significant and cannot be only attributed to uncertainties in the measurements.

“but differences between the drift rate of the first storm (WS1) and the zonal winds are significant and can not be only attributed to the uncertainties in the measurements.”

The error bars of the differences between zonal winds and bright features are too large to distinguish if the bright features are advected by the wind or are a manifestation of a phase speed of the ribbon.

“These differences have error bars that are too large to distinguish if the bright features are advected by the wind or are a manifestation of a phase speed of the ribbon.”

The three different velocities at the SEZ correspond to the same epoch and constitute strong evidence of an intense vertical wind shear in the SEZ.

“The three different velocities at the SEZ correspond to the same epoch and constitute a strong evidence of an intense vertical wind shear in the SEZ.”

The difference of 35 ms^-1 in the zonal wind of the shear-like cloud and the zonal winds may represent a phase speed of the wave, which has remained largely stable at least since 2016.

“In that case, the difference of 35 ms -1 in the zonal wind of the shear-like cloud and the zonal winds may represent a phase speed of the wave, which has remained largely stable at least since 2016.”

More recent simulations showed that small perturbations to an eastward jet can grow and form a meandering jet when the jet latitude, amplitude, curvature and vertical structure are appropriate.

“More recent simulations showed that small perturbations to an eastward jet can grow and form a meandering jet when the jet latitude, amplitude, curvature and vertical structure are appropiate (Morales-Juber´ ıas et al., 2015).”

• cites Morales-Juber´ ıas et al., 2015Not checked yet

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The small positive drift rates in 2007-2011 combined with negative drift rates in 1990-1991 and 1980-1981 imply a complex variation of the hexagon's drift rate in time.

“The small positive drift rates in 2007-2011 combined with negative drift rates in 1990-1991 and 1980-1981 in table 3 imply a complex variation of the hexagon's drift rate in time.”

These simulations successfully reproduce the hexagon's characteristics by imposing conditions to the zonal jet at deep levels (at least 2 bar with a constant behavior in depth until 10 bar).

“These simulations do not require nearby vortices and they succesfull reproduce the hexagon's characteristics by imposing conditions to the zonal jet at deep levels (at least 2 bar with a constant behavior in depth until 10 bar)”

• cites Morales-Juber´ ıas et al., 2015Not checked yet

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The hexagon's drift rate, when interpreted as a phase speed with respect to the environment wind, is consistent with a Rossby wave.

“This drift rate, when interpreted as a phase speed with respect to the environment wind, is consistent with a Rossby wave (Allison et al., 1990; S´ anchez-Lavega et al., 2014).”

• cites Allison et al., 1990 sourceNot checked yet

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• cites S´ anchez-Lavega et al., 2014Not checked yet

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The interpretation that the bright white spot is probably a deeply rooted atmospheric system moving at zonal speeds characteristic of levels below the typically observed levels in visible images of the Equatorial Zone still holds.

“The interpretation in S´ anchez-Lavega et al. (2016) that the bright white spot is probably a deeply rooted atmospheric system moving at zonal speeds characteristics of levels bellow the typically observed levels in visible images of the Equatorial Zone still holds”

• cites S´ anchez-Lavega et al. (2016)Not checked yet

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The presence of the NPS and coincident drift rate in 1980-1981 and 1990-1991, and its absence since the first images of the north polar area in 2007, indicate the NPS is important in modifying the hexagon's drift rate.

“Still, the hexagon's drift rate changes in time and the presence of the NPS and coincident drift rate in 19801981 and 1990-1991 periods, and its absence since the first images of the north polar area in 2007 seem to indicate the importance of this feature in modifying the hexagon's drift rate.”

The lower troposphere seems stable.

“However, the lower troposphere seems stable.”

The variation in the hexagon's drift rate is not seasonal.

“The comparison of the different drift rates found in Voyager and Cassini data for the same season implies that the variation in the hexagon's drift rate is not seasonal.”

The fast eastward jet where the hexagon is located has remained stable since the Voyager 1 in 1980 imaged this area.

“For instance, the fast eastward jet where the hexagon is located has remained stable since the Voyager 1 in 1980 imaged this area (see Antu˜ nano et al. 2018 and references therein).”

• cites Antu˜ nano et al. 2018Not checked yet

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Seasonal changes have been observed in the upper troposphere and stratosphere, with temperatures in the tropopause changing from ~78 to 91 K from 2004 to 2017.

“Certainly, seasonal changes have been observed in the upper troposphere and stratosphere, with temperatures in the tropopause changing from ∼ 78 to 91 K from 2004 to 2017 (Fletcher et al., 2018).”

• cites Fletcher et al., 2018 sourceNot checked yet

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The varied values of velocities are interpreted as a consequence of the vertical cloud structure and vertical wind shear that should be similar in the period 2015-2018.

“We interpret these varied values of velocities as a consequence of the vertical cloud structure and vertical wind shear that should be similar in the period 2015-2018.”

The morphology of this feature in filters penetrating below the upper hazes corresponds to a sheared structure.

“The morphology of this feature in filters penetrating below the upper hazes corresponds to a sheared structure.”

Local equatorial winds outside the region of the white spot in 2015 are similar to those obtained by Cassini.

“Winds obtained at the visible cloud at this latitude by S´ anchez-Lavega et al. (2016) over HST images acquired in 2015 indicate that local equatorial winds outside the region of the white spot are similar to those obtained by Cassini.”

• cites S´ anchez-Lavega et al. (2016)Not checked yet

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This implies clouds located below the levels affected by seasonal variations.

“This implies clouds located below the levels affected by seasonal variations.”

The 35 m s−1 slower velocity of this feature could be a manifestation of a phase speed of the wave disturbance.

“The 35 ms -1 slower velocity of this feature could be a manifestation of a phase speed of the wave disturbance.”

The polar jet extends deep in Saturn's atmosphere, well below levels affected by seasonal variations.

“The survival of the hexagon during the polar night, the stability of the polar jet to the varying solar irradiation, the indirect determination of winds extending down at least to the ∼ 10 bar (S´ anchez-Lavega et al., 2011) level, and the recent determination of deep winds in Saturn's mid to low latitudes (Galanti et al., 2019) convincingly agree that the polar jet extends deep in the atmosphere,”

• cites S´ anchez-Lavega et al., 2011Not checked yet

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• cites Galanti et al., 2019 sourceNot checked yet

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These bright features are most probably related with the activity of the ribbon wave at 47° N.

“They are most probably related with the activity of the ribbon wave at 47 ◦ N.”

The long-term variations of this small drift rate are not in agreement with a simple seasonal variation.

“The long-term variations of this small drift rate are not in agreement with a simple seasonal variation”

• AssumptionThe authors state this as an assumption: The identification of hexagon vertices in Figure 23 is correct.

For the ACA system, this implies a coherent lifetime of the three vortices of at least 7 years, only slightly smaller than the AV.

“For the ACA system this implies a coherent lifetime of the three vortices of at least 7 years, only slightly smaller than the AV.”

A comparison with Cassini ISS images and their position with respect to the zonal winds suggests these features are a chain of cyclones moving at very close latitudes.

“and a comparison with Cassini ISS images and their position with respect to the zonal winds suggests they are a chain of cyclones moving at very close latitudes.”

The long-term variations of this small drift rate reinforce the role of the NPS in forcing the hexagon at the time of its discovery and the decade after.

“and reinforce the role of the NPS in forcing the hexagon at the time of its discovery and the decade after.”

• AssumptionThe authors state this as an assumption: The identification of hexagon vertices in Figure 23 is correct.

The hexagon does not require a nearby polar anticyclone to maintain its stability.

“While the hexagon does not require a nearby polar anticyclone to maintain its stability”

The identification of hexagon vertices in 1980-81 and 1990-1991 was determined by testing several permutations of the vertices and computing polynomial fits to the data.

“The identification of vertices of the hexagon in 1980-81 and 1990-1991 was found after testing several permutations of the vertices and computing polynomial fits to the data.”

The longevity and characteristics of these features imply a non-convective nature.

“Their longevity and characteristics imply a non convective nature”

This is in agreement with the zonal winds measured after the fade of the GWS and those predicted from thermal wind arguments.

“This is in agreement with the zonal winds measured after the fade of the GWS (Sayanagi et al., 2013), and those predicted from thermal wind arguments (Achterberg et al., 2014; Fletcher et al., 2016).”

• cites Sayanagi et al., 2013 sourceNot checked yet

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• cites Achterberg et al., 2014 sourceNot checked yet

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• cites Fletcher et al., 2016 sourceNot checked yet

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