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F. M. Flasar - One of the best experts on this subject based on the ideXlab platform.
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The formation and evolution of Titan’s winter Polar Vortex
Nature Communications, 2017Co-Authors: Nicholas A Teanby, Bruno Bézard, Sandrine Vinatier, Melody Sylvestre, Remco De Kok, Simon Calcutt, Conor Nixon, Patrick Irwin, F. M. FlasarAbstract:Saturn's largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter Polar vortices. Following Titan's 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-Polar subsidence and winter Polar Vortex formation. Throughout 2010-2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the Polar Vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2-6 years post-equinox. The long time-frame to reach a stable Vortex configuration results from the high infrared opacity of Titan's trace gases and the relatively long atmospheric radiative time constant. Winter Polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan.
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The formation and evolution of Titan's winter Polar Vortex.
Nature communications, 2017Co-Authors: Nicholas A Teanby, Bruno Bézard, Sandrine Vinatier, Melody Sylvestre, Conor A. Nixon, Patrick G. J. Irwin, Remco De Kok, Simon Calcutt, F. M. FlasarAbstract:Saturn’s largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter Polar vortices. Following Titan’s 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-Polar subsidence and winter Polar Vortex formation. Throughout 2010–2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the Polar Vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2–6 years post-equinox. The long time-frame to reach a stable Vortex configuration results from the high infrared opacity of Titan’s trace gases and the relatively long atmospheric radiative time constant. Winter Polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan.
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The formation and evolution of Titan’s winter Polar Vortex
Nature Communications, 2017Co-Authors: Nicholas A Teanby, Bruno Bézard, Sandrine Vinatier, Melody Sylvestre, Conor A. Nixon, Patrick G. J. Irwin, Simon Calcutt, F. M. FlasarAbstract:Saturn’s largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter Polar vortices. Following Titan’s 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-Polar subsidence and winter Polar Vortex formation. Throughout 2010–2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the Polar Vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2–6 years post-equinox. The long time-frame to reach a stable Vortex configuration results from the high infrared opacity of Titan’s trace gases and the relatively long atmospheric radiative time constant. Winter Polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan. The Polar hot-spot appeared in Titan after equinox in 2010 suddenly cooled in early 2012, which wasn’t predicted by models. Here the authors use observations to show that the increase in trace gases during the hot-spot resulted in radiative cooling feedback.
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Titan's Winter Polar Vortex
2008Co-Authors: F. M. Flasar, R. K. Achterberg, Paul J. SchinderAbstract:Titan's atmosphere has provided an interesting study in contrasts and similarities with Earth's. While both have N$_2$ as the dominant constituent and comparable surface pressures $\sim1$ bar, Titan's next most abundant molecule is CH$_4$, not O$_2$, and the dissociative breakup of CH$_4$ and N$_2$ by sunlight and electron impact leads to a suite of hydrocarbons and nitriles, and ultimately the photochemical smog that enshrouds the moon. In addition, with a 15.95-day period, Titan is a slow rotator compared to Earth. While the mean zonal terrestrial winds are geostrophic, Titan's are mostly cyclostrophic, whipping around the moon in as little as 1 day. Despite the different dynamical regime, Titan's winter stratosphere exhibits several characteristics that should be familiar to terrestrial meteorologists. The cold winter pole near the 1 -mbar level is circumscribed by strong winds (up to 190 m/s) that act as a barrier to mixing with airmasses at lower latitudes. There is evidence of enhancement of several organic species over the winter pole, indicating subsidence. The adiabatic heating associated with this subsidence gives rise to a warm anomaly at the 0.01-mbar level, raising the stratopause two scale heights above its location at equatorial latitudes. Condensate ices have been detected in Titan's lower stratosphere within the winter Polar Vortex from infrared spectra. Although not always unambiguously identified, their spatial distribution exhibits a sharp gradient, decreasing precipitously across the Vortex away from the winter pole. The interesting question of whether there is important heterogeneous chemistry occurring within the Polar Vortex, analogous to that occurring in the terrestrial Polar stratospheric clouds in the ozone holes, has not been addressed. The breakup of Titan's winter Polar Vortex has not yet been observed. On Earth, the Polar Vortex is nonlinearly disrupted by interaction with large-amplitude planetary waves. Large-scale waves have not been identified in Titan's atmosphere, so the decay of its Polar Vortex may be more gradual than on Earth. Observations from an extended Cassini mission into late northern spring should provide critical data indicating whether the Vortex goes away with a bang or just fades away.
Hans-f. Graf - One of the best experts on this subject based on the ideXlab platform.
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Observational constraints on the tropospheric and near-surface winter signature of the Northern Hemisphere stratospheric Polar Vortex
Climate Dynamics, 2014Co-Authors: Hans-f. Graf, Davide Zanchettin, Claudia Timmreck, Matthias BittnerAbstract:A composite analysis of Northern Hemisphere’s mid-winter tropospheric anomalies under the conditions of strong and weak stratospheric Polar Vortex was performed on NCEP/NCAR reanalysis data from 1948 to 2013 considering, as additional grouping criteria, the coincidental states of major seasonally relevant climate phenomena, such as El Niño-Southern Oscillation (ENSO), Quasi Biennial Oscillation and strong volcanic eruptions. The analysis reveals that samples of strong Polar Vortex nearly exclusively occur during cold ENSO states, while a weak Polar Vortex is observed for both cold and warm ENSO. The strongest tropospheric and near-surface anomalies are found for warm ENSO and weak Polar Vortex conditions, suggesting that internal tropospheric circulation anomalies related to warm ENSO constructively superpose on dynamical effects from the stratosphere. Additionally, substantial differences are found between the continental winter warming patterns under strong Polar Vortex conditions in volcanically-disturbed and volcanically-undisturbed winters. However, the small-size samples obtained from the multi-compositing prevent conclusive statements about typical patterns, dominating effects and mechanisms of stratosphere-troposphere interaction on the seasonal time scale based on observational/reanalysis data alone. Hence, our analysis demonstrates that patterns derived from observational/reanalysis time series need to be taken with caution as they not always provide sufficiently robust constraints to the inferred mechanisms implicated with stratospheric Polar Vortex variability and its tropospheric and near-surface signature. Notwithstanding this argument, we propose a limited set of mechanisms that together may explain a relevant part of observed climate variability. These may serve to define future numerical model experiments minimizing the sample biases and, thus, improving process understanding.
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The North Atlantic variability structure, storm tracks, and precipitation depending on the Polar Vortex strength
Atmospheric Chemistry and Physics, 2005Co-Authors: Katrin Walter, Hans-f. GrafAbstract:Motivated by the strong evidence that the state of the northern hemisphere Vortex in boreal winter influences tropospheric variability, teleconnection patterns over the North Atlantic are defined separately for winter episodes where the zonal wind at 50hPa and 65° N is above or below the critical velocity for vertical propagation of zonal planetary wave 1. We argue that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the Polar Vortex, while this is not the case at sea level. If the Polar Vortex is strong, there exists one meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the Polar Vortex is weak, there exist two such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong Polar Vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results suggest that it is important to include the state of the Polar Vortex strength in any study of the variability over the North Atlantic.
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Polar Vortex controls coupling of North Atlantic Ocean and atmosphere
Geophysical Research Letters, 2005Co-Authors: Hans-f. Graf, Katrin WalterAbstract:[1] The structure of the North Atlantic leading atmospheric winter variability mode strongly depends on the state of the Polar stratospheric Vortex. If the Polar Vortex is strong, one teleconnection pattern emerges in the upper troposphere, while two mostly independent ones appear when the Vortex is weak. The anomaly patterns associated with the different Polarities of these modes show strong differences in the wind fields and in the correlation of atmospheric variability with the sea surface temperature of the North Atlantic. Only when the Polar Vortex is strong, does a basin-wide tripole correlation pattern exist between tropospheric variability and sea surface temperature. Under weak Vortex conditions one of the variability modes correlates with the subtropical, the other with the subPolar gyre. These results suggest that a NAO index based on near surface pressure that fails to account for the state of the Polar Vortex is a suboptimal representation of the tropospheric circulation variability.
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The North Atlantic variability structure, storm tracks, and precipitation depending on the Polar Vortex strength
Atmospheric Chemistry and Physics Discussions, 2004Co-Authors: Katrin Walter, Hans-f. GrafAbstract:There is ample evidence that the state of the northern Polar stratospheric Vortex in boreal winter influences tropospheric variability. Therefore, the main teleconnection patterns over the North Atlantic are defined separately for winter episodes in which the zonal mean wind at 50 hPa and 65° N is above or below the critical Rossby velocity for zonal planetary wave one. It turns out that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the Polar Vortex, while this is not the case at sea level. If the "Polar Vortex is strong", there exists "one" meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the "Polar Vortex is weak", there exist "two" such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong Polar Vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results show that it is essential to include the state of the upper dynamic boundary conditions (the Polar Vortex strength) in any study of the variability over the North Atlantic. Climate forecast based solely on the forecast of a "classic NAO" and further statistical downscaling may lead to the wrong conclusions if the state of the Polar Vortex is not considered as well.
Nicholas A Teanby - One of the best experts on this subject based on the ideXlab platform.
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The formation and evolution of Titan’s winter Polar Vortex
Nature Communications, 2017Co-Authors: Nicholas A Teanby, Bruno Bézard, Sandrine Vinatier, Melody Sylvestre, Remco De Kok, Simon Calcutt, Conor Nixon, Patrick Irwin, F. M. FlasarAbstract:Saturn's largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter Polar vortices. Following Titan's 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-Polar subsidence and winter Polar Vortex formation. Throughout 2010-2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the Polar Vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2-6 years post-equinox. The long time-frame to reach a stable Vortex configuration results from the high infrared opacity of Titan's trace gases and the relatively long atmospheric radiative time constant. Winter Polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan.
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The formation and evolution of Titan's winter Polar Vortex.
Nature communications, 2017Co-Authors: Nicholas A Teanby, Bruno Bézard, Sandrine Vinatier, Melody Sylvestre, Conor A. Nixon, Patrick G. J. Irwin, Remco De Kok, Simon Calcutt, F. M. FlasarAbstract:Saturn’s largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter Polar vortices. Following Titan’s 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-Polar subsidence and winter Polar Vortex formation. Throughout 2010–2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the Polar Vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2–6 years post-equinox. The long time-frame to reach a stable Vortex configuration results from the high infrared opacity of Titan’s trace gases and the relatively long atmospheric radiative time constant. Winter Polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan.
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The formation and evolution of Titan’s winter Polar Vortex
Nature Communications, 2017Co-Authors: Nicholas A Teanby, Bruno Bézard, Sandrine Vinatier, Melody Sylvestre, Conor A. Nixon, Patrick G. J. Irwin, Simon Calcutt, F. M. FlasarAbstract:Saturn’s largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter Polar vortices. Following Titan’s 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-Polar subsidence and winter Polar Vortex formation. Throughout 2010–2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the Polar Vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2–6 years post-equinox. The long time-frame to reach a stable Vortex configuration results from the high infrared opacity of Titan’s trace gases and the relatively long atmospheric radiative time constant. Winter Polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan. The Polar hot-spot appeared in Titan after equinox in 2010 suddenly cooled in early 2012, which wasn’t predicted by models. Here the authors use observations to show that the increase in trace gases during the hot-spot resulted in radiative cooling feedback.
Katrin Walter - One of the best experts on this subject based on the ideXlab platform.
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The North Atlantic variability structure, storm tracks, and precipitation depending on the Polar Vortex strength
Atmospheric Chemistry and Physics, 2005Co-Authors: Katrin Walter, Hans-f. GrafAbstract:Motivated by the strong evidence that the state of the northern hemisphere Vortex in boreal winter influences tropospheric variability, teleconnection patterns over the North Atlantic are defined separately for winter episodes where the zonal wind at 50hPa and 65° N is above or below the critical velocity for vertical propagation of zonal planetary wave 1. We argue that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the Polar Vortex, while this is not the case at sea level. If the Polar Vortex is strong, there exists one meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the Polar Vortex is weak, there exist two such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong Polar Vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results suggest that it is important to include the state of the Polar Vortex strength in any study of the variability over the North Atlantic.
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Polar Vortex controls coupling of North Atlantic Ocean and atmosphere
Geophysical Research Letters, 2005Co-Authors: Hans-f. Graf, Katrin WalterAbstract:[1] The structure of the North Atlantic leading atmospheric winter variability mode strongly depends on the state of the Polar stratospheric Vortex. If the Polar Vortex is strong, one teleconnection pattern emerges in the upper troposphere, while two mostly independent ones appear when the Vortex is weak. The anomaly patterns associated with the different Polarities of these modes show strong differences in the wind fields and in the correlation of atmospheric variability with the sea surface temperature of the North Atlantic. Only when the Polar Vortex is strong, does a basin-wide tripole correlation pattern exist between tropospheric variability and sea surface temperature. Under weak Vortex conditions one of the variability modes correlates with the subtropical, the other with the subPolar gyre. These results suggest that a NAO index based on near surface pressure that fails to account for the state of the Polar Vortex is a suboptimal representation of the tropospheric circulation variability.
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The North Atlantic variability structure, storm tracks, and precipitation depending on the Polar Vortex strength
Atmospheric Chemistry and Physics Discussions, 2004Co-Authors: Katrin Walter, Hans-f. GrafAbstract:There is ample evidence that the state of the northern Polar stratospheric Vortex in boreal winter influences tropospheric variability. Therefore, the main teleconnection patterns over the North Atlantic are defined separately for winter episodes in which the zonal mean wind at 50 hPa and 65° N is above or below the critical Rossby velocity for zonal planetary wave one. It turns out that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the Polar Vortex, while this is not the case at sea level. If the "Polar Vortex is strong", there exists "one" meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the "Polar Vortex is weak", there exist "two" such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong Polar Vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results show that it is essential to include the state of the upper dynamic boundary conditions (the Polar Vortex strength) in any study of the variability over the North Atlantic. Climate forecast based solely on the forecast of a "classic NAO" and further statistical downscaling may lead to the wrong conclusions if the state of the Polar Vortex is not considered as well.
Gunter Stober - One of the best experts on this subject based on the ideXlab platform.
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the extraordinarily strong and cold Polar Vortex in the early northern winter 2015 2016
Geophysical Research Letters, 2016Co-Authors: Vivien Matthias, Andreas Dörnbrack, Gunter StoberAbstract:The Arctic Polar Vortex in the early winter 2015/16 was the strongest and coldest o f the la st 68 years. Using global reanalysis data, satellite observations, and mesospheric radar wind measurements over northern Scandinavia we investigate the characteristics of the early sta g e Polar Vortex and relate them to previous winters. We found a correlation between the planetary wave (PW) activity and the strength and temperature of the northern Polar vor- tex in the stratosphere and mesosphere. In Nov/D ec 2015, a reduced PW generation in the troposphere and a stronger PW filtering in the troposphere and stratosphere, caused by stronger zonal winds in mid-latitudes, resulted in a stronger Polar Vortex. Thi s effect was strengthened by the equator ward shift of PWs due to the strong zonal wind in Polar latitudes resulting in a southward shift of the Eliassen-Palm flux divergence and hence inducinga decreased deceleration of th e Polar Vortex by PWs.
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The extraordinarily strong and cold Polar Vortex in the early northern winter 2015/2016
Geophysical Research Letters, 2016Co-Authors: Vivien Matthias, Andreas Dörnbrack, Gunter StoberAbstract:The Arctic Polar Vortex in the early winter 2015/16 was the strongest and coldest o f the la st 68 years. Using global reanalysis data, satellite observations, and mesospheric radar wind measurements over northern Scandinavia we investigate the characteristics of the early sta g e Polar Vortex and relate them to previous winters. We found a correlation between the planetary wave (PW) activity and the strength and temperature of the northern Polar vor- tex in the stratosphere and mesosphere. In Nov/D ec 2015, a reduced PW generation in the troposphere and a stronger PW filtering in the troposphere and stratosphere, caused by stronger zonal winds in mid-latitudes, resulted in a stronger Polar Vortex. Thi s effect was strengthened by the equator ward shift of PWs due to the strong zonal wind in Polar latitudes resulting in a southward shift of the Eliassen-Palm flux divergence and hence inducinga decreased deceleration of th e Polar Vortex by PWs.