Landmasses cover a large portion of the northern hemisphere, and nearly one-half of Eurasia and N... more Landmasses cover a large portion of the northern hemisphere, and nearly one-half of Eurasia and North America are extensively covered with snow in the cold season (Dery and Brown, 2007). Snowcovered land plays a key role in the climate system, owing to the snow radiative and thermodynamical properties, such as high albedo, high emissivity and low thermal conductivity, and its effect on surface fluxes of moisture and heat. Snow covered land can hence impact climate in a variety of ways. The snow-albedo feedback plays an important role in the spring (e.g. Schlosser and Mocko, 2003) when an early seasonal retreat of the snow cover acts as a positive feedback on spring temperatures. However, the climate response to high-latitude snow cover could also involve thermodynamical feedbacks in the surface energy balance, as well as large-scale dynamical feedbacks. Eastern Eurasia for example, is a region where the Asian jet establishes a near-zonal waveguide for propagating Rossby waves arisin...
Eastward-propagating planetary waves (EPWs) were investigated prior to the boreal January 2009 ma... more Eastward-propagating planetary waves (EPWs) were investigated prior to the boreal January 2009 major sudden stratospheric warming (SSW) event simulated by the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model with specified dynamics. About 22 days before SSW onset, a background flow with jet maxima around the upper polar stratosphere and subtropical mesosphere developed due to the net forcing by gravity and planetary waves. The mesospheric wind structure was largely unstable and supported a wave geometry conducive to overreflection. With a zonal phase speed of ∼10 m s−1, EPWs appeared near their turning and critical layers as wavenumber-2 perturbations in the stratosphere and mesosphere. Accompanied by upward EPW activity from the lower stratosphere, EPW growth exhibited characteristics of wave instability and overreflection
Journal of Atmospheric and Solar-Terrestrial Physics, 2020
Energetic particle precipitation is one of the main processes by which the sun influences atmosph... more Energetic particle precipitation is one of the main processes by which the sun influences atmospheric composition and structure. The polar middle atmosphere is chemically disturbed by the precipitation-induced production of nitric oxides (NO x) and hydrogen oxides (HO x) and the associated ozone (O 3) loss, but the importance for the dynamics is still debated. The role of precipitating medium energy electrons (MEEs), which are able to penetrate into the mesosphere, has received increased attention, but has only recently begun to be incorporated in chemistry-climate models. We use the NCAR Whole Atmosphere Community Climate Model (WACCM) to study the climate impact from MEE precipitation by performing two idealized ensemble experiments under preindustrial conditions, with and without the MEE forcing, over the period of the solar cycle 23 (only full calendar years, 1997-2007). Each experiment includes 20 11-year ensemble members, total 220 years. Our results indicate a strong month-to-month variability in the dynamical response to MEE throughout the winter period. We find a strengthening of the polar vortex in the northern hemisphere during December, but the signal decays rapidly in the following months. The polar vortex strengthening is likely attributable to planetary wave reduction due to increased zonal symmetries in upper stratospheric ozone heating, initially triggered by MEE-induced NO x advected into the sunlit regions. We also find a similar early winter polar vortex strengthening in the southern hemisphere during June. Changes in mean meridional circulation accompany these anomalous wave forcings, leading to dynamically-induced vertical temperature dipoles at high latitudes. The associated weakening of the stratospheric mean meridional circulation results in an upper stratospheric polar ozone deficit in early winter. This polar cap ozone deficit is strongest in the southern hemisphere and contributes to a polar vortex weakening in late winter, in concert with increased planetary wave forcing. In both hemispheres, the stratospheric polar vortex signal seems to migrate downwards into the troposphere and to the surface.
Journal of Atmospheric and Solar-Terrestrial Physics, 2018
Sudden Stratospheric Warmings (SSW) affect the chemistry and dynamics of the middle atmosphere. M... more Sudden Stratospheric Warmings (SSW) affect the chemistry and dynamics of the middle atmosphere. Major warmings occur roughly every second winter in the Northern Hemisphere (NH), but has only been observed once in the Southern Hemisphere (SH), during the Antarctic winter of 2002. Observations by the Global Ozone Monitoring by Occultation of Stars (GOMOS, an instrument on board Envisat) during this rare event, show a 40% increase of ozone in the nighttime secondary ozone layer at subpolar latitudes compared to non-SSW years. This study investigates the cause of the mesospheric nighttime ozone increase, using the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model with specified dynamics (SD-WACCM). The 2002 SH winter was characterized by several reductions of the strength of the polar night jet in the upper stratosphere before the jet reversed completely, marking the onset of the major SSW. At the time of these wind reductions, corresponding episodic increases can be seen in the modelled nighttime secondary ozone layer. This ozone increase is attributed largely to enhanced upwelling and the associated cooling of the altitude region in conjunction with the wind reversal. This is in correspondence to similar studies of SSW induced ozone enhancements in NH. But unlike its NH counterpart, the SH secondary ozone layer appeared to be impacted less by episodic variations in atomic hydrogen. Seasonally decreasing atomic hydrogen plays however a larger role in SH compared to NH.
Journal of Geophysical Research: Atmospheres, 2019
Using meteor wind data from the Super Dual Auroral Radar Network (SuperDARN) in the Northern Hemi... more Using meteor wind data from the Super Dual Auroral Radar Network (SuperDARN) in the Northern Hemisphere, we (1) demonstrate that the migrating (Sun‐synchronous) tides can be separated from the nonmigrating components in the mesosphere and lower thermosphere (MLT) region and (2) use this to determine the response of the different components of the semidiurnal tide (SDT) to sudden stratospheric warming (SSW) conditions. The radars span a limited range of latitudes around 60°N and are located over nearly 180° of longitude. The migrating tide is extracted from the nonmigrating components observed in the meridional wind recorded from meteor ablation drift velocities around 95‐km altitude, and a 20‐year climatology of the different components is presented. The well‐documented late summer and wintertime maxima in the semidiurnal winds are shown to be due primarily to the migrating SDT, whereas during late autumn and spring the nonmigrating components are at least as strong as the migrating...
Journal of Atmospheric and Solar-Terrestrial Physics, 2017
Using simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analys... more Using simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analyses, global satellite observations of nitrogen oxide (NO) and water vapour by the Sub-Millimetre Radiometer instrument (SMR), of temperature by the Microwave Limb Sounder (MLS), as well as local radar observations, this study examines the recent major stratospheric sudden warming accompanied by an elevated stratopause event (ESE) that occurred in January 2013. We examine dynamical processes during the ESE, including the role of planetary wave, gravity wave and tidal forcing on the initiation of the descent in the mesosphere-lower thermosphere (MLT) and its continuation throughout the mesosphere and stratosphere, as well as the impact of model eddy diffusion. We analyse the transport of NO and find the model underestimates the large descent of NO compared to SMR observations. We demonstrate that the discrepancy arises abruptly in the MLT region at a time when the resolved wave forcing and the planetary wave activity increase, just before the elevated stratopause reforms. The discrepancy persists despite doubling the model eddy diffusion. While the simulations reproduce an enhancement of the semi-diurnal tide following the onset of the 2013 SSW, corroborating new meteor radar observations at high northern latitudes over Trondheim (63.4°N), the modelled tidal contribution to the forcing of the mean meridional circulation and to the descent is a small portion of the resolved wave forcing, and lags it by about ten days.
Atmospheric Chemistry and Physics Discussions, 2016
A Sudden Stratospheric Warming (SSW) affects the chemistry and dynamics of the middle atmosphere.... more A Sudden Stratospheric Warming (SSW) affects the chemistry and dynamics of the middle atmosphere. The major warmings occur roughly every second year in the Northern Hemispheric (NH) winter, but has only been observed once in the Southern Hemisphere (SH), during the Antarctic winter of 2002. Using the National Center for Atmospheric Research's (NCAR) Whole Atmosphere Community Climate Model with specified dynamics (WACCM-SD), this study investigates the effects of this rare warming event on the ozone layer located around the SH mesopause. This secondary ozone layer changes with respect to hydrogen, oxygen, temperature, and the altered SH polar circulation during the major SSW. The 2002 SH winter was characterized by three zonal-mean zonal wind reductions in the upper stratosphere before a fourth wind reversal reaches the lower stratosphere, marking the onset of the major SSW. At the time of these four wind reversals, a corresponding episodic increase can be seen in the modeled ni...
This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW)... more This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere-lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150 • W to 25 • E and latitudes from 51 to 66 • N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000-2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal.
ABSTRACT The primary modes of wintertime variability, such as the North Atlantic Oscillation (NAO... more ABSTRACT The primary modes of wintertime variability, such as the North Atlantic Oscillation (NAO) have a marked signature on the ozone layer. The recent realisation that the NAO is part of a more global pattern, termed the Arctic Oscillation (AO), which extends from the surface upwards into the stratosphere, highlighted the coupling of the NAO with the stratosphere and the ozone layer. We examine nearly 20 years of global TOMS observations, and diagnose the signatures of the leading patterns of climatic variability upon total ozone. Our main emphasis is on the E uropean and Atlantic sector. These signatures are twofold. We seek for influences on both the seasonal-mean (quasi-stationary) ozone, and else on the fast transient (eddy) ozone variability linked to passing weather systems. We hence systematically examine how leading climate patterns influence seasonal-mean ozone in winter and spring, through a statistical analysis using empirical orthogonal functions. In addition, we carried out an analysis of ozone synoptic variability. Satellite column ozone observations indicate a strong signature of storm tracks in ozone, with marked asymm etries between the Pacific and Atlantic storm tracks. Of particular interest are the large amplitude ozone "mini-hole" events that frequently develop over the Atlantic in winter. We examine the relationship between the occurrences of such ozone minihole events and the NAO or other climate patterns.
ABSTRACT Using the newly analyzed mesospheric water vapor and temperature observations from the S... more ABSTRACT Using the newly analyzed mesospheric water vapor and temperature observations from the SMR microwave instrument aboard the Odin research satellite, we present evidence for an anomalously strong descent of dry mesospheric air from the lower mesosphere into the upper stratosphere in the late winter of 2004, 2006, and 2009. In these cases, the descent follows the recovery of the upper stratospheric polar vortex from a mid-winter stratospheric sudden warming. This downward progression is also accompanied by the rapid formation of an anomalously warm polar mesospheric layer, giving rise to an elevated stratopause (near 75km) and its eventual return to pre-warming level (near 1 hPa) over 1.5-2 months. These three winters stand out in the record of Odin/SMR observations spanning the period July 2001 to June 2009.
ABSTRACT Three-hourly WACCM simulations are used to characterize a composite life cycle of the ma... more ABSTRACT Three-hourly WACCM simulations are used to characterize a composite life cycle of the major Stratospheric Sudden Warming (SSW) event from the surface up to 130 km. The life cycle reveals the intimate coupling between the stratosphere and mesosphere, in addition to the stratosphere-troposphere connection much studied in the past. During the SSW onset, the polar zonal-mean wind reversal occurs at nearly all model levels: eastward to westward direction below 80 km and vice versa above. Concurrent anomalous warming appears throughout the stratosphere and above 90 km with cooling in the intervening mesospheric layer, consistent with stratospheric and thermospheric downwelling as well as mesospheric upwelling. As SSW matures, these anomalous patterns migrate downward in time greatly affecting the distributions of polar NOx and local ozone maxima layers in ways observed by satellites. We further analyze the behavior of planetary waves and the variability in the secondary ozone maximum, associated with the abrupt reformation of a high-altitude stratopause in the polar mesosphere.
Quarterly Journal of the Royal Meteorological Society, 1997
SUMMARK A series of lower stratospheric ozone profiles taken by a lidar during February 1995 in n... more SUMMARK A series of lower stratospheric ozone profiles taken by a lidar during February 1995 in northern Norway are examined. The instrument location allowed continuous monitoring of rapidly evolving layered ozone structures, or laminae, near the polar vortex edge. Observations under the vortex edge revealed laminae extending up to 20 km. Moreover, when the lidar was sampling vortex air, a thicker layer, characterized by unusually low mixing ratios, was also observed, thereby confirming satellite observations of Arctic ozone depletion during the winter 1994/95. To unravel the respective role of dynamics and chemistry in generating these ozone-depleted layers requires better understanding of trace constituent lamination and filamentation through numerical modelling. High-resolution modelled realistic transport of idealized tracers and satellite-derived ozone on many isentropes is therefore used to reconstruct fine-scale three-dimensional tracer fields. Tilted tracer or ozone sheets, peeled off near the vortex edge, lead to the formation of laminae in profiles. The model shows remarkable success in reproducing laminae intensification, thickening and lofting, analogous to the ones observed by the lidar over the course of a few hours.
Journal of the Meteorological Society of Japan. Ser. II, 2008
Using an ensemble of wintertime hindcasts with a high-resolution (T106L60) Atmospheric General Ci... more Using an ensemble of wintertime hindcasts with a high-resolution (T106L60) Atmospheric General Circulation Model (AGCM) forced by observed sea surface temperatures (SSTs) and extending into the stratosphere, we investigate the formation and lifecycle of the Aleutian-Icelandic low Seesaw (AIS) during the 1978 to 1993 period. The AIS has been newly proposed to be an important mode of variability, linking the major wintertime surface lows, the Icelandic Low and the Aleutian Low, in late winter, and thereby linking climate variability over the North Pacific and the North Atlantic. We demonstrate for the first time with a stratosphere-troposphere model, that a coherent, ensemble-mean AIS extension into the stratosphere exists, where its presence modulates ultra-long planetary wave propagation and the polar night jet intensity. The model AIS peaks in February, when the Aleutian and Icelandic Low anti-correlation maximizes at −0.59. The AIS provides a new way to describe the El Niño-Southern Oscillation (ENSO) phenomenon influence into the stratosphere. For example, El-Nino conditions correspond to a deeper than normal Aleutian Low, extending its influence into the Icelandic sector as an AIS negative phase (weakened Icelandic Low), hence enhanced planetary wave vertical propagation and a weakened stratospheric polar vortex. This maturation of the AIS in late winter explains the intra-seasonal variability of the stratospheric response to ENSO, which peaks in late winter. Internal model variability is large and enhanced potential predictability is found primarily in the western hemisphere, with a western Atlantic maxima being more pronounced in the stratosphere than in the upper troposphere.
The two-day wave is observed in the Upper Atmosphere Research Satellite Microwave Limb Sounder te... more The two-day wave is observed in the Upper Atmosphere Research Satellite Microwave Limb Sounder temperature data around 40-58 km. Between December 1991 and September 1994, the two-day wave temperature signature is most significant after each solstice when the derived easterly winds near the stratopause extend across the equator to at least 20Њ latitude in the winter hemisphere, and the zonal mean winds near the equator are inertially unstable with observed inertial instability disturbances. The observed two-day wave consists of a 2.0-day period zonal wavenumber-3 and a 1.8-day period zonal wavenumber-4 component, named (3, 2.0) and (4, 1.8), respectively. The (3, 2.0) component is dominant during two of the three available austral summers, but its amplitude is much weaker than the (4, 1.8) component during the two observed boreal summers. During the austral summers, correspondence between amplification of the two-day wave temperature signatures, regions of reversed potential vorticity gradient due to meridional curvature of the zonal mean flow, and the critical lines for the (3, 2.0) and (4, 1.8) modes suggest barotropic instability as a source of both wave components. Momentum redistribution by observed inertial instability appears to barotropically destabilize the equatorward flank of the easterly jet where the wave components subsequently grow. During the boreal summers, the (4, 1.8) component appears to be excited by instability that is associated with vertical shear and curvature of the flow seated above the observational domain. The boreal (3, 2.0) mode appears unrelated to the zonal flow instability within the observational domain and may reflect a normal-mode-like response during these periods.
Using newly analyzed mesospheric water vapor and temperature observations from the Sub-Millimeter... more Using newly analyzed mesospheric water vapor and temperature observations from the Sub-Millimeter Radiometer instrument aboard the Odin research satellite over the period 2001-2009, we present evidence for an anomalously strong descent of dry mesospheric air from the lower mesosphere into the upper stratosphere in the winters of 2004, 2006, and 2009. In the three cases, the descent follows the recovery of the upper stratospheric polar vortex from a major midwinter stratospheric sudden warming. It is also accompanied by the rapid formation of an anomalously warm polar mesospheric layer, i.e., an elevated polar stratopause, near 75 km, and its slower descent to prewarming level (near 1 hPa) over 1.5-2 months. These three winters stand out in the current record of Odin/Sub-Millimeter Radiometer observations started in July 2001.
Landmasses cover a large portion of the northern hemisphere, and nearly one-half of Eurasia and N... more Landmasses cover a large portion of the northern hemisphere, and nearly one-half of Eurasia and North America are extensively covered with snow in the cold season (Dery and Brown, 2007). Snowcovered land plays a key role in the climate system, owing to the snow radiative and thermodynamical properties, such as high albedo, high emissivity and low thermal conductivity, and its effect on surface fluxes of moisture and heat. Snow covered land can hence impact climate in a variety of ways. The snow-albedo feedback plays an important role in the spring (e.g. Schlosser and Mocko, 2003) when an early seasonal retreat of the snow cover acts as a positive feedback on spring temperatures. However, the climate response to high-latitude snow cover could also involve thermodynamical feedbacks in the surface energy balance, as well as large-scale dynamical feedbacks. Eastern Eurasia for example, is a region where the Asian jet establishes a near-zonal waveguide for propagating Rossby waves arisin...
Eastward-propagating planetary waves (EPWs) were investigated prior to the boreal January 2009 ma... more Eastward-propagating planetary waves (EPWs) were investigated prior to the boreal January 2009 major sudden stratospheric warming (SSW) event simulated by the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model with specified dynamics. About 22 days before SSW onset, a background flow with jet maxima around the upper polar stratosphere and subtropical mesosphere developed due to the net forcing by gravity and planetary waves. The mesospheric wind structure was largely unstable and supported a wave geometry conducive to overreflection. With a zonal phase speed of ∼10 m s−1, EPWs appeared near their turning and critical layers as wavenumber-2 perturbations in the stratosphere and mesosphere. Accompanied by upward EPW activity from the lower stratosphere, EPW growth exhibited characteristics of wave instability and overreflection
Journal of Atmospheric and Solar-Terrestrial Physics, 2020
Energetic particle precipitation is one of the main processes by which the sun influences atmosph... more Energetic particle precipitation is one of the main processes by which the sun influences atmospheric composition and structure. The polar middle atmosphere is chemically disturbed by the precipitation-induced production of nitric oxides (NO x) and hydrogen oxides (HO x) and the associated ozone (O 3) loss, but the importance for the dynamics is still debated. The role of precipitating medium energy electrons (MEEs), which are able to penetrate into the mesosphere, has received increased attention, but has only recently begun to be incorporated in chemistry-climate models. We use the NCAR Whole Atmosphere Community Climate Model (WACCM) to study the climate impact from MEE precipitation by performing two idealized ensemble experiments under preindustrial conditions, with and without the MEE forcing, over the period of the solar cycle 23 (only full calendar years, 1997-2007). Each experiment includes 20 11-year ensemble members, total 220 years. Our results indicate a strong month-to-month variability in the dynamical response to MEE throughout the winter period. We find a strengthening of the polar vortex in the northern hemisphere during December, but the signal decays rapidly in the following months. The polar vortex strengthening is likely attributable to planetary wave reduction due to increased zonal symmetries in upper stratospheric ozone heating, initially triggered by MEE-induced NO x advected into the sunlit regions. We also find a similar early winter polar vortex strengthening in the southern hemisphere during June. Changes in mean meridional circulation accompany these anomalous wave forcings, leading to dynamically-induced vertical temperature dipoles at high latitudes. The associated weakening of the stratospheric mean meridional circulation results in an upper stratospheric polar ozone deficit in early winter. This polar cap ozone deficit is strongest in the southern hemisphere and contributes to a polar vortex weakening in late winter, in concert with increased planetary wave forcing. In both hemispheres, the stratospheric polar vortex signal seems to migrate downwards into the troposphere and to the surface.
Journal of Atmospheric and Solar-Terrestrial Physics, 2018
Sudden Stratospheric Warmings (SSW) affect the chemistry and dynamics of the middle atmosphere. M... more Sudden Stratospheric Warmings (SSW) affect the chemistry and dynamics of the middle atmosphere. Major warmings occur roughly every second winter in the Northern Hemisphere (NH), but has only been observed once in the Southern Hemisphere (SH), during the Antarctic winter of 2002. Observations by the Global Ozone Monitoring by Occultation of Stars (GOMOS, an instrument on board Envisat) during this rare event, show a 40% increase of ozone in the nighttime secondary ozone layer at subpolar latitudes compared to non-SSW years. This study investigates the cause of the mesospheric nighttime ozone increase, using the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model with specified dynamics (SD-WACCM). The 2002 SH winter was characterized by several reductions of the strength of the polar night jet in the upper stratosphere before the jet reversed completely, marking the onset of the major SSW. At the time of these wind reductions, corresponding episodic increases can be seen in the modelled nighttime secondary ozone layer. This ozone increase is attributed largely to enhanced upwelling and the associated cooling of the altitude region in conjunction with the wind reversal. This is in correspondence to similar studies of SSW induced ozone enhancements in NH. But unlike its NH counterpart, the SH secondary ozone layer appeared to be impacted less by episodic variations in atomic hydrogen. Seasonally decreasing atomic hydrogen plays however a larger role in SH compared to NH.
Journal of Geophysical Research: Atmospheres, 2019
Using meteor wind data from the Super Dual Auroral Radar Network (SuperDARN) in the Northern Hemi... more Using meteor wind data from the Super Dual Auroral Radar Network (SuperDARN) in the Northern Hemisphere, we (1) demonstrate that the migrating (Sun‐synchronous) tides can be separated from the nonmigrating components in the mesosphere and lower thermosphere (MLT) region and (2) use this to determine the response of the different components of the semidiurnal tide (SDT) to sudden stratospheric warming (SSW) conditions. The radars span a limited range of latitudes around 60°N and are located over nearly 180° of longitude. The migrating tide is extracted from the nonmigrating components observed in the meridional wind recorded from meteor ablation drift velocities around 95‐km altitude, and a 20‐year climatology of the different components is presented. The well‐documented late summer and wintertime maxima in the semidiurnal winds are shown to be due primarily to the migrating SDT, whereas during late autumn and spring the nonmigrating components are at least as strong as the migrating...
Journal of Atmospheric and Solar-Terrestrial Physics, 2017
Using simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analys... more Using simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analyses, global satellite observations of nitrogen oxide (NO) and water vapour by the Sub-Millimetre Radiometer instrument (SMR), of temperature by the Microwave Limb Sounder (MLS), as well as local radar observations, this study examines the recent major stratospheric sudden warming accompanied by an elevated stratopause event (ESE) that occurred in January 2013. We examine dynamical processes during the ESE, including the role of planetary wave, gravity wave and tidal forcing on the initiation of the descent in the mesosphere-lower thermosphere (MLT) and its continuation throughout the mesosphere and stratosphere, as well as the impact of model eddy diffusion. We analyse the transport of NO and find the model underestimates the large descent of NO compared to SMR observations. We demonstrate that the discrepancy arises abruptly in the MLT region at a time when the resolved wave forcing and the planetary wave activity increase, just before the elevated stratopause reforms. The discrepancy persists despite doubling the model eddy diffusion. While the simulations reproduce an enhancement of the semi-diurnal tide following the onset of the 2013 SSW, corroborating new meteor radar observations at high northern latitudes over Trondheim (63.4°N), the modelled tidal contribution to the forcing of the mean meridional circulation and to the descent is a small portion of the resolved wave forcing, and lags it by about ten days.
Atmospheric Chemistry and Physics Discussions, 2016
A Sudden Stratospheric Warming (SSW) affects the chemistry and dynamics of the middle atmosphere.... more A Sudden Stratospheric Warming (SSW) affects the chemistry and dynamics of the middle atmosphere. The major warmings occur roughly every second year in the Northern Hemispheric (NH) winter, but has only been observed once in the Southern Hemisphere (SH), during the Antarctic winter of 2002. Using the National Center for Atmospheric Research's (NCAR) Whole Atmosphere Community Climate Model with specified dynamics (WACCM-SD), this study investigates the effects of this rare warming event on the ozone layer located around the SH mesopause. This secondary ozone layer changes with respect to hydrogen, oxygen, temperature, and the altered SH polar circulation during the major SSW. The 2002 SH winter was characterized by three zonal-mean zonal wind reductions in the upper stratosphere before a fourth wind reversal reaches the lower stratosphere, marking the onset of the major SSW. At the time of these four wind reversals, a corresponding episodic increase can be seen in the modeled ni...
This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW)... more This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere-lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150 • W to 25 • E and latitudes from 51 to 66 • N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000-2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal.
ABSTRACT The primary modes of wintertime variability, such as the North Atlantic Oscillation (NAO... more ABSTRACT The primary modes of wintertime variability, such as the North Atlantic Oscillation (NAO) have a marked signature on the ozone layer. The recent realisation that the NAO is part of a more global pattern, termed the Arctic Oscillation (AO), which extends from the surface upwards into the stratosphere, highlighted the coupling of the NAO with the stratosphere and the ozone layer. We examine nearly 20 years of global TOMS observations, and diagnose the signatures of the leading patterns of climatic variability upon total ozone. Our main emphasis is on the E uropean and Atlantic sector. These signatures are twofold. We seek for influences on both the seasonal-mean (quasi-stationary) ozone, and else on the fast transient (eddy) ozone variability linked to passing weather systems. We hence systematically examine how leading climate patterns influence seasonal-mean ozone in winter and spring, through a statistical analysis using empirical orthogonal functions. In addition, we carried out an analysis of ozone synoptic variability. Satellite column ozone observations indicate a strong signature of storm tracks in ozone, with marked asymm etries between the Pacific and Atlantic storm tracks. Of particular interest are the large amplitude ozone "mini-hole" events that frequently develop over the Atlantic in winter. We examine the relationship between the occurrences of such ozone minihole events and the NAO or other climate patterns.
ABSTRACT Using the newly analyzed mesospheric water vapor and temperature observations from the S... more ABSTRACT Using the newly analyzed mesospheric water vapor and temperature observations from the SMR microwave instrument aboard the Odin research satellite, we present evidence for an anomalously strong descent of dry mesospheric air from the lower mesosphere into the upper stratosphere in the late winter of 2004, 2006, and 2009. In these cases, the descent follows the recovery of the upper stratospheric polar vortex from a mid-winter stratospheric sudden warming. This downward progression is also accompanied by the rapid formation of an anomalously warm polar mesospheric layer, giving rise to an elevated stratopause (near 75km) and its eventual return to pre-warming level (near 1 hPa) over 1.5-2 months. These three winters stand out in the record of Odin/SMR observations spanning the period July 2001 to June 2009.
ABSTRACT Three-hourly WACCM simulations are used to characterize a composite life cycle of the ma... more ABSTRACT Three-hourly WACCM simulations are used to characterize a composite life cycle of the major Stratospheric Sudden Warming (SSW) event from the surface up to 130 km. The life cycle reveals the intimate coupling between the stratosphere and mesosphere, in addition to the stratosphere-troposphere connection much studied in the past. During the SSW onset, the polar zonal-mean wind reversal occurs at nearly all model levels: eastward to westward direction below 80 km and vice versa above. Concurrent anomalous warming appears throughout the stratosphere and above 90 km with cooling in the intervening mesospheric layer, consistent with stratospheric and thermospheric downwelling as well as mesospheric upwelling. As SSW matures, these anomalous patterns migrate downward in time greatly affecting the distributions of polar NOx and local ozone maxima layers in ways observed by satellites. We further analyze the behavior of planetary waves and the variability in the secondary ozone maximum, associated with the abrupt reformation of a high-altitude stratopause in the polar mesosphere.
Quarterly Journal of the Royal Meteorological Society, 1997
SUMMARK A series of lower stratospheric ozone profiles taken by a lidar during February 1995 in n... more SUMMARK A series of lower stratospheric ozone profiles taken by a lidar during February 1995 in northern Norway are examined. The instrument location allowed continuous monitoring of rapidly evolving layered ozone structures, or laminae, near the polar vortex edge. Observations under the vortex edge revealed laminae extending up to 20 km. Moreover, when the lidar was sampling vortex air, a thicker layer, characterized by unusually low mixing ratios, was also observed, thereby confirming satellite observations of Arctic ozone depletion during the winter 1994/95. To unravel the respective role of dynamics and chemistry in generating these ozone-depleted layers requires better understanding of trace constituent lamination and filamentation through numerical modelling. High-resolution modelled realistic transport of idealized tracers and satellite-derived ozone on many isentropes is therefore used to reconstruct fine-scale three-dimensional tracer fields. Tilted tracer or ozone sheets, peeled off near the vortex edge, lead to the formation of laminae in profiles. The model shows remarkable success in reproducing laminae intensification, thickening and lofting, analogous to the ones observed by the lidar over the course of a few hours.
Journal of the Meteorological Society of Japan. Ser. II, 2008
Using an ensemble of wintertime hindcasts with a high-resolution (T106L60) Atmospheric General Ci... more Using an ensemble of wintertime hindcasts with a high-resolution (T106L60) Atmospheric General Circulation Model (AGCM) forced by observed sea surface temperatures (SSTs) and extending into the stratosphere, we investigate the formation and lifecycle of the Aleutian-Icelandic low Seesaw (AIS) during the 1978 to 1993 period. The AIS has been newly proposed to be an important mode of variability, linking the major wintertime surface lows, the Icelandic Low and the Aleutian Low, in late winter, and thereby linking climate variability over the North Pacific and the North Atlantic. We demonstrate for the first time with a stratosphere-troposphere model, that a coherent, ensemble-mean AIS extension into the stratosphere exists, where its presence modulates ultra-long planetary wave propagation and the polar night jet intensity. The model AIS peaks in February, when the Aleutian and Icelandic Low anti-correlation maximizes at −0.59. The AIS provides a new way to describe the El Niño-Southern Oscillation (ENSO) phenomenon influence into the stratosphere. For example, El-Nino conditions correspond to a deeper than normal Aleutian Low, extending its influence into the Icelandic sector as an AIS negative phase (weakened Icelandic Low), hence enhanced planetary wave vertical propagation and a weakened stratospheric polar vortex. This maturation of the AIS in late winter explains the intra-seasonal variability of the stratospheric response to ENSO, which peaks in late winter. Internal model variability is large and enhanced potential predictability is found primarily in the western hemisphere, with a western Atlantic maxima being more pronounced in the stratosphere than in the upper troposphere.
The two-day wave is observed in the Upper Atmosphere Research Satellite Microwave Limb Sounder te... more The two-day wave is observed in the Upper Atmosphere Research Satellite Microwave Limb Sounder temperature data around 40-58 km. Between December 1991 and September 1994, the two-day wave temperature signature is most significant after each solstice when the derived easterly winds near the stratopause extend across the equator to at least 20Њ latitude in the winter hemisphere, and the zonal mean winds near the equator are inertially unstable with observed inertial instability disturbances. The observed two-day wave consists of a 2.0-day period zonal wavenumber-3 and a 1.8-day period zonal wavenumber-4 component, named (3, 2.0) and (4, 1.8), respectively. The (3, 2.0) component is dominant during two of the three available austral summers, but its amplitude is much weaker than the (4, 1.8) component during the two observed boreal summers. During the austral summers, correspondence between amplification of the two-day wave temperature signatures, regions of reversed potential vorticity gradient due to meridional curvature of the zonal mean flow, and the critical lines for the (3, 2.0) and (4, 1.8) modes suggest barotropic instability as a source of both wave components. Momentum redistribution by observed inertial instability appears to barotropically destabilize the equatorward flank of the easterly jet where the wave components subsequently grow. During the boreal summers, the (4, 1.8) component appears to be excited by instability that is associated with vertical shear and curvature of the flow seated above the observational domain. The boreal (3, 2.0) mode appears unrelated to the zonal flow instability within the observational domain and may reflect a normal-mode-like response during these periods.
Using newly analyzed mesospheric water vapor and temperature observations from the Sub-Millimeter... more Using newly analyzed mesospheric water vapor and temperature observations from the Sub-Millimeter Radiometer instrument aboard the Odin research satellite over the period 2001-2009, we present evidence for an anomalously strong descent of dry mesospheric air from the lower mesosphere into the upper stratosphere in the winters of 2004, 2006, and 2009. In the three cases, the descent follows the recovery of the upper stratospheric polar vortex from a major midwinter stratospheric sudden warming. It is also accompanied by the rapid formation of an anomalously warm polar mesospheric layer, i.e., an elevated polar stratopause, near 75 km, and its slower descent to prewarming level (near 1 hPa) over 1.5-2 months. These three winters stand out in the current record of Odin/Sub-Millimeter Radiometer observations started in July 2001.
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Papers by Yvan Orsolini