Aura Microwave Limb Sounder Observations Research Articles

Impact of Strong and Weak Stratospheric Polar Vortices on the Mesosphere and Lower Thermosphere

AbstractThe impact of strong and weak Northern Hemisphere stratospheric polar vortices on the zonal mean and atmospheric tides in the mesosphere and lower thermosphere (MLT) is investigated using Specified Dynamics Whole Atmosphere Community Climate Model (WACCM) with thermosphere‐ionosphere eXtension (SD‐WACCM‐X) simulations and Aura Microwave Limb Sounder observations. The observed and simulated zonal mean anomalies during strong and weak vortices are in good agreement. Zonal mean and tidal anomalies during weak polar vortex events are consistent with those known to occur during sudden stratospheric warmings. Generally opposite, though weaker, anomalies are found to occur when the stratospheric polar vortex is strong. The most notable impact in the MLT is a reduction in the migrating semidiurnal tide of ∼25%–35% during strong polar vortex times. The results demonstrate that, in addition to weak polar vortex events, a strong stratospheric polar vortex can also have an appreciable impact on the middle and upper atmosphere.

Impact of intensity variability of the Asian summer monsoon anticyclone on the chemical distribution in the upper troposphere and lower stratosphere

During the Asian summer monsoon (ASM) season, the process of stratosphere–troposphere exchange significantly affects the concentration and spatial distribution of chemical constituents in the upper troposphere and lower stratosphere (UTLS). However, the effect of the intensity of the Asian summer monsoon anticyclone (ASMA) on the horizontal distribution of chemical species within and around the ASMA, especially on the daily time scale, remains unclear. Here, the authors use the MERRA-2 reanalysis dataset and Aura Microwave Limb Sounder observations to study the impact of ASMA intensity on chemical distributions at 100 hPa during the ASM season. The intraseasonal variation of ASMA is classified into a strong period (SP) and weak period (WP), which refer to the periods when the intensity of ASMA remains strong and weak, respectively. The relatively low ozone (O3) region is found to be larger at 100 hPa during SPs, while its mixing ratio is lower than during WPs in summer. In June, analysis shows that the O3 horizontal distribution is mainly related to the intensity of AMSA, especially during SPs in June, while deep convections also impact the O3 horizontal distribution in July and August. These results indicate that the intraseasonal variation of the ASMA intensity coupled to deep convection can significantly affect the chemical distribution in the UTLS region during the ASM season.摘要亚洲夏季风期, 平流层–对流层物质交换过程能显著影响上对流层下平流层化学成分的浓度变化和空间分布. 然而, 亚洲夏季风反气旋强度的季节内变化对其内部和周围地区化学成分水平分布的影响尚不清楚. 本文将亚洲夏季风反气旋划分为季节内强周期和弱周期, 发现当亚洲夏季风反气旋更强时, 100 hPa O3低值区的面积更大, O3浓度更低. 但是这种影响主要体现在6月份, 7, 8月的O3水平分布还受东南亚地区深对流的影响. 这些结果表明亚洲夏季风反气旋强度和深对流的季节内变化可以显著影响亚洲夏季风期上对流层下平流层的化学分布.

Seasonal and interannual variations of water vapor in the upper troposphere and lower stratosphere over the Asian Summer Monsoon region- in perspective of the tropopause and ocean-atmosphere interactions

In this study, we examine distributions of water vapor in the upper troposphere and lower stratosphere (UTLS) over the Asian Summer Monsoon (ASM) anticyclone during summer using thirteen years of Aura Microwave Limb Sounder observations. We also present the role of tropical tropopause and ocean-atmosphere interactions in controlling the UTLS water vapor over the ASM anticyclone region. Water vapor accumulated inside the central part of ASM anticyclone is primarily transported from the low-latitude oceanic region and very less amount of water vapor is pumped directly from the lower to the upper troposphere. It is concluded water vapor in the UTLS region inside the central part of ASM anticyclone is mostly controlled by horizontal advection and very less from the local process and tropopause temperature in both summer and winter. The tropopause altitude is highest, and the pressure is lowest inside the central part of ASM anticyclone as compared to the other tropical ASM regions during summer. In addition, there is an increase of 1–1.3 ppmb water vapor along with the warm tropopause in the UTLS region during El-Niño events over the entire ASM region, however, more water vapor is observed over the southern (tropical) part of ASM region. These observations help in understanding the controlling factor for the distribution of water vapor in the UTLS water vapor in different seasons.

Increased humidity in the stratosphere as a possible factor of ozone destruction in the Arctic during the spring 2011 using Aura MLS observations

ABSTRACTAnalysis of data from Microwave Limb Sounder (MLS) on Aura satellite showed that the temperature in the stratosphere of the Arctic was much lower than normal during the entire period of January–March 2011 in the altitude range of 20–35 km. This had led to a considerable spread of polar stratospheric clouds (PSCs) that presumably formed most intensely in periods and at heights of minimal temperatures (maximal temperature drops below the PSC formation threshold). Water vapour and ozone deviations from multiyear averages are found to correlate strongly both between themselves and with deviations in the temperature. The largest ozone losses were observed in March, probably due to a photochemical release of chlorine that avoided deactivation in view of nitrogen deficit caused by denitrification. Presumably, elevated humidity in the stratosphere had raised the threshold of formation of PSCs that persisted until late March; this promoted the chlorine activation and, thereby, delayed the chlorine deactivation, resulting in an even higher level of overall ozone losses during March 2011.

Decline in Antarctic Ozone Depletion and Lower Stratospheric Chlorine Determined From Aura Microwave Limb Sounder Observations

AbstractAttribution of Antarctic ozone recovery to the Montreal protocol requires evidence that (1) Antarctic chlorine levels are declining and (2) there is a reduction in ozone depletion in response to a chlorine decline. We use Aura Microwave Limb Sounder measurements of O3, HCl, and N2O to demonstrate that inorganic chlorine (Cly) from 2013 to 2016 was 223 ± 93 parts per trillion lower in the Antarctic lower stratosphere than from 2004 to 2007 and that column ozone depletion declined in response. The mean Cly decline rate, ~0.8%/yr, agrees with the expected rate based on chlorofluorocarbon lifetimes. N2O measurements are crucial for identifying changes in stratospheric Cly loading independent of dynamical variability. From 2005 to 2016, the ozone depletion and Cly time series show matching periods of decline, stability, and increase. The observed sensitivity of O3 depletion to changing Cly agrees with the sensitivity simulated by the Global Modeling Initiative chemistry transport model integrated with Modern Era Retrospective Analysis for Research and Applications 2 meteorology.

Radiative Effects of Stratospheric Seasonal Cycles in the Tropical Upper Troposphere and Lower Stratosphere

Water vapor and ozone are powerful radiative constituents in the tropical lower stratosphere, impacting the local heating budget and nonlocally forcing the troposphere below. Their near-tropopause seasonal cycle structures imply associated “radiative seasonal cycles” in heating rates that could affect the amplitude and phase of the local temperature seasonal cycle. Overlying stratospheric seasonal cycles of water vapor and ozone could also play a role in the lower stratosphere and upper troposphere heat budgets through nonlocal propagation of radiation. Previous studies suggest that the tropical lower stratospheric ozone seasonal cycle radiatively amplifies the local temperature seasonal cycle by up to 35%, while water vapor is thought to have a damping effect an order of magnitude smaller. This study uses Aura Microwave Limb Sounder observations and an offline radiative transfer model to examine ozone, water vapor, and temperature seasonal cycles and their radiative linkages in the lower stratosphere and upper troposphere. Radiative sensitivities to ozone and water vapor vertical structures are explicitly calculated, which has not been previously done in a seasonal cycle context. Results show that the water vapor radiative seasonal cycle in the lower stratosphere is not sensitive to the overlying water vapor structure. In contrast, about one-third of ozone’s radiative seasonal cycle amplitude at 85 hPa is associated with longwave emission above 85 hPa. Ozone’s radiative effects are not spatially homogenous: for example, the Northern Hemisphere tropics have a seasonal cycle of radiative temperature adjustments with an amplitude 0.8 K larger than the Southern Hemisphere tropics.

Zonally resolved impact of ENSO on the stratospheric circulation and water vapor entry values

AbstractBased on simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the period 1979–2013, with model transport driven by the ECMWF ERA‐Interim reanalysis, we discuss the impact of the El Niño Southern Oscillation (ENSO) on the variability of the dynamics, water vapor, ozone, and mean age of air (AoA) in the tropical lower stratosphere during boreal winter. Our zonally resolved analysis at the 390 K potential temperature level reveals that not only (deseasonalized) ENSO‐related temperature anomalies are confined to the tropical Pacific (180–300°E) but also anomalous wave propagation and breaking, as quantified in terms of the Eliassen‐Palm (EP) flux divergence, with strongest local contribution during the La Niña phase. This anomaly is coherent with respective anomalies of water vapor (±0.5 ppmv) and ozone (±100 ppbv) derived from CLaMS being in excellent agreement with the Aura Microwave Limb Sounder observations. Thus, during El Niño a more zonally symmetric wave forcing drives a deep branch of the Brewer‐Dobson (BD) circulation. During La Niña this forcing increases at lower levels (≈390 K) over the tropical Pacific, likely influencing the shallow branch of the BD circulation. In agreement with previous studies, wet (dry) and young (old) tape recorder anomalies propagate upward in the subsequent months following El Niño (La Niña). Using CLaMS, these anomalies are found to be around +0.3 (−0.2) ppmv and −4 (+4) months for water vapor and AoA, respectively. The AoA ENSO anomaly is more strongly affected by the residual circulation (≈2/3) than by eddy mixing (≈1/3).

Evaluation of UTLS carbon monoxide simulations in GMI and GEOS-Chem chemical transport models using Aura MLS observations

Abstract. This study evaluates the distribution and variation of carbon monoxide (CO) in the upper troposphere and lower stratosphere (UTLS) during 2004–2012 as simulated by two chemical transport models, using the latest version of Aura Microwave Limb Sounder (MLS) observations. The simulated spatial distributions, temporal variations and vertical transport of CO in the UTLS region are compared with those observed by MLS. We also investigate the impact of surface emissions and deep convection on CO concentrations in the UTLS over different regions, using both model simulations and MLS observations. Global Modeling Initiative (GMI) and GEOS-Chem simulations of UTLS CO both show similar spatial distributions to observations. The global mean CO values simulated by both models agree with MLS observations at 215 and 147 hPa, but are significantly underestimated by more than 40 % at 100 hPa. In addition, the models underestimate the peak CO values by up to 70 % at 100 hPa, 60 % at 147 hPa and 40 % at 215 hPa, with GEOS-Chem generally simulating more CO at 100 hPa and less CO at 215 hPa than GMI. The seasonal distributions of CO simulated by both models are in better agreement with MLS in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH), with disagreements between model and observations over enhanced CO regions such as southern Africa. The simulated vertical transport of CO shows better agreement with MLS in the tropics and the SH subtropics than the NH subtropics. We also examine regional variations in the relationships among surface CO emission, convection and UTLS CO concentrations. The two models exhibit emission–convection–CO relationships similar to those observed by MLS over the tropics and some regions with enhanced UTLS CO.

The upward branch of the Brewer‐Dobson circulation quantified by tropical stratospheric water vapor and carbon monoxide measurements from the Aura Microwave Limb Sounder

AbstractThe vertical distributions of water vapor (H2O) and carbon monoxide (CO) in the tropical lower stratosphere are controlled largely by their mixing ratios near the tropopause and by ascending motions as part of the Brewer‐Dobson circulation (BDC). The upward propagation of seasonal variations imprinted on H2O and CO vertical profiles, often referred to as the tropical “tape recorder,” can be used to derive the mean vertical velocity, , in this region of the lower stratosphere where quasi‐horizontal mixing is not strong enough to erase the seasonal tape recorder signals. We used Aura Microwave Limb Sounder observations of the tropical tape recorders from 2004 to 2014 to derive values of at pressures between 90 and 16 hPa (about 18 to 28 km altitude). Mean vertical profiles of are consistent with calculated velocities derived from net radiative heating rates based on observed temperature, humidity, cloud, and trace gas amounts. Temporal variations in are dominated by a quasi‐biennial oscillation (QBO) and seasonal cycles, with maximum upwelling coinciding with easterly phases of the QBO in zonal wind shear and during the November–December period of the seasonal cycle. Both the QBO and annual modes emphasize the importance of wave phenomena in modulating the strength of tropical upwelling in the BDC. Interannual anomalies in are correlated with variations in the El Niño–Southern Oscillation (ENSO), with enhanced stratospheric upwelling during El Niño phases and reduced upwelling during La Niña. A small decreasing linear trend (~6%/decade) in is observed from 2005 to 2014, although confidence is low in identifying such a trend as part of a long‐term change due to the influence of ENSO over this period.

Radiative Impacts of the 2011 Abrupt Drops in Water Vapor and Ozone in the Tropical Tropopause Layer

Abstract An abrupt drop in tropical tropopause layer (TTL) water vapor, similar to that observed in 2000, recently occurred in 2011, and was concurrent with reductions in TTL temperature and ozone. Previous studies have indicated that such large water vapor variability can have significant radiative impacts. This study uses Aura Microwave Limb Sounder observations, the Stratospheric Water Vapor and Ozone Satellite Homogenized dataset, and two radiative transfer models to examine the radiative effects of the observed changes in TTL water vapor and ozone on TTL temperatures and global radiative forcing (RF). The analyses herein suggest that quasi-isentropic poleward propagation of TTL water vapor reductions results in a zonal-mean structure with “wings” of extratropical water vapor reductions, which account for about half of the 2011 abrupt drop global radiative impact. RF values associated with the mean water vapor concentrations differences between 2012/13 and 2010/11 are between −0.01 and −0.09 W m−2, depending upon the altitude above which perturbations are considered. TTL water vapor and ozone variability during this period jointly lead to a transient radiative cooling of ~0.25–0.5 K in layers below the tropopause. The 2011 abrupt drop also prolonged the reduction in stratospheric water vapor that followed the 2000 abrupt drop, providing a longer-term radiative forcing of climate. Water vapor concentrations from 2005 to 2013 are lower than those from 1990 to 1999, resulting in a RF between these periods of about −0.045 W m−2, approximately 12% as large as, but of opposite sign to, the concurrent estimated CO2 forcing.

An assessment of upper troposphere and lower stratosphere water vapor in MERRA, MERRA2, and ECMWF reanalyses using Aura MLS observations

AbstractGlobal water vapor (H2O) measurements from Microwave Limb Sounder (MLS) are used to evaluate upper tropospheric (UT) and lower stratospheric (LS) H2O products produced by NASA Modern‐Era Retrospective Analysis for Research and Applications (MERRA), its newest release MERRA2, and European Centre for Medium‐Range Weather Forecasts (ECMWF) Interim Reanalyses. Focusing on the H2O amount and transport from UT to LS, we show that all reanalyses overestimate annual global mean UT H2O by up to ~150% compared to MLS observations. Substantial differences in H2O transport are also found between the observations and reanalyses. Vertically, H2O transport across the tropical tropopause (16–20 km) in the reanalyses is faster by up to ~86% compared to MLS observations. In the tropical LS (21–25 km), the mean vertical transport from ECMWF is 168% faster than the MLS estimate, while MERRA and MERRA2 have vertical transport velocities within 10% of MLS values. Horizontally at 100 hPa, both observation and reanalyses show faster poleward transport in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Compared to MLS observations, the H2O horizontal transport for both MERRA and MERRA2 is 106% faster in the NH but about 42–45% slower in the SH. ECMWF horizontal transport is 16% faster than MLS observations in both hemispheres. The ratio of northward to southward transport velocities for ECMWF is 1.4, which agrees with MLS observation, while the corresponding ratios for MERRA and MERRA2 are about 3.5 times larger.

Variability in Antarctic ozone loss in the last decade (2004–2013): high-resolution simulations compared to Aura MLS observations

Abstract. A detailed analysis of the polar ozone loss processes during 10 recent Antarctic winters is presented with high-resolution MIMOSA–CHIM (Modèle Isentrope du transport Méso-échelle de l'Ozone Stratosphérique par Advection avec CHIMie) model simulations and high-frequency polar vortex observations from the Aura microwave limb sounder (MLS) instrument. The high-frequency measurements and simulations help to characterize the winters and assist the interpretation of interannual variability better than either data or simulations alone. Our model results for the Antarctic winters of 2004–2013 show that chemical ozone loss starts in the edge region of the vortex at equivalent latitudes (EqLs) of 65–67° S in mid-June–July. The loss progresses with time at higher EqLs and intensifies during August–September over the range 400–600 K. The loss peaks in late September–early October, when all EqLs (65–83° S) show a similar loss and the maximum loss (> 2 ppmv – parts per million by volume) is found over a broad vertical range of 475–550 K. In the lower stratosphere, most winters show similar ozone loss and production rates. In general, at 500 K, the loss rates are about 2–3 ppbv sh−1 (parts per billion by volume per sunlit hour) in July and 4–5 ppbv sh−1 in August–mid-September, while they drop rapidly to 0 by mid-October. In the middle stratosphere, the loss rates are about 3–5 ppbv sh−1 in July–August and October at 675 K. On average, the MIMOSA–CHIM simulations show that the very cold winters of 2005 and 2006 exhibit a maximum loss of ~ 3.5 ppmv around 550 K or about 149–173 DU over 350–850 K, and the warmer winters of 2004, 2010, and 2012 show a loss of ~ 2.6 ppmv around 475–500 K or 131–154 DU over 350–850 K. The winters of 2007, 2008, and 2011 were moderately cold, and thus both ozone loss and peak loss altitudes are between these two ranges (3 ppmv around 500 K or 150 ± 10 DU). The modeled ozone loss values are in reasonably good agreement with those estimated from Aura MLS measurements, but the model underestimates the observed ClO, largely due to the slower vertical descent in the model during spring.

A Match-based approach to the estimation of polar stratospheric ozone loss using Aura Microwave Limb Sounder observations

Abstract. The well-established "Match" approach to quantifying chemical destruction of ozone in the polar lower stratosphere is applied to ozone observations from the Microwave Limb Sounder (MLS) on NASA's Aura spacecraft. Quantification of ozone loss requires distinguishing transport- and chemically induced changes in ozone abundance. This is accomplished in the Match approach by examining cases where trajectories indicate that the same air mass has been observed on multiple occasions. The method was pioneered using ozonesonde observations, for which hundreds of matched ozone observations per winter are typically available. The dense coverage of the MLS measurements, particularly at polar latitudes, allows matches to be made to thousands of observations each day. This study is enabled by recently developed MLS Lagrangian trajectory diagnostic (LTD) support products. Sensitivity studies indicate that the largest influence on the ozone loss estimates are the value of potential vorticity (PV) used to define the edge of the polar vortex (within which matched observations must lie) and the degree to which the PV of an air mass is allowed to vary between matched observations. Applying Match calculations to MLS observations of nitrous oxide, a long-lived tracer whose expected rate of change is negligible on the weekly to monthly timescales considered here, enables quantification of the impact of transport errors on the Match-based ozone loss estimates. Our loss estimates are generally in agreement with previous estimates for selected Arctic winters, though indicating smaller losses than many other studies. Arctic ozone losses are greatest during the 2010/11 winter, as seen in prior studies, with 2.0 ppmv (parts per million by volume) loss estimated at 450 K potential temperature (~ 18 km altitude). As expected, Antarctic winter ozone losses are consistently greater than those for the Arctic, with less interannual variability (e.g., ranging between 2.3 and 3.0 ppmv at 450 K). This study exemplifies the insights into atmospheric processes that can be obtained by applying the Match methodology to a densely sampled observation record such as that from Aura MLS.

Modulation of Antarctic vortex composition by the quasi‐biennial oscillation

AbstractUsing a decade of Aura Microwave Limb Sounder observations, we show distinctly different N2O distributions in Southern Hemisphere winter that depend on the phase of the quasi‐biennial oscillation (QBO). Composites of the nitrous oxide (N2O) anomalies calculated for westerly and easterly phases show that QBO‐generated variability originating in the subtropical middle stratosphere fills the midlatitude surf zone by late winter. After the spring vortex breakup, the anomaly is transported to the Antarctic where it remains until the next vortex forms in fall. Trapped in the newly formed vortex, the anomaly descends in isolation through fall and winter, arriving in the Antarctic lower stratosphere in September—about 1 year after it formed. This transport pathway explains previously reported variability of N2O and inorganic chlorine (Cly) inside the Antarctic vortex and demonstrates that the middle stratosphere QBO affects ozone depletion by modulating Antarctic Cly.

The contributions of chemistry and transport to low arctic ozone in March 2011 derived from Aura MLS observations

Stratospheric and total columns of Arctic O3 (63–90oN) in late March 2011 averaged 320 and 349 DU, respectively, 50–100 DU lower than any of the previous 6 years. We use Aura Microwave Limb Sounder (MLS) O3 observations to quantify the roles of chemistry and transport and find there are two major reasons for low O3 in March 2011: heterogeneous chemical loss and a late final warming that delayed the resupply of O3 until April. Daily vortex‐averaged partial columns in the lowermost stratosphere (p > 133 hPa) and middle stratosphere (p < 29 hPa) are largely unaffected by local heterogeneous chemistry, according to model calculations. Very weak transport into the vortex between late January and late March contributes to the observed low ozone. The lower stratospheric (LS) column (133–29 hPa, ~370–550 K) is affected by both heterogeneous chemistry and transport. Because MLS N2O data show strong isolation of the vortex, we estimate the contribution of vertical transport to LS O3 using the descent of vortex N2O profiles. Simulations with the Global Modeling Initiative (GMI) chemistry and transport model (CTM) with and without heterogeneous chemical reactions show 73 DU vortex averaged O3 loss; the loss derived from MLS O3 is 84 ± 12 DU. The GMI simulation reproduces the observed O3 and N2O with little error and demonstrates credible transport and chemistry. Without heterogeneous chemical loss, March 2011 vortex O3 would have been at least 40 DU lower than climatology due to the late final warming that did not resupply O3 until mid‐April.

Record-breaking ozone loss in the Arctic winter 2010/2011: comparison with 1996/1997

Abstract. We present a detailed discussion of the chemical and dynamical processes in the Arctic winters 1996/1997 and 2010/2011 with high resolution chemical transport model (CTM) simulations and space-based observations. In the Arctic winter 2010/2011, the lower stratospheric minimum temperatures were below 195 K for a record period of time, from December to mid-April, and a strong and stable vortex was present during that period. Simulations with the Mimosa-Chim CTM show that the chemical ozone loss started in early January and progressed slowly to 1 ppmv (parts per million by volume) by late February. The loss intensified by early March and reached a record maximum of ~2.4 ppmv in the late March–early April period over a broad altitude range of 450–550 K. This coincides with elevated ozone loss rates of 2–4 ppbv sh−1 (parts per billion by volume/sunlit hour) and a contribution of about 30–55% and 30–35% from the ClO-ClO and ClO-BrO cycles, respectively, in late February and March. In addition, a contribution of 30–50% from the HOx cycle is also estimated in April. We also estimate a loss of about 0.7–1.2 ppmv contributed (75%) by the NOx cycle at 550–700 K. The ozone loss estimated in the partial column range of 350–550 K exhibits a record value of ~148 DU (Dobson Unit). This is the largest ozone loss ever estimated in the Arctic and is consistent with the remarkable chlorine activation and strong denitrification (40–50%) during the winter, as the modeled ClO shows ~1.8 ppbv in early January and ~1 ppbv in March at 450–550 K. These model results are in excellent agreement with those found from the Aura Microwave Limb Sounder observations. Our analyses also show that the ozone loss in 2010/2011 is close to that found in some Antarctic winters, for the first time in the observed history. Though the winter 1996/1997 was also very cold in March–April, the temperatures were higher in December–February, and, therefore, chlorine activation was moderate and ozone loss was average with about 1.2 ppmv at 475–550 K or 42 DU at 350–550 K, as diagnosed from the model simulations and measurements.

Aura Microwave Limb Sounder observations of the polar middle atmosphere: Dynamics and transport of CO and H2O

[1] The vertical structure and evolution of the wintertime annular modes are studied using 6 years of geopotential height, carbon monoxide (CO), and water vapor (H2O) data from Aura Microwave Limb Sounder. The Northern Hemisphere annular mode (NAM) and the Southern Hemisphere annular mode (SAM) reveal a strong coupling of the dynamics in the stratosphere and mesosphere between 316 hPa (∼9 km) and 0.002 hPa (∼90 km). CO is a good tracer throughout the middle atmosphere, while variable vertical gradients of H2O limit the regions where it is useful as a dynamical tracer. The maximum of the CO NAM and SAM (CNAM and CSAM) indices is used to monitor and characterize the evolution of wintertime polar dynamics as a function of time and height. The CNAM analysis reveals reformation of a stronger mesospheric polar vortex after significant stratospheric sudden warmings in 2006, 2009, and 2010. There is a significant anticorrelation between the mesospheric and stratospheric CNAM indices during 2005–2010 winters, supporting the hypothesis of mesosphere-stratosphere coupling through planetary-gravity wave interactions.

Seasonal variation of the Hough modes of the diurnal component of ozone heating evaluated from Aura Microwave Limb Sounder observations

The global distribution of atmospheric ozone from August 2004 to December 2008 observed by Aura Microwave Limb Sounder is used to calculate the daily ozone heating rate. Comparison with a photochemical model shows that the Strobel/Zhu parameterized model of the solar heating rate gives an accurate diurnal cycle of the heating in the 10–70 km altitude range. The heating calculated using the Strobel/Zhu model is decomposed into Hough modes and the annual (AO), semiannual (SAO), and quasi‐biennial (QBO) periodicities of each Hough mode are presented. The results show that the majority of the O3 heating goes into the symmetric (1,−2), (1,1), and (1,−4) modes. The largest propagating mode (1,1) and the largest trapped mode (1,−2) have obvious SAO signatures near 47 km, where the heating magnitude is largest. The forcing of the propagating (1,1) mode by ozone heating is larger during solstices than during equinoxes. There are also AO in the (1,−2) and (1,1) modes. The QBO signatures are relatively weak in the two modes in the region of large heating rate. The heating rate of the strongest propagating mode (1,1) reaches maximum near December/January solstice; that for the strongest trapped mode (1,−2) reaches maximum near equinoxes. The strongest annual variation takes place in the (1,−1) mode; it reaches 3.6 K/d near 45 km at the December/January solstice. A new parameterization of the diurnal component of the heating rate, which covers the vertical range from 10 km to 70 km, is developed based on the seasonal variations in each Hough mode.

Aura Microwave Limb Sounder observations of the Northern Annular Mode: From the mesosphere to the upper troposphere

Vertical structure and evolution of the wintertime Northern Hemisphere Annular Mode (NAM), the first Empirical Orthogonal Function (EOF) of geopotential height anomalies, are constructed from the 2005–2009 Aura Microwave Limb Sounder (MLS) measurements in the entire middle atmosphere between 316 hPa (∼9 km) and 0.001 hPa (∼90 km). This is the first report of NAM structure extending into the mesosphere. The mode appears to be robust and it accounts for up to 70% of middle‐atmospheric variance before decreasing in the upper mesosphere. The vertical connection of the NAM modes suggests strong dynamic coupling between the mesosphere and stratosphere. Time evolution of the NAM suggests that the significant NAM anomalies typically appear first in the mesosphere and progress downward. NAM patterns derived from MLS observations are consistent with those derived from long‐term reanalysis below the middle stratosphere. The NAM indexes show the mesospheric cooling signals during the major stratospheric sudden warmings (SSWs) in 2006 and 2009.

Aura Microwave Limb Sounder observations of dynamics and transport during the record‐breaking 2009 Arctic stratospheric major warming

A major stratospheric sudden warming (SSW) in January 2009 was the strongest and most prolonged on record. Aura Microwave Limb Sounder (MLS) observations are used to provide an overview of dynamics and transport during the 2009 SSW, and to compare with the intense, long‐lasting SSW in January 2006. The Arctic polar vortex split during the 2009 SSW, whereas the 2006 SSW was a vortex displacement event. Winds reversed to easterly more rapidly and reverted to westerly more slowly in 2009 than in 2006. More mixing of trace gases out of the vortex during the decay of the vortex fragments, and less before the fulfillment of major SSW criteria, was seen in 2009 than in 2006; persistent well‐defined fragments of vortex and anticyclone air were more prevalent in 2009. The 2009 SSW had a more profound impact on the lower stratosphere than any previously observed SSW, with no significant recovery of the vortex in that region. The stratopause breakdown and subsequent reformation at very high altitude, accompanied by enhanced descent into a rapidly strengthening upper stratospheric vortex, were similar in 2009 and 2006. Many differences between 2006 and 2009 appear to be related to the different character of the SSWs in the two years.