Authors response to reviewers and community comment

The mesosphere and lower thermosphere (MLT) is a dynamic layer of the earth’s atmosphere. This region marks the interface at which neutral atmosphere dynamics begin to influence the ionosphere and space weather. However, our understanding of this region and our ability to accurately simulate it in global circulation models (GCMs) is limited by a lack of observations, especially in remote locations. To this end, a meteor radar was deployed on the remote mountainous island of South Georgia (54° S, 36° W) in the Southern Ocean from 2016 to 2020. The goal of this study is to use these new measurements to characterise the fundamental dynamics of the MLT above South Georgia including large-scale winds, solar tides, planetary waves (PWs) and mesoscale gravity waves (GWs). We first present an improved method for time-height localisation of radar wind measurements and characterise the large-scale MLT winds. We then explore the amplitudes and phases of the diurnal (24 h), semidiurnal (12 h) and terdiurnal (8 h) solar tides at this latitude. We also explore PW activity and find very large amplitudes up to 30 ms−1 for the quasi-2 day wave in summer and show that the dominant modes of the quasi-5, 10 and 16 day waves are westward W1 and W2. We investigate wind variance due to GWs in the MLT and use a new method to show an east-west tendency of GW variance of up to 20 % during summer and a weaker north-south tendency of 0–5 % during winter. This is contrary to the expected tendency of GW directions in the winter stratosphere below, which is a strong suggestion of secondary GW (2GW) observations in the MLT. Lastly, comparison of radar winds to a climatological Whole Atmosphere Community Climate Model (WACCM) simulation reveals a simulated summertime mesopause and zonal wind shear that occur at altitudes around 10 km lower than observed, and southward winds during winter above 90 km altitude in the model that are not seen in observations. Further, wintertime zonal winds above 85 km altitude are eastward in radar observations but in WACCM they are found to weaken and reverse to westward. Recent studies have linked this discrepancy to the impact of 2GWs on the residual circulation which are not included in WACCM. These measurements therefore provide vital constraints that can guide the development of GCMs as they extend upwards into this important region of the atmosphere.

Previous observations and simulations are controversial as to whether El Niño will increase or decrease the diurnal tide (DW1) in the upper mesosphere and lower thermosphere (MLT) region. This study revisited the linear response of the MLT DW1 to El Niño during the winter (December-January-February) based on 19-year satellite observations of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). The MLT DW1 temperature amplitudes decreased by ~10 % during four El Niño winters from 2002 to 2020, consistent with the results from the simulation of the Specified-Dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM). According to the multiple linear regression analysis, the linear effects of El Niño-Southern Oscillation (ENSO) on tropical MLT DW1 are negative in both SABER observations and SD-WACCM simulations. In the SD-WACCM simulation, Hough mode (1, 1) dominates the DW1 tidal variation in the tropical MLT region. The consistency between the (1, 1) mode in the tropopause region and in the MLT region, as well as the downward phase progression from 15 to 100 km, indicates the direct upward propagation of DW1 from the excitation source in the troposphere. During 7 of 8 El Niño winters from 1979 to 2014, the anomalous amplitudes of the (1, 1) mode are negative in both the tropopause region and MLT region. The suppressed DW1 heating rates in the tropical troposphere (average over ~0–16 km and 35° S–35° N) during the El Niño events contribute to the decreased DW1 tide. The mesospheric latitudinal zonal wind shear anomalies during El Niño winters would lead to a narrower waveguide and prevent the vertical propagation of the DW1 tide. The gravity wave drag excited by convection also plays a role in modulating the MLT DW1 amplitude.

In the equatorial region, the influence of the stratospheric quasi-biennial oscillation (QBO) extends into the mesosphere and lower thermosphere (MLT) region.This work presents the characteristics of this response to the QBO in terms of the semiannual oscillations (SAOs) around the stratopause and the mesopause, and the tides, using the longterm global reanalysis for the whole neutral atmosphere over 19 years of 2004-2023.The results indicate that the response is observed at a wide height region of the mesosphere and lower thermosphere, with dominance in two separate height regions, at altitudes around 80 km and 100 km.The lower one is observed as a vertical shift of the easterly shear during equinoxes, which can be explained by the difference in the QBO phase-dependent filtering of waves in the stratosphere.The upper one is observed as a stronger easterly during the QBO westerly phase at 10 hPa, in all seasons, associated with larger amplitude and the forcing due to the migrating diurnal tide (DW1).The DW1-propagatable latitude region, i.e., the region where the sum of relative and planetary vorticity is equal to or smaller than the frequency of DW1, is relatively wider in the middle mesosphere during the QBO westerly phase.These results suggest that there is a two-step mechanism: first, direct modulation of the upper mesospheric zonal wind by the QBO through selective wave transmission, and second, the modulation of the lower thermospheric zonal wind due to the influence of modulated upper mesospheric zonal wind on the DW1.(249/250)

Abstract. The mesosphere and lower thermosphere (MLT) wind structure over Wuhan (30° N, 114° E) in 2000/2001 winter and over Langfang (39.4° N, 116.6° E) in 2009/2010 winter are examined to reveal the effects of stratospheric sudden warming (SSW) in mid-low-latitude MLT region. The result shows that the MLT daily zonal wind over these two sites reversed from eastward wind to westward wind for several days during the SSW events. The reversals were almost coincident with the polar stratospheric temperature reaching its maximum at 10 hPa, 90° N and were about ten days prior to the reversal of high latitude stratospheric zonal wind at 10 hPa, 60° N. The temporal variations of tides, gravity waves and 2-day planetary waves in the mid-latitude MLT showed different behavior during the two SSW events. During the 2001 SSW event, MLT diurnal tide reached its maximum when the MLT zonal wind decreased rapidly and SSW event began in polar stratosphere; the activity of 2-day waves decreased after the onset of the 2001 SSW, while the gravity wave increased when the 2001 SSW developed into a major warming. However, in the 2009/2010 winter, the semidiurnal tide and 2-day wave in MLT over Langfang reached a peak about two days earlier than zonal wind reversal at 10 hPa, 60° N; no significant features were found in diurnal tides, terdiurnal tides and gravity waves related to the 2010 SSW event.

Preliminary observations and simulation of nocturnal variations of airglow temperature and emission rates at Pune (18.5°N), India

Winds and tides of the Antarctic mesosphere and lower thermosphere: One year of meteor-radar observations over Rothera (68°S, 68°W) and comparisons with WACCM and eCMAM

The main aim of the paper is to describe mesosphere and lower thermosphere (MLT) wind field observed between June 2000 and May 2002 by medium frequency (MF) radars at two locations near the equatorial region and at tropical latitude. We have presented and compared observations of mean horizontal winds in the mesosphere and lower thermosphere (MLT) region over Kolhapur (16.8°N, 74.2°E) and Tirunelveli (8.7°N, 77.8°E). Our analysis reveals annual oscillation in the zonal wind and semi-annual oscillation in the meridional wind over Kolhapur. On the other hand, results over Tirunelveli reveal semi-annual oscillation (SAO) in the zonal wind and annual oscillation in the meridional wind. Also we have observed enhanced magnitude of wind speed in spring equinox period of 2002 exhibiting the signatures of the quasi-biennial oscillation (QBO) over Tirunelveli.

We present the long‐term trends of the Arctic stratospheric polar vortex (SPV) using the Modern Era Retrospective Research analysis for Research and Application‐2 (MERRA‐2) reanalysis data, and assess its impact on the mesosphere and lower thermosphere (MLT) using Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (SD‐WACCM‐X) simulations. Our findings reveal that the SPV showed a weakening trend from 1980 until the early 2000s, but a strengthening trend after the 2000s. This change is attributed to the weakening of the planetary wave (PW) activities, particularly the wavenumber‐1, which can be driven by long‐term Arctic surface changes. A strong link has been found between the observed PWs activity and the increasing sea surface temperature (SST) over/nearby the Barents‐Kara Sea before early 2000s and over the Central North Pacific region after the 2000s. Furthermore, we observe that the SPV trend influences the zonal mean zonal winds and migrating solar semidiurnal tide (SW2) in the MLT. The SW2 exhibits a ∼13%/decade positive trend from 1980 to the early 2000s and a ∼14%/decade negative trend after 2000s. These results provide evidence of the impact of Arctic surface variabilities, including sea‐ice, on the MLT dynamics, especially SW2 tide.

The variability of the upper atmosphere is largely influenced by dynamical forcing from the lower and middle atmosphere. The Mesosphere and Lower Thermosphere (MLT) is the transition region between the middle atmosphere and the upper atmosphere, and it determines dynamical forcing to the upper atmosphere from below. It is thus of high importance to know, describe, and understand the dynamical processes within the MLT to quantify dynamics. Therein, General Circulation Models (GCMs) have been a significant tool to explain MLT processes.However, developing the right parameterizations that allow to accurately model near-to-realistic states of the MLT by GCMs is challenging, which is reflected in a large diversity of results from different models in comparison to observations, e.g., of winds and temperatures at the MLT.In recent years, the community model ICON (Icosahedral Nonhydrostatic Weather and Climate Model) has been expanded into altitudes up to 150 km, named the UA (Upper Atmosphere) branch. UA-ICON is increasingly being applied to model and to understand MLT processes and how they are controlled by the lower and middle atmosphere.We present newly developed capabilities of UA-ICON. Examples are mesospheric cooling during stratospheric warming events, low summer mesopause temperatures through appropriate specification of gravity wave parameterizations and runs of high spatial resolution. Results are discussed in comparison with observations and with predictions by other GCMs.

Behaviour of monthly tides from meteor radar winds at 22.7°S during declining phases of 23 and 24 solar cycles

<div> <p>Atmospheric gravity waves generated in the troposphere by a number of sources (convection, frontogenesis, orography etc.) can travel great vertical distances, propagating upwards to 80 - 120km where they influence the chemical and dynamical structure of the Mesosphere and Lower Thermosphere (MLT).</p> </div><div> <p>Current chemistry-climate models represent gravity waves, and their impact on the temperature and the chemical composition of the atmosphere, by means of parameterizations that take into account the turbulence and the mixing caused by breaking waves but largely neglect the dynamical and chemical constituent transport by vertically propagating non-breaking waves.</p> </div><div> <p>We present initial results from the WAVECHASM (Wave-Induced Transport of Chemically Active Species in the Mesosphere and Lower Thermosphere) project. By making use of a recent novel theoretical approach, where the effective wave diffusivity is expressed as a function of the eddy diffusivity and of the variances of the temperature perturbation and lapse rate fluctuations, the WAVECHASM project aims to incorporate the missing transport processes into global atmospheric chemistry models. We will show here that it is possible to modify the current gravity wave drag parameterization of NCAR’s Whole Atmosphere Community Climate Model (WACCM) to explicitly account for the wave-driven vertical mixing associated with non-breaking gravity waves. This additional source of vertical mixing is expected to induce significant constituent transport in the upper atmosphere.</p> </div>

We present analysis of the chemical and dynamical variability in the mesosphere and lower thermosphere (MLT) during the 2018-2019 sudden stratospheric warming (SSW) as simulated by the high resolution (~25 km horizontal and 0.1 scale height vertical resolution) version of the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X). The WACCM-X simulations make use of new capabilities, including the spectral element dynamical core and the ability to constrain the lower atmosphere meteorology in WACCM-X at high-resolutions. Compared to standard resolution (~200 km horizontal and 0.25 scale height vertical resolution) WACCM-X simulations, the high-resolution simulations are in better agreement with Thermosphere Ionosphere Mesosphere Energetics Dynamics-Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED-SABER) and Atmosphere Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) observations. In particular, the high-resolution simulations better reproduce the Northern Hemisphere middle-high latitude winds in the MLT. The downward transport of nitric oxide (NO) following the SSW is also better reproduced in the high-resolution simulations. The results demonstrate the importance of capturing mesoscale processes for accurately simulating the chemistry and dynamics of the MLT.      

Using version 1.0 of TIMED/SABER nighttime O(3P) density data in the mesosphere and lower thermosphere (MLT) retrieved from 2.0 and 1.6 μm radiances, we conducted a study on the semiannual oscillation (SAO), annual oscillation (AO) and quasi-biennial oscillation (QBO) of the atomic oxygen volume mixing ratio at 96 km, from 40° S to 40° N, for 2002–2019. We first analyzed the altitude profiles of the atomic oxygen volume mixing ratio and kinetic temperature, and chose to study the daily average of the atomic oxygen volume mixing ratio at 96 km. For the analysis of SAO and AO, we fitted two sinusoidal functions with periods of 6 and 12 months to the daily mean atomic oxygen volume mixing ratio to obtain the annual and semiannual amplitude. The SAO amplitudes had two peaks of 1.68 × 10−3 and 1.63 × 10-3 at about 25° S and 25° N, and displayed a clear hemispheric symmetry. The AO amplitude increased with the latitude and showed distinct minima (valleys) of 3.36 × 10−4 around the equator, as well as a clear hemispheric asymmetry. The correlation coefficient between the atomic oxygen volume mixing ratio QBO with equatorial stratospheric QBO was higher in the tropics than the mid latitudes.

Long‐term observations from medium‐frequency and meteor radars (1993–2012) and rocket soundings (1979–1990 and 2002–2007) are used to study mesosphere and lower thermosphere (MLT) zonal wind variations in relation to the stratospheric winds over northern low latitudes. The combined data set provides a complete height profile of amplitude of semiannual oscillation (SAO) up to 100 km, with an exception around 75–80 km. The SAO signal has maxima around 50 km and 82 km and a minimum around 65 km. The MLT zonal winds show remarkable interannual variability during northern hemispheric spring equinox and much less during fall equinox. Zonal wind mesospheric spring equinox enhancements (MSEE) appear with a periodicity of 2–3 years, suggesting a modulation by the quasi‐biennial oscillation, which we identified with the strength of stratospheric westward winds. Out of 20 years of observations, the stratospheric westward winds are strong during 11 years (non‐MSEE) and weak during 9 years. Six of these 9 years show large MLT winds (MSEE), and 3 years (1999, 2004, and 2006) show small MLT winds (missing MSEE). These unexpected small winds occur in years with global circulation anomalies associated with strong sudden stratospheric warmings and an early spring transition of zonal winds. With the proposed three MSEE classes, we take into account local and global forcing factors.

A two‐dimensional nonlinear numerical model was used to simulate large winds (≥ 100 m s−1) and wind shears (≥ 40 m s−1 km−1) in the mesosphere and lower thermosphere (MLT) that are caused by the interaction between gravity waves (GWs) and migrating tidal background. By varying the wavelengths of GWs and the phases of diurnal and semidiurnal tides, 64 numerical experiments were performed. Our numerical experiments indicate that both migrating diurnal and semidiurnal tides strongly modulate the occurrence of GW breaking, and the resulted large winds and wind shears. The simulated large winds and wind shears are in good agreement with those from the rocket‐sounding chemical release measurements. Moreover, the occurrence of large wind shears highly depends on the phases of migrating tides in local time, which is in agreement with the reported lidar observations. The local time dependence of large wind shears is mainly attributed to the filtering and/or hindering effects of diurnal and semidiurnal tidal winds on GWs. Our simulation reveals that the nonlinear interactions between GWs breaking and the migrating diurnal and semidiurnal tides may play an important role in driving the large winds and wind shears in the MLT region and their local time dependence.