1. Introduction
In the middle atmosphere, atmospheric waves, such as gravity waves (GWs), Rossby waves (RWs), and tides (TWs), which mainly propagate from the lower atmosphere, are dominant and largely affect the momentum budget. The momentum deposition caused by these waves is the driving force of the mean meridional circulation in the middle atmosphere, which maintains a thermal structure that is partly much different from that expected from the radiative balance. Thus, to better understand the dynamics of the middle atmosphere, particularly the mesosphere and lower thermosphere (MLT), it is important to examine the relative contributions of these waves to the momentum budget. However, only a few studies on the momentum budget in the MLT have been conducted from this point of view (McLandress et al. 2006; Becker 2012).
GWs originating from the troposphere have the largest impact on the momentum budget in the MLT region. GWs originating from the lower atmosphere have significantly large wind and temperature amplitudes in the MLT region, with low atmospheric density, and hence easily break and deposit the momentum into the mean flow. Such GW forcing acts to maintain a weak-zonal-wind layer and drives the summer-to-winter-pole meridional circulation near the mesopause (e.g., Matsuno 1982; Holton 1983; Plumb 2002). Most general circulation models (GCMs) include the GW forcing using parameterization schemes. The GW forcing is negative (positive) in the winter (summer) upper mesosphere, and the magnitude reaches 100–160 m s−1 day−1 at ~60° in both hemispheres depending on the season and the model (Fomichev et al. 2002; Miyoshi et al. 2014; Becker 2012). The GW forcing around the mesopause in the GCMs is consistent with a recent observational estimate (~120 m s−1 day−1 at z = ~87 km) in summer obtained using the mesosphere–stratosphere–troposphere/incoherent scatter (MST/IS) radar at Syowa Station in Antarctica (69.0°S, 39.4°E), which is also called the Program of the Antarctic Syowa MST/IS (PANSY) radar (Sato et al. 2014, 2017).
On the other hand, two specific kinds of RWs, which are called quasi-2-day waves (QTDWs) and 4-day waves, are frequently observed in the summer and winter mesosphere, respectively. According to observational studies conducted using different satellites (e.g., Wu et al. 1996; Lieberman 1999; Limpasuvan and Wu 2003; Garcia et al. 2005; Baumgaertner et al. 2008; Ern et al. 2013) and radars (e.g., Salby and Roper 1980; Murphy et al. 2007), and according to model studies (e.g., Gu et al. 2016), the QTDWs have zonal wavenumbers s = 2–4 and propagate westward, and their activity is maximized slightly after the summer solstice. Recently, Pancheva et al. (2016) conducted a comprehensive study on the QTDWs using global analysis data obtained by a data assimilation system called the Navy Operational Global Atmospheric Prediction System-Advanced Level Physics High Altitude (NOGAPS-ALPHA) over 14 months. They showed that the QTDWs are not due to a single type of waves but are composed of several kinds depending on the latitude and season. The 4-day waves were first reported by Venne and Stanford (1979) using the Nimbus-5 selective chopper radiometer in the winter stratosphere and are characterized as a mixture of components with s = 1–4 and with the same phase velocity (Prata 1984; Lawrence and Randel 1996; Garcia et al. 2005). According to previous modeling studies (McLandress et al. 2006; Watanabe et al. 2009; Yue et al. 2012), both QTDWs and 4-day waves can provide significant wave forcing, reaching a few tens of meters per second per day in the MLT region. The QTDW forcing estimated by the models is consistent with a recent study by Ern et al. (2013), who used data from the High Resolution Dynamics Limb Sounder and the Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) satellite instruments. In addition, Ern et al. (2013) indicated large interannual variability of the QTDWs. However, these model studies were based on simulations covering a few to several years. Thus, climatological features of the wave forcing should be examined using data covering as long a period as possible.
One of the most likely excitation mechanisms of these RWs is the barotropic (BT)/baroclinic (BC) instability (e.g., Salby and Callaghan 2001; Manney and Randel 1993). Plumb (1983) examined the BC instability around the strato-mesospheric westward wind and indicated that the most unstable mode has a zonal wavenumber s = 3, which is consistent with observations. In addition, the result obtained via model studies (McLandress et al. 2006; Watanabe et al. 2009) that positive Eliassen–Palm (EP) flux divergence associated with these RWs is observed in the region of negative latitudinal PV gradient is consistent with the RW radiation by the BT/BC instability. A few mechanisms causing the necessary condition of BT/BC instability were proposed by previous studies. Strong RW breaking, such as that which induces the sudden stratospheric warming in winter, significantly reduces the latitudinal gradient of the PV (Baldwin and Holton 1988; Greer et al. 2013). Forcing by gravity waves propagating from the lower atmosphere is another strong candidate (Watanabe et al. 2009; Ern et al. 2013; Norton and Thuburn 1999; McLandress et al. 2006). Most previous studies on forcing discussed the intensification of the mean wind shear due to wave forcing by waves originating from the lower atmosphere. However, consideration of PV yields a more comprehensive and direct interpretation of the BT/BC instability condition formed by these waves. Sato and Nomoto (2015) showed, based on a case study of the Northern Hemisphere (NH) in winter and utilizing a gravity wave resolving general circulation model, that the BT/BC instability condition is characterized as a PV maximum at midlatitudes. This maximum is attributable to an increase in the static stability (or Brunt–Väisälä frequency squared) N2 due to the upwelling caused by GW forcing.
Tides are dominant at the low and middle latitudes of the MLT region. Large amplitudes of zonal wind components of diurnal westward-migrating tides with s = 1 (DW1) are confined to low latitudes below a height of z = 100 km, while those of semidiurnal westward-migrating tides with s = 2 (SW2) extend to high latitudes (Miyahara et al. 1999; Wu et al. 2006). Zhu et al. (2008) showed, using SABER observation data, that the Eliassen–Palm flux divergence (EPFD) associated with DW1 is negative (~−5 m s−1 day−1) at low latitudes and is positive (10–20 m s−1 day−1) at middle latitudes at z = ~95 km.
Although the RW, GW, and TW are distinguished by their physical properties, their forcings may not be independent. Cohen et al. (2013) showed that excess parameterized GW forcing can be compensated for by the BT/BC instability. This means that accurate observational constraints on GW parameterizations and/or confirmation of resolved waves by observations are important for examining the momentum budget. Because the expression of GWs by current parameterizations is not adequate (e.g., Geller et al. 2013), future projections of the relative contributions of GWs and RWs to the Brewer–Dobson circulation will not be easy (e.g., Butchart 2014). However, the compensation may truly occur in the real atmosphere.
A method for tackling the dynamics of the compensation mechanism is the utilization of a GW-resolving high-resolution GCM without GW parameterizations. Sato and Nomoto (2015) used such a GCM, called the KANTO model (Watanabe et al. 2008), and elucidated an interesting compensation via a case study of the NH in winter, which is regarded as an interplay of the GWs and RWs. The event starts with a strong RW intrusion from the troposphere to the stratosphere. The RWs break in the upper stratosphere and cause strong wave forcing, which shifts the strato-mesospheric eastward jet poleward. The region of strong GW forcing above the jet also shifts poleward following the jet shift. The GW forcing drives upwelling at the middle latitudes and causes an increase in N2. This results in the PV maximum at the middle latitudes satisfying the necessary condition of BT/BC instability. New RWs are radiated to weaken the PV maximum. Such interplay of the GWs and RWs must be present in the real atmosphere; hence, its role in the climatological momentum budget of the middle atmosphere is an interesting topic.
The present study will examine the relative contributions of RWs, GWs, and TWs to the momentum budget in the middle atmosphere, particularly focusing on the MLT region, using simulation data from a whole-atmosphere model [i.e., the Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA); e.g., Jin et al. (2011)]. Surprisingly and interestingly, resolved GWs appear in the MLT region of the model in spite of the relatively low horizontal resolution. Thus, both resolved and parameterized forcings are examined as the GW contribution. It is also shown that in situ generation of RWs due to the BT/BC instability is a significant factor of the momentum budget. A plausible mechanism of the instability formation in the mesosphere is discussed based on a PV analysis. Satellite observation data from the Aura Microwave Limb Sounder (MLS) are also analyzed for validation of the model data. The generation of GWs in the MLT region and its cause will be shown in more detail and discussed in a companion paper (Yasui et al. 2018, hereafter Part II).
The remainder of this paper is organized as follows. A brief description of the data from GAIA and the Aura MLS used in this study is given in section 2. The method of analysis is described in section 3. Section 4 shows the climatology of the momentum budget in the middle atmosphere. The characteristics of RWs generated in situ in the mesosphere are examined in section 5. Section 6 discusses the formation of the BT/BC instability in the mesosphere and its relation to the parameterized GW forcing. A summary and the concluding remarks are given in section 7.
2. Data description
Two kinds of data are used for the analysis of the dynamics in the middle atmosphere, particularly in the MLT region. One is simulation data from GAIA, and the other is observation data from the Aura MLS (Waters et al. 2006). The time period analyzed in this study is approximately 11 years, from 8 August 2004 to 19 June 2015. The quantitative analysis regarding the momentum budget is performed using mainly the model data. The model results are validated using the observation data as much as possible.
a. GAIA
GAIA is a whole-atmosphere model that covers an altitude range from the surface to the thermosphere/ionosphere. This model is composed of three submodels, namely, a GCM, an ionospheric model, and an electrodynamical model, which communicate via a coupler module. Only outputs from the GCM component (e.g., Miyoshi and Fujiwara 2003) are used in this study. The model resolution is T42L150, corresponding to a horizontal resolution of ~2.8° and vertical grid intervals of 0.2 scale height from the surface (997.5 hPa) to z = ~600 km (1.017 × 10−9 hPa). The model is nudged toward the JRA-25/JMA Climate Data Assimilation System (JCDAS) data (Onogi et al. 2007) for the altitude range from the surface to ~30 km (12 hPa) in order to realistically simulate the quasi-biennial oscillation (QBO) in the equatorial stratosphere and planetary waves originating from the troposphere. Daily values of the 10.7-cm solar radio flux (F10.7) index are included as a proxy of the solar UV–EUV variation. Parameterization schemes for orographic GWs developed by McFarlane (1987) and for nonorographic GWs developed by Lindzen (1981) are implemented as shown in Garcia et al. (2007). The nonorographic GWs are launched at the ~200-hPa level and wave phase speeds of launched GWs are −30 to +30 m s−1 at an interval of 10 m s−1. The source stress spectrum is specified as a Gaussian, where
An analysis is carried out for an altitude range from the surface to an altitude of
b. Aura MLS
3. Methods of analysis
a. Analysis of wave activity flux and wave forcing
b. Classification of RWs, GWs, and TWs
As mentioned in section 1, resolved GWs with significant amplitudes appear in the model in spite of its coarse horizontal resolution. Thus, the EPFD associated with the resolved GWs and the parameterized GW forcing are examined (separately) as the GW contribution to the momentum budget. The definitions of the RWs, resolved GWs, and TWs in the present study are as follows. First, migrating tides with zonal wavenumbers
c. Potential vorticity analysis regarding barotropic and/or baroclinic instability
4. Comparison of the mean and wave fields between the model and observation
Figure 1 shows latitude–height sections of the climatology of
Figures 1c and 1f show the EP flux and EPFD for January from Aura MLS and GAIA, respectively. Because the Aura MLS data do not include the
It should be also noted that TWs simulated by GAIA are also realistic. The maximum amplitudes of temperature, zonal wind, and meridional wind for diurnal migrating tides are ~7 K, ~15 m s−1, and ~20 m s−1 at
In summary, roughly speaking, the model fields are consistent with observations for the zonal mean field and RW fields. Thus, further detailed analysis regarding the momentum budget in the middle atmosphere is carried out using model data in the following sections.
5. Momentum budget in the middle atmosphere
a. Total wave forcing
Figure 2 shows the climatology of total wave forcing, which is the sum of the EPFD, including the term
However, there are a few notable differences between the two hemispheres. As is well known, the total wave forcing in the winter stratosphere is stronger in the NH than in the SH, reflecting the difference in wave activity of stationary RWs (usually called planetary waves). In addition, the total wave forcing in spring is stronger in the SH than in the NH. This is due to the longer duration of the polar night jet in the SH, which provides more preferable conditions for RWs to penetrate into the stratosphere.
Here, we note two important correspondences between the wave forcing and zonal wind in the MLT region, which are related to the discussion later in this paper. First, strong positive (eastward) wave forcing in the summer mesosphere is distributed along the low-latitude side of the summer westward jet, tilting poleward with height. This feature is consistent with the SABER observations, although the analysis by the SABER observations provided only absolute values of the wave forcing (e.g., Ern et al. 2011). This suggests that the strong latitudinal and vertical shear of the westward jet is maintained by the wave forcing. Second, negative wave forcing in the winter hemisphere is maximized at middle and high latitudes around
b. Each wave forcing
Figure 3 separately shows the latitude–height sections of the climatology of the EP flux and EPFD associated with RWs, resolved GWs, and TWs and the parameterized GW forcing (GWFP) for January, April, July, and October. First, the characteristics of each wave contribution will be shown; second, the contribution of each wave to the total wave forcing will be discussed.
The characteristic features of RWs in January were already discussed in section 4. The strong upward and slightly equatorward EP fluxes in the stratosphere in January in the NH for RWs are also observed in July and October in the SH, and they are quite weak in April in the NH. However, there are a few regions with positive EPFD even in the stratosphere, suggesting the presence of nonlinear processes. The divergence of the upward EP fluxes is strongly negative at middle latitudes in the lower mesosphere slightly above
Significant EP fluxes associated with resolved GWs are observed in the MLT region in all seasons. We considered that the components designated “resolved GWs” are GWs because the EP fluxes are mainly due to
The contribution of TWs to the momentum budget is large at low latitudes, particularly in equinoctial seasons. EP fluxes and the EPFD are negative near the equator for
GWFP is mainly positive (negative) in the summer (winter) MLT region, similar to the EPFD caused by resolved GWs. Positive GWFP in the summer upper mesosphere has two maxima located at low to middle latitudes and at high latitudes. The high-latitude maximum reflects the total wave forcing and should be neglected because it is likely too strong a source of fluxes given in the GW parameterization, as indicated by (Geller et al. 2013) based on a comparison of parameterized GW forcing with observations and GW-resolving model simulations. In contrast, negative GWFP in the winter hemisphere has a single maximum at middle latitudes. It is interesting that the maxima of the positive and negative GWFP in the solstitial seasons are located at lower altitudes than those of EPFD caused by resolved GWs. The features during the equinoctial seasons are interesting. The GWFP in autumn is negative, and its distribution is similar to that in winter. In contrast, GWFP in spring is negative below
Next, the contribution of each wave to the total wave forcing is examined by comparing features observed in Figs. 2 and 3. During solstitial seasons, the wave forcing in the MLT region (i.e., z > 50 km) is mainly contributed to by resolved and parameterized GWs. The EPFD caused by RWs tends to mitigate the GW forcing in the summer MLT region, except for the positive EPFD observed at middle to high latitudes around z = 70 km. In contrast, the EPFD caused by RWs reinforces the GW forcing in the winter MLT region. The EPFD caused by RWs has magnitudes that are comparable to those caused by resolved GWs for both the summer and winter seasons, indicating that the EPFD due to RWs generated in the mesosphere is not negligible in the MLT region. In the stratosphere, the total wave forcing is large in winter and mostly attributable to the EPFD due to RWs and partly to GWFP, particularly for the upper stratosphere. The total wave forcing and each wave forcing in the summer stratosphere are small.
Contributions by respective waves in the stratosphere and MLT region in autumn are similar to those in winter, although the magnitude is relatively small. The total wave forcing in spring is mainly negative for z < 70 km and positive for z > 70 km. This structure is mainly attributable to the GWFP, but the EPFD due to resolved GWs contributes largely in the upper region, and the contribution of RWs is large in the lower region. It may also be worth noting for equinoctial seasons that the EPFD associated with TWs at low latitudes is largely canceled by that associated with RWs.
In summary, the total wave forcing, as a measure of the momentum budget, is basically determined by RWs in the winter stratosphere and by parameterized GWs in the MLT region. The contribution of parameterized GWs is large in the equinoctial stratosphere. However, RWs and resolved GWs in the MLT region, which are likely generated in situ in the mesosphere, make a significant contribution to the momentum budget. The TW contribution is limited to the low-latitude region of the lower thermosphere. Note that resolved GWs in GAIA may be largely different from GWs in the real atmosphere because of the relatively coarse horizontal resolution of the model. However, considering that the mean fields simulated in GAIA are in good agreement with observations (Figs. 1b,e) and that the features of EP flux and EPFD associated with RWs simulated in GAIA are also realistic (Figs. 1c,f), to simulate the realistic mean fields, it is likely that the EPFD of resolved GWs is realistic in GAIA.
6. Rossby waves generated in the MLT region
As discussed in the last section, some evidence exists suggesting the in situ generation of RWs in the mesosphere. First, RWs generated in the troposphere hardly propagate upward in summer because the stratospheric mean wind is westward. Second, positive EPFD is observed at high latitudes of the mesosphere in both the summer and winter seasons. A likely mechanism of the RW generation in the mesosphere is BT/BC instability. In this section, the instability condition is examined in terms of PV from a climatological viewpoint regarding the robustness, seasonal variability, and maintenance mechanism. Moreover, characteristics of the in situ generated RWs in the mesosphere and their relation to the anomalous PV structure are studied.
a. Modified potential vorticity in the MLT region
Figure 5 shows the
The characteristic features of the
Next, seasonal variations of the
Figure 7b shows the time–latitude section of the EPFD climatology, averaged for
b. Rossby wave characteristics in the MLT region
In this section, we examine the characteristics of the in situ generated RWs in both the summer and winter mesospheres. First, we show the zonal wavenumber and frequency power spectra of geopotential fluctuations at
Figure 11 shows the result for the 4-day waves, which are extracted as components with 1 <
c. Physical mechanisms of PV maximum formation in the mesosphere
According to the case study by Sato and Nomoto (2015) using the KANTO model, a gravity wave–resolving GCM, GW forcing is responsible for the PV maximum at middle latitudes of the winter mesosphere: GW forcing causes an upwelling at middle latitudes, increases
It is also worth noting that the magnitude of the positive
7. Summary and concluding remarks
This study examined the climatology of the momentum budget in the MLT region using simulation data from a whole-atmosphere model (GAIA) covering ~11 years, from August 2004 to June 2015, in terms of the respective wave contributions. Surprisingly, resolved GWs with large amplitudes appeared in the mesosphere and lower thermosphere, regardless of the relatively coarse horizontal resolution of the model whose truncation wavenumber is 42. Thus, four components were analyzed: contributions by 1) RWs, 2) resolved GWs, 3) TWs, and 4) parameterized GW forcing.
It was found that resolved and parameterized GW forcings are comparable and the main contributors to the total wave forcing in the MLT region in all seasons, although the dominant region is slightly higher for resolved GW forcing. TW forcing is dominant at low latitudes of the lower thermosphere, particularly for equinoctial seasons. It is also interesting that large EP flux associated with RWs is observed in both summer and winter MLT regions. The characteristics of the EPFD associated with RWs and resolved GWs suggest that these waves are in situ generated in the mesosphere and significantly contribute to the momentum budget in the MLT region.
Next, the generation mechanisms of RWs in the mesosphere were examined. It was shown that a positive (negative)
In summary, it was shown that the momentum budget of the MLT region is attributable not only to RWs and GWs originating from the troposphere but also to contributions made by RWs and GWs that are generated in situ in the mesosphere. The generation of RWs in the mesosphere is largely due to the BT/BC instability caused by the forcing due to GWs from the troposphere. It is also worth noting that the nonlinear dynamics may also be important, particularly in the NH winter, during which strong RW activity was observed. In addition, RW generation in the mesosphere due to filtered GWs and the stratospheric RWs may also be important (Smith 2003). The generation mechanism of GWs in the mesosphere is also an important and interesting issue. The characteristics and generation mechanisms of the resolved GWs in our model will be closely examined in the companion paper (Part II). The importance of the forcing of GWs from the lower atmosphere will again be highlighted.
It is also worth noting here that currently, most of the GW parameterization schemes are applied with some oversimplifications, including the assumption of a steady-state wave field and background flow, instantaneous GW propagation, and one-dimensional vertical propagation. Many authors (Bühler and McIntyre 1999, 2003, 2005; Dosser and Sutherland 2011; Bölöni et al. 2016; Amemiya and Sato 2016) have suggested that important aspects of wave–flow interaction could be neglected due to the above oversimplifications. Thus, there is still room for reducing the model errors due to the oversimplifications of GW parameterizations. However, the significant consistency between Aura MLS observations and GAIA model simulation shown in this study indicates that the overall features of the GW momentum transport in the middle atmosphere revealed by this study must be close to the real world.
Acknowledgments
This work was supported by JST CREST JPMJCR1663. The figures were drawn by the GFD-DENNOU library. The numerical simulation in this work was performed using the Hitachi SR16000/M1 and the NICT Science Cloud System, Japan. The Aura MLS data were obtained from the Jet Propulsion Laboratory, California Institute of Technology (from
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