2 and 3-dimensional structure of the descent of mesospheric trace constituents after the 2013 SSW elevated stratopause event

We use the Specified Dynamics version of the Whole Atmosphere Community Climate Model Extended (SDWACCMX) to model the descent of nitric oxide (NO) and other mesospheric tracers in the extended, elevated stratopause phase of the 2013 Sudden Stratospheric Warming (SSW). The dynamics are specified with a high altitude version of the Navy Global Environmental model (NAVGEM-HA). Consistent with our earlier published results, we find that using a high altitude meteorological analysis to nudge WACCMX allows for a realistic simulation of the descent of lower thermospheric nitric oxide 5 down to the lower mesosphere, near 60 km. This is important because these simulations only included auroral electrons, and did not consider additional sources of NO from higher energy particles, for example, medium energy electron precipitation (> 30 keV). This suggests that the so-called energetic particle precipitation indirect effect (EPP-IE) can be accurately simulated, at least in years of low geomagnetic activity, such as 2013, without the need for additional NO production, provided the meteorology is accurately constrained. Despite the general success of WACCMX in simulating mesospheric NO, a detailed comparison 10 of the WACCMX fields with the analyzed NAVGEM-HA H2O and satellite NO and H2O data from the Solar Occultation for Ice Experiment (SOFIE) and the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) reveals significant differences in the latitudinal and longitudinal distributions in the 45-55 km region. This stems from the tendency for WACCMX descent to maximize at sub-polar latitudes and while such sub-polar descent is seen in the NAVGEM-HA analysis, it is more transient than in the WACCMX simulation. These differences are linked to differences in the Transformed Eulerian 15 Mean (TEM) circulation between NAVGEM-HA and WACCMX, most likely arising from small differences in how gravity wave forcing is represented. To attempt to compensate for the differing distributions of model vs. observed NO and to enable us to quantify the total amount of upper atmospheric NO delivered to the stratopause region, we use potential vorticity and equivalent latitude coordinates. Preliminary results suggest both model and observations are generally consistent with NO totals in the range of 0.1-0.25 gigamoles (GM). 20 1 https://doi.org/10.5194/acp-2021-68 Preprint. Discussion started: 27 January 2021 c © Author(s) 2021. CC BY 4.0 License.


Introduction
One of the more interesting and challenging problems in middle atmospheric science has been to accurately represent the descent of upper mesospheric and thermospheric nitric oxide (N O) down to the stratopause and below during polar winter conditions. The idea is that since NO is copiously produced in the lower thermosphere by auroral and photo-electrons (Gerard and Barth, 1977;Barth et al., 1988;Siskind et al., 1989Siskind et al., , 1990, if some fraction of this NO were to be transported to the middle 5 atmosphere, it could play a role in the stratospheric ozone budget. This would signify that energetic electron precipitation (EEP) in the upper atmosphere could affect middle atmospheric (and possibly lower atmospheric) (Seppala et al., 2009) chemistry and climate. That such upper-to-middle atmospheric coupling might occur was proposed many years ago by Solomon et al. (1982). They noted that the most likely place for this to occur would be in the winter polar regions where the mean circulation favors descent and also where photolysis would naturally be weak. Observational validation of this idea remained course these events were not all equally intense or appropriately timed during the winter season to yield maximal NO descent (cf. discussion by Holt et al., 2013); for example some dry mesospheric air was seen to descend after a SSW in 2010 (Straub et al., 2012) but there was no observed descent of upper mesospheric NO (Perot et al., 2014). The 2013 event, which is the focus of this paper, was one of the strongest in terms of bringing down enhanced NO from the upper mesosphere to the upper stratosphere.
Ultimately, the challenge associated with these events is in accurately simulating this mechanism of long range atmospheric coupling by whole atmosphere models. This has proven difficult. Simulations of mesospheric and lower thermospheric (MLT) NOx descent using the Whole Atmosphere Community Climate Model (WACCM, or its extended version WACCMX) have greatly underestimated the amount of NOx. This underestimate has been reported both for the Southern Hemisphere (Pettit et al., 2019) and for the Northern Hemisphere Orsolini et al., 2017). A similar deficit was also reported 10 with the Hamburg model of Neutral and Ionized Atmosphere (HAMMONIA) (Meraner et al., 2016) when operated in a free-running mode (i.e. unconstrained by observations). A comprehensive overview of models of NO descent is given by Funke et al (2017). They concluded that "the magnitude of the simulated NO tongue is generally underestimated by these models". This underestimate has raised questions about whether the deficiency lies in the neglect of photochemical sources from, for example, medium energy electrons or from an incomplete specification of mesospheric dynamics. In our previous 15 work ( Siskind et al., 2015), we supported the latter hypothesis, at least in years with low geomagnetic activity.. Specifically, we showed that nudging WACCM with a meteorological analysis that extended up to the upper mesosphere and which presumably provided more realistic dynamical fields, yielded dramatic improvements in the representation of NO descent from the MLT to the lower mesosphere (Siskind et al., 2015). Pedatella et al. (2018) reached a similar conclusion, although their combined WACCMX/DART system still underestimated the descent. Here, we follow up on those works to consider in more detail the 20 ability of such a nudged model to capture this descent accurately. As part of our analysis, we will consider the two and three dimensional manifestation of the descent; most previous modeling studies have been limited to coarser averages over longitude and latitudes (although see Salmi et al., 2011). Section 2 below discusses the modeling approach, Section 3 shows zonally averaged results, Section 4 presents results as a function of both latitude and longitude, Section 5 attempts to quantify the deposition of MLT NO in the upper stratosphere both as observed and as simulated by WACCMX and Section 6 discusses the 25 implication of these results and provides conclusions.

WACCMX
In this paper, we use the Whole Atmosphere Community Climate Model, extended version (WACCMX; Liu et al., 2010). The domain of this model extends from the ground to about 500 km. It is divided into 108 vertical levels such that the vertical 30 resolution is about 2 km in the upper stratosphere and lower mesosphere, and set to be one-fourth of the local pressure scale height in the thermosphere. WACCMX can be configured to use atmospheric specifications to constrain its meteorology (winds and temperature) from the ground to any altitude; this model configuration is referred to as Specified Dynamics (SD); see 3 https://doi.org /10.5194/acp-2021-68 Preprint. Discussion started: 27 January 2021 c Author(s) 2021. CC BY 4.0 License. Sassi et al., (2013) for some details of the initial implementation. Note, the NO descent study of Siskind et al. (2015), to which we referred above, used WACCM, not WACCMX. More recently, McDonald et al. (2018) have documented significant improvements to the representation of tidal amplitudes when 3-hourly meteorological fields from NAVGEM-HA (see next section) are supplied to WACCMX. Sassi et al. (2018), using atmospheric specifications from NAVGEM-HA up to 90 km altitude in SD-WACCMX, showed a significant influence on the wave driving of the thermospheric circulation compared with 5 using meteorological input only up to lower altitudes (e.g. 0-50 km). The simulation used in this study is exactly the same "hybma" (hybrid data assimilation with middle atmospheric observations) model run described in Sassi et al., (2020) that uses the NAVGEM-HA output. As in the previous studies referred to above, the nudging is applied to the WACCMX winds and temperatures, but not the trace constituents. 10 To constrain WACCMX dynamical fields, we use a meteorological analysis of winds, temperatures and constituents from a high altitude version of the Navy Global Envirnomental Model (NAVGEM-HA). NAVGEM-HA is the middle atmospheric extension of the Navy's operational weather forecast system (Hogan et al., 2014). Details of the high altitude extension are provided by Eckermann et al., (2018);McCormack et al (2017) andHoppel et al., (2013). Briefly, to supplement the operational troposheric and stratospheric observations used in the operational forecast system, middle atmosphere conditions in 15 NAVGEM-HA are constrained by the additional assimilation of three satellite datasets. These include (1) Microwave Limb Sounder (MLS) temperature, ozone and water vapor (Schwartz et al., 2008) (2) temperature profiles from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) (Remsberg et al., 2008;Rezac et al., 2015) and (3) microwave radiances from the upper atmosphere sounding channels of the Special Sensor Microwave Imager/Sounder (SSMIS) on the Defense Meteteorological Satellite Program (DMSP) platforms (Swadley et al., 2008). Synoptic analyses of horizontal winds 20 and temperatures are produced at a 6 hourly cadence and are used to initialize shortterm forecasts, ultimately producing a 3-hourly analysis/forecast product each day up to ≈ 100 km. Validation of NAVGEM-HA wind fields against independent ground based wind measurements have been provided by McCormack et al.,(2017), Eckermann et al. (2018) andJones et al. (2020). Also the tides derived from these winds have been compared with independent satellite data (Dhadly et al., 2018).

NAVGEM-HA
Finally, Eckermann et al. (2018) successfully compared NAVGEM-HA temperatures against AIM SOFIE data. However, there 25 has not yet been published comparison of NAVGEM-HA tracer (i.e. H 2 O) analyses with independent observations; this will be provided in Section 3 below.

Model Results
Our baseline comparisons are shown in Figures 1 and 2 a pressure grid. In interpreting this figure it is important to note that both observations and model are zonal averages. As we shall see, there is considerable longitudinal variability in the data such that values of, for example, NO equal to 100-200 ppbv in the zonal average actually reflect longitude sectors where much greater values are observed combined with many longitude sectors where the data were too low to allow for a retrieval and were set to 1E-5 ppbv. The top of the left hand column shows the progression of the occultation latitudes and the model is sampled at the same latitudes.  between SOFIE and WACCMX NO of a factor of 2-3 that may reflect either how energetic particle precipitation is represented or how transport is handled in the model. However, since neither the SOFIE nor WACCMX results indicate a substantial contribution of air from above 0.01 hPa to below 0.2 hPa during the period from mid-January to mid-February, it is unclear to what extent these higher altitude differences are relevant to the present study. Note, Orsolini et al. (2017) showed ODIN Sub- 15 millimeter radiometer (SMR) H 2 O data and the pattern of descent, averaged over a broader latitude band than SOFIE, appears similar to what we see in Figure 1 (see their Figure 7B, averaged from 70-90 • ).
Overall, the general good agreement between calculated and observed NO at 0.1-0.2 hPa, and the lack of evidence for a substantial contribution of air from the uppermost mesosphere, where medium energy electrons (MEE, energies greater than 30 keV) deposit their energy, combine to have implications for whether or not MEE ionization is important for simulating the 20 EPP IE in early 2013. Ionization from MEE can create NO between 0.01 and 0.001 hPa, but since neither the NO nor the H 2 O contours show any significant contribution of air from those altitudes, it would not be possible for this ionization to compensate for the inability of previously published simulations to bring down enhanced NO from the upper mesosphere (Orsolini et al., 2017;Pedatella et al., 2018). Our results therefore suggest that for this specific period under consideration, an additional odd nitrogen source from MEE precipitation is not required. 25 At the same time, despite the apparent success of the model in delivering NO abundances of > 100 ppbv to the midmesosphere, Figure 1 clearly shows some interesting differences between the model and SOFIE at higher pressures. Here, both the NO and the H 2 O suggest that the model descent is much less than in the data. For example, SOFIE shows a narrow tongue of H 2 O less than 3 ppmv continuously descending to near 2 hpa by the end of March. By contrast, the isolines of H 2 O in WACCMX are much more nearly horizontal and the dry mesospheric air does not penetrate to such low altitudes in the model.

30
Regarding the NO, SOFIE also shows the enhancement descending to about 2 hpa while the enhanced layer in WACCMX remains above the 1 hpa level. Thus from mid-February onwards, the descent in WACCMX appears much less than what is observed. Indeed, based upon the relatively flat slopes of the NO and H 2 O contours (see for example, the 4.0 to 5.0 ppmv H 2 O isolines) in WACCMX, the descent in the model appears to almost stall from early/mid February to the end of the month. In subsequent sections of the paper, we will explore the details of and possible reasons for these differences and their implications for quantifying the delivery of MLT NO to the upper stratosphere.
Our second baseline comparison is given in Figure 2, which presents high latitude averaged mixing ratio profiles of NO for two specific dates, 19 Feb and 5 March, corresponding to the dates given in Figure 3 of Orsolini et al., 2017 (note, they referenced their profiles by the onset of the SSW on January 5 as Day 0; those day numbers are shown in the Figure). They 5 compared WACCM simulations nudged by MERRA with ODIN/SMR data and showed that their calculated NO descent underpredicted the observations by over an order of magnitude. They show the SMR data as clearly presenting a tongue of enhanced NO descending throughout the period, but their model shows no evidence for such an enhancement. We interpret the difference between our model results and Orsolini et al.'s as due to our use of a high altitude analysis to better constrain the meteorology of the mid/upper mesosphere. (Note, Orsolini used MERRA to nudge WACCM; subsequently MERRA-2 (Gelaro 10 et al., 2017) has been shown to provide a much improved representation of elevated stratopause events). While we do not attempt to make a detailed comparison with the SMR data, there is one additional feature which can be gleaned from Figure   2 here, namely the altitude of the model NO tongue on February 19. Figure 2 shows it to be at 0.1 hpa, while an inspection of Orsolini et al's figure indicates that SMR measured it at a lower altitude, closer to 0.2 hpa. Admittedly, this is only a very qualitative comparison; however, it does appear consistent with the slower descent in WACCMX relative to the SOFIE data 15 that is more clearly demonstrated in Figure 1.
We can more precisely see the difference in descent rate by tracking the evolution of the vertical profiles of H 2 O from SOFIE, WACCMX and also the NAVGEM-HA analysis, all sampled at the SOFIE occultation latitudes. This is seen in Figure   3   To understand these behaviors more fully requires a multi-dimensional comparison between observed and calculated descent. This is the focus of the next section.  It is immediately clear that in the WACCMX results for February 1 and 15, the strongest descent occurs not at the pole but near 70 • N, i.e. where a noticeable dip in the contour lines is seen. By March 01, the sub polar descent in WACCMX has faded and there is a suggestion that the overall descent has noticeably weakened, i.e. the very lowest values of H 2 O have receded upwards somewhat (look at the pressure level associated with the 5.0 ppmv contour). This weakening of the descent may be at 10 least partially consistent with the apparent stalling of the descent we saw in Figure 3. A second reason for this apparent stalling of descent in WACCMX is the sampling. With a postive gradient of model H 2 O going from sub-polar to polar latitudes, as the SOFIE occultation latitudes move to higher latitudes, it will tend to sample model regions with somewhat higher values of H 2 O. Thus we see that on February 15, the SOFIE sampling has moved poleward of the minimum value of the H 2 O in the lower mesosphere. This will be also be seen clearly below when we show polar plots. 15 In the bottom row of Figure  The existence and consequence of off-polar descent, most notably in WACCM but also to some degree in NAVGEM-HA as well, can be dramatically visualized in the polar plots presented in Figure 5. These are surfaces of H 2 O mixing ratio, for the 30 first 3 dates shown in Figure 4, at the 0.32 hpa level that was shown as a reference line in Figure 4. Also shown as thick black rings, are the SOFIE occultation latitudes for these dates. The off-polar descent is manifested as a ring of circumpolar dry air, with a local mixing ratio maximum centered at the pole. This is present in WACCMX on February 1st and in both WACCMX the SOFIE sampling in WACCMX can been seen by the fact that the smaller black ring in the WACCMX February 15 panel completely misses the dry annulus of H 2 O even though the actual value of H 2 O in this annulus is lower on February 15 than on February 1st. Therefore we see that descent has occurred in WACCMX, but the SOFIE sampling completely misses it and continues to sample only air with mixing ratios > 5 ppmv (the yellow colors). This demonstrates the need for care in sampling general circulation models to correspond to occultation data at single latitudes. 5 By March 1st, the WACCMX and NAVGEM-HA fields are very different. Thus the annulus in WACCMX is still present, but displaced such that the local maximum is over Northern Greenland. By contrast, these longitudes are precisely where the deep minimum in NAVGEM-HA H 2 O is seen. The relative phasing of WACCMX and NAVGEM-HA H 2 O and temperature can be most clearly seen in Figure 6 which presents line plots of the longitudinal variability of these WACCMX and NAVGEM-HA fields, and additionally, shows the SOFIE data for comparison. On February 15, in general, all three temperature products agree; 10 there is a small wave with a maximum near 80-100 • E and a minimum near 220-240 • E. The H 2 O fields show all little variation for this date (the absolute abundance in WACCMX is higher, consistent with what was discussed in regards to Figure 4). By March 1, the situation is different. The temperature variation is greater for all three products and there are some interesting differences in the water vapor variation. The WACCMX field shows a general anticorrelation between the longitudinal variation in H 2 O and temperature while NAVGEM-HA and SOFIE show a positive correlation. These differences are, at first glance, 15 confusing. In general we'd expect the downward advection of dry mesospheric air to be associated with the descending warm stratopause, i.e. an anticorrelation, consistent with observations (Manney et al., 2008). WACCMX conforms to this expectation but NAVGEM-HA and SOFIE do not and are almost 180 degrees out of phase-they show the lowest H 2 O corresponding to the coldest temperatures. Here, having the SOFIE data as an independent validation is useful for what otherwise would be a puzzling difference between WACCMX and NAVGEM-HA. 20 We suggest that the differences between WACCMX and NAVGEM-HA are likely due to differences in the relative roles of vertical and horizontal advection. We can gain some insights into this by looking at the Transformed Eulerian Mean (TEM) circulations inferred from the WACCMX and NAVGEM-HA wind fields. This can be seen in Figure 7 which compares the monthly averaged (for February) TEM horizontal and vertical winds calculated from WACCMX and NAVGEM-HA for the mesosphere plotted poleward of 30 • N. The approach is the same as discussed by Siskind et al (2010Siskind et al ( , 2015. whereby the 25 meridional motion, v*, is defined by equation (4)  In general, the residual mean circulation is driven by wave forcing, either from large scale planetary waves or small scale gravity waves (Andrews et al., 1987;Smith, 2012). Since the larger scale forcing is presumably constrained by NAVGEM-HA, any differences in v* and w* between WACCMX and NAVGEM-HA are most likely due to unresolved small scale gravity waves that are not exactly captured either by the nudging procedure or by the WACCMX gravity wave parameterization. A 15 truly comprehensive examination of the causes of these differences is beyond the scope of the present study. The differences in the distribution of H 2 O do suggest that nudging to realistic meteorology, while a necessary step, may in and of itself be insufficient to capture the details of mesospheric descent in the polar vortex. Further, as we will discuss in the next section, the persistence of an annular descent pattern in WACCMX complicates the comparison of WACCMX NO and occultation data such as SOFIE. The differences between WACCMX and SOFIE in the H 2 O variation will have analogs in our model-data nitric 20 oxide comparison. However, despite these differences, we will show that the overall net transport of NO in WACCMX down to the stratopause is probably not far off from that suggested by the observations.

Delivery of MLT NO into the stratosphere
Here we attempt to evaluate the net deposition of WACCMX MLT NOx into the middle atmosphere and compare the model with satellite observations. In doing this comparison we must take into account the complex WACCMX tracer descent pattern 25 outlined above, whereby vortex-centered descent appears to encounter a zero wind line and thus diverges into an annulus. This pattern of descent in WACCMX means that a straightforward comparison of WACCMX with satellite data can be misleading.
The satellite data that we use to illustrate this are from SOFIE and ACE. ACE is similar to SOFIE in that it is an occultation experiment; however, its orbit differs from AIM so the latitudinal sampling is different.

81
• N and diverge as the month progresses. We will perform our comparisons for March 1-3, when ACE and SOFIE were roughly coincident and also for the period March 23-25 when ACE was sampling around 68-72 • N and SOFIE was near the pole (87-88 • N). After this time ACE moved too far south to observe enhanced polar NO. Figure 8 shows an overall comparison of NO altitude profiles from SOFIE and ACE for the two periods we consider here.
Since both instruments are observing the same latitude for March 1-3, the left panel can be considered the more valid intercomparison. Both data products are provided on a very fine altitude grid (1 km for ACE, 200 meters for SOFIE) and the plot reflects that; however, their native vertical resolutions are coarser and also different for each experiment (3.5 km for ACE and 2.5 km for SOFIE (Hervig et al., 2019)). This appears to show up in the figure where it seems that the NO layer recorded by SOFIE 5 is narrower and perhaps peaked slightly more sharply than in ACE. With this consideration in mind, the intercomparison for March 1-3 shows good agreement. Note that both instruments show descent from peaks centered near 0.32 hpa in early March descending to about 1 hpa for March 23-25. We will present an additional comparison below using vortex centered coordinates (equivalent latitudes) which will provide useful additional context for the March 23-25 comparison when the two instruments were not sampling the same latitude. The use of equivalent latitude is important because it places the data and the model in a vortex centered framework. It shows that the equivalent latitude can vary by a significant amount as a function of longitude around a single latitude circle (e.g. see Randall et al., 2002;Randall et al., 2005b). This means that even though ACE and SOFIE are only sampling a single latitude on 20 a given day, they actually are sampling different regions of the vortex. This will be helpful in comparing the satellite data to the model. Both ACE and AIM tend to maximize at equivalent latitudes greater than 75-80 (i.e. at longitudes less than 30 or greater than 200-220). Note that these are the same longitudes that NAVGEM-HA and SOFIE showed the lowest H 2 O in Figure  geographic locations in WACCMX as it is in the observations. Figure 10 presents a global polar cap overview of the relevant quantities we used in our analysis. The figure shows polar stereographic views of equivalent latitude, geopotential height, CH 4 mixing ratio (ppmv), and NO x mixing ratio (log 10 (ppbv)). Note, Figure 10 shows NO x (=NO + NO 2 ), not simply NO as in Figures 8 and 9. This is because NO has a pronounced diurnal variation whereby it forms NO 2 at night and is reformed rapidly each day due to NO 2 photolysis at visible wavelengths. On the other hand, NO x is much longer lived (the discussion 5 regarding Figure 14 below will quantify this in more detail) and is more useful as a dynamical tracer, as it is presented in Figure 10. During daylight hours, for the pressures shown here, the difference between NO and NO x is small (< 5%) and we have confirmed this by looking both at the ACE NO 2 data (SOFIE does not measure NO 2 so cannot be used for this specific assessment) as well as the WACCM NO and NO 2 fields. Thus when we evaluate the total WACCMX budget of NO x , this will differ little from the total NO budget. Finally, on all panels, the thick black line indicates the vortex edge as defined by One of our approaches to quantifying net descent will be vortex centered, i.e. how much WACCMX NO x is contained inside the polar vortex defined by the black line. A second approach will account for the fact that, as seen in Figure 9, and evident here, vortex, to and below 60 • equivalent latitude. This is accompanied by lower values in the vortex core. To attempt to account for this varied dependence of NO x on equivalent latitude, we will evaluate the amount of WACCMX NO x that is bounded by different ranges of equivalent latitude. Finally, the third approach will be to use regions of low CH 4 as a flag for enhanced NO x . This will also have complications as we discuss below. These three approaches to quantifying the descending NO in WACCMX will then be compared to a simple geometric model of the latitudinal variation of the NO based upon the SOFIE 25 and ACE data.
A summary of these different results is presented in Table 1 where the total NOx is expressed in gigamoles (GM), for the two periods presented in Figure 8. As shown in Figure 8 (and also Figure 1) including molecules that were not of MLT origin. Use of equivalent latitudes allows us to account for both possibilities. The problem with this approach is that it is hard to define a single objective criterion for which range of equivalent latitudes to use.
Thus we select three bands to show the sensitivity of the answer to the different ranges-these are given in columns 2-4 of the Table. In general, the values obtained with this approach are roughly consistent with the vortex-only approach. The third and final approach to estimating the amount of MLT NO in WACCMX is to use the anticorrelation between CH 4 and MLT NO.

5
This has been long used in observational analyses, starting with the Halogen Occultation Experiment (HALOE) data on the UARS satellite, (cf. Callis et al., 1996;Siskind and Russell, 1996;Siskind et al., 2000) and relies on the idea that in general, CH 4 and NO x should be positively correlated in the stratosphere since both are ultimately of tropospheric origin. However, for those situations where air from the MLT has descended to the stratopshere, CH 4 and NO x will be anticorrelated since MLT air is depleted in CH 4 but enriched in NO x . With HALOE data, there typically was a single value of CH 4 that could to used as a 10 threshold to distinguish between the two air masses (cf Figure 4 of Siskind and Russell, 1996). The problem is that in WACCM, the CH 4 -NO x relationship is not necessarily single valued and the correlation between CH 4 and NO x is often ambiguous.
The complexity of the WACCMX CH 4 and NO x relationship in WACCMX is shown in Figure 11 for two pressures that correspond to the location of the WACCMX NO layer in Figure 1 for   Table 1 can be compared with our geometric model estimates for the two periods that are, in turn, compared with ACE and SOFIE data. Our geometric approach to estimate the MLT NO implied by the observations is different than with the model since we do not have complete global coverage. However, we can make some 30 plausible estimates by looking at the variation of ACE and SOFIE data with equivalent latitude. This is shown in Figure 12.
The data in the left column can be compared the WACCMX data seen in Figures 9 and 10. The data in the right column can be compared with Figure 13 which shows model results for March 23-25 for the three pressures which roughly bracket the enhanced NO in WACCMX seen in Figure 1. In general, for both dates, it appears that the measurments do not spread to such low equivalent latitudes as suggested by the model. Whereas Figure 10 indicated the presence of MLT NOx in WACCMX over latitudes less than about 73 • for this period (note, both AIM and SOFIE were taking samples at these locations but if the signal is too low for a good retrieval, they will report near zero values in their databases). Likewise for the March 23-25 period, Figure   13 shows that the model NO often maximizes at equivalent latitudes between 65-70 • while the data show clear fall offs for equivalent latitudes less than 70. Our specific approach to estimating how much NO might be consistent with the observations 5 is to make the very simple assumption that it is constant as per the horizontal lines in the figure, over the indicated range of equivalent latitudes (i.e. down to 73N for March 1-3 and 70N for March 23-35), for a 5 km thick layer. The resulting estimates from this approach are given in the last column of Table 1. It is interesting that our estimates are of the same order as, albeit perhaps a factor of 1.5-2 lower than, our estimates from the model.
The advantage of this approach is that it is ultimately based upon simple geometry and potential uncertainties can be readily 10 assessed. For example, one uncertainty is the actual thickness of the enhanced NO layer. The data suggest that the layer is narrow and near the limit of the vertical resolution of both the satellites and of WACCMX (recall WACCMX gridding corresponds to about 2 km resolution near the stratopause). With our geometric approach, we can easily scale our answers.
Thus if the layer were actually 6 km thick, our estimates would be low by 20%; if the layer were 4 km thick, our estimates might be equivalently too high. To double our estimated value for March 23-25 (i.e. to bring it more to the middle of the model 15 estimates) by expanding the area would require 20 ppbv to be present at equivalent altitudes as low as 60N. Enhanced NO at such low equivalent latitudes does not appear to be present in the data and thus we give an approximate upper limit of a factor of 2 for the accuracy of our estimates.
Ultimately, all these comparisons and indirect approaches reinforce the same point, namely that the MLT NO (or NOx, as discussed above, the difference is immaterial) in WACCMX does not present at the same altitudes, latitudes or longitudes as 20 indicated by the SOFIE and ACE data. However, the global totals are quite close. Certainly the immediate conclusion one draws is that it is hard to argue that the WACCMX/NAVGEM-HA system is underestimating the NOx descent such that we would require additional sources of NO.

Discussion and Conclusions
We draw two conclusions from our study. The first deals with the 2 and 3D morphology of the mesospheric descent. It appears 25 that subtle differences in the residual mean circulation, presumably due to equivalently subtle differences in gravity wave forcing, can have surprisingly large effects on tracer transport near the stratopause. The existence of off-polar descent was seen many years ago in analyses of the Antarctic polar vortex (Manney et al., 1994); our results here, reflecting the MLS observations assimilated into NAVGEM-HA, provide the first observational support for this morphology at lower mesospheric altitudes. At the same time, it is clear that this effect is overestimated in WACCMX. Another consequence of the differences 30 in tracer transport between WACCMX and observations is that MLT NOx in WACCMX spreads equatorward more than what ACE and SOFIE suggest. This then could have some consequences for the ability of WACCMX to incorporate MLT NOx into the stratospheric NOx layer; namely NO may photodissociate too quickly in the model. This is illustrated by Figure 14 13 https://doi.org/10.5194/acp-2021-68 Preprint. Discussion started: 27 January 2021 c Author(s) 2021. CC BY 4.0 License.
which presents the diurnally averaged lifetime of NO against photodissociation using the parameterization of Minschwaner and Siskind, (1993) as a function of latitude for equinox conditions. It shows that for latitudes equatorward of 70 • N, where much of the NO in WACCMX is found, the lifetime against dissociation is on the order of 2-4 weeks at 1 hpa. By contrast, for the regions where the NO in the observations is found, in the vortex core, poleward of 75 • N, the lifetime is much longercloser to 2-3 months. Nitric oxide here would see much less sunlight than in the model, and could continue descending lower 5 into the stratosphere.
Our second main conclusion follows from our earlier study (Siskind et al., 2015) and is that we continue to not require any added chemical production from medium energy electrons. This conclusion echoes that reached by Shepherd et al., (2014)   Res. Lett.,15, 92-94, 1988.          Figure 13. WACCMX NO, sampled according to the SOFIE (diamonds) and ACE (pluses) data shown in the right hand column in Figure   12. The pressures differ from those in Figure 12 due to the fact that the tongue of enhanced NO in WACCMX does not descend as low as is observed (see Figure 1).