Mesospheric rotational temperatures from O₂(0-1) and OH(6-2) band nightglow emissions that originate from 94 and 87 km altitudes, respectively, were obtained from a low-latitude location, Mount Abu (24.6°N, 72.8°E), in India using a high spectral resolution grating spectrograph, which showed significant enhancements during the major sudden stratospheric warming (SSW) event of January 2013. To investigate the relationship of these enhancements in the context of SSW occurrences, a detailed study was carried out for 11 SSW events that occurred during 2004–2013 using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) data. In addition to SABER, Optical Spectrograph and Infrared Imaging System and Solar Occultation For Ice Experiment mesospheric temperatures were also used which showed similar latitudinal behavior as obtained by SABER. The longitudinal mean mesospheric temperatures at different latitudes of Northern and Southern Hemispheres have been derived. It is found that during SSW events the well-known mesospheric cooling over the Northern Hemispheric high latitudes turns to heating over midlatitudes and then reverts to cooling closer to equatorial regions. This trend continues into the Southern Hemisphere as well. These variations in the mesospheric temperatures at different latitudes have been characterized based on northern hemispheric stratospheric temperature enhancements at high latitudes during SSW periods. In comparison with the COSPAR International Reference Atmosphere-86-derived temperatures, the SABER temperatures show an increase/decrease in Southern/Northern Hemisphere. Such a characterization in mesospheric temperatures with respect to latitudes reveals a hitherto unknown intriguing nature of the latitudinal coupling in the mesosphere that gets set up during the SSW events.

Key Points

Mesospheric heating over low latitudes observed during SSW events
Latitudinal variations in mesospheric temperatures have been characterized
“Double-humped” structure in mesospheric temperatures revealed during SSW

1 Introduction

Sudden stratospheric warming (SSW) is a large-scale phenomenon that occurs mostly in the northern hemispheric polar stratosphere during winter time and is characterized by a drastic rise in temperature within a few days. An SSW event is called “minor” if the mean temperature at or below the 10 hPa pressure level (~32 km) poleward of 60° rises by at least 25 K within a period of a week. An event is considered as “major” if in addition to increased temperatures, zonal mean eastward winds reverse at these altitude and latitude regions [McInturff, 1978; Labitzke, 1981]. SSW is attributed to the interaction of zonal winds with vertically propagating planetary waves (PWs) forced from the troposphere, and their effect in terms of quasi 16 day waves has also been seen to be present over low- and equatorial-latitude E and F regions of the ionosphere [e.g., Laskar et al., 2014]. It has also been shown that the westward acceleration of zonal winds over northern hemispheric high latitudes occurs during SSW periods in the presence of PWs [Matsuno, 1971]. Further, it has been shown that during SSW events mesospheric cooling occurs at high latitudes [Labitzke, 1972, 1981], which has also been confirmed by the OH airglow temperature measurements over both Northern and Southern Hemispheres (NH and SH) [e.g., Walterscheid et al., 2000; Azeem et al., 2005]. In the NH observation from Eureka, Canada (80°N, 86°W), mesospheric cooling of ~ 25 K was observed during 13 to 14 February 1993 [Walterscheid et al., 2000], while measurements over South Pole station in Antarctica showed a decrease in mesospheric temperatures of ~ 15 K prior to the stratospheric warmings of May 1995 and July 2002 [Azeem et al., 2005]. Moreover, by comparison of SABER temperatures at different pressure levels, it was noted that while the stratospheric temperature at 10 hPa pressure level increased over several days, the mesospheric temperature (in the 0.3 to 0.01 hPa pressure range) decreased simultaneously during 2002 SSW period over SH at 80°S latitude [Siskind et al., 2005]. Further, a new meridional circulation has been found to get set in the mesosphere/lower thermosphere (MLT) region during SSW events owing to the concomitant increase in the lower thermospheric temperatures in the altitudes above cooler mesopause in high latitudes [Laskar and Pallamraju, 2014]. Thus, it is imperative that a comprehensive investigation on latitudinal distribution of mesospheric temperatures be carried out, especially during SSW events.

Greater information on mesospheric temperatures and winds are available at high latitudes compared with those at equatorial, low, and middle latitudes for the periods during SSW events. Lower mesospheric temperatures at equatorial latitudes were first obtained using rocketsondes from Thumba (8.5°N, 76°E), in India, wherein it was reported that occasional warmings occur in the upper stratosphere and lower mesosphere during SSW periods [Mukherjee and Ramana Murty, 1972]. Recently, a warm mesospheric condition has been reported [Sridharan et al., 2010] during the 2009 SSW event over Gadanki (13.5°N, 79.2°E), a low-latitude location in India, using Rayleigh lidar observations. These observations were also found to be consistent with the SABER-derived temperatures obtained over the same location. In all these studies the temperature behavior at altitudes below 80 km was investigated. Due to the paucity of continuous ground-based measurements of mesospheric temperatures over equatorial to middle latitudes, the behavior of the mesosphere during SSW periods at these latitudes is not well understood.

To understand the low-latitude mesospheric temperature behavior, measurements of neutral temperatures corresponding to 94 km and 87 km altitudes are being carried out using O₂(0-1) and OH(6-2) band nightglow emissions, respectively, with a ground-based grating spectrograph called Near-Infrared Imaging Spectrograph (NIRIS) from Mount Abu (24.6°N, 72.8°E), in India. In this study, we have used high cadence NIRIS-derived mesospheric temperatures from a low-latitude location, SABER-, Optical Spectrograph and Infrared Imaging System (OSIRIS-), and Solar Occultation For Ice Experiment (SOFIE)-derived mesospheric temperatures at different latitudes and SSW temperatures and zonal winds as derived from Modern Era Retrospective-Analysis for Research and Applications (MERRA) data sets to characterize the behavior of mesospheric temperatures along the north-south meridian in both hemispheres. Stratospheric temperatures and winds at high latitudes (>60°N) are considered as tracers of the stratospheric behavior in the NH. In that regard the observed stratospheric temperature enhancements and zonal wind reversals at NH high latitudes during SSW periods would indicate the prevalent planetary wave activity.

2 Measurement Techniques

2.1 NIRIS

NIRIS is a high spectral resolution (0.21 nm at 840 nm) large field-of-view (FOV) (~80°) imaging spectrograph which has a spectral coverage of 823–894 nm. NIRIS was commissioned at Physical Research Laboratory's optical aeronomy observatory at Gurushikhar, Mount Abu (24.6°N, 72.8°E), in January 2013. The details of this spectrograph are described in Pallamraju et al. [2014]. The spectrograph has been used for balloon-borne ultraviolet measurements of OI 297.2 nm emission and has been suitably modified for measurements in the near-infrared spectral region. NIRIS yields O₂(0-1) atmospheric and OH(6-2) vibrational band spectra centered at 864.5 nm and 840 nm, respectively, at a high data cadence of ~5 min. From these nighttime emission line spectra, rotational temperatures are derived by taking the spectral line ratio of P₁(2) and P₁(4) rotational lines for the OH(6-2) vibrational band centered at 840 and 846.5 nm, respectively. This method for deriving mesospheric temperatures assumes that the underlying gas is in local thermodynamic equilibrium (LTE) [Mies, 1974; Meriwether, 1984] which is valid for OH(6-2) vibrational band having rotational levels N′ < 5 [e.g., Pendleton et al., 1993]. In this study as rotational levels corresponding to N′ = 1 and N′ = 3 are used, they satisfy LTE condition. To calculate the temperature using O₂(0-1) atmospheric bands, similar procedure was applied in which PP and PQ pairs of rotational lines for K″ = 7 and 15 centered at 866 and 868 nm wavelengths, respectively, are considered. The spectroscopic constants and line strength values are taken from Schlapp [1937]. The accuracy of temperatures derived by NIRIS is ±3 K. For the study reported in this paper zenith temperature measurements are used.

2.2 MERRA

MERRA has been developed by NASA's Global Modeling and Assimilation Office focusing on the satellite era (1979 to present), the details of which are provided by Rienecker et al. [2011]. In this study, MERRA data have been used to obtain the stratospheric temperatures poleward of 60°N and longitudinally averaged zonal wind at 60°N at 10 hPa pressure level (~32 km).

2.3 SABER

SABER onboard Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite is a 10 channel broadband limb-viewing, infrared radiometer that measures daytime and nighttime Earth limb emissions (~20–450 km tangent height range) in the spectral range of 1.27 µm to 17 µm [Russell et al., 1999]. SABER yields kinetic temperatures as a function of latitude and longitude in the altitude range of 20 to 140 km that are derived using the 15 µm CO₂ emissions, which account for non-LTE conditions in the region. The estimated uncertainty in the SABER temperatures in the altitude range of 80–100 km is ±4 K [Mertens et al., 2001].

2.4 OSIRIS

The OSIRIS instrument onboard Odin satellite is a limb-viewing instrument, which observes scattered solar radiation and airglow emissions in the wavelength range of 275–810 nm for tangent heights ranging from 10 to 100 km. Retrieval algorithms have been developed to recover the stratosphere and MLT temperatures from limb radiances [Haley and McDade, 2002; Sheese et al., 2010]. For the study reported in this paper we have used OSIRIS-derived mesospheric temperature data during 2009 and 2010 major SSW events.

2.5 SOFIE

SOFIE on board the Aeronomy of Ice in the Mesosphere satellite provides information on temperatures in the polar atmosphere with 15 sunset/sunrise measurements at latitudes from 65°–85°S/65°–85°N each day [Gordley et al., 2009]. We have used level 2a data to extract mesospheric temperatures at polar latitudes in both hemispheres during all the major SSW events considered in this study.

3 Observations and Results

The mesospheric nocturnal temperatures derived from O₂ and OH using NIRIS in the month of January 2013 during day of the year (DOY) 7–21 are shown in Figures 1a and 1b, respectively. From Figure 1a it may be noted that the mesospheric temperatures at 94 km show higher values for DOY 7, 8, 16, 18, and 19 before midnight and gradually reduce later. From Figure 1b it can be seen that on 8 January 2013 the mesospheric temperature enhancement is in the postmidnight period. On 16 January 2013 OH temperature enhancements are seen during premidnight hours and seem to be reduced during postmidnight. All these observations show modulations due to the presence of gravity, tidal, and planetary waves in the mesospheric temperatures at these altitudes. On 17 and 18 January the OH mesospheric temperatures are elevated for the duration of the data available in comparison with the nocturnal mean temperature value for this month (discussed below). The dissimilar behavior in the temperatures at these two altitudes is a result of vertically propagating gravity waves. The effects of PW-type scale sizes on these altitudes are discussed in this paper using these temperature values, while the characterization of temperatures at these two altitudes in the time scales of gravity and tidal wave regimes will be presented in a separate study.

Details are in the caption following the image — **Figure 1**
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(a) NIRIS-derived O₂ rotational temperatures (at 94 km) over Gurushikhar, Mount Abu, for a few selected nights during SSW period are shown together with the estimated uncertainty (~ ± 3 K) at each of the datum points. (b) Similar plot as in Figure 1a but for OH temperature (at 87 km). (c) Nocturnal mean mesospheric temperatures derived from O₂(0-1) atmospheric band for the month of January 2013 (black line) and monthly mean (blue line) are shown. The red vertical bar shows the range of the derived temperatures for a given night. (d) Similar plot as in Figure 1c but for OH-derived temperatures. It may be noted that mesospheric temperatures are elevated throughout the night (as shown in the range of temperatures) during the SSW period.

Figures 1c and 1d show nocturnal mean O₂ and OH rotational temperatures during 1–21 January and 1–29 January, respectively, together with the range of the temperature on a given night. From these figures it can be seen that mesospheric temperatures are elevated for several nights when compared with the monthly mean values. Especially, the nightly average together with the nightly range in the O₂ and OH temperatures is enhanced during 16 to 19 January and 16 to 26 January, respectively, when compared with the monthly mean. Interestingly, the enhancement in the mesospheric temperatures as seen from NIRIS data described here was found to be occurring during a major SSW event (7–27 January 2013; Figure 2h) which occurs at polar latitudes. In this context it is relevant to note that OH band emissions from higher vibrational levels peak at higher altitudes [von Savigny et al., 2012]. Using Scanning Imaging Absorption spectrometer for Atmospheric Cartography, on Envisat data, it was shown that the OH (6-2) emission rate profile is typically vertically shifted upward by 1–2 km relative to the OH (3-1) band, and the OH (8-3) band is generally found to peak about 1 km higher than the OH (6-2) band [von Savigny et al., 2012]. The O₂(0-1) emission layer is considered to be located at 94 km altitude [Murtagh et al., 1990], and the altitude of peak OH emission can be considered as 86.8 ± 2.6 km [Baker and Stair, 1988]. Further, using OSIRIS data, it has been shown that ground-based OH temperatures can be affected by changes in the OH emission layer height [Sheese et al., 2014] which could lead to an uncertainty in temperature of the order of ±2–4 K (and up to ~7 K in extreme case as seen during 2009 major SSW event over high latitudes). However, as can be seen from Figures 1c and 1d the enhancement in the NIRIS-derived temperatures is much larger and therefore is believed to be due to SSW-related effects and not by layer movement. This interpretation is confirmed by satellite-based measurements (as shown later).

These observed features in the NIRIS data provided motivation for a wider investigation to understand the behavior of global large-scale mesospheric temperature distribution that exists during SSW events. In this study, we have used SSW temperatures and winds at 10 hPa pressure level from MERRA- and SABER-derived mesospheric temperatures during 11 SSW events that occurred during the past 10 years (2004–2013) in the months of January and February. Of these 11 SSW events, six were major and five were minor in nature. In Figure 2, the temperature in the polar cap (T_S) poleward of 60°N and longitudinally averaged zonal wind (U_S) at 60°N from MERRA are shown for the years 2004–2013 (from DOY 1 to 65). Six major warming events occurred during the years in 2004, 2006, 2008, 2009, 2010, and 2013 where the zonal mean zonal winds at 60°N at 10 hPa pressure level reversed from eastward to westward. Five minor events were observed in the years 2008 (three events), and one each in 2011 and 2012, where the temperature in the polar cap poleward of 60°N at 10 hPa pressure level increased by more than 25 K without any reversal of zonal mean zonal winds. In Figure 2 the duration of all these events is marked by vertical dashed lines. A list of the duration together with the peak temperature and the minimum zonal wind during these periods is given in Table 1.

Table 1. List of SSW Events Considered for This Study

Year	DOY Range	T_Smax (K)	U_Smin (m s⁻¹)
2004	2–14	232.37	−14.67
2006	17–35	236.96	−26.15
2008 (I)	21–28	234.69	19.42
2008 (II)	32–41	230.42	29.03
2008 (III)	43–50	228.12	10.77
2008 (IV)	51–60	241.06	−14.58
2009	17–56	251.83	−31.08
2010	23–56	235.77	−6.92
2011	29–37	226.41	23.61
2012	14–26	234.77	6.57
2013	7–27	233.53	−12.29

The latitudinal coverage of the TIMED satellite is 135°, (83° and 52° in opposite hemispheres). Every 60 days the coverage of latitudes reverses due to periodic yaw maneuver of the satellite [Russell et al., 1999]. The latitudinal coverage of the SABER-derived temperature data is approximately from 55°S to 85°N for the 11 SSW events considered in this study. Thus, the data are organized into a total of 15 ranges of 10° latitude from 80°–90°N to 60°–50°S in the NH and SH. The SABER mean mesospheric temperatures (longitudinally averaged in the range of 2°–142° and in the altitude range of 85–89 km) are obtained at each of the 10° latitudinal intervals for all the SSW events considered in this study. Similarly, OSIRIS-derived mean mesospheric temperatures (with similar longitudinal averaging as for the SABER data but in the altitude range of 86–89 km) are obtained at each of the 10° latitudinal intervals for all major SSW events. For all the given latitudinal ranges the daily mean mesospheric temperature from OSIRIS is derived wherever the number of data points is more than 10 for a given DOY. The zonal mean mesospheric temperatures from SOFIE data set are derived with the latitude ranges poleward of 65° in both hemispheres for all the major SSW events considered here. In Figures 3a and 3b the blue colored lines show SABER-derived daily mean mesospheric temperature values at all the 15 latitude ranges as a function of DOY for the year 2013 and 2009, respectively, while the purple colored lines show the SOFIE-derived mean mesospheric temperatures over polar latitudes in both hemispheres. The dashed vertical lines in Figures 3a and 3b show the duration of the SSW events (DOY 7–27 in 2013 and DOY 17–56 in 2009). The dashed horizontal lines in Figures 3a and 3b show the mean mesospheric temperature (for a given latitude range) during the DOY ranges as indicated above for the 2013 and 2009 SSW events. From Figure 3a mesospheric cooling at high latitudes (>60°) over the NH is clearly seen during the SSW period especially during DOY 9–20 in both SABER and SOFIE data. As one moves from high to low latitudes, the mesospheric cooling turned into mesospheric heating around midlatitudes (during DOY 7–25 in 10°–50°N latitude range) before turning again to cooling over equatorial latitudes (during DOY 7–15). The red colored line in Figure 3a shows the NIRIS-derived nocturnal mean OH temperatures obtained at Gurushikhar, Mount Abu, which is overlaid on the longitudinal mean mesospheric temperatures obtained from SABER data sets for the latitude range of 20°–30°N. From Figure 3a the NIRIS-derived mesospheric temperatures are seen to be higher. This is not unexpected as the temperatures obtained by NIRIS are over a small FOV (zenith) and at a high data cadence as opposed to the SABER temperatures that are obtained by averaging a large longitude range (2°–142°). An enhancement in temperature is also noticed in the SH during DOY 10–23 (depending on the latitude). In Figure 3b similar behavior in the SABER- and SOFIE-derived mesospheric temperatures is seen for the major SSW events of 2009. The mesospheric cooling at higher latitudes (>50°N) is clearly observed in both SABER- and SOFIE-derived mesospheric temperatures. The OSIRIS-derived mesospheric temperatures are shown by magenta colored lines in Figure 3b for the 2009 major SSW event since OSIRIS data are not available for 2013. For 2009 SSW event the OSIRIS temperatures are available in the SH from 10°S to 90°S for the altitude, latitude, and longitude ranges as discussed above. As can be seen from Figure 3b, the OSIRIS temperature trends for the given latitudinal range match well with the SABER and SOFIE data with some differences in the absolute vales. It may be noted that the transitions from cooling to warming from high to low latitudes in the NH and SH occur with some finite time delay indicating a physical transport of energy from high latitudes in the NH to different latitudes extending to even those in the SH. As the SABER measurements have a better data cadence compared with that of OSIRIS, further analysis has been carried out using SABER-derived mesospheric temperatures. The observed latitudinal behavior of variability in the mesospheric temperatures has been characterized with respect to stratospheric temperatures poleward of 60°N at 10 hPa pressure level during SSW events and is explained below.

The mean mesospheric temperature for a given latitude range was obtained by averaging the temperature values over that latitude range for a week before and after the SSW period. The differences in mesospheric temperatures at a given latitude range from the mean mesospheric temperatures (ΔT_M) are calculated for all the 15 latitude ranges for a given SSW duration. Similarly, the deviations in stratospheric temperatures (ΔT_S) from the mean value (calculated by averaging T_S over 60°–90°N for 1 week before and after the SSW event) are obtained from the MERRA data set. As discussed earlier, there is a time delay in mesospheric response to the SSW events at high latitudes that is different at different latitudes. Therefore, we have defined the post-SSW period when the longitudinally averaged zonal wind at 60°N at 10 hPa pressure level for the major SSW events becomes close to zero. The SSW periods are summarized in Table 1 and are also shown by vertical dashed lines in Figure 2 for all the 11 SSW events considered in this study.

As seen in Figures 3a and 3b the mesospheric temperatures at NH high latitudes show cooling up to 60°–70°N during the SSW event with different behavior at other latitudes. Correlation analyses have been performed between ΔT_S at high latitude and ΔT_M obtained at different latitudes to investigate their interrelationships, if any. Figures 4a and 4b show the result of such analyses for some chosen latitude ranges for the 2013 and 2009 major SSW events, respectively. Here the horizontal and vertical axes represent stratospheric temperature deviation, ΔT_S, at high latitude and mesospheric temperature difference, ΔT_M, at different latitudes as discussed above, and each “plus” stands for a day during SSW period. The slopes and the correlation coefficient for each of the latitude ranges are also shown in the respective panels. The high latitudes (panels (i) and (ii) of Figures 4a and 4b) show a negative slope which indicates that increase in stratospheric temperatures, ΔT_S, is correlated with a decrease in mesospheric temperature, ΔT_M—which is consistent with general understanding. It is clearly seen (panel (iii)) that for 20°–30°N latitude range ΔT_M increases with increase in ΔT_S, indicating a mesospheric heating with respect to stratospheric warming. For the 0°–10°N latitude range ΔT_M decreases with increasing values of ΔT_S (panel (iv)). These observations show similar trends for all the major SSW events with different values of the slope for individual major SSW events. While the mesospheric cooling over high latitudes during SSW has been reported in the literature, the excursions in the mesospheric temperatures over other latitudes as detailed here have not been reported before.

The analyses relating the observed ΔT_M at different latitudes with ΔT_S at high latitude as shown in Figure 4 for a few latitude ranges have been extended to all the 15 latitude ranges in Figure 5a. Figure 5a includes the three major SSW events of 2009, 2010, and 2013 as these were stronger and extended over a longer duration among the six major events considered in this study. Similarly, in Figure 5b the combined behavior of ΔT_M with respect to ΔT_S for all five minor events at different latitude range is shown. Each plus in Figures 5a and 5b corresponds to a day during SSW period. From these scatterplots, correlation analyses have been performed and the slopes of the best fit lines obtained. The correlation coefficients (R) between ΔT_S and ΔT_M together with the slope of the best fit lines (S) are shown in each of the subplots in Figures 5a and 5b. It is apparent from the mesospheric temperature behavior as shown in Figure 3 that the transitions in temperature from high to low latitudes occur during different days (DOY as discussed above) and is a nonlinear phenomenon. Therefore, the linear fits shown in Figure 5 are a first-order approximation of the variability between ΔT_S and ΔT_M and hence show a smaller “R” values than expected for a linearly varying phenomena. Nevertheless, the slopes obtained by these linear fits do provide a broad “visual” picture of the mesospheric temperature variability at different latitudes with respect to the stratospheric temperature changes at high latitudes during SSW events. The slope (S) is positive if R is positive and vice versa. Thus, the slopes of the best fit lines indicate the effect of the stratospheric temperature enhancements (ΔT_S) on mesospheric temperature differences (ΔT_M) at different latitudes. As mentioned above, a negative/positive slope indicates that there is a cooling/warming in the mesosphere in that latitude region. It is seen from Figure 5a that the slope is negative to begin with in the NH high latitudes (mesospheric cooling) and gradually turns positive (mesospheric heating) in middle to tropical latitudes. At latitudes closer to the equator we see a small negative slope, which, after crossing the equatorial latitudes, turns positive before again turning negative over SH high latitudes. The change of slopes from −1.05 at 85°N farther away from equator as compared to −0.37 at 55°S indicates that the latitudinal temperature behavior is asymmetric, with respect to the equator. For major events (Figure 5a) it is seen that in the NH, mesospheric cooling is seen up to 65°N, and the peak in mesospheric heating occurs at 25°N. In the SH, the mesospheric warmings peak at ~35°S and change to mesospheric cooling at ~45°S. It may be noted that for major warming events (Figures 5a) there are stronger and systematic variations in the slopes as compared to the minor events (Figures 5b). For minor events the excursions of the slopes are in and around zero and the mesospheric cooling at the higher latitudes is not apparent. For both major and minor events, the SH mesospheric temperatures show smaller changes as compared to those in the NH.

Using SOFIE data, ΔT_M was calculated in a similar way as done for SABER data discussed in the previous section. Figure 6a shows ΔT_S versus ΔT_M scatterplot using SOFIE data, which includes all three major SSW events of 2009, 2010, and 2013. As can be seen in this figure the mesosphere in the 60°–80° latitude range for both NH and SH shows mesospheric cooling during major SSW events, which is in accordance with the trends observed using SABER data (Figure 5a for the NH). The data from SOFIE extend the coverage by 20° latitude in the SH when compared with that of SABER.

As discussed earlier OSIRIS-derived mesospheric temperatures are available from 20° to 85°S. ΔT_M values have been calculated for every 10° latitudinal intervals from 80°–90°S to 20°–30°S for 2009 and 2010 major SSW events and are shown in Figure 6b with respect to ΔT_S. The trends at different latitudinal coverage are similar to those estimated by SABER (Figure 5a). However, there are minor differences in values of slopes which could be due to the fact that the OSIRIS data are available for alternate days and data for 2013 SSW event are not included in Figure 6b. Nevertheless, OSIRIS provides data poleward of 55°S latitude which are not available from SABER, and the values of slope show a somewhat constant behavior poleward of 55°S. This is in contrast to the steep slopes in the NH and is understood to be a consequence of SSW being a NH phenomenon.

It is important to note that independently obtained mesospheric temperatures from satellites do show an increase in mesospheric temperatures over tropical to middle latitudes during several major SSW events. These add credence to the ground-based NIRIS measurements that indicated the existence of such a feature during the major SSW event of 2013.

To investigate the characteristic behavior of individual events, plots such as those shown in Figure 5 have been made for each of the 11 events considered in this study (plots not shown here) and the variations in their slopes with respect to latitude are plotted in Figures 7a and 7b for major and minor events, respectively. The geographic latitudes marked in Figure 7 are the midpoints of the 15 latitude ranges. The values of slope as obtained from Figures 5a and 5b are also shown as green colored lines in Figures 7a and 7b, respectively, to indicate the typical behavior for major and minor SSW events. From Figure 7a it is clear that the NH polar mesosphere shows a cooling trend during major SSW events, and this can even extend to 50°N (as seen for the 2009 SSW event). However, as one moves equatorward, the midlatitude mesospheric temperature shows warming effects. Closer to the equator, the mesospheric temperatures again indicate cooling. This trend is similar for all the major SSW events studied. Away from the equator toward midlatitudes in the SH, the mesospheric temperatures again start increasing. A transition from mesospheric warming to mesospheric cooling is observed beyond the midlatitudes in the SH as well, similar to the NH. SABER-measured mesospheric temperature data in the SH are available only up to 55°S, but from OSIRIS data (Figure 6b) it can be seen that the mesospheric cooling farther poleward of 55°S is constant. The mesospheric warmings in the NH seem to be greater and distributed over a larger spatial extent when compared with the SH. This is seen in the characteristic “double-humped” structure in the slopes (ΔT_M/ΔT_S) with respect to the latitude. During minor SSW events (Figure 7b) no significant trend is noticed in the slopes derived for different geographic latitudes. During one of the minor SSW events of 2008 (marked as 2008_III in Figure 7b) the high latitudes show warming, which turns to a cooling in the midlatitude in NH, whereas high latitude over SH shows warmings. During the 2012 minor event the midlatitude NH shows significant mesospheric warming. These two seem anomalous, and at present the reason is unknown.

As can be seen from Figure 7, the variation in slopes at different latitudes for different SSW events is organized in a similar fashion. However, to investigate if there is any order in this behavior with respect to the strength of SSW events, the latitudes of slope reversals from negative to positive values in both hemispheres as a function of peak stratospheric (at 10 hPa pressure level) temperature (T_Smax) poleward of 60°N are shown in Figure 8a. The slope reversal latitudes (SRLs) from positive to negative or vice versa are very distinct during major SSW events (as shown in Figure 7a). However, slope reversals are not so evident during minor SSW events (Figure 7b). For the minor SSW events the latitudes of slope reversals are found by considering either the latitudes at which the slope is zero or latitudes of trend reversals, if the slope does not cross the zero level. The result of correlation between the peak SSW temperatures (T_Smax) and the latitudes of slope reversals in both hemispheres in the poleward directions is plotted in Figure 8a. Red/blue color shows the best fit plots for NH/SH together with the R values. From Figure 8a we find a linear relationship (R = −0.66) between T_Smax and the SRL in the poleward side of NH. It indicates that when T_Smax is large, the mesospheric cooling over high latitudes in the NH extends farther toward midlatitudes. From the same figure we find a linear relationship (R = −0.45) between T_Smax and the SRL in the poleward side of SH. The R values are better in the NH as compared to the SH, possibly due to the fact that the SSW events are NH phenomena. The relationship between SRL in term of SSW strengths, T_Smax, in the poleward direction in both NH and SH considering all the 11 SSW events (including major and minor) is given below:

$urn:x-wiley:21699380:media:jgra51688:jgra51688-math-0001$ (1a)

$urn:x-wiley:21699380:media:jgra51688:jgra51688-math-0002$ (1b)

where T_Smax and SRL are measured in K and degrees, respectively. Equatorward transition latitudes have not been considered as they are not structured as that of poleward transitions.

From Figure 7 it is seen that the latitudes of peak positive slopes in both hemispheres are different for all the SSW events considered as the strengths of each event differs from the other. T_Smax during an SSW event and the difference, D, between the latitudes of peak positive slopes in the NH and SH (of the “double-humped structure”) are plotted in Figure 8b for all the 11 SSW events. It is interesting to note that when T_Smax is greater, the latitudinal difference between NH and SH maxima in mesospheric warming at midlatitudes decreases and vice versa. T_Smax and D are related linearly as

$urn:x-wiley:21699380:media:jgra51688:jgra51688-math-0003$ (2)

where T_Smax and D are measured in K and degrees, respectively.

Thus, through the characterization in this work, it can be appreciated that just by the knowledge of T_Smax (which is available in the public domain) a first-order approximation on the behavior of latitudinal mesospheric temperature structure can be made. From equations 1a and 1b the poleward transition latitudes of mesospheric temperature (from cooling to heating) can be obtained in both hemispheres. From equation 2 the latitudinal difference, D, between the peak positive slopes can be obtained for any SSW event. It is known that the mesospheric temperature behavior and its variation with respect to latitudes (and altitude) is a nonlinear process. In all the characterizations attempted in this study, the behavior of mesospheric temperature has been made considering only a linear approximation. Nevertheless, this empirical characterization is an attempt to obtain a broad picture of mesospheric temperature variation with respect to latitudes during SSW events.

Figure 9a shows SABER-derived mean mesospheric temperature during the periods shown in Table 1 for all the 11 SSW events as a function of geographic latitudes and are compared with the COSPAR International Reference Atmosphere (CIRA-86) model [Fleming et al., 1990]. CIRA model outputs provide an estimation of temperature, zonal wind, and geopotential/geometric height as a function of altitude from 0 to 120 km altitudes in the latitudinal range of 80°S to 80°N. The CIRA zonal mean temperatures have been averaged for the altitude range from 84.8 to 89.2 km and over 10° latitudes for the months of January and February and are shown as the red dashed line in Figure 9a together with all the 11 SSW events that occurred during these 2 months. The temperature difference (ΔT) between the observed (SABER) and the modeled (CIRA-86) values for the months in which SSW events occurred has been obtained and is shown in Figure 9b as a function of geographic latitude. As seen from Figures 9a and 9b the CIRA-86 seems to overestimate the mesospheric temperatures in the NH and underestimate them in the SH, especially over high latitudes. As can be seen from Figure 9b, extreme ΔT values can be up to +20 K in the SH and −20 K in the NH showing a large range, which is due to the fact that different SSW events have different background conditions. A linear relationship has been obtained between the correction to be applied, ΔT (in K) and geographic latitudes (in degrees) as shown below:

$urn:x-wiley:21699380:media:jgra51688:jgra51688-math-0004$ (3)

It should be mentioned here that in obtaining the equation 3, we have not considered the 2006 event as that event is used to validate this relationship. In Figure 9c the SABER-derived mesospheric temperature values for the year 2006 are shown as a black curve together with the standard error. As can be seen, the standard error is larger at NH polar latitudes showing large range of variations in mesospheric temperatures during SSW period. The red dashed curve shows the zonal mean mesospheric temperatures as obtained from CIRA model for the month of January (averaged for the altitude range from 84.8 to 89.2 km and over 10° latitudes). To these CIRA values, the latitude-dependent correction shown by equation 3 has been applied and the resulting values are shown by the blue colored line. It can be seen that the correction shown by equation 3 brings the CIRA-modeled mesospheric temperature values closer to the SABER-measured values.

4 Summary and Conclusions

Ground-based spectroscopic measurement of mesospheric rotational temperatures from O₂ and OH bands originating at 94 km and 87 km altitudes as obtained from NIRIS from Gurushikhar (24.6°N, 72.8°E), a low-latitude location in India, showed enhanced temperatures (compared to their monthly mean values) during the SSW event of January 2013. This led to a wider investigation to understand mesospheric temperature changes over low latitudes and to characterize the behavior of mesospheric temperature variation as a function of geographic latitude during an SSW period with respect to the stratospheric (at 10 hPa pressure level) temperatures over high latitude (60°–90°N). In this regard, SABER-derived mesospheric temperatures for the 11 SSW events which occurred during 2004–2013 have been considered. OSIRIS- and SOFIE-measured mesospheric temperatures are also considered, when available, and both showed broad similarity in their behavior with SABER data. A detailed analysis of all of these independent measurements has revealed that there is a mesospheric heating over tropical to middle latitudes, more so in the NH, during major SSW events. All the major SSW events studied showed well-known mesospheric cooling over NH high latitudes. Closer to the equatorial latitudes the mesospheric heating turns into cooling and again turns to heating over midlatitudes in the SH before turning to cooling over the SH high-latitude regions. The double-humped structure in the mesospheric to stratospheric temperature ratios (ΔT_M/ΔT_S) versus latitude is very clear during major SSW events with two crests over tropical to middle latitudes and a trough over the geographic equator. Mesospheric temperatures during minor events do not show formation of such double-humped structure. A relationship between slope reversal latitudes (SRL) toward poleward side in terms of the peak SSW temperature, T_Smax, is obtained for both hemispheres [SRL_NH = −1.3 × T_Smax + 380.67 and SRL_SH = −0.44 × T_Smax + 151.25]. Further, it is found that the latitudinal difference (D) between the two crests in the observed double-humped structure is related linearly with T_Smax as D = −2.62 × T_Smax + 709.5. It is also found that the CIRA-86-derived mesospheric temperatures overestimate the values in the NH and underestimate them in the SH as compared to the SABER-measured temperatures, which is more prominent at the higher latitudes. A linear relationship has been obtained between the differences in SABER- and CIRA-derived mesospheric temperatures (ΔT) as a function of latitude ΔT = −0.13 × Lat − 0.37, wherein 10 SSW events have been considered. Thus, with just the knowledge of the peak SSW temperatures, T_Smax, the mesospheric temperature behavior with respect to latitude can be determined and their magnitudes with respect to latitudes can be found by applying the above mentioned correction to the CIRA model temperatures. Moreover, from the results presented in this study, a hitherto unknown aspect of the relationship between stratospheric temperature at high latitude and mesospheric temperatures at different latitudes has been brought to light. These results strongly support the interactions in stratospheric-mesospheric coupling and high- to low-latitude coupling of mesosphere lower thermosphere region, especially during SSW events.

Acknowledgments

We thank the SABER and SOFIE teams for providing the level 2a products (obtained from http://saber.gats-inc.com/browse_data.php and http://sofie.gats-inc.com/sofie/index.php), which have been used in this study. We thank Patrick Sheese for providing OSIRIS temperature data. We acknowledge the Global Modeling and Assimilation Office (GMAO) for providing MERRA data (http://acdb-ext.gsfc.nasa.gov/Data_services/met/ann_data.html). We thank R. Sekar and R. Sridharan for going through a draft of this manuscript and making useful suggestions for improving the communication. This work is supported by Department of Space, Government of India.

Michael Liemohn thanks Edward Llewellyn and another reviewer for their assistance in evaluating this paper.

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Citing Literature

Volume120, Issue4

April 2015

Pages 2926-2939

On the latitudinal distribution of mesospheric temperatures during sudden stratospheric warming events

Abstract

Key Points

1 Introduction