Oblique propagation of gravity waves (GWs) refers to the latitudinal propagation (or vertical propagation away from their source) from the low-latitude troposphere to the polar mesosphere. This propagation is not included in current gravity wave parameterization schemes, but may be an important component of the global dynamical structure. Previous studies have revealed a high correlation between observations of GW pseudomomentum flux (GWMF) from monsoon convection and Polar Mesospheric Clouds (PMCs) in the northern hemisphere. In this work, we report on data and model analysis of the effects of stratospheric sudden warmings (SSWs) in the northern hemisphere, on the oblique propagation of GWs from the southern hemisphere tropics, which in turn influence PMCs in the southern summer mesosphere. In response to SSWs, the propagation of GWs at the midlatitude winter hemisphere is enhanced. This enhancement appears to be slanted toward the equator with increasing altitude and follows the stratospheric eastward jet. The oblique propagation of GWs from the southern monsoon regions tends to start at higher altitudes with a sharper poleward slanted structure toward the summer mesosphere. The correlation between PMCs in the summer southern hemisphere and the zonal GWMF from 50°N to 50°S exhibits a pattern of high-correlation coefficients that connects the winter stratosphere with the summer mesosphere, indicating the influence of Interhemispheric Coupling mechanism. Temperature and wind anomalies suggest that the dynamics in the winter hemisphere can influence the equatorial region, which in turn, can influence the oblique propagation of monsoon GWs.

Key Points

Polar Mesospheric Clouds in the southern hemisphere can be influenced by obliquely propagating monsoon gravity waves
Stratospheric sudden warmings enhance the GW propagation at the midlatitude winter hemisphere which appear to be slanted equatorward
Stratospheric sudden warmings appear to alter the GW momentum flux distribution along the oblique GW propagation path in summer hemispheres

Plain Language Summary

Propagation of waves throughout the Earth's atmosphere is a key phenomenon to understanding atmospheric dynamics, as it changes temperature, pressure, density, and composition. Due to the exponentially decreasing density, the amplitude and energy carried by these waves increase exponentially as they propagate vertically. When the waves break, their energy is released and transferred to the background flow. Gravity waves (GWs) can propagate up to the middle atmosphere but are too small to be resolved by most global-scale atmospheric models. The deep convection from monsoon regions is known to be a major source of mesospheric GWs and previous studies on summer northern hemisphere have shown that monsoon GWs tend to propagate obliquely from the low-latitude stratopause up to the high-latitude mesopause. We focus this study on the summer southern hemisphere and the Interhemispheric Coupling (IHC) between the summer mesopause, where Polar Mesospheric Clouds (PMCs) form, and the winter stratosphere where sudden warmings occur. PMCs are excellent indicators of atmospheric changes. Their correlations with wind, temperature, and GW pseudomomentum flux highlight the consequences of anomalies in the winter stratosphere, such as warmings, on the oblique propagation of GWs that influence the PMC formation in the summer southern hemisphere.

1 Introduction

The dynamics significant to the coupling between atmospheric regions involve the generation, propagation, and modulation of tides, planetary waves (PWs), and gravity waves (GWs). GWs are the least understood due to their small scales and are often parameterized in global climate models. This study contributes to the understanding of the coupling between atmospheric regions, specifically between the tropical stratosphere, a source of monsoon GWs, and the high-latitude mesosphere, where Polar Mesospheric Clouds (PMCs) form (Rapp et al., 2002). Sato et al. (2009) first suggested that obliquely propagating GWs from monsoon regions could be an important source of mesospheric GWs. More recently, Thurairajah et al. (2017, 2020) used satellite observations of PMCs and GWs to study the effect of obliquely propagating monsoon generated GWs on PMCs, both in the northern hemisphere (NH). Thurairajah et al. (2020) showed a conceptual figure of the vertical and obliquely propagating GW paths in the NH. This work further investigates this topic focusing on the southern hemisphere (SH). PMCs exist due to a dynamical refrigeration process of the summer mesopause region, driven by GWs. In the winter hemisphere, Rossby waves from the troposphere induce a poleward flow called Brewer-Dobson circulation. The components of GWs not filtered out by this stratospheric circulation, can propagate up to winter mesosphere and drive a poleward circulation that leads to an equatorward circulation in the summer mesosphere. This pole-to-pole circulation implies an adiabatic expansion of the summer pole that cools the summer mesopause down enough to form PMCs (Karlsson & Shepherd, 2018). While the propagation and the breaking processes of these GWs are responsible for the cold summer mesopause, GWs have also been shown to cause the sublimation of cloud particles leading to the destruction of PMC layers (e.g., Chandran et al., 2012; Chu et al., 2009; Gerrard et al., 2004; Jensen & Thomas, 1994; Rapp et al., 2002) and enhancement of PMCs (Gao et al., 2018).

While there are several sources of GWs, Sato et al. (2009) suggested that the largest source of mesospheric GWs in summer is the deep convection from monsoon regions. From model simulations, Sato et al. (2009) showed that the latitudinal shear in the prevailing westward jet, that has a slanted structure from the tropical stratosphere to the polar mesosphere, could refract these monsoon generated GWs to the high-latitude mesosphere. The oblique propagation (or latitudinal but vertical propagation away from the source) has been reported in model studies (e.g., Kalisch et al., 2014) and observations (e.g., Thurairajah et al., 2017, 2020; Yasui et al., 2016). Yasui et al. (2016) used mesospheric wind data from Antarctica and precipitation data from the tropics and found that a significant component of the mesospheric GWs in the high-latitude summer SH originates from tropical convection (i.e., monsoon regions). Thurairajah et al. (2017, 2020) used data from two satellite instruments and showed a high correlation between observations of the GW pseudomomentum flux (GWMF) (and GW amplitude) from monsoon GWs and PMCs, in the summer NH. This oblique propagation of GWs, from the low-latitude troposphere to the high-latitude mesosphere, is not included in current gravity wave parameterization schemes but may be an important component of the global dynamical structure of the mesosphere.

Karlsson et al. (2007) found correlations between the temperature in the winter polar stratosphere and the PMC Occurrence Frequency (PMC OF) observed in the opposite summer hemisphere during Sudden Stratospheric Warmings (SSWs). SSWs are a consequence of interactions between the atmospheric PWs and the mean flow in the polar stratosphere (Matsuno, 1971). During SSWs, PWs induce a reversal of the polar stratospheric jet from eastward to westward in the winter hemisphere. The changes in the background wind alter the filtering of GWs, and consequently, the direction of GW drag from westward to eastward in the middle to high latitudes (∼60–90°) (e.g., Liu et al., 2002). The resulting equatorward circulation in the upper mesosphere yields an upward flow in the mesosphere and a downward flow in the lower thermosphere, respectively, resulting in an adiabatic cooling and warming (Cullens et al., 2015; Liu et al., 2002). In the Interhemispheric Coupling (IHC) model presented by Körnich and Becker (2010), amplification of PWs and associated changes in GWs in the winter polar region alter the global residual circulation, changing the filtering of GWs in the summer hemisphere.

In this study, we analyze the influence of IHC mechanisms on PMCs by considering the effect of SSWs, occurring in the winter stratosphere, on the dynamics of the summer SH and on the PMC activity in the summer mesosphere. We investigate the combined influence of IHC and oblique propagation of monsoon GWs on PMCs using data from November to March of 2010/2011 (a no-SSW year) and 2012/2013 (a major-SSW year). This paper is organized as follows. Section 2 presents the data and methods used in the derivation of GWMF, the process of locating the monsoon regions in the summer SH, the calculation of PMC OF, and the process of identifying a season with and without SSW events. Section 3 presents a comparison of PMC activity and GWMF activity above the monsoon regions from 2008 to 2014, in both the NH and the SH. Section 3 also describes the monsoon regions in the summer SH, the zonal mean zonal wind structure, the zonal mean GWMF, the correlation between PMC OF and GWMF, and the IHC analysis using wind and temperature information. Section 4 contains a summary and conclusions.

2 Data and Methodology

2.1 Monsoon Convection and Gravity Waves

In this study, the location of the low-latitude source of GWs in the summer SH is investigated using two parameters: the rainfall rate (i.e., precipitation) and the Outgoing Longwave Radiation (OLR). Both data have been shown to be a good proxy to estimate the strength of the monsoon convection (Wright & Gille, 2011). The Tropical Rainfall Monitoring Mission (TRMM) was designed to monitor and study tropical rainfall. It has been operated for 17 years, including several mission extensions, before being decommissioned on April 15, 2015. The rainfall rate data set is collected using the Dual-frequency Precipitation Radar (DPR) instrument. The DPR instrument is an electronically scanning radar, operating at 13.8 GHz that measures the 3D rainfall distribution over both land and ocean, and define the layer depth of the precipitation. The daily (noninterpolated) OLR information are collected by the National Oceanic and Atmospheric Administration (NOAA) satellite using the Advanced Very High Resolution Radiometer (AVHRR). NOAA-18 is a weather forecasting satellite run by NOAA and launched in 2005, into a sun-synchronous orbit at an altitude of 854 km above the Earth. OLR data at the top of the atmosphere are observed globally from the AVHRR instrument aboard NOAA-18 (Liebmann & Smith, 1996). Negative OLR are indicative of enhanced convection and hence more cloud coverage. More convective activity implies higher, colder cloud tops, which emit much less infrared radiation into space.

GW variability is derived from temperature observations from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite (Russell et al., 1999). Since 2002, the satellite TIMED is focused on understanding the energy transfer into and out of the Mesosphere and Lower Thermosphere/Ionosphere (MLTI) region of the Earth's atmosphere (energetics), as well as the basic structure (i.e., pressure, temperature, and winds) that results from the energy transfer into the region (dynamics). SABER is a limb-scanning infrared radiometer that has provided global atmospheric measurements of temperature, pressure, and geopotential height and trace species in the altitude range of 10–110 km. Due to the yaw cycle of TIMED, SABER can perform continuous measurements over the latitude range of 50°N–50°S.

As explored by Wei et al. (2019), one of the two approaches for GW parameterization is the pseudomomentum scheme that exploits the fact that, in a Lagrangian-mean reference frame, the response of a large-scale flow can only be due to forcing momentum. Present-day GW parameterizations follow this method which, transformed into an Eulerian reference frame, leads to a pseudomomentum-flux convergence (Fritts & Alexander, 2003)

(1)

where F_Px and F_Py are the zonal and meridional components of the vertical flux of horizontal pseudomomentum, respectively,

is the density of the background atmosphere, f is the inertial frequency (i.e., the Coriolis parameter),

is the intrinsic frequency, and

and

are the zonal, meridional, and vertical wind perturbations, respectively. The pseudomomentum scheme is opposed to the direct scheme, which forces the large-scale flow both in the momentum equation by an elastic momentum-flux convergence and in the entropy equation by the entropy-flux convergence (Wei et al., 2019). Ern et al. (2004) showed that limb-viewing satellite temperature measurements can be used to derive the total vertical flux of horizontal momentum (referred here as the pseudomomentum) of GWs. Under the midfrequency approximation

, where N is the Brunt Väisälä (i.e., buoyancy) frequency, the total vertical flux of horizontal pseudomomentum, _FPh

(2)

can be expressed by a GW pseudomomentum flux given by

(3)

where k_h is the horizontal wavenumber, m is the vertical wavenumber, g is the acceleration due to gravity, is the temperature amplitude (after removing the PW wavenumber 1–5 components), and T₀ is the background temperature. We note that Equation 3 only calculates the absolute values of GWMF and provides an estimate for the vertical propagation of GWs, but does not provide the zonal and the meridional components (i.e., the direction of the GW propagation). This is because the satellite measurement track and the wave vector of the observed GW are not aligned, and therefore, the values of the horizontal wavelength will usually overestimate the true wavelength of the GW (Ern et al., 2011). Only the projection k of the horizontal wave vector can be determined, not the wave vector itself (Preusse et al., 2009). We use the version 2.0 level 2A SABER temperature data. Previous studies have shown that in the vertical wavelength range of 4–20 km, SABER is sensitive to horizontal wavelengths greater than 500 km (Thurairajah et al., 2020 and references therein). However, SABER can observe GWs with horizontal wavelengths as short as 100 km depending on the distance between two subsequent altitude profiles (e.g., Ern et al., 2018). Albeit these limitations, previous studies have shown that the above technique is reliable for GW related studies (e.g., Thurairajah et al., 2017; Yamashita et al., 2013). Using GW amplitudes that are not affected by any errors associated with horizontal wavelength calculations, both Ern et al. (2011) and Thurairajah et al. (2017) have shown the same zonal mean GW characteristics (e.g., vertical and oblique GW propagation) indicating that the results from GWMF calculation are real, giving us confidence in our results.

The background conditions including winds and temperature are obtained from Modern-Era Retrospective Analysis for Research and Applications (MERRA-2), an NASA atmospheric reanalysis for the satellite era using the Goddard Earth Observing System Model, Version 5 (GEOS-5) with its Atmospheric Data Assimilation System (ADAS), version 5.12.4 (Rienecker et al., 2011). The MERRA-2 reanalysis process uses a forecast model to combine observations and produce gridded data sets of a large number of variables. While the model top of ∼77 km (∼0.01 hPa) might not be reliable, MERRA-2 data have been validated in the stratosphere and lower mesosphere (Gelaro et al., 2017) and our results give a sense of the changes in temperature and zonal wind in the IHC mechanism between the high-latitude winter stratosphere and the high-latitude summer mesosphere.

2.2 Polar Mesospheric Clouds

PMC information are collected from the Cloud Imaging and Particle Size (CIPS) experiment on the Aeronomy of Ice in the Mesosphere (AIM) satellite (McClintock et al., 2008; Rusch et al., 2009). The version used is v05.10 level 3C (summary files) data product that provide season-long zonal averages of PMC occurrence. Since 2007, the primary goal of the AIM mission is to explore PMCs and to understand whether the clouds' ephemeral nature, and their variation over time, is related to Earth's changing climate. The mission collects data on cloud abundance, space distribution, and size of particles. CIPS is an ultraviolet imager that has provided PMC data (albedo, ice water content, occurrence frequency) in the latitude range of ∼40–85° for both hemispheres (McClintock et al., 2008). To understand the seasonal variability in PMCs, we calculated the PMC OF by taking the sum of observed clouds over the total performed observations (Equation 4). The zonal mean PMC OF were daily-averaged over the high-latitude region 65–85°N/S for the purpose of this study

(4)

For the IHC study, Becker and Fritts (2006) and Karlsson et al. (2009) found a significant correlation between the vertical component of the Eliassen-Palm (EP) flux in the winter lower stratosphere and the temperature at the summer mesopause, but with a lag time that was altitude-dependent. Following the method used by Karlsson et al. (2009), the time-lagged correlation between the PMC OF anomalies () at the summer mesosphere and the zonal mean zonal wind anomalies at the winter lower stratosphere resulted in two lag times, at the two highest correlation coefficients (i.e., one for each half of the PMC season). Then, we used the PMC altitudes from the Solar Occultation for Ice Experiment (SOFIE) instrument onboard AIM. AIM/SOFIE measures ice extinction profiles with a vertical resolution of ∼1–2 km and the PMC altitude is determined to be at the peak ice extinction (Hervig et al., 2009). Using the two lags and the linear fit of the PMC altitudes, we interpolated the lag in PMC response to to create a series of lag times corresponding to each day of the PMC season. This method is detailed and illustrated in Section 3.5.

2.3 Stratospheric Sudden Warmings

To identify SSWs, Charlton and Polvani (2007) used an algorithm that identifies SSWs based on the reversal of the zonal mean zonal wind from eastward to westward, at 60°N and at 10 hPa. In addition to this wind condition, SSW years can be grouped by major-SSW, minor-SSW, and no-SSW using the condition of a positive zonal mean temperature gradient between 60°N and 85°N at 10 hPa (e.g., Cullens et al., 2015). If both conditions are satisfied (westward wind and positive temperature gradient), a major-SSW occurred. If one of the two conditions is satisfied, a minor-SSW occurred. If none of the conditions is satisfied, no-SSW occurred in the winter hemisphere for that particular year. Using the wind speed from MERRA-2, Figure 1 shows the zonal mean wind at 60°N from ∼30-km to ∼80-km altitude during winter 2012/2013 (left) where a major-SSW has been reported. There is a clear reversal of the polar jet from eastward to westward (negative in blue) between ∼January 7th and ∼January 28th, 2013, Days Since Solstice (DSS) +17 and +38. To understand the effects of SSWs on the IHC pattern and on the propagation of GWs, we use as a case study, a comparison of the 2010/2011 (no-SSW) and 2012/2013 (major-SSW) years (Figure 1, right panel).

Details are in the caption following the image — **Figure 1**
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Zonal mean wind speed at 60°N from ∼30- to ∼77-km altitude during winter NH 2012/2013 (left) and at ∼32 km (10 hPa) altitude during winter seasons in NH for 2010/2011 and 2012/2013 (right). The wind reversal is indicated by the blue area (negative = westward) in the left panel and by the triangular marker in the right panel. NH, northern hemisphere.

3 Results

3.1 PMC Activity

To understand the variability in the occurrence of PMCs, we use AIM/CIPS observations from years 2008 to 2014, over the summer of both hemispheres. We compute the daily-averaged PMC OF over the latitude range of ∼65–85°. Figure 2 shows the PMC OF over six PMC seasons in the summer NH (Figure 2a) and six PMC seasons in the summer SH (Figure 2b) from DSS −30 to +70. For these years, one can notice the uniformity in the seasonal distribution of PMCs in the summer NH compared to the SH. The seasons tend to start over a 10-days window between May 21st and June 1st and end between August 20th and August 28th. The average of these six seasons (Figure 2c) follows a normal distribution with a daily standard deviation σ of ±7% in the first half and σ of ±4% in the second half of the season. This consistency seen in the summer NH is not present in the summer SH for the same range of years. Although the PMC seasons tend to end over a 10-days window between February 11th and February 21st, the start of the PMC season varies along years (Figure 2b). PMC seasons start either around November 21st (2009, 2012, and 2013) or around mid-December (2008, 2010, and 2011). The resulting daily standard deviation (Figure 2d) presents an asymmetric distribution along the PMC season, from November 21st (DSS −30) to February 29th/March 1st (DSS +70), with σ of ±12% in the first half and σ of ±7% in the second half of the season. The peak of PMC activity for both hemispheres tends to occur ∼15 days after solstice (July 6th in NH, January 5th in SH) but the PMC OF is significantly lower in the summer SH than in the summer NH (∼20% less PMC OF).

Looking closely at the no-SSW and major-SSW years we use for our detailed study (i.e., 2010/2011 and 2012/2013, respectively), both PMC seasons end on February 12th (DSS +53). However, while the SH PMC season 2010/2011 (Figure 2b, cyan line) starts on solstice, the SH PMC season 2012/2013 (Figure 2b, black line) starts 25 days earlier, on November 24th (DSS −27). The average PMC OF amplitude for the SH 2012/2013 (major-SSW) PMC season is twice that of the SH 2010/2011, but we observe a significant decrease from ∼January 10th (DSS +20), when PMC OF is maximum, to ∼January 20th (DSS +30), during 2012/2013.

PMCs in the NH tend to be larger and brighter, extending to lower latitudes and exhibiting less day-to-day and year-to-year variation than their SH counterparts (Karlsson & Shepherd, 2018). Alexander and Rosenlof (1996) showed that the summer stratosphere is also warmer in the SH relative to the NH due to greater gravity wave induced forcing in the southern summer. Stratospheric hemispheric asymmetries have mesospheric counterparts whereby there would be weaker gravity wave drag in the southern upper mesosphere, implying a warmer summer mesopause (Siskind et al., 2011). This has been suggested as a possible cause of the lower PMC OF in the summer SH. Using the Solar Backscatter Ultraviolet (SBUV) satellite instruments, Benze et al. (2012) also found that, while the NH and SH PMC seasons on average start at the same time, variability in the SH onset date is twice as high compared to the NH onset date. Gumbel and Karlsson (2011) made the same conclusion using nine years of PMC observations by the Odin satellite, where PMC seasons last from DSS −26 ± 3 to DSS 63 ± 3 in the NH and from DSS −24 ± 9 to DSS 58 ± 2 in the SH. Gumbel and Karlsson (2011) confirmed the role played by IHC from the winter stratosphere on the seasonal, interannual, and hemispheric variability of PMCs. The authors also showed that the vertical propagation of GWs from the summer stratosphere is influenced by polar vortex conditions, which in turn can influence the summer mesosphere. Delayed start of PMC seasons can be explained by a persistent SH stratospheric jet, beyond DSS −30, and the late onset of PMC season in the summer SH 2010/2011 seen in Figure 2b coincides with a long-lasting polar vortex conditions in the Antarctic stratosphere (Gumbel & Karlsson, 2011).

3.2 Monsoon Regions in the SH

In order to locate monsoon regions in the SH, we evaluate the strength of the monsoon convection by looking at the daily-averaged OLR from NOAA/AVHRR and the daily-averaged precipitation from TRMM/DPR. Figure 3 depicts both the OLR (top panel) and the precipitation (bottom panel) for the month of January, averaged from 2008 to 2014. More convective activity implies higher, colder cloud tops, which emit much less infrared radiation into space. Therefore, a negative OLR is indicative of enhanced convection. From these two analyses, three highly convective regions have been identified in the SH: (1) Indonesia [∼0–20°S, ∼90–160°E], (2) Central Africa [∼0–20°S, ∼15–50°E], and (3) Amazonia [∼0–20°S, ∼40–80°W]. The location of these regions in the summer SH is consistent for individual years (not shown here) and agrees with the results obtained by Wright and Gille (2011), using the High Resolution Dynamics Limb Sounder (HIRDLS) onboard the NASA's Aura satellite (see Figure 2 and Table 1 in Wright and Gille [2011]). A parallel study on summer NH also showed the ∼0–20°N latitude bin to be the most convective zonal area and a consistent monsoon region for the summer NH (not shown here).

3.3 GWMF and Background Winds

TIMED/SABER performs continuous measurements over the latitude range of 50°N–50°S, which covers the monsoon regions. Due to the yaw cycle of TIMED, SABER observes the high latitudes only for about half the PMC season. In the summer hemisphere, monsoon generated GWs have been shown to vertically propagate from their source in the troposphere up to the stratopause (∼50 km) where they focus into the mesospheric jet and can obliquely propagate to the high-latitude mesosphere (e.g., Sato et al., 2009; Thurairajah et al., 2017). Looking at 50 km above the monsoon regions (0–20°N/S) for both hemispheres, we explore the seasonal variability in the zonal mean GWMF from DSS −30 to DSS +70 in the summer NH (Figure 4a) and the summer SH (Figure 4b) for years 2008–2014. Figures 4c and 4d show the corresponding average and 1 − σ standard deviation of the six seasons in the summer NH and the summer SH, respectively. In both hemispheres, the momentum flux carried by GWs tends to increase until it reaches its maximum ∼50 days after solstice. Note that, like the daily-averaged PMC OF (Figure 2d), the daily standard deviation of GWMF above monsoon regions in SH (Figure 4d) exhibits an asymmetric distribution with a distinct transition at solstice from large (σ ∼ 0.09 log₁₀ hPa at DSS −5) to small (σ ∼ 0.02 log₁₀ hPa at DSS +5) standard deviation. Despite this asymmetric pattern, both hemispheres present a relatively similar GWMF activity at the stratopause above their respective monsoon regions. Although monsoon regions in the widely studied summer NH present high momentum fluxes, the amplitude of GWMF above the monsoon regions in the summer SH is of equal if not higher than its NH counterpart for the same range of years and latitudes, consistent with results from Wright and Gille (2011).

Looking closely at the no-SSW and major-SSW years (i.e., 2010/2011 and 2012/2013, respectively) that we focus on in the next sections, both years exhibit a similar GWMF seasonal distribution. The no-SSW season 2010/2011 (Figure 4b, cyan line) presents a slightly higher GWMF (+0.1 log₁₀ hPa) than the major-SSW season 2012/2013 (Figure 4b, black line) between DSS −30 and −10 and between DSS +50 and +60.

In order to investigate the effects of SSWs on GWMF, we show the superposition of the zonal mean zonal wind speed from MERRA-2 on the zonal mean GWMF for the no-SSW year 2010/2011 and the major-SSW year 2012/2013, respectively, in Figures 5a and 5b. Both data are averaged over a 21-day period from January 7th, when SSW was triggered in winter NH 2012/2013 (see Figure 1), to January 27th, 20 days after the SSW. The altitude range for the zonal wind is limited to ∼30–77 km by the model's upper limit and the latitude range for GWMF is limited to ∼50°S–85°N due to TIMED's yaw cycle during this 21-day period. If vertically propagating GWs were propagating conservatively, GWMF would be constant with increasing altitude (e.g., Ern et al., 2011). However, Figures 5a and 5b show a decrease in GWMF with altitude indicating dissipation. Previous ray-tracing studies have confirmed the SABER GW climatology (Preusse et al., 2009). We denote the general GW vertical propagation by using the maximum GWMF at each altitude. The maximum GWMF calculated at each 1-km altitude step is depicted by white dots in the summer SH and black dots in the winter NH. Figure 5c shows the subtraction of the GWMF in the no-SSW season 2010/2011 (Figure 5a) from the GWMF in the major-SSW season 2012/2013 (Figure 5b). In this difference plot, a positive [negative] value indicates an increase [decrease] in GWMF during the 20 days after SSW is triggered in the high-latitude winter NH.

In the winter hemisphere, the stratospheric eastward jet that prevails in the high latitudes (∼60–90°) can be attributed to the polar vortex, which is an important source of GWs (Ern et al., 2011). When no-SSW is reported (winter NH 2010/2011, Figure 5a), an enhanced GWMF in the winter NH is found in the polar jets with strong eastward wind (>50 m/s). The maximum GWMF is at high latitudes (∼70°N) and has a distinct peak up to ∼65-km altitude. When a major-SSW occurs (winter NH 2012/2013, Figure 5b), a reversal of the polar stratospheric jet is observed between latitudes ∼55–85°N at 10 hPa altitude which reduces the strength of the eastward jet in the winter tropical NH (<30 m/s). The poleward flow (i.e., Brewer-Dobson circulation) at this altitude is strongly enhanced resulting in warmer temperature and weaker eastward zonal wind that causes the westward GWs to break at lower altitudes (Becker, 2012). The GWMF in the high-latitude winter stratosphere and winter mesosphere is therefore lower compared to the no-SSW winter NH (Figure 5c). The reduction of ∼0.5 log₁₀ hPa at ∼55-km altitude and 75°N corresponds to a 68% decrease in GWMF (hPa). This reduced GW activity associated with SSW has been reported in previous studies (e.g., Siskind et al., 2010). In the NH, the GWMF enhancement centered at ∼70°N in the 2010/2011 winter and extending to ∼65 km (Figure 5a) is suppressed in the 2012/2013 winter. In the 2012/2013 winter hemisphere, there is an increase in GWMF at midlatitudes (∼50°N) which slants toward the equator with increasing altitude. This increase in midlatitude GWMF appears to follow the stratospheric eastward jet (Figure 5b). Figure 5c also shows this increase in GWMF of almost 100% (+0.3 log₁₀ hPa) from the tropical winter stratosphere (∼30 km, ∼50°N) to the equatorial lower mesosphere (∼60 km, ∼0°). The difference plot also suggests that this winter midlatitude enhancement may be connected to the oblique propagation path from the SH monsoon region. Given that the maximum GWMF in the equatorial region above ∼65 km is present in both years, these GWs may have been generated by a different source.

Above the monsoon regions (latitudes ∼0–20°S), GW propagation is quasi vertical up to the stratopause (∼50 km) and follows the jet peak as in the NH vertical GW propagation. In the SH, as the GWs propagate vertically, the GWMF decreases with altitude due to dissipation, decelerates the jet and contributes to the slanted structure of the westward wind (Sato et al., 2009). This westward wind associated with the monsoon circulation is slanted toward the high latitudes (above ∼40-km altitude) and allows the oblique propagation of GWs along the westward jet from the low-latitude monsoon regions to the high-latitude mesosphere. This general structure of the slanted zonal mean westward wind is similar for all years from 2008 to 2014 (not shown here). In response to the SSW event and for the 20 days that follow, the summer SH sees a significant increase in GWMF concentrated at ∼30-km altitude above the equator (Figure 5c) of about 82% (+0.26 log₁₀ hPa). Although the path depicted by the maximum GWMF (white dots) remains similar between summer SH 2010/2011 and summer SH 2012/2013 (Figures 5a and 5b, respectively), we observe an increase of between 7% and 17% (+0.03 and +0.07 log₁₀ hPa) in GWMF over a larger region, from ∼30-km to ∼80-km altitude and above the latitude bin ∼30–50°S (Figure 5c).

3.4 Correlation Between PMCs and GWMF

Obliquely propagating GWs generated from low latitudes have been shown to have an influence on the polar summer mesosphere (Thurairajah et al., 2017, 2020), and PMCs are sensitive indicators of such changes (Karlsson & Shepherd, 2018). While Thurairajah et al., (2017, 2020) presented results in the summer NH, here we study the instantaneous correlation between the time series of PMC OF in southern summer upper mesosphere (latitudes ∼65–85°S) and GWMF measured over the latitude range 50°S–50°N and the altitude range 30–90 km. Figures 6a and 6b depict the zonal mean zonal wind superposed on the correlation coefficients, for 2010/2011 (no-SSW) and 2012/2013 (major-SSW), respectively, using data from November 21st (DSS −30) to March 1st (DSS +70).

In both the 2010/2011 and 2012/2013 seasons, the high-correlated region (r > 0.5, 90% significant) in the midlatitude summer mesosphere is assumed (based on Figure 5) to be associated to the oblique propagation of GWs above monsoon regions in the low-latitude stratopause and slanted poleward to the high-latitude mesopause. We also observe a positive correlation in the winter hemisphere at lower altitudes that could confirm the link between the wintertime dynamics and the summer mesopause in the opposite hemisphere. A positive correlation between PMC OF and GWMF means that an increase in GW activity correlates with an increase in PMCs activity, and vice versa.

When SSWs occur in the winter stratosphere, the small region of high correlation (r > 0.5, 90% significant) between PMCs and GWMF in the winter stratopause (∼50-km altitude, ∼32°N latitude) seen in Figure 6a for the no-SSW year is replaced by a significantly larger area (∼30–60-km altitude, ∼20–40°N latitude) of high correlation (Figure 6b) for the SSW year. It exhibits the same pattern seen in the GWMF maxima (Figure 5b, black dots) and the GWMF difference between no-SSW and major-SSW (Figure 5c), starting at midlatitudes and slanted toward the equator as altitude increases. This suggests that although the GW activity in the winter stratosphere is strongly reduced during SSW events, with breaking occurring at lower altitudes, the PMC seasonal variations are more correlated with the GWMF variations associated with the SSW dynamics. It agrees with (Karlsson & Becker, 2016) who demonstrated the impact of winter GWs on the global mean meridional circulation and the summer mesopause cooling.

The highly correlated region (r > 0.5, 90% significant) in the midlatitude summer mesosphere is replaced by a larger area of higher coefficients when major-SSW occurs in the winter NH 2012/2013 (Figure 6b). If we evaluate the angle of this pattern of high-correlation coefficients using a hypothetical straight line slanted poleward, the resulting straight line stays in summer SH for the no-SSW season (Figure 6a), connecting the low-latitude summer stratosphere (∼30-km altitude, ∼10°S latitude) with midlatitude mesopause (∼80-km altitude, ∼30°S). However, the same method in Figure 6b exhibits a diagonal line that connects the midlatitude winter stratosphere (∼30-km altitude, ∼50°N) with the midlatitude summer mesosphere (∼80-km altitude, ∼40°S latitude). This diagonal of positive correlations between PMCs and GWMF is between two large anticorrelated regions (r < 0.5, 90% significant): the equatorial upper mesosphere (∼75-km altitude, ∼0° latitude) and the low-latitude upper stratosphere (∼50-km altitude, ∼20–40°S latitude). These observations suggest that despite a similar zonal mean westward wind structure prevailing in the summer SH for a 20-day period between 2010/2011 and 2012/2013 (see Figures 5a and 5b), the major-SSW occurring in the winter stratosphere could change the GW activity in the SH. For the no-SSW season (2010/2011), the GWs from monsoon convection are presumed to propagate vertically and reach ∼50-km altitude, and then obliquely propagate following the poleward tilt of the easterly jet that prevails in the summer SH. This agrees with the results obtained in the summer NH by Thurairajah et al. (2017). For the major-SSW season (2012/2013), the oblique propagation of monsoon GWs appears to be modified. The high correlation region above the monsoon regions is at a higher altitude above the stratopause (∼65 km) and slanted poleward with a sharper angle in mesosphere.

In addition to monsoon generated GWs, GWs can also be generated by jet stream (e.g., Zhang, 2004; Wei & Zhang, 2014). Although the changes in GWMF may be caused by GWs from jet/front systems, GWs generated by deep convection from monsoon region is considered to be the main source of summer mesospheric GWs (Sato et al., 2009). Given that the oblique propagation of these monsoon generated GWs is possible due to the latitudinal shear in the westward jet (and that the high correlation is along this jet in Figure 6), our results likely indicate changes in oblique propagation of GWs.

3.5 IHC Analysis

Due to the yaw cycle of TIMED, no GWMF information is available in the high-latitude summer SH from January ∼15th to March ∼15th, when SSWs usually occur in the opposite winter stratosphere (see Figure 5). Therefore, we investigate the effect of SSWs on the PMC region by comparing 2010/2011 (no-SSW season) with 2012/2013 (major-SSW season) within an IHC analysis, applying the method used by Karlsson et al. (2009b) and graphically described by Figure 7. Here, we use the zonal mean zonal wind (U) and the temperature (T) from MERRA-2 in the available altitude range (∼0–77 km). Although the top altitude does not include the PMC altitudes, this analysis gives us a sense of changes in U and T as seen in the IHC mechanism.

Following Karlsson et al. (2009b), we first compare the PMC OF, averaged over latitudes 65–85°S, with the zonal mean zonal wind, averaged over latitudes 59–61°N and altitudes 10-5 hPa. At ∼60°N, this altitude region is a good indicator of the variability in the winter stratosphere (Karlsson et al., 2009b). Figure 7a shows the PMC OF from AIM/CIPS for SH 2010/2011. Since the IHC of the middle atmosphere general circulation is characterized by a global anomaly pattern of the zonal mean temperature (Körnich & Becker, 2010), and to remove the seasonal variability, this analysis uses the anomaly fields of PMCs, wind, and temperature data (, and , respectively) which we derive by subtracting a sixth order polynomial fitting to the data. Figure 7b shows the SH and the NH for 2010/2011. By computing a time-lagged correlation between these two parameters, the highest correlation coefficient indicates two lag times. There is a 9-days lag in the first half of the PMC season, and a 13-days lag in the second half of the PMC season. Halves of the PMC season are indicated by the dashed vertical lines in Figure 7. Karlsson et al. (2009b) noted that the lag changes during the PMC season due to the associated change in PMC altitudes. Therefore, the resulting lag times are altitude-dependent. In Figure 7c, we show the PMC altitudes from the AIM/SOFIE data. Using a linear fit of the PMC altitude variations, the two lag times are then used as two points on DSS +14 and +40 (i.e., the median dates) to obtain an interpolated time varying lag along the PMC season.

From this method, we compare the correlation of the lagged PMC OF anomaly with the global zonal mean zonal wind and the zonal mean temperature anomaly fields from MERRA-2 for both 2010/2011 and 2012/2013 (see Figures 8 and 9, respectively). This correlation analysis is focused on data from November 21st (DSS −30) to March 1st (DSS +70) (i.e., summer in the SH and winter in the NH).

In Figures 8a and 9a, both the no-SSW and the major-SSW seasons show large areas of high correlation (r > 0.5, 90% significant) between PMC OF' and U'. The area with the highest coefficients is located in the winter stratosphere, highly correlated with PMCs in the summer mesopause and agreeing with the IHC mechanism as described by Karlsson et al. (2009b). The westerly winds that prevail in the winter stratosphere allows waves with westward phase speeds to propagate up to the mesosphere. When these waves break, a poleward and downward flow is induced and the resulting adiabatic heating of the high-latitude winter mesosphere involves the mean circulation from summer pole to winter pole in the upper mesosphere (Karlsson et al., 2009b). The second area of positive correlation between PMC OF' and U' is located in the opposite summer stratosphere and is due to the dynamics in the winter stratosphere affecting the summer stratospheric flow (Karlsson et al., 2009a). During SSW, these two positive correlation areas are enhanced (Figure 9a). Although the westerly wind prevailing in high-latitude winter NH is weakened by SSW, the highest correlation between PMC OF' and U' suggests that the PMC day-to-day variations are significantly more correlated (within the 90% confidence level) with anomalies that cross the equator via the meridional circulation and that these anomalies are caused by SSW.

In Figures 8b and 9b, both the no-SSW and the major-SSW seasons show a negative correlation between PMC OF' and T' in the high-latitude winter stratosphere, a positive correlation in the equatorial stratosphere and a positive correlation in the polar winter mesosphere. This quadrupole structure agrees with previous IHC analyses. Karlsson et al. (2007) showed this positive/negative winter dipole pattern in the correlation between noctilucent cloud properties and stratospheric temperatures in the winter stratosphere from July 2002 to January 2007. Due to higher PW activity in the winter troposphere and stratosphere, the high-latitude stratosphere and low-latitude mesosphere experience warming while the high-latitude mesosphere and low-latitude stratosphere experience cooling. The deceleration of the zonal wind by PWs leads to a reduction of the net GW drag, responsible for driving the mesospheric meridional circulation (Becker & Fritts, 2006). Therefore, the meridional circulation in the winter mesosphere is weaker, the adiabatic heating at the winter pole is reduced and the high-latitude winter mesosphere is cooler during high PW activity. It also reduces the upwelling and increases temperature in the equatorial mesosphere. In stratospheric altitudes, the Brewer-Dobson circulation warms the high latitudes up and cools the equatorial stratosphere down. In both seasons, the correlation between PMC OF' and T' tend to exhibit this quadrupole structure in the winter hemisphere. However, the most important step for IHC consists of how anomalies induced by SSWs cross the equator. As the zonal wind does not change in the summer stratosphere, the GW filtering remains the same between no-SSW and major-SSW season, and it allows large phase speed GWs to propagate up to mesosphere. In this region, the U' anomaly has an impact on the height of the GW breaking, because the background wind is closer to the GW phase speed. The wave breaking at lower altitudes creates a downward shift of the GW drag associated with a downward shift of the upper branch of the residual circulation (Körnich & Becker, 2010). This leads to a positive T' anomaly in the summer polar mesopause during SSWs.

In addition to a strongly enhanced quadrupole structure in the major-SSW season 2012/2013, we observe a strong positive correlation between PMC OF' and T' below the PMC region in Figure 9b. This high-correlation area, concentrated only in the equatorial stratosphere for SH 2010/2011 (Figure 8b), extends toward the high-latitude summer mesosphere for SH 2012/2013, depicting a pattern slanted poleward from the low-latitude stratopause to the high-latitude mesosphere (Figure 9b). Knowing that a positive correlation with temperature can be associated to the destruction of PMC layers (Gumbel & Karlsson, 2011), this high positive correlation could explain the decrease in PMC OF occurring later in the SH 2012/2013 season at DSS +20 (see Figure 2b). Siskind et al. (2011) have shown that a similar decrease in PMC activity at midseason NH 2007 likely resulted from IHC due to enhanced PWs in the SH winter. Karlsson and Becker (2016) showed that winter GW activity reduces the net GW drag in the winter mesosphere, which then leads to a weaker winter residual circulation and a warmer summer polar mesosphere. The change in dynamics, induced by the major-SSW and associated with the temperature patterns in Figures 8b and 9b, could also explain the change in the correlation pattern between PMC OF and GWMF in Figures 6a and 6b.

4 Summary and Conclusions

Oblique propagation of GWs refers to the latitudinal propagation of GWs, from the summer stratosphere above the tropical monsoon convection source to the high-latitude summer mesosphere. Previous studies have been conducted in the summer NH using a large range of PMC seasons and revealed a high correlation between observations of GWMF from monsoon GWs and PMCs. Although this oblique propagation plays an important role in the global dynamical structure of the mesosphere, it is not included in GW parameterization schemes. Motivated by these studies, here, we presented a combination of satellite observations and model to understand this atmospheric phenomenon in the summer SH. We compared six PMC seasons in summer NH and six PMC seasons in summer SH from 2008 to 2014. PMC OF in the summer NH tends to exhibit a normal distribution from DSS −30 to +70, but this consistency and symmetry was not present in the PMC OF in the summer SH. PMCs in NH tend to be larger and brighter, extending to lower latitudes and exhibiting less day-to-day and year-to-year variation than their SH counterparts (Karlsson & Shepherd, 2018).

Knowing the largest source of GWs in the summer troposphere to be deep convection from monsoon regions (Sato et al., 2009), we measured the convection strength in the summer SH. We identified three high-convective regions: (1) Indonesia [∼0–20°S, ∼90°E−160°E], (2) Central Africa [∼0–20°S, ∼15–50°E], and (3) Amazonia [∼0–20°S, ∼40–80°W]. We then analyzed the seasonal variability in the zonal mean GWMF at 50 km above these monsoon regions in the SH (0–20°S) for years 2008–2014. We replicated this analysis for the opposite NH (0–20°N). Despite an asymmetric distribution in SH, which was also present in the daily-averaged PMC OF, GWMF amplitudes above the monsoon regions in SH are as significant as its widely more studied NH counterpart.

In addition to this hemispheric comparison, we were interested in the effects of the seasonal variability in the opposite winter NH. We identified years when SSWs (major and minor) occurred by looking at the polar jet reversal (from eastward to westward) at 60°N latitude and ∼32 km (10 hPa) altitude using MERRA-2 winds. As a case study, we focused the rest of our analysis on two PMC seasons in SH, 2010/2011 (no-SSW was observed in the NH) and 2012/2013 (major-SSW occurred in the NH). We then compared the zonal mean GWMF and the zonal mean zonal wind speed between the two PMC seasons, averaged for 21 days starting on January 7th, when major-SSW occurred in 2013. For no-SSW year (2010/2011), results confirmed the enhanced GWs in the strong stratospheric eastward jet. In addition, the oblique propagation of GWs from the southern monsoon regions to the southern mesosphere was consistent with previous work on oblique propagation of GWs in the NH. During the SSW in 2013, the eastward jet in winter NH is reversed to westward, and westward GWs tend to break at lower altitudes. The resulting lower GW activity in high-latitude stratosphere is shown by a 68% decrease in GWMF (−0.5 log₁₀ hPa), at ∼55 km and ∼75°N, compared to the no-SSW year. This decrease is accompanied by a significant increase in GWMF (+0.3 log₁₀ hPa, +100% hPa) at ∼30 km and ∼50°N. This increase in midlatitude GWMF appears to be slanted toward the equator with increasing altitude and follows the stratospheric eastward jet. Although the oblique propagation of GWs from the southern monsoon regions, depicted by the maximum GWMF, is seen in both seasons (no-SSW and major-SSW), the GWMF at ∼30 km above the equator is shown to be 82% greater (+0.26 log₁₀ hPa) in the major-SSW season than in the no-SSW season. This increase in GWMF extended from ∼30-km to ∼80-km altitude and above the latitude bin ∼30–50°S. The GWMF was 7–17% greater (+0.03 to +0.07 log₁₀ hPa) during the major-SSW season compared to the no-SSW season.

By investigating the correlation between daily-averaged PMC OF and zonal mean GWMF, three observations can be made regarding the effects of SSW. (1) Although the GWMF contribution in the winter high-latitude stratosphere is strongly reduced during SSWs, the PMC seasonal variations in the summer SH are highly correlated with the GWMF seasonal variations in NH. (2) This highly correlated region between the PMC OF and the zonal mean GWMF shows a similar distribution to that of the GWMF maxima. The GWMF maxima occurs at midlatitudes in the winter hemisphere lower stratosphere and slants toward the equator as altitude increases, following the stratospheric eastward jet. (3) Despite a similar westward zonal wind structure in the summer SH in both seasons, the pattern of high correlation between the PMC OF and GWMF in the summer, which was associated to the propagation of monsoon generated GWs, changes in the year when SSW occurs in the winter hemisphere. The major-SSW summer SH 2012/2013 shows a pattern that starts at a higher altitude (∼65 km) and is slanted poleward with a sharper angle in mesosphere, compared to the oblique propagation described by Thurairajah et al. (2017) and shown in the no-SSW summer SH 2010/2011.

Extending this study beyond the range of SABER at higher latitudes, we performed an IHC analysis following the method used by Karlsson et al. (2009b), to investigate the correlation of the day-to-day variability in PMC OF with the variability in the zonal mean zonal wind and temperature from MERRA-2. The comparison of both seasons showed agreements with the IHC mechanisms described in previous studies (Karlsson et al., 2007, 2009, 2016) and highlighted the influence of major-SSWs in the winter NH on PMCs in the summer SH. The results obtained in 2012/2013 are similar to results obtained by Karlsson et al. (2009b) for 2007/2008, where a major-SSW was also reported in the winter NH. (1) The strong correlation of adjusted-lag PMC OF' with zonal wind (U') is largely enhanced for the major-SSW season (|r| +14%), suggesting that the day-to-day variability in PMCs is more correlated to SSW-induced anomalies that cross the equator via the meridional circulation. (2) The quadrupole structure of correlation and anticorrelation between PMC OF' and T' is significantly enhanced for the major-SSW season, presenting a higher absolute value of the correlation coefficient when major-SSWs occur (|r| +21%). This suggests that the day-to-day PMC variability in summer SH is significantly more correlated, within the 90% confidence level, to variabilities in U' and T' during SSW events. (3) The positive correlation between PMC OF' and T', depicted by a large structure slanted poleward from the equatorial stratosphere to the high-latitude summer mesosphere for SH 2012/2013, could explain the decrease seen in PMC OF for the same season, occurring later at DSS +20. Although the permanent effect of IHC is a cooling of the high-latitude summer mesosphere, shown by Karlsson and Becker (2016) to be determined by the strength of the westward GW drag in the winter mesosphere, the increased GW activity in winter stratosphere can lead to a warmer summer polar mesosphere via a weaker residual circulation (Karlsson & Becker, 2016).

In conclusion, we showed that during the major-SSW in the winter NH 2012/2013, the GWMF in the summer SH increased, the correlation between PMCs and GWs which are assumed to be from monsoon regions is enhanced, resulting in changes in the dynamics of the global atmosphere. We also showed by using the adjusted-lag PMC OF, that major-SSWs can play a role in the destruction of PMC layers, days later in the PMC season. While the intrahemispheric connection between low-altitude and high-altitude summer hemisphere has been shown to influence the start of PMC season due to the persistency of the polar vortex (Gumbel & Karlsson, 2011), this study showed enhancement in IHC between winter stratosphere in NH and summer stratosphere in SH during SSWs, that in turn reduces the seasonal PMC activity. We expect to verify these observations regarding the effects of SSWs on GW propagation from the troposphere in both hemispheres, in the future, using ray-tracing simulations.

Acknowledgments

This research was supported by the NASA Grant 80NSSC18K0650 and by the NSF award 1855476.

Open Research

Data Availability Statement

The TIMED/SABER data (version 2.0 level 2A) are available from the SABER website (http://saber.gats-inc.com/data.php). The precipitation data from TRMM/DPR are available from NASA's Data and Information Services Center website (https://disc.gsfc.nasa.gov/datasets/TRMM_3B42_Daily_7/summary). The radiation data from NOAA/AVHRR are available from NOAA's Physical Sciences Laboratory website (https://psl.noaa.gov/data/gridded/data.uninterp_OLR.html). MERRA-2 simulations (version 5.12.4) are available from NASA's Data and Information Services Center website (https://disc.gsfc.nasa.gov/datasets/M2I6NVANA_5.12.4/summary). The PMC data from AIM/CIPS are available from the Laboratory for Atmospheric and Space Physics website (http://lasp.colorado.edu/aim/download-data-L3C.php) and PMC data from AIM/SOFIE are available from the SOFIE website (http://sofie.gats-inc.com/sofie/index.php).

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Citing Literature

Volume126, Issue5

16 March 2021

e2020JD033970

The Influence of Obliquely Propagating Monsoon Gravity Waves in the Southern Polar Summer Mesosphere After Stratospheric Sudden Warmings in the Winter Stratosphere

Abstract

Key Points

Plain Language Summary

1 Introduction