The Role of Vertically and Obliquely Propagating Gravity Waves in Influencing the Polar Summer Mesosphere

Abstract

Using an 8-year (2007-2014) data set from two different limb-viewing instruments, we evaluate the relative roles of vertically versus obliquely propagating gravity waves (GWs) as sources of GWs in the polar summer mesosphere. Obliquely propagating waves are of interest because they are presumed to be generated by the summer monsoons. In the high-latitude upper mesosphere, the correlation coefficient between the time series of ice water content (IWC) and GW amplitude is 0.48, indicating that the observed GWs enhance polar mesospheric clouds (PMCs). For vertically propagating waves, the correlation coefficient between IWC and stratospheric/lower mesospheric (20-70 km) GW amplitude at the same high latitudes becomes more negative with increasing altitude. This change in correlation from negative in the lower mesosphere to positive at PMC altitudes suggests the presence of another source of GWs. The positive correlation coefficient between the time series of IWC and GW amplitude from 0-50°N, 20-90 km shows a slanted structure suggesting oblique propagation. This slanted structure is more robust in some seasons compared to others, and this interannual variability may be due to the latitudinal gradient of the mesospheric easterly jet where steeper gradients allow for low-latitude tropospheric GWs to be refracted to the high-latitude mesosphere more efficiently. Gravity-Wave Regional or Global Ray Tracer (GROGRAT) ray tracing simulations show that more GWs propagate obliquely compared to vertically propagating waves that reach PMC altitudes. For obliquely propagating waves, GROGRAT simulations indicate that nonorographic tropospheric GWs with faster phase speed (>20 m/s) and longer horizontal wavelength (>400 km) have a higher probability of reaching the polar summer mesosphere.

SABER Observations of Gravity Wave Responses to the Madden‐Julian Oscillation From the Stratosphere to the Lower Thermosphere in Tropics and Extratropics

“…These summer hemisphere MLT maximums of GWs over high‐latitude region is consistent with the results from the gravity wave regional or global ray tracer (GROGRAT) model (Preusse et al, 2009). One possible source of these waves is obliquely propagating GWs that originate from the lower latitude (e.g., Thurairajah et al, 2020).…”

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

“…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, 2017Thurairajah et al, , 2020Yasui 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).…”

“…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).…”

“…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).…”

Formation of a Mesospheric Inversion Layer and the Subsequent Elevated Stratopause Associated With the Major Stratospheric Sudden Warming in 2018/19

“…However, using the KANTO model, Sato et al (2009Sato et al ( , 2012 showed evidence of lateral propagation of GWs and provided theoretical explanations of the mechanisms involved. Conducting ray-tracing simulations, Yamashita et al (2013) also suggested that high GW activity in the MLT during ES events observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on the Thermosphere, Ionosphere, Mesosphere Energetics Dynamics (TIMED) satellite is caused by poleward propagating GWs (Thurairajah et al, 2020). To understand the middle atmosphere dynamics in which behavior of the GWs is one of the key processes, state-of-the-art GW-permitting GCMs provide an effective approach.…”