Revisiting internal gravity waves analysis using GPS RO density profiles: comparison with temperature profiles and application for wave field stability study

Abstract

Abstract. We revise selected findings regarding the utilization of Global Positioning System radio occultation (GPS RO) density profiles for the analysis of internal gravity waves (IGW), introduced by Sacha et al. (2014). Using various GPS RO datasets, we show that the differences in the IGW spectra between the dry-temperature and dry-density profiles that were described in the previous study as a general issue are in fact present in one specific data version only. The differences between perturbations in the temperature and density GPS RO profiles do not have any physical origin, and there is not the information loss of IGW activity that was suggested in Sacha et al. (2014). We investigate the previously discussed question of the temperature perturbations character when utilizing GPS RO dry-temperature profiles, derived by integration of the hydrostatic balance. Using radiosonde profiles as a proxy for GPS RO, we provide strong evidence that the differences in IGW perturbations between the real and retrieved temperature profiles (which are based on the assumption of hydrostatic balance) include a significant nonhydrostatic component that is present sporadically and might be either positive or negative. The detected differences in related spectra of IGW temperature perturbations are found to be mostly about ± 10 %.The paper also presents a detailed study on the utilization of GPS RO density profiles for the characterization of the wave field stability. We have analyzed selected stability parameters derived from the density profiles together with a study of the vertical rotation of the wind direction. Regarding the Northern Hemisphere the results point to the western border of the Aleutian high, where potential IGW breaking is detected. These findings are also supported by an analysis of temperature and wind velocity profiles. Our results confirm advantages of the utilization of the density profiles for IGW analysis.

Interannual variability in the gravity wave drag – vertical coupling and possible climate links

“…It can be argued that the OGWD variability results are caused simply by the variations in the stratosphere or upper troposphere (e.g., jet shift, meandering due to anomalous planetary wave (PW) activity) possibly leading to Doppler shifting effects or variations in critical lines for the orographic GW propagation (e.g., the relevance of the occurrence of the Aleutian High for the EA/NP hotspot - Pisoft et al, 2018). The modulation of GWs by PWs receives great attention in the scientific community (e.g., Cullens et al, 2015), and considering this causality mechanism, the dynamical influence of the OGWD variations would be of secondary importance only.…”

Extratropical age of air trends and causative factors in climate projection simulations

“…Ploeger et al (2015b; Lagrangian model study) found that AbM corresponds well with the local mixing integrated along the air parcel pathway. Garny et al (2014) and Ploeger et al (2015a) found that the AbM trends in the lowest part of the stratosphere are affected predominantly by changes in the local mixing intensity, but those further above are strongly coupled to the residual mean circulation. This complicates the analysis of possible AbM drivers in the Ex regions, because the Ex regions are located in the transition region between these two regimes.…”

“…In several studies a physical connection between changes in AoA, AbM and RCTTs; the speeding up of the residual mean circulation (see Birner and Bönisch, 2011;Li et al, 2012;Garny et al, 2014;Ploeger et al, 2015a); and increasing wave driving by changes in the resolved and unresolved extratropical wave forcing (e.g., Okamoto et al, 2011;Shepherd and McLandress, 2011;Butchart, 2014) has been postulated. In the present study, after the correction to the vertical shift of pressure levels, we could not find a simple link between trends of wave driving (TD c , GWD c , EPFD c and UP c DC ); net upwelling (UP c wstar and UP c vstar , Fig.…”

“…The isoline widening (not to be confused with the circulation widening) is due to a combination of the upward shift itself and a decrease in AoA. A decomposition of AoA into residual circulation transit times (RCTTs; Birner and Bönisch, 2011) and aging by mixing (AbM; Garny et al, 2014;Ploeger et al, 2015a) shows an additional contribution to the AoA isoline widening by the AbM reduction.…”

“…In the final section of the results, we investigate possible causative factors of AbM and RCTT trends with a focus on the hypothesis of a strengthening residual mean circulation in the shallow BDC branch (e.g., Li et al, 2012;Garny et al, 2014;Ploeger et al, 2015a) driven by changes in resolved and unresolved extratropical wave forcing (e.g., Okamoto et al, 2011;Shepherd and McLandress, 2011;Butchart, 2014). However, after the correction to the vertical shift of pressure layers, a clear connection between the acceleration of the residual circulation, stronger wave driving in the extratropical lower to middle stratosphere, net tropical upwelling, and the time evolution of the RCTT and especially AbM trends is not found.…”