Movement of decaying quasi-2-day wave in the austral summer-time mesosphere

Abstract

The quasi-2-day wave (Q2DW) is a large temperature disturbance in the austral summer-time mesosphere. Its decay-phase movement and phase-speed are poorly understood. Q2DW events observed by the microwave limb sounder (MLS) onboard the NASA Aura satellite reveal that during the temperature Q2DW's decay-phase, the Q2DW temperature disturbance is still substantial, but its phase-speed reduces exponentially, on average, from around -70 to -20 m/s in around 30-days. Observations also reveal significant interannual variability in these phase-speed values. Q2DW events simulated by the extended Canadian middle atmosphere model (eCMAM) reveal that during the Q2DW decay-phase, the temperature Q2DW phase-speed is strongly correlated with the summer mesosphere easterly jet (SMEJ) wind values. eCMAM also shows that the phase-speed interannual variabilities partially reflect the interannual variabilities of the SMEJ. Planetary-wave diagnostics indicate that during the Q2DW's decay phase, there is still an active transfer of momentum from the SMEJ into Q2DW. The model simulations therefore indicate that the SMEJ plays a substantial role in the decaying temperature Q2DW phase-speed variabilities observed by MLS. This adds to our knowledge of how the SMEJ affects the Q2DW.

Figure 1

(a) Snap-shots of eCMAM southern hemispheric temperature at 82 km on 00UT, day 23 in year 2002. (b) Amplitude spectra of quasi-2-day wave with wavenumber 3 at latitude 40S and altitude 82 km. (c) 18-year average of MLS observed Q2DW amplitude zonal profile on the 30th day after peak-Q2DW-day. (d) 18-year average of MLS observed Q2DW amplitude and phase-speed from 2005 till 2023 at latitude 40S and altitude 82 km. Both time-series spans from the 5th day till the 60th day of each year.

Figure 2

(a) 31-year average of the yearly correlation zonal-profile between decaying Q2DW phase-speed and decay-phase SMEJ wind values. (b) 31-year average of the yearly RMS zonal-profile between decaying Q2DW phase-speed and decay-phase SMEJ wind values. (c) 30-year average of decaying Q2DW amplitude. See the main text for more details.

Figure 3

(a) Correlation between decaying Q2DW phase-speed and SMEJ wind values averaged between the day of peak Q2DW amplitude and the 30th day after this peak. (b) RMS between the same parameters in (a). Contour-filled regions in (a) and (b) are areas of statistically significant correlation at 95% level. (c) Sample time-series located inside the box in (a) and (b). (d) Scatter plot of the two time-series in (c).

Figure 4

(a) Correlation between eCMAM decay-phase SMEJ average wind values and its reconstruction using ENSO index, QBO index and F10.7 index. (b) Correlation between eCMAM decaying-Q2DW average phase-speed and its reconstruction using ENSO index, QBO index and F10.7 index. (c) Sample SMEJ average wind values time-series and its reconstruction. (d) Sample decaying-Q2DW average phase-speed and its reconstruction. Both sample time-series in (c) and (d) are average of time-series found inside black boxes in (a) and (b), respectively. Contour-filled regions in (a) and (b) are areas of statistically significant correlation at 95% level.

Figure 5

(a) Correlation between MLS decaying-Q2DW average phase-speed and its reconstruction using ENSO index, QBO index and F10.7 index. (b) Sample decaying-Q2DW average phase-speed and its reconstruction. Sample time-series in (b) is average of time-series found inside black boxes in (a). Contour-filled regions in (a) are areas of statistically significant correlation at 95% level.

Figure 6

Left, middle and right column are 30-year averages of planetary-wave diagnostics on the 10th day, 20th day and 30th day after peak of Q2DW amplitude, respectively. (a–c) Zonal profiles of Q2DW amplitude and EP Flux vectors. Contour-filled regions are regions where EP Flux divergence is positive. (d–f) Zonal profiles of EP Flux vectors and EP Flux divergence. Contour-filled regions are regions where EP Flux divergence is positive. (g–i) Zonal profiles of daily-mean zonal-mean zonal winds, ${\overline{q}}_{\varphi }$ and critical lines. Contour-filled regions of the background zonal wind are areas where ${\overline{q}}_{\varphi }<0$ which is a crucial but insufficient condition for baroclinic/barotropic instability.