# Presentation: 095281

## BACKGROUND

**BACKGROUND**

- Recent studies, e.g. *Siskind et al* *(2007)*, describe a raising of the 2006 Arctic winter stratopause. Observations show a large drop in polar stratopause height, followed by the breakdown of the polar vortex. After a short period, a cool high-latitude stratopause reformed at high altitude, remaining separate from that at midlatitudes. Based on a series of simulations, Siskind et al (2007) concluded that this was related to reduced levels of GW drag.
- HIRDLS allows us to measure atmospheric temperature profiles globally with unprecedentedly high vertical resolution, allowing us to directly observe a significant      ⁮⁮⁮⁮⁮proportion of the GW spectrum of

- the middle atmosphere. Here we analyse HIRDLS GW MF results in the stratosphere during this period by comparison with other years, and discuss the results obtained using wind data from European Centre for Medium-Range Weather Forecasting (ECMWF) operational analyses.

**Figure 1: Arctic 2006 stratopause as observed by HIRDLS. Contours marked in K**

- GW MF is calculated by the method of* Alexander* *et al* (*2008).* Daily vertical temperature profiles are binned to 2.5° latitude, 12° longitude, removing the mean and zonal wavenumbers 1-3. The S-Transform (*Stockwell et al, 1996*), is then applied to the residual, which returns the vertical wavenumbers as a complex-valued function    of    height.    For    each

- adjacent profile pair the cospectrum and covariance spectrum are then computed, and the maximum in the covariance spectrum located, giving the dominant vertical wavelength as a function of height. From the phase differences between adjacent profiles horizontal wavenumbers *k**h* are obtained. This underestimates the true *k**h* and consequently MF.

**GRAVITY WAVE DETECTION IN HIRDLS DATA**

**OBSERVED MOMENTUM FLUXES**

- Figure 4 shows zonal mean MF values for the Northern Hemisphere, averaged over the Februaries of 2005- 2007, illustrating a sharp reduction of measured values during 2006. This agrees with the hypothesis of *Siskind et al* (2007) of reduced MF levels during this period. Figure 5 shows time series of MF at 60N, 10hPa over the same three Arctic winters. The line for 2006 shows a sharp reduction from the values for the other two years, with the beginning of the period of sharply reduced MF coinciding with the period of anomalously low wind speed in figure 3.

**Figure 4: Monthly zonal mean MF for February 2005, 2006 and 2007**

**Figure 5: Time series of 10hPa MF at 60N over three Arctic winters**

**CONCLUSIONS**

- HIRDLS data give us the opportunity to directly observe small-scale GW activity on a global scale, allowing us to make detailed estimates of GW MF. This allows us to confirm the hypothesis of Siskind et al (2007) that the raised stratopause of Arctic winter 2006 was due to anomalously low GW activity. Comparing our results with

- ECMWF analyses showing  anomalous easterly and  weak westerly zonal winds in the lower stratosphere during this period provides evidence of wave filtering of the GW spectrum, reducing the intensity of GW MF reaching the middle stratosphere. This leads to a strong reduction in the overall GW MF reaching the stratopause.