Response of the Mesopause Region Dynamics to the February 2001 Stratospheric Warming

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1 Response of the Mesopause Region Dynamics to the February 21 Stratospheric Warming Ch. Jacobi 1, D. Kürschner 2, H.G. Muller 3, D. Pancheva 4, N.J. Mitchell 5, B. Naujokat 6 1. Institute for Meteorology, University of Leipzig, Stephanstr. 3, 413 Leipzig, Germany 2. Institute for Geophysics and Geology, University of Leipzig, Observatory, 4779 Wermsdorf, Germany 3. Department of Aerospace Power and Sensors, Cranfield University, Swindon SN6 8LA, U.K. 4. University of Wales, Aberystwyth, Ceredigion SY23 3BZ, U.K. 5. The Department of Electronic and Electrical Engineering, The University of Bath, BA2 7AY, UK. 6. Stratospheric Research Group, Free University of Berlin, Carl-Heinrich-Becker-Weg 6-1, 2165 Berlin, Germany Poster available at

2 Summary Mesopause region horizontal winds have been measured over three stations (,, ESRANGE) in Europe during winter 2/21 to investigate the response of the mesosphere/lower thermosphere region to a major stratospheric warming. Although different measuring methods have been applied (LF D1, meteor radar) the measured temporal and vertical wind structure was very similar. Also the stratospheric warming effect was visible, and very similar, in both midlatitude and high-latitude sites. The warming resulted in a reversal of both the zonal and meridional wind. In the zonal component, this reversal was due to a planetary oscillation with period 1 days, which was even more pronounced in the vertical gradients, so that the stratospheric warming effect on the mesopause was owing to the superposition of an intensifying planetary wave and a slow overall decrease of the zonal prevailing winds.

3 The February 21 stratospheric warming as seen at 3 hpa Figure 1: Northern hemisphere geopotential height distribution during January and February 21 at 3 hpa. The stratospheric warming is well visible in February, when the polar vortex is strongly disturbed.

4 Time series of zonal prevailing winds Figure 2: Zonal prevailing winds over km 98 km (52 N, 2 W), (52 N, 15 E) and ESRANGE (68 N, 21 E) at two different heights, respectively. v oz in ms km 98 km Wave structures as well as changes in vertical gradients are visible. The peak effect of the warming is seen around day N o 5. 3 ESRANGE km 94 km Days, 1 =

5 Time series of meridional prevailing winds Figure 3: Meridional prevailing winds over km 98 km ESRANGE (52 N, 2 W), (52 N, 15 E) and (68 N, 21 E) at two different heights, respectively. v om in ms km 98 km Well-defined wave structures are not clearly visible. During the warming, strong southward winds are found km 97 km ESRANGE Days, 1 =

6 Time series of zonal prevailing wind vertical gradients Figure 4: 4 Vertical gradient of the zonal prevailing wind. Over and (lower curves) clear wave structures are visible. v oz / z in ms -1 /km ESRANGE v oz / z in ms -1 /km Days, 1 =

7 Amplitude spectra Figure 5: 1,2 1, Mean zonal wind Amplitude spectra of the zonal prevailing wind and the vertical zonal wind gradient, calculated using the data from January 15 through February 24, day wave is only visible in the gradients. Normalised Amplitude,8,6,4,2,,8 Vertical zonal wind gradient,6,4,2,,,5,1,15,2,25,3,35,4 Frequency (1/d) ESRANGE

8 97 The 1-day wave over ESRANGE Height in km Days, 1 = Figure 6: Height-time cross-sections of the zonal wind over ESRANGE. During February (right part of the figure) a clear upward propagating 1-day wave is visible.

9 Vertical structure of the 1-day wave Figure 7: Amplitudes and phases of the 1-day oscillation over ESRANGE, and change rate du/dt of the zonal prevailing winds at the different height gates. Over ESRANGE a clear decrease of the zonal winds during the entire stratospheric warming period is found. Height in km du/dt in ms -1 d -1 -,8 1 -,6 -,4 -,2, Amplitude of the 1-day wave zonal prevailing wind change Phase of the 1-day wave Amplitude in ms -1 Phase in degr.

10 Results and Conclusions During February 21 a major stratospheric warming occurred. This warming was visible in the mesosphere/lower thermosphere: 1. A clear 1-day wave was measured during the warming. 2. The warming results in a decrease of the zonal prevailing wind, which is superposed by the wave. 3. The meridional prevailing wind was negative, which is due to a residual circulation above the region of wave-mean flow interaction. Acknowledgements: This study was partly supported by BMBF within the AFO2 programme under 7 ATF1 (MEDEC).

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