Climatology of the IGW's contribution into the height-seasonal structure of the mean circulation in the midlatitude lower thermosphere

A. N. Fahrutdinova and O. G. Khutorova

Kazan State University, Kazan, Tatarstan, Russia

Received November 11, 1998

Contents

Abstract

Introduction

Currently a top priority is given to studies of wave motion. At the upper atmosphere of middle latitudes, internal gravity waves provide an important contribution into the atmospheric momentum budget. The wave momentum is transferred to the environment under the attenuation of an IGW [Fritts, 1985; Lindzen, 1981]. It is important to study the influence of long-period waves.

Radiometeor method is promising for wave researches in the 80-110 km height region. Radiometeor measurements of wind velocity in the Kazan University (54^o N, 49^o E) at the meteor radar with an altimeter during the previous 10 years allow us to investigate IGW's influence on the dynamics of the lower thermosphere [Fahrutdinova et al., 1997]. The wind observations were carried out by the azimutal methods at the distance of 300 km relatively a the point of observation. The radar installation makes altitude measurements in the 80-110 km range, the mean square error being one km. A data bank has been created. The data for three two-year cycles of the measurements are used in this work. The January 1986-December 1987 and July 1993-June 1995 periods were adjacent to minima of solar activity and the May 1988-April 1990 period was near its maximum.

In the Kazan University the method of IGW derivation from radiometeor data has been developed. This method allows us to study IGW with the periods of above 1 hour and the horizontal wave lengths of above 100 km, and also to determine the vertical wave numbers and changes of the IGW amplitude with height in the 80-110 km interval.

The wind and coordinates measured were used to create time series of the zonal and meridional wind averaged over the meteor zone with the horizontal and vertical sizes of 100 km and 3 km, respectively, the step in the horizontal and vertical directions being 50 km and 1.5 km, respectively. The time series obtained were exposed to a high-frequency filtration using averaging of the data within 30 min or 60 min. Then monthly mean prevailing wind and tidal components were calculated and subtracted from the initial series of the data. Then wave disturbances (WD) with periods 1-24 h were derived.

The summary effect of the influence of WD with temporal scales of 1-24 hour on the zonal and meridional circulation is presented in this paper. In each spatial interval we obtain the amplitude and phase of WD. The selection of WD is performed using a check of a linearity of the phase's spatial regression dependence, the signal exceeding noise with a probability of more than 75%. We have created the databank of WD parameters: zonal u', j_u and meridional v', j_v wind amplitude and phase, their horizontal l_x,y and vertical l_z wavelengths, horizontal c_x,y and vertical c_z phase velocities (the coordinates x, y, z corresponding to the Eastward, Northward, and upward directions), and the coefficient g of the amplitude exponential decay with height. The magnitude of g shows that the wave energy decreases with height. Most probable values of these parameters are presented in Table 1 in dependently on solar activity phases.

The values of l_x, c_x, u', v' agree with the results of Gavrilov and Kalov [1986] obtained in radiometeor measurements. The magnitude of g shows that the wave energy decreases with height. To study the WD influence on the upper atmosphere dynamics, vertical fluxes of the horizontal momenta F_yz and F_xz were calculated using the parameters obtained by the radiometeor method, and also using the method and expressions for evaluation of wave fluxes for waves with the temporal scales of IGW and tidal movements proposed by Gavrilov [1984]. Within the limits of the linear theory neglecting the molecular and turbulent viscosity, we have

Here W is the Doppler wave frequency, k_x = l_x^-1, k_z = l_z^-1, c is the ratio of the heat capacities under constant pressure and constant volume, W_B is the Brunt-Vaisala frequency, and W_z is the vertical component of the angular velocity of the Earth's rotation.

Results and Discussion

The comparison of the seasonal variations of the wave flux of the zonal momentum for WD with scales of 1-24 hour at heights of 80-110 km obtained in the radiometeor measurements [Fahrutdinova and Khutorova, 1992] with the measurements at the MU-radar of the Kyoto University at heights of 11-17 km [Murayama et al., 1994] show the presence of annual variations of WD intensity at heights of the middle troposphere-lower stratosphere and upper mesosphere-lower thermosphere with a maximum in winter and a minimum in summer.

If the annual oscillations of WD intensity prevail at stratospheric heights, then at mesospheric heights [Tsuda et al., 1990; Wilson et al., 1991] and in the upper mesosphere-lower thermosphere [Fahrutdinova and Khutorova, 1992; Gavrilov and Kalov, 1986] there are semi-annual oscillations comparable with annual oscillations. The results of the comparison confirm a tropo-stratospheric origin of WD and modulation of their intensity by the mean wind in the middle atmosphere and lower thermosphere.

The amendments for the mean circulation in a geostrofical approach were evaluated as:

where h₀ = H/(1-2gH) is the attenuation decrement of the wave energy and H is the atmospheric scale height. We have calculated additional velocities du and dv to the zonal u and meridional v wind velocities induced by wave disturbances with periods of 1-24 hours.

Presented in Figures 1 and 2 are the results of calculations of the wind velocities induced by WD with periods of 1-24 hour, du and dv , and also the zonal u and meridional v wind for the periods January 1986-December 1987, May 1988-April 1990, and July 1993-June 1995. Plus and minus sign correspond to the western and eastern wind, respectively, in Figure 1 and to the southern and northern wind, respectively, in Figure 2.

One can see that the monthly mean values of du during a solar activity cycle change from +5 ms ^-1 to -20 ms ^-1, the most probable values of the ratio du / u being 0.1-0.5. In the years of solar activity minimum the drag of the zonal circulation is mainly observed, except spring and summer of 1987 and 1994 when the acceleration of the zonal circulation was detected.

Due to WD a strong eastern flow du equal to - (15-20) ms ^-1 is observed in November-December 1994 and January 1995 in the 80-100 km height region. Weaker eastern flow du equal to -10 ms ^-1 is observed in October 1986. In both cases a drag of the zonal circulation occurs.

During the period of solar activity maximum, the monthly mean values of du do not exceed +5 ms ^-1 and cause mainly a drag of the zonal circulation, except in November-December 1989 and January-February 1990 when an acceleration of the zonal circulation is observed and the du / u ratio is approximately equal to 1.

Figure 2 shows a complicated height-temporal structure of the dv values manifesting both the acceleration and drag of the meridional circulation by wave flows. The acceleration and drag may be observed at different heights at the same season.

Due to WD a considerable southern wind is observed during the minimum of solar activity. In the spring, summer, and autumn of 1986, autumn of 1993, and spring-summer of 1995, dv reached values equal to 15 ms ^-1 and caused a drag of the meridional circulation. This fact agrees with Gavrilov et al. [1995], who stated that the intensity of IGW with periods of 2-10 hours shows a negative correlation with solar activity.

At the period of solar activity maximum the height-temporal structure of dv is more homogeneous. The values of dv do not exceed +5 ms ^-1 and cause a drag and acceleration of meridional circulation in the summer-autumn and winter-spring periods, respectively.

The extremum values of dv during the 11-years cycle of solar activity are equal to +15 and -10 ms ^-1, the most probable value of the dv / v ratio being equal to 0.2-0.9.

Fahrutdinova and Khutorova [1992] and Gavrilov et al. [1995] showed that the monthly characteristics of the gravity waves have annual and semi-annual variations. We think that both averaged wind speed and the additional speed have seasonal fluctuations. We have performed the harmonic analysis of u, du, v, and dv for each height. Variations with periods of 12 and 6 months were revealed for all parameters.

Estimations of the amplitudes and phases of annual A1, T1 and semi-annual A2, T2 oscillations of du and dv were obtained. Different intensity of the annual and semi-annual oscillations in different periods of the 11-year solar cycle was found.

A1(du) increases with height in the interval 80-100 km. The ratio A1(du)/A1(u) varies from 0.3 to 0.9 in the periods of solar activity maximum and minimum, respectively.

Ratio A2(du)/A2(u) is equal to 0.2 at heights of 95-110 km during 1986-1987 and 1988-1990. An increase of A2(du) and A2(u) is observed in the period 1993-1995 and the ratio A2(du)/A2(u) is equal to 0.6 at heights of 95-110 km.

Lower values of A1(dv) and A2(dv) were found in the years of solar activity maximum. The difference is best pronounced for the A2(dv) values.

The analysis of the phase of the annual and semi-annual oscillations showed the following.

For the meridional circulation the difference in phases of the annual and semi-annual oscillations is equal to 0-5 months and 0-1.5 months for the dv and v values, respectively. That means that the annual oscillations may be both out of phase and in phase, whereas the semi-annual variations are nearly in phase always.

For the zonal circulation, the difference in phases of the annual and semi-annual oscillations of the du and u values is within 4-6 months and 0-2.5 months, respectively. That means that the annual oscillations are close to being out of phase and the semi-annual oscillations are nearly in phase, but the condition of being in phase is fulfilled somewhat worse than that for the oscillations of dv and v.

Conclusion

The vertical and temporal structure in dv and du, cells of the circulation generated by WD were found with the maximum values of the southern wind equal to 10-15 ms ^-1 in spring and summer of 1986 and 1995 and of the eastern wind equal to - 20 ms ^-1 in winter in the periods of solar activity minimum. At the periods of solar activity maximum, the values of the wind generated by WD do not exceed + 5 ms ^-1.

Wave flows formed semi-annual oscillations of the zonal and especially meridional circulation. In these periods the amplitudes of the semi-annual oscillations are higher during solar activity minima than during maxima.

The amplitudes of the semi-annual oscillations of the amendments to the circulation induced by WD ( dv ) are higher than the amplitudes of the annual oscillations of the meridional wind and less than the oscillations of the zonal wind ( du ).

Acknowledgments

References

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