3.1. Horizontal Distribution of Ep
 Figure 1 shows the distribution of the monthly mean Ep in 2002 averaged at 12–33 km in the Northern Hemisphere. In this study we focus on the behavior of Ep in the polar region (higher than 50°N), but it may be useful to describe the general latitude distribution of Ep here. Because of the active convection in the tropics Ep appears generally large at low latitudes [Tsuda et al., 2000]. It is also noteworthy that the tropical tropopause is located at about 15–18 km at low latitudes; therefore the sharp temperature structure around the tropopause could contaminate Ep in our analysis. So, we do not show in Figure 1 the results of Ep at latitudes lower than 35°N in order to focus on the Ep distribution in the polar region.
 In the Arctic region, Ep is enhanced from November to March, with the largest Ep values are seen in winter months (December–February). We can recognize in Figure 1 localization of Ep, such that the large Ep values exceeding 4 J/kg are seen around west Eurasia (0–40°E, 50–70°N) in January and December, middle Eurasia (40–120°E, 60–80°N) in January and Greenland (20–60°W) in January. On the other hand, small Ep value is seen around western North America (50–80°N, 120–160°W), over the Pacific Ocean (160–220°E, 50–60°N) and over the North Pole (80–90°N).
3.2. Year-to-Year Variations of Ep in 2001–2005
 Figure 2 shows the monthly variations of the zonal mean Ep at 50–80°N at 12–19 km, 19–26 km and 26–33 km. The mean Ep in winter and summer are estimated in each season, and these values together with the winter/summer ratio of Ep are summarized in Table 1. In all of the three height ranges the monthly variations of Ep are similar, having maximum in winter and minimum in summer. Note that in December 2001 and December 2003 Ep is enhanced at all heights. At 12–19 km and 26–33 km, the winter peak of Ep was narrower than that at 19–26 km.
Figure 2. Monthly variation of zonal mean Ep at 50–80°N in three altitude regions: 12–19 km, 19–26 km, and 26–33 km.
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Table 1. Zonal Mean Ep at 50–80°N in the Arctic Regiona
|Altitude Range||Annual Mean Ep, J/kg||Ep in Winter, J/kg||Ep in Summer, J/kg||Winter/Summer Ratio|
 At 26–33 km, the mean Ep is the largest (2.78 J/kg), which is about twice as large as the values at 19–26 km and 12–19 km (1.43–1.45 J/kg). The mean Ep in winter is the largest at 26–33 km (4.56 J/kg), and the smallest at 19–26 km (2.15 J/kg). The mean Ep in summer is large (1.48 J/kg) at 26–33 km, while at 12–19 km and 19–26 km Ep was smaller and comparable (0.79 J/kg). The ratio of mean Ep in winter to that in summer is large and comparable at 12–19 km and 26–33 km (3.07–3.08) and smallest at 19–26 km (2.72). This result shows that the contrast of Ep between summer and winter is large at 12–19 km and 26–33 km and is smaller at 19–26 km.
 Figure 3 shows the monthly variation of the zonal mean Ep at 12–33 km in the four latitude regions: 50–60°N, 60–70°N, 70–80°N and 80–90°N. We use Ep in the entire height region of the analysis, i.e., 12–33 km, because the time variation of Ep is similar in all height regions. The annual mean Ep value and the Ep averaged in winter and summer months are shown in Table 2. The ratio of mean Ep between summer and winter is also shown in Table 2.
Figure 3. Monthly variation of zonal mean Ep at 12–33 km in four latitude regions:50–60°N, 60–70°N, 70–80°N, and 80–90°N.
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Table 2. Zonal Mean Ep at 12–33 km in the Arctic Regiona
|Latitude Range||Annual Mean Ep, J/kg||Ep in Winter, J/kg||Ep in Summer, J/kg||Winter/Summer Ratio|
 In all latitude regions, Ep shows a similar annual cycle with a maximum in winter. Note that gravity waves are enhanced largely in December 2001 at 50–60°N, 60–70°N, and 70–80°N. The peaks of Ep appear differently, for example, between winter in 2001/2002 and 2004/2005 such that a single intense peak existed in the former period, while a broad enhancement with three Ep peaks is seen in the latter. The peak of Ep mostly coincided between the different latitude regions. However, a time lag sometimes occurred. For example, a large peak was simultaneously recognized in December 2001 at 50–60°N, 60–70°N and 70–80°N, but the peak occurred in January 2002 at 80–90°N. Another example is the peak in March 2003 at 60–70°N, which did not coincide with the peak at other latitude regions.
 The 5 year mean Ep values are similar at 50–60°N, 60–70°N and 70–80°N ranging from 1.85 to 1.90 J/kg, but it is smaller (1.39 J/kg) at 80–90°N. The mean Ep in winter months is larger at 50–60°N and 60–70°N (3.07–3.13 J/kg), slightly smaller (2.89 J/kg) at 50–60°N, and the smallest at 80–90°N (2.32 J/kg). The mean Ep in summer months shows a tendency for the value to gradually increase from 0.74 J/kg at 80–90°N to 1.17 J/kg at 50–60°N. The ratio of mean Ep between winter and summer is the largest at 70–80°N (3.37) and smallest 50–60°N (2.47 J/kg). This result shows that the contrast of the mean Ep between winter and summer is the largest at 70–80°N and smallest at 50–60°N.
3.3. Comparison of Ep With Mean Horizontal Wind, Planetary Wave Activity, and Surface Wind
 Time variations of zonal mean Ep are compared with mean horizontal wind amplitude, the planetary wave amplitude, the planetary wave amplitude fluctuation, the vertical component of E-P flux, the divergence of E-P flux and the mean horizontal surface wind. Ep is averaged over 12–33 km at 50–80°N. We have calculated the zonal mean horizontal winds, V, at 50–80°N in the height range between 250 and 7 hPa. The planetary wave amplitude is estimated daily by using the geopotential height, ϕ. We have analyzed the zonal variations of ϕ by means of FFT, and extracted the planetary wave amplitude corresponding to wave numbers 1 and 2, then we defined the daily planetary wave intensity, Ap as the root mean square of the two components as follows;
where PW1 and PW2 are the planetary wave amplitudes corresponding to the wave number 1 and 2. Then, we have further smoothed Ap using a 7-day running mean, and defined this as 〈Ap〉. We have calculated Ap′ = Ap − 〈Ap〉, and regarded it as the magnitude of the short-term fluctuating components of planetary waves. We also refer to the vertical component of E-P flux Fz, determined from the four dimensional global objective analysis data produced by JMA. The divergence of E-P flux F, defined as ∇ · F, is also analyzed. Here, we plot absolute values of ∇ · F/ρ(≡ΔF), where ρ is density. Fz is a measure of planetary wave activity, whereas ΔF is that of rapid changes of planetary wave activity due to planetary wave transience and/or breaking. The wave transience is produced by wave growth and/or damping in the course of planetary wave propagation. The planetary wave breaking intermittently occurs near an equatorial critical surface where the phase velocity of the wave coincides with the zonal wind speed, i.e., the surf zone [McIntyre and Palmer, 1983]. The horizontal wind amplitude 10 m above the surface, Vs is averaged zonally at 50–80°N.
 Figure 4 shows the time variations of Ep together with the variations of other variables. Asterisks in Figure 4 show the mean values of individual parameters in winter months. We also analyzed the cross correlation function between Ep and other reference variables and investigated the time lag between the parameters (results not shown here).
Figure 4. Time variation of zonal mean Ep at 12–33 km and 50–80°N (first panel), V averaged zonally at 250–7 hPa in altitude and 50–80°N in latitude (second panel), Ap at 10 hPa and 50–60°N (third panel), Ap′ at 10 hPa and 50–60°N (fourth panel), Fz at 50–80°N and 10 hPa (fifth panel), absolute value of ΔF at 50–80°N and 10 hPa (sixth panel), and mean Vs averaged zonally and at 50–80°N (seventh panel). Asterisks show the mean value of each variable in winter (DJF).
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 Ep shows a clear annual cycle, with a broad peak from November to March. Annual variations of V and Ap correlate well with Ep with coefficients of 0.83 and 0.90, and a small time lag. Ap′ correlates with Ep (correlation coefficient = 0.77). Fz correlated well with Ep without time lag (correlation coefficient = 0.86). Some peaks of ΔF match with those of Ep (correlation coefficient = 0.68). Time variations of Vs and Ep are similar (correlation coefficient = 0.79).
 The mean values in winter are estimated for all variables. In Table 3 we summarize these variables in each year that are normalized relative to the values in 2001, which can be used to test year-to-year variability. In Table 3, we can recognize that Ep is large in winter in 2001–2002, while in other seasons Ep are comparable. V, Ap and Ap′ have larger values in 2002–2003 and 2004–2005 in winter than those in 2001–2002. Fz has large value in 2001 and in other year the Fz values are comparable. Vs has no clear differences between years. Year-to-year variation of Fz is consistent with Ep.
Table 3. Ratio of the Mean Value of the Parameters in the Arctic Normalized by Mean Value in 2001
 The cross correlation coefficient between Ep and V is as high as 0.79, which might suggest gravity wave generation by topography. However, the cross correlation function was calculated for the zonal mean variables, which does not certify the relative locations of these disturbances.
 In order to examine the possibility of gravity wave generation by topography (orographic effects) in the polar region (>50°N), we check the horizontal distribution of Ep, and compare it with topography. Figure 5 shows the 5 year averaged distribution of Ep in winter months during 2001–2005 and the topography distribution. In Figure 5, large Ep values are not recognized over most of the large mountainous regions, such as the Rocky Mountains. Some enhancement of Ep is seen around Europe (Scandianvia). However, over the flat area like central Eurasia Ep is also large. So this result suggests that gravity waves are not mainly generated by topography at high latitudes (>50°N).
Figure 5. (left) Five year mean distribution of Ep from 2001 to 2005 in December, January and February and (right) the topographical distribution in the Arctic region.
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 Radiosonde results at the Northern Hemisphere indicated that Ep became larger over Scandinavia in winter, and this enhancement was explained as the effect by local topography [Yoshiki and Sato, 2000]. This result is contrary to our results. However, radiosonde observation sites are limited around Europe (Scandinavia) and Greenland only. Our study covering the entire Northern Hemisphere has suggested that the topography does not seem to be the largest generation source of gravity waves in the Arctic region (>50°N).
 Figure 6 shows the distribution of Ep at 19–33 km, ϕ at 50–7 hPa and V in January 2004 in the Arctic region. The gravity wave enhancement occurred in central Europe (0–20°E and 50°N) and middle Eurasia at 80–120°E and 60°N. In winter the structure of Ep is similar to that of V, that is, Ep is enhanced along an ellipse over Eurasia at 50°N and 0–150°E. This distribution is similar to V and ϕ.
Figure 6. Distribution of (left) Ep at 19–33 km in January 2004 in the Arctic region, (middle) ϕ at 50–7 hPa in January 2004 and (right) V at 50–7 hPa.
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 On the basis of our analysis, we investigate below three possibilities for the generation mechanism of gravity waves:
 1. The first possibility is planetary wave activity and geostrophic adjustment. Near the polar night jet, planetary waves could become active; then, the amplification of planetary waves distorts the polar vortex. Moreover, rapid changes of planetary wave activity due to planetary wave transience and/or breaking lead to rapid changes of such distortion. In general, strong planetary wave activity would be accompanied with its rapid change, so that both the mechanisms could not be absolutely separated each other. Under such conditions, unbalanced flow of the polar vortex would be brought about, which can excite gravity waves through geostrophic adjustment. The annual cycle of Fz is consistent with that of Ep. The good correlation between Ep and ΔF also suggests that planetary wave transience and/or breaking are effective in generating gravity waves.
 2. The second possibility is orographic generation of gravity waves. Planetary waves are generally generated by topography and the thermal contrast difference between sea and land. Because a good correlation is seen between Ep and Ap, we consider the possibility that gravity waves are generated simultaneously with planetary waves by the same excitation source, i.e., topography. However, the horizontal scales are considerably different between planetary waves and gravity waves; therefore the effectiveness of topography in exciting these waves may also be different. Moreover, we do not find that gravity waves are localized over mountain ranges. It is unlikely, therefore, that gravity waves and planetary waves are generated simultaneously.
 3. The third possibility is wave-mean flow interaction. Gravity waves are generated in the troposphere and the wave propagation is controlled by the mean horizontal winds. Because we cannot determine the phase velocity of individual gravity wave events, it is hard to examine the critical level interaction of the gravity waves with the background winds. Provided the gravity waves are generated in a random manner, such that the phase velocity and propagation direction are uniformly distributed, the interannual variations of Ep could appear similar to that of the background wind.
 Our study has clarified that the gravity waves in the Arctic region (50–90°N) in winter are most likely to be generated by geostrophic adjustment in association with planetary wave transience and/or breaking under the strong activity of planetary waves. However, we cannot clearly deny a possibility that the wave-mean flow interaction is responsible for the annual and interannual variations of Ep.