Semidiurnal solar tide differences between fall and spring transition times in the Northern Hemisphere

J Federico Conte,Fazlul I Laskar,Peter Brown,Gunter Stober,Hauke Schmidt,Jorge L Chau

doi:10.5194/angeo-36-999-2018

Abstract

Abstract. We present a study of the semidiurnal solar tide (S2) during the fall and spring transition times in the Northern Hemisphere. The tides have been obtained from wind measurements provided by three meteor radars located at Andenes (69∘ N, 16∘ E), Juliusruh (54∘ N, 13∘ E) and Tavistock (42∘ N, 81∘ W). During the fall, S2 is characterized by a sudden and pronounced decrease occurring every year and at all height levels. The spring transition also shows a decrease in S2, but not sudden and that ascends from lower to higher altitudes during an interval of ∼ 15 to 40 days. To assess contributions of different semidiurnal tidal components, we have examined a 20-year free-run simulation by the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA). We found that the differences exhibited by the S2 tide between equinox times are mainly due to distinct behaviors of the migrating semidiurnal and the non-migrating westward-propagating wave number 1 tidal components (SW2 and SW1, respectively). Specifically, during the fall both SW2 and SW1 decrease, while during the springtime SW2 decreases but SW1 remains approximately constant or decreases only slightly. The decrease shown by SW1 during the fall occurs later than that of SW2 and S2, which indicates that the behavior of S2 is mainly driven by the migrating component. Nonetheless, the influence of SW1 is necessary to explain the behavior of S2 during the spring. In addition, a strong shift in the phase of S2 (of SW2 in the simulations) is also observed during the fall. Our meteor radar wind measurements show more gravity wave activity in the fall than during the spring, which might be indicating that the fall decrease is partly due to interactions between SW2 and gravity waves.

Highlights

It is well known that the mesosphere and lower thermosphere (MLT) variability is strongly influenced by a large variety of waves that dynamically interact and couple different regions of the terrestrial atmosphere
We have analyzed wind measurements provided by three meteor radars located at Andenes (69.3◦ N, 16◦ E), Juliusruh (54.6◦ N, 13.3◦ E) and Tavistock (42.3◦ N, 80.8◦ W)
We assume that one can use the analysis of the simulated tides to explain the behavior of S2 in the observations. Both equinox time periods are characterized by a reduction in the activity of the S2 tide, the decrease observed in the fall is abrupt, more pronounced and it happens at all height levels at approximately the same time (Fig. 1)

Summary

Introduction

It is well known that the mesosphere and lower thermosphere (MLT) variability is strongly influenced by a large variety of waves that dynamically interact and couple different regions of the terrestrial atmosphere. Global-scale waves include planetary waves (PWs), which have periods of ∼ 2– 30 days, as well as thermal tides, which have periods that are harmonics of the solar day (e.g., Rossby, 1939; Forbes, 1984). The non-migrating tides are primarily excited by tropospheric latent heat release and may be westward or eastward propagating (Hagan and Forbes, 2002, 2003). Another source of non-migrating tides is the non-linear interaction between migrating tides and stationary planetary waves (e.g., Angelats i Coll and Forbes, 2002)

Results

Discussion

Conclusion