Lunar tide amplification during the January 2009 stratosphere warming event: Observations and theory

Amplification of the lunar gravitational M-2 and N-2 tides during the January 2009 sudden stratosphere warming (SSW) event are explored using SABER temperature data at 110 km, CHAMP and GRACE densities at 360 and 480 km, and the Global-Scale Wave Model (GSWM). Utilizing background temperatures and winds characteristic of individual days during January and February 2009 the GSWM is used to establish the frequency response of the atmosphere and its dependence on the zonal mean atmospheric state, and to provide daily estimates of the steady state response to lunar gravitational forcing. SABER M-2 temperature amplitudes are found to achieve values of order 10-15 K at 110 km, roughy 5 days after the peak warming at 10 hPa (day 23 of 2009), and represent a factor of 3 or so amplification over January climatological values. This amplification is attributable to a shifting of the so-called Pekeris resonance peak of the atmosphere to 12.42 hours (the M (2) period) due to changes in the zonal mean wind distribution in connection with the SSW. N-2 responses of order 4-6 K are similarly found near days 25 and 40, when the Pekeris peak passes near to 12.66 hours (the N-2 period). The GSWM predicts temperature perturbations of order 20-30 K for M-2 and 3-4 K for N-2, reasonably consistent with the above results, given nuances of the data analysis and recognized shortcomings of utilizing a steady state model. An enhanced M-2 density response of order 15-20% is found in the CHAMP measurements between days 30 and 40, as compared with a GSWM estimate of order 10%. The upper thermosphere N-2 response could not be extracted from CHAMP measurements, and both M-2 and N-2 response determinations using the GRACE data were deemed unreliable. The GSWM estimates lunar tidal wind responses of order 40-60 ms(-1) in the dynamo region (ca. 100-150 km) in connection with the SSW. These constitute large perturbations to the nominal tidal winds at these altitudes, and are thus expected to carry the lunar tide signature into the ionosphere vis-a-vis dynamo electric fields, consistent with observations and interpretations made in the recent literature. Citation: Forbes, J. M., and X. Zhang (2012), Lunar tide amplification during the January 2009 stratosphere warming event: Observations and theory, J. Geophys. Res., 117, A12312, doi:10.1029/2012JA017963.