The structure of the lunar semi‐diurnal pressure tide L 2

The Hough structure of the lunar semi‐diurnal tide L 2( p ) in surface pressure is estimated using joint weightings by area and probable‐error structure, on annual and Lloyd seasonal scales, with new data. Global representations, on seasonal and annual scales of the L 2 wave, in both spherical harmonic and Hough function forms, are presented for the Haurwitz–Cowley dataset enlarged by the more recent tidal determinations of Hutchings and Palumbo. These are mainly for southern hemiphere island locations. The asymmetric Hough eigenfunction terms H 3 2 are considerably larger than previously estimated, both annually and seasonally, a possible artefact of the earlier data analysis technique. They are consistent and identified with the well‐known difference in phasing and amplitude of L 2 by hemisphere. The dependence of the Hough structure on the order of the fitting and on the more extreme residuals is examined. It is stable for the smallest models (spherical harmonic terms up to degree 6 and order 3), to removal of the largest residuals from a first fitting, and to the two largest datasets. Seasonal changes in the Hough structure are discussed. The asymmetric results appear consistent with early frictional/thermal interpretations on the seasonal variation of L 2. But the sensitivity of L 2 to upper‐air temperature structure is not consistently shown in the analyses for the main mode H (2, 2). Copyright © 2005 Royal Meteorological Society.