Semi-Diurnal Pressure Fluctuations in the ERA40 Data

In a regional analysis the removal of the globally present semi-diurnal pressure signal could be necessary in order to isolate the pressure signal of regional features. As soon as global reanalysis data is used an essential prerequisite is to determine the characteristic signal of this data. The semi-diurnal surface pressure fluctuations between 1958 and 2001, deduced from the ERA40 reanalysis data provided by the European Centre for Medium Range Weather Forecasts, are approximatively determined. Annual mean global structure and magnitude of the ERA40 related semi-diurnal pressure tide compare reasonably well with the structure derived from theory and other reanalysis data. However, the comparison with observed data shows regional shortcomings due to the low temporal resolution of the six-hourly ERA40 data. Along the equator the annually mean amplitude of the semi-diurnal pressure fluctuation in the ERA40 data amounts to 1.34 hPa. The seasonal values are 1.23 hPa for northern hemispheric summer and 1.39 hPa for winter