Trends in and closure of the atmospheric angular momentum budget in the 20th century in ERA-20C

It is well known that global warming in the 20th century has influenced the global circulation of the atmosphere. Atmospheric angular momentum (AAM), a measure of the rotation of the atmosphere around the Earth's axis, is a useful quantity to investigate changes in the global atmospheric circulation. In this study, 20th century trends in the AAM budget are determined using the ERA-20C reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF). In addition, the closure of the AAM budget is determined to assess the ability of ERA-20C to conserve angular momentum. The total AAM has increased in the 20th century, associated mainly with an increasing relative (zonal wind) AAM in most of the stratosphere and the tropical upper troposphere, and a poleward redistribution in the midlatitudes. These trends can be related to the warming in the troposphere and cooling in the lower stratosphere found in this study, likely caused by increasing atmospheric CO2 concentrations. The Ω-AAM, representing the rotation of the atmosphere along with the Earth, shows no clear trend, but a spurious peak around 1920. This peak is caused by a global increase in surface pressure and is considered an artefact of changes in the amount of assimilated observations. It is also found that the AAM budget is not well closed in ERA-20C, which is mainly the result of the assimilation of observations during production of the reanalysis. The trends in the AAM budget in ERA-20C are likely affected by changes in the number of assimilated observations and should be validated with other reanalyses in further research