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

Abstract

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.

Changes in the Global Climate: Atmospheric Angular Momentum and Pacific Ocean Temperatures

“…An earlier study by Paek and Huang (2012) compared five different reanalyses and found very different trends in the datasets. For the longer Twentieth Century Reanalysis (Compo et al 2006(Compo et al , 2011 there appears to be a positive trend especially in the ERA-20C reanalysis (Veerman and Heerwaarden 2019), but overall, the trends in reanalysis data are not yet consistent. By contrast, Paek and Huang (2012) find that interdecadal variations in global AAM are well reproduced in the reanalysis datasets.…”

“…Atmospheric angular momentum (AAM) has been used to study changes in the global circulation on multiple time scales ranging from synoptic to millennial (Paek and Huang 2012;Veerman and Heerwaarden 2019). AAM is a measure of the rotation of the atmosphere about Earth's axis and is altered through exchanges with the solid Earth also rotating about its axis (Peixoto and Oort 1992).…”

“…Interdecadal and decadal variations of the atmosphere and ocean have been studied extensively dating back to at least the late 1800s (e.g., Nigam et al 2020;Dong and Dai 2015;Paek and Huang 2012;Veerman and Heerwaarden 2019;Yang et al 2020;Moron et al 1998;Meehl et al 2016a). For the IPO, Henley et al (2015) suggest warm or positive phases were observed during roughly 1924-44 and 1977-99, and negative or cold phases during 1945-76 and 1999-2013.…”