A positive trend in the mass term of global atmospheric angular momentum (AAM), designated as \(M_{\Omega }\) , has been reported in previous studies but mostly analyzed from a single reanalysis dataset. In this paper, we examine the statistical significance of this trend over the period of 1950 to 2020 using four reanalysis datasets, i.e., ERA5, ERA-20C, JRA55, and NCEP/NCAR. We confirm that all of the datasets produce long-term positive trends despite some discrepancies. Moreover, all the datasets are in agreement on a positive trend over the last decade, especially over the equatorial region, suggesting an increase in equatorial air mass over the past five decades. We use a Lagrangian trajectory model to simulate changes in the interhemispheric air mass transport, whose results indicate that a mass accumulation in the equatorial region has occurred since the 1970s due mainly to transports from the Southern Hemisphere (SH). We also computed the change in the water vapor component of \(M_{\Omega }\) ( \(\Delta M_{\Omega w}\) ), and found that global \(\Delta M_{\Omega w}\) contributes around 25 to 50% of the changes in global \(M_{\Omega }\) ( \(\Delta M_{\Omega }\) ), with the largest contribution originating from the equatorial region. Although both \(M_{\Omega }\) and \(M_{\Omega w}\) exhibit long-term positive trends, their statistical correlation varies across the interdecadal timescale. Particularly, a decrease in \(M_{\Omega w}\) is found to be inconsistent with an increase in \(M_{\Omega }\) from the mid-1980s to the 2000s. By estimating dry air surface pressure from the difference between total surface pressure, \(p_{s}\) , and water vapor surface pressure, \(p_{sw}\) , we identify that a persistent increase in dry atmosphere mass over the equatorial region might have been responsible for maintaining the positive trend of global \(M_{\Omega }\) over the last five decades, while the contribution of \(M_{\Omega w}\) is also increasing over the last decade. Thus, the large positive trend in \(M_{\Omega }\) over the last decade is likely a combined effect of the increase in dry atmosphere mass that occurred in previous decades and the current moistening of the equatorial atmosphere. These findings offer new insight into how long-term changes in atmospheric mass distribution—particularly over the equatorial region—may serve as key drivers of global \(M_\Omega\) variability, with potential implications for large-scale circulation and Earth’s angular momentum balance.
