Long-term positive trend in the mass term of global atmospheric angular momentum and its relationships with the moistening of the equatorial atmosphere

Abstract A positive trend in the mass term of global atmospheric angular momentum (AAM), designated as $$M_{\Omega }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>Ω</mml:mi> </mml:msub> </mml:math> , 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 $$M_{\Omega }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>Ω</mml:mi> </mml:msub> </mml:math> ( $$\Delta M_{\Omega w}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mi>Ω</mml:mi> <mml:mi>w</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ), and found that global $$\Delta M_{\Omega w}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mi>Ω</mml:mi> <mml:mi>w</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> contributes around 25 to 50% of the changes in global $$M_{\Omega }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>Ω</mml:mi> </mml:msub> </mml:math> ( $$\Delta M_{\Omega }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>Ω</mml:mi> </mml:msub> </mml:mrow> </mml:math> ), with the largest contribution originating from the equatorial region. Although both $$M_{\Omega }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>Ω</mml:mi> </mml:msub> </mml:math> and $$M_{\Omega w}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mi>Ω</mml:mi> <mml:mi>w</mml:mi> </mml:mrow> </mml:msub> </mml:math> exhibit long-term positive trends, their statistical correlation varies across the interdecadal timescale. Particularly, a decrease in $$M_{\Omega w}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mi>Ω</mml:mi> <mml:mi>w</mml:mi> </mml:mrow> </mml:msub> </mml:math> is found to be inconsistent with an increase in $$M_{\Omega }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>Ω</mml:mi> </mml:msub> </mml:math> from the mid-1980s to the 2000s. By estimating dry air surface pressure from the difference between total surface pressure, $$p_{s}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>p</mml:mi> <mml:mi>s</mml:mi> </mml:msub> </mml:math> , and water vapor surface pressure, $$p_{sw}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>p</mml:mi> <mml:mrow> <mml:mi>sw</mml:mi> </mml:mrow> </mml:msub> </mml:math> , 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 $$M_{\Omega }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>Ω</mml:mi> </mml:msub> </mml:math> over the last five decades, while the contribution of