21 Mar 2022
21 Mar 2022
Status: this preprint is currently under review for the journal ACP.

Variability of air mass transport from the boundary layer to the Asian monsoon anticyclone

Matthias Nützel1, Sabine Brinkop1, Martin Dameris1, Hella Garny1,2, Patrick Jöckel1, Laura L. Pan3, and Mijeong Park3 Matthias Nützel et al.
  • 1Deutsches Zentrum für Luft- und Raumfahrt, Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 2Meteorologisches Institut München, Ludwig-Maximilians-Universität München, Munich, Germany
  • 3National Center for Atmospheric Research, Boulder, Colorado, USA

Abstract. Air masses within the Asian monsoon anticyclone (AMA) show anomalous signatures in various trace gases. In this study, we analyze how air masses are transported from the planetary boundary layer (PBL) to the AMA via multiannual trajectory anlyses. While previous studies analyzed the PBL to AMA transport mainly for individual monsoon seasons or particular periods, we focus on the climatological perspective and on the interannual and intraseasonal variability.

To this end we employ backward trajectories, which were computed using reanalysis data. Based on these trajectories, we analyze air mass transport from the PBL to the AMA during northern summer (June–August) for 14 summer seasons. Further, we backtrack forward trajectories from a free-running chemistry-climate model (CCM) simulation, which includes parametrized Lagrangian convection. The analysis of this additional model data set helps us to carve out robust or sensitive features of PBL to AMA transport with respect to the employed model.

Results from both the trajectory model and the Lagrangian CCM emphasize the robustness of the three-dimensional transport pathways from the PBL to the AMA. Air masses are transported upwards on the eastern side of the AMA and are uplifted within the full AMA domain above. While this is in agreement with previous modelling studies, we refine the picture of the so-called "conduit" (Bergman et al., 2013). The contributions from the Tibetan Plateau (TP; 17 % vs. 15 %) and the West Pacific (around 12 %) are similar in both model results. However, the contributions from the Indian subcontinent and South-East Asia are considerably larger in the Lagrangian CCM data, which might point towards the importance of convective transport for PBL to AMA transport for these regions.

The analysis of both model data sets highlights the interannual and intraseasonal variability with respect to PBL source regions of the AMA. Additionally, we analyze the relation of the interannual east-west displacement of the AMA – which we find to be related to the monsoon Hadley index – to the transport behaviour and find that there are differences for "east" and "west years", the main transport characteristics, however, are comparable.

Regarding the intraseasonal variability our trajectory model results show that transport from the PBL over the Tibetan Plateau (TP) to the AMA is weak in early June (less than 4 % of the AMA air masses), whereas in August TP air masses contribute considerably (roughly 24 %). The evolution of the contribution from the TP is supported by data from the Lagrangian CCM and is related to the northward shift of the subtropical jet and the AMA during this period. This result may help to reconcile previous results and further highlights the need of taking the subseasonal (and interannual) variability of the AMA and associated transport into account.

Matthias Nützel et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-143', Anonymous Referee #1, 13 Apr 2022
  • RC2: 'Comment on acp-2022-143', Anonymous Referee #2, 24 Apr 2022
  • AC1: 'Author Comment on acp-2022-143', Matthias Nützel, 23 Jun 2022

Matthias Nützel et al.

Data sets

The ERA-Interim reanalysis dataset, Copernicus Climate Change Service (C3S) European Centre for Medium-Range Weather Forecast (ECMWF)

Matthias Nützel et al.


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Short summary
During the Asian summer monsoon season a large high-pressure system is present at levels close to the tropopause above Asia. We analyze how air masses are transported from surface levels to this high pressure system, which shows distinct features from the surrounding air masses. To achieve this, we employ multiannual data from two complementary models that allow us analyze these transport pathways. With this method we investigate the interannual and intraseasonal variability.