27 Nov 2023
 | 27 Nov 2023

Changes in the tropical upper tropospheric zonal momentum balance due to global warming

Abu Bakar Siddiqui Thakur and Jai Sukhatme

Abstract. The zonal momentum budget of the deep upper tropics is studied in the context of present and future climates. In the zonal mean, as is known, a robust balance exists between the acceleration by the horizontal eddy momentum flux convergence and the deceleration by the mean meridional momentum advection. During summer, climatological stationary Rossby waves over the Asian monsoon longitudes converge westerly momentum into the tropics and are the primary contributors to the eddy term. During winter, anomalous westerly winds over the tropical East Pacific allow extratropical waves to propagate into the deep tropics, where they tend to break and decelerate the flow. When integrated over all longitudes, eddies from these two regions sum constructively in summer and destructively in winter, always yielding a net positive momentum forcing that balances the mean flow term. The state-of-the-art CMIP6 suite qualitatively captures these features in the historical run and shows that the momentum fluxes change due to global warming. In summer, stationary eddy circulations in the Asian monsoon zone weaken in the upper troposphere (UT) but strengthen in the lower stratosphere (LS). Greater upward mass flux from the troposphere forces a stronger divergence and a more intense circulation in the LS following a Sverdrup vorticity balance. This strengthening of summertime tropical and subtropical stationary waves in the LS is observed over all longitudes and is verified in an idealized aquaplanet general circulation model experiment. In winter, eddy westerlies over the East Pacific longitudes decrease in strength due to the expected weakening of the subtropical stationary waves with warming. This causes a significant decrease in the propagation of extratropical waves into this region, along with a drop in eddy potential vorticity fluxes associated with these waves. Thus, apart from the mean meridional flux, which weakens due to the projected weakening of the Hadley Cells, our analysis of warming simulations clearly suggests significant and robust changes in the eddy momentum fluxes in the deep tropics. Potential implications of these changes in the context of the zonal mean flow and regional circulations are discussed.

Abu Bakar Siddiqui Thakur and Jai Sukhatme

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2779', Anonymous Referee #1, 03 Jan 2024
    • AC1: 'Reply on RC1', Abu Bakar Siddiqui Thakur, 20 Feb 2024
  • RC2: 'Comment on egusphere-2023-2779', Anonymous Referee #2, 26 Jan 2024
    • AC2: 'Reply on RC2', Abu Bakar Siddiqui Thakur, 20 Feb 2024
Abu Bakar Siddiqui Thakur and Jai Sukhatme
Abu Bakar Siddiqui Thakur and Jai Sukhatme


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Short summary
We analyze the present and future states of the tropical upper troposphere. Observations and climate model simulations suggest that interactions between disparate families of waves and the mean flow maintain present-day upper-level winds, and each component undergoes complex changes due to global warming. While the net east-west flow of the atmosphere may remain unaltered, this study indicates robust changes to local circulations that may influence tropical precipitation and regional climate.