The detailed dynamics of the June–August Hadley Cell

Document identifier: oai:DiVA.org:ltu-76824
Access full text here:10.1002/qj.3702
Keyword: Natural Sciences, Rymd- och flygteknik, Atmospheric science, WMRG, Tropical–extratropical vorticity, Tropical convection, OLR, Hadley Cell, Filaments, Angular momentum, Maskinteknik, Earth and Related Environmental Sciences, Teknik och teknologier, Aerospace Engineering, Mechanical Engineering, Engineering and Technology, Meteorologi och atmosfärforskning, Geovetenskap och miljövetenskap, Naturvetenskap, Meteorology and Atmospheric Sciences, Atmosfärsvetenskap
Publication year: 2020
Relevant Sustainable Development Goals (SDGs):
SDG 13 Climate actionSDG 14 Life below water
The SDG label(s) above have been assigned by OSDG.ai

Abstract:

The seminal theory for the Hadley Cells has demonstrated that their existence is necessary for the reduction of tropical temperature gradients to a value such that the implied zonal winds are realisable. At the heart of the theory is the notion of angular momentum conservation in the upper branch of the Hadley Cells. Eddy mixing associated with extra‐tropical systems is invoked to give continuity at the edge of the Hadley Cell and to reduce the subtropical jet by a factor of 3 or more to those observed. In this paper a detailed view is presented of the dynamics of the June–August Hadley Cell, as given by ERA data for the period 1981–2010, with an emphasis on the dynamics of the upper branch. The steady and transient northward fluxes of angular momentum have a very similar structure, both having a maximum on the equator and a reversal in sign near 12°S, with the transient flux merging into that associated with eddies on the winter sub‐tropical jet. In the northward absolute vorticity flux, the Coriolis torque is balanced by both the steady and transient fluxes. The overturning circulations that average to give the Hadley Cell are confined to specific longitudinal regions, as are the steady and transient momentum fluxes. In these regions, both intra‐seasonal and synoptic variations are important. The dominant contributor to the Hadley Cell is from the Indian Ocean and W Pacific regions, and the maxima in OLR variability and meridional wind in these regions have a characteristic structure associated with the Westward‐moving Mixed Rossby‐Gravity wave. Much of the upper tropospheric motion into the winter hemisphere occurs in filaments of air from the summer equatorial region. These filaments can reach the winter sub‐tropical jet, leading to the strengthening of it and of the eddies on it, implying strong tropical‐extratropical interaction.

Authors

Brian Hoskins

Department of Meteorology, University of Reading, Reading, UK Grantham Institute—Climate Change and the environment, Imperial College, London, U
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G.Y Yang

Department of Meteorology, University of Reading, Reading, UK. Climate Directorate, National Centre for Atmospheric Science, University of Reading, Reading, UK
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Ricardo Fonseca

Luleå tekniska universitet; Rymdteknik; Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates
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