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The role of boundary layer momentum advection in the mean location of the ITCZ

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Abstract

The inter-tropical convergence zones (ITCZ) form closer to the equator during equinoxes while they form well away from the equator during the boreal summer. A simple three-way balance between the pressure gradients, Coriolis force and effective Rayleigh friction has been classically used to diagnose the location of maximum boundary layer convergence in the near equatorial ITCZ. If such a balance can capture the dynamics of off-equatorial convergence was not known. We used idealized aqua planet simulations with fixed, zonally symmetric sea surface temperature boundary conditions to simulate the near equatorial and off-equatorial ITCZ. As opposed to the convergence of inter-hemispheric flows in the near equatorial convergence, the off-equatorial convergence forms due to the deceleration of cross-equatorial meridional flow. The detailed momentum budget of the off-equatorial convergence zone reveals that the simple balance is not sufficient to capture the relevant dynamics. The deceleration of the meridional flow is strongly modulated by the inertial effects due to the meridional advection of zonal momentum in addition to the terms in the simple balance. The simple balance predicts a spurious near equatorial convergence and a consistent off-equatorial convergence of the meridional flow. The spurious convergence disappears when inertial effects are included in the balance. As cross equatorial meridional flow decelerates to form convergence, the inertial effects cancel the pressure gradient effects near the equator while they add away from the equator. The contribution to the off-equatorial convergence induced by the pressure gradients is significantly larger than the contribution due to the inertial effects and hence pressure gradients appear to be the primary factor in anchoring the strength and location of the off-equatorial convergence.

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Acknowledgements

The authors were supported partially by the Divecha Centre for Climate Change, Indian Institute of Science (IISc), Bengaluru. They thank Dr Ashwin Seshadri for helpful discussions. The super-computer facility provided by Centre for Atmospheric and Oceanic Sciences at IISc – ‘Mandhan’ cluster is acknowledged.

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Author notes

  1. Vishal Dixit

    Present address: Climate Change Research Centre, University of New South Wales, Sydney, NSW, 2052, Australia

Authors and Affiliations

  1. Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bengaluru, 560 012, India

    Vishal Dixit & J Srinivasan

  2. Divecha Centre for Climate Change, Indian Institute of Science, Bengaluru, 560 012, India

    Vishal Dixit & J Srinivasan

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  1. Vishal Dixit
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  2. J Srinivasan
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Correspondence to Vishal Dixit.

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Corresponding editor: N V Chalapathi Rao

Supplementary materials pertaining to this article are available on the Journal of Earth Science Website (http://www.ias.ac.in/Journals/Journal_of_Earth_System_Science).

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Dixit, V., Srinivasan, J. The role of boundary layer momentum advection in the mean location of the ITCZ. J Earth Syst Sci 126, 78 (2017). https://doi.org/10.1007/s12040-017-0856-5

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  • DOI: https://doi.org/10.1007/s12040-017-0856-5

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