A dynamical framework for the origin of the diagonal South Pacific and South Atlantic convergence zones

van der Wiel, Karin, Matthews, Adrian ORCID: https://orcid.org/0000-0003-0492-1168, Stevens, David ORCID: https://orcid.org/0000-0002-7283-4405 and Joshi, Manoj ORCID: https://orcid.org/0000-0002-2948-2811 (2015) A dynamical framework for the origin of the diagonal South Pacific and South Atlantic convergence zones. Quarterly Journal of the Royal Meteorological Society, 141 (691). pp. 1997-2010. ISSN 0035-9009

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Abstract

The South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ) are diagonal bands of precipitation that extend from the equator southeastward into the Southern Hemisphere over the western Pacific and Atlantic Oceans, respectively. With mean precipitation rates over 5 mm day−1, they are a major component of the tropical and global climate in austral summer. However, their basic formation mechanism is not fully understood. Here, a conceptual framework for the diagonal convergence zones is developed, based on calculations of the vorticity budget from reanalysis and Rossby wave theory. Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi-circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest-southeast tilt. Ray tracing diagnostics in a non-divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to destabilisation and ascent ahead of the cyclonic vorticity anomaly in the wave, triggering deep convection because of the high sea surface temperatures in this region. Latent heat release then forces additional ascent and strong upper-tropospheric divergence, with an associated anticyclonic vorticity tendency. A vorticity budget shows that this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is consequently comprised of the sum of these pulses of diagonal bands of precipitation. Similar mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone.

Item Type: Article
Uncontrolled Keywords: spcz,sacz,tropical-extratropical interaction,rossby waves,sdg 13 - climate action ,/dk/atira/pure/sustainabledevelopmentgoals/climate_action
Faculty \ School: Faculty of Science > School of Environmental Sciences
Faculty of Science
Faculty of Science > School of Natural Sciences (former - to 2024)
University of East Anglia Research Groups/Centres > Theme - ClimateUEA
Faculty of Science > School of Mathematics (former - to 2024)
UEA Research Groups: Faculty of Science > Research Groups > Centre for Ocean and Atmospheric Sciences
Faculty of Science > Research Groups > Fluid and Solid Mechanics
Faculty of Science > Research Groups > Climatic Research Unit
University of East Anglia Schools > Faculty of Science > Tyndall Centre for Climate Change Research
Faculty of Science > Research Centres > Tyndall Centre for Climate Change Research
Depositing User: Pure Connector
Date Deposited: 01 Dec 2015 07:20
Last Modified: 25 Sep 2024 11:39
URI: https://ueaeprints.uea.ac.uk/id/eprint/55513
DOI: 10.1002/qj.2508

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