Why the South Pacific Convergence Zone is diagonal

During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest–southeast) oriented SPCZ are determined through a series of experiments with an atmo...

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Bibliographic Details
Published in:Climate dynamics 2016-03, Vol.46 (5-6), p.1683-1698
Main Authors: van der Wiel, Karin, Matthews, Adrian J., Joshi, Manoj M., Stevens, David P.
Format: Article
Language:English
Subjects:
Atmospheric circulation
Atmospheric temperature
Boundary conditions
Climatology
Convection
Convergence
Earth and Environmental Science
Earth Sciences
Geophysics/Geodesy
Marine
Methods
Moisture content
Observations
Oceanography
Orography
Potential energy
Precipitation
Precipitation (Meteorology)
Sea surface temperature
Summer
Weather forecasting
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Summary:During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest–southeast) oriented SPCZ are determined through a series of experiments with an atmospheric general circulation model. Continental configuration and orography do not have a significant influence on SPCZ orientation and strength. The key necessary boundary condition is the zonally asymmetric component of the sea surface temperature (SST) distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that, on its western flank, transports warm moist air from the equator into the SPCZ region. This moisture then intensifies (diagonal) bands of convection that are initiated by regions of ascent and reduced static stability ahead of the cyclonic vorticity in Rossby waves that are refracted toward the westerly duct over the equatorial Pacific. The climatological SPCZ is comprised of the superposition of these diagonal bands of convection. When the zonally asymmetric SST component is reduced or removed, the subtropical anticyclone and its associated moisture source is weakened. Despite the presence of Rossby waves, significant moist convection is no longer triggered; the SPCZ disappears. The diagonal SPCZ is robust to large changes (up to ±6 °C) in absolute SST (i.e. where the SST asymmetry is preserved). Extreme cooling (change
ISSN:0930-7575
1432-0894
DOI:10.1007/s00382-015-2668-0