Tropical and extratropical influences on variability of the Southern Hemisphere wintertime subtropical jet

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  • 1 ARC Centre of Excellence for Climate Extremes, and School of Earth, Atmosphere and Environment, Monash University, Melbourne, Australia
  • 2 Bureau of Meteorology, Melbourne, Australia.
  • 3 National Center for Atmospheric Research, Boulder, USA
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Abstract

Interannual variability of the Southern Hemisphere subtropical jet (STJ) is assessed using atmospheric reanalyses during 1979-2018. The focus is on the austral winter season when the STJ is strongest and most distinct from the midlatitude eddy-driven jet (EDJ). Variations in the intensity and latitudinal position of the STJ are diagnosed using an index developed to discriminate between variations associated with the EDJ. STJ intensity and position variations are found to be tied to different mechanisms. An intensification of the STJ is associated with enhanced divergent outflow from diabatic heating over the equatorial Pacific Ocean, primarily resulting from eastern Pacific or canonical El Niño. This intensification is associated with a narrowing of the STJ and an in-place weakening of the EDJ. An equatorward shifted STJ, however, appears to be eddy-driven and is associated with an acceleration and poleward displacement of the EDJ, which projects onto the positive polarity of the Southern Annular Mode. As has previously been reported, El Niño Modoki (or central Pacific El Niño) can act to shift the EDJ poleward during austral winter; thus, a possible pathway for changes in the position of the STJ is via tropically-forced changes in the position of the EDJ. In contrast to previous studies, we also highlight a weakening and poleward shift of the STJ in association with an expansion of the Hadley circulation.

Corresponding author: Zoe E. Gillett, School of Earth, Atmosphere and Environment, 9 Rainforest Walk, Monash University, VIC 3800, Australia. E-mail: zoe.gillett@monash.edu

Abstract

Interannual variability of the Southern Hemisphere subtropical jet (STJ) is assessed using atmospheric reanalyses during 1979-2018. The focus is on the austral winter season when the STJ is strongest and most distinct from the midlatitude eddy-driven jet (EDJ). Variations in the intensity and latitudinal position of the STJ are diagnosed using an index developed to discriminate between variations associated with the EDJ. STJ intensity and position variations are found to be tied to different mechanisms. An intensification of the STJ is associated with enhanced divergent outflow from diabatic heating over the equatorial Pacific Ocean, primarily resulting from eastern Pacific or canonical El Niño. This intensification is associated with a narrowing of the STJ and an in-place weakening of the EDJ. An equatorward shifted STJ, however, appears to be eddy-driven and is associated with an acceleration and poleward displacement of the EDJ, which projects onto the positive polarity of the Southern Annular Mode. As has previously been reported, El Niño Modoki (or central Pacific El Niño) can act to shift the EDJ poleward during austral winter; thus, a possible pathway for changes in the position of the STJ is via tropically-forced changes in the position of the EDJ. In contrast to previous studies, we also highlight a weakening and poleward shift of the STJ in association with an expansion of the Hadley circulation.

Corresponding author: Zoe E. Gillett, School of Earth, Atmosphere and Environment, 9 Rainforest Walk, Monash University, VIC 3800, Australia. E-mail: zoe.gillett@monash.edu
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