February drying in Southeastern Brazil and the Australian Monsoon: Global mechanism for a regional rainfall feature

Patrick Kelly, Brian E Mapes

Research output: Contribution to journalArticle

2 Scopus citations

Abstract

A dry February rainfall signal in southeastern Brazil is shown to be a robust, repeatable feature of climatology. This February depression or minimum in climatological rain curves has an amplitude of about 30% of the seasonal mean and coincides with a poleward excursion of tropical barotropic easterlies to about 25°S in austral midsummer. Momentum budget decomposition indicates that stationary eddy momentum flux [u*υ*] near 150 hPa in Australian longitudes is a main sink in that latitude belt's zonal momentum budget. A physical linkage among these phenomena is suggested by statistically significant interannual correlations among February anomalies of southeastern Brazil rainfall, zonal-mean zonal wind, and indices of the Australian monsoon. To test a causality hypothesis that the sharply peaked Australian region monsoon drives the sharp climatological dry signal over Brazil, an observation-inspired tropospheric heating signal near Australia is added to the temperature equation of the full-physics Community Atmosphere Model. Results indicate the near linearity of the global subtropical responses for the modest (roughly 1 and 2 K day-1) magnitudes and scales of imposed heating. Consistent with the hypothesis, this imposed heating robustly causes easterly changes to subtropical, barotropic, zonal-mean momentum, a westward displacement of the mean synoptic-scale pattern in the western Atlantic (the western edge of the subtropical high), and reduced rainfall in southeastern Brazil. These results are closely analogous to the previous findings on a related boreal summer subtropical signal.

Original languageEnglish (US)
Pages (from-to)7529-7546
Number of pages18
JournalJournal of Climate
Volume29
Issue number20
DOIs
StatePublished - 2016

Keywords

  • Anticyclones
  • Atmospheric circulation
  • Barotropic flows
  • Monsoons
  • South Atlantic convergence zone
  • Subtropics

ASJC Scopus subject areas

  • Atmospheric Science

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