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 language | English (US) |
|---|---|
| Pages (from-to) | 7529-7546 |
| Number of pages | 18 |
| Journal | Journal of Climate |
| Volume | 29 |
| Issue number | 20 |
| DOIs | |
| State | Published - 2016 |
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Keywords
- Anticyclones
- Atmospheric circulation
- Barotropic flows
- Monsoons
- South Atlantic convergence zone
- Subtropics
ASJC Scopus subject areas
- Atmospheric Science
Cite this
February drying in Southeastern Brazil and the Australian Monsoon : Global mechanism for a regional rainfall feature. / Kelly, Patrick; Mapes, Brian E.
In: Journal of Climate, Vol. 29, No. 20, 2016, p. 7529-7546.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - February drying in Southeastern Brazil and the Australian Monsoon
T2 - Global mechanism for a regional rainfall feature
AU - Kelly, Patrick
AU - Mapes, Brian E
PY - 2016
Y1 - 2016
N2 - 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.
AB - 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.
KW - Anticyclones
KW - Atmospheric circulation
KW - Barotropic flows
KW - Monsoons
KW - South Atlantic convergence zone
KW - Subtropics
UR - http://www.scopus.com/inward/record.url?scp=84993969542&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84993969542&partnerID=8YFLogxK
U2 - 10.1175/JCLI-D-15-0838.1
DO - 10.1175/JCLI-D-15-0838.1
M3 - Article
AN - SCOPUS:84993969542
VL - 29
SP - 7529
EP - 7546
JO - Journal of Climate
JF - Journal of Climate
SN - 0894-8755
IS - 20
ER -