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| Titel |
Large scale features associated with strong frontogenesis in equivalent potential temperature in the South American subtropics east of the Andes |
| VerfasserIn |
J. M. Arraut, H. M. J. Barbosa |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7340
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| Digitales Dokument |
URL |
| Erschienen |
In: 4th EGU Alexander von Humboldt Conference "The Andes: Challenge for Geosciences" ; Nr. 22 (2009-10-13), S.73-78 |
| Datensatznummer |
250014731
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| Publikation (Nr.) |
 copernicus.org/adgeo-22-73-2009.pdf |
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| Zusammenfassung |
| South American subtropics east of the Andes exhibit a region of
intense climatological frontogenesis in equivalent potential
temperature (EPT) in the December to March season, mostly produced by
deformation of the wind field. The goal of this paper is to
investigate the large scale features associated with intense and weak
frontogenesis by deformation (FGD) in EPT in the region where it
attains its climatological maximum. This can be approximately
delimited by 32–42° S and 66–69° W, which is small enough as to contain
only one synoptic perturbation at a time. The spatial average of the
positive values of frontogenesis at 850 hPa over the whole region
(DFG+) is used to represent the strength of the perturbation. ECMWF
ERA-40 reanalysis data set is used to calculate DFG+ at six hour
intervals for 21 seasons (1981–2002). Compositing analysis is carried
out for strong (above the 0.75 quantile) and weak (below the 0.25 quantile)
events. For strong events the geopotential field at 850 hPa
exhibits the North Argentinean Low (NAL), a transient trough and the
Low Pressure Tongue East of the Andes (LPT). Upon comparison with the
composite field of FGD it can be observed that FGD exhibits a strong
maximum over the Argentinean Col (AC) which separates the NAL and the
trough. These features are absent in the weak frontogenesis composite,
which exhibits a stronger South Pacific Subtropical High close to the
continent. At 250 hPa the strong FGD composite exhibits a trough over
the Andes with a wind speed maximum to its east. Both of these
features are associated with the deepening of the NAL in the
literature. These are not present in the weak FGD composites. Strong
events show an intense quasi meridional corridor of water vapor
transport from the Amazon to the subtropics that encounters westerly
flow in the neighborhood of the AC. This is absent in weak events. A
preliminary analysis of precipitation is carried out using the GPCP
daily data set. An intense precipitation nucleus appears slightly
northeast of the AC, with maximum intensity in the day that follows
the strong events. Weak events exhibit a drying of the subtropics
instead, between one and three days after the events. Higher
precipitation over the oceanic South Atlantic Convergence Zone can be
also observed. Analogous composites were constructed for the presence
and absence of both the AC and the LPT, showing similar
characteristics to the strong and weak FGD event composites
respectively, but with lower intensities. This shows that by selecting
strong FGD events, intense NAL and LPT events are also singled out. |
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