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| Titel |
Multi-scale forcing and the formation of subtropical desert and monsoon |
| VerfasserIn |
G. X. Wu, Y. Liu, X. Zhu, W. Li, R. Ren, A. Duan, X. Liang |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 9 ; Nr. 27, no. 9 (2009-09-29), S.3631-3644 |
| Datensatznummer |
250016657
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| Publikation (Nr.) |
 copernicus.org/angeo-27-3631-2009.pdf |
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| Zusammenfassung |
This study investigates three types of atmospheric forcing across the
summertime subtropics that are shown to contribute in various ways to the
occurrence of dry and wet climates in the subtropics. To explain the
formation of desert over the western parts of continents and monsoon over
the eastern parts, we propose a new mechanism of positive feedback between
diabatic heating and vorticity generation that occurs via meridional
advection of planetary vorticity and temperature. Monsoon and desert are
demonstrated to coexist as twin features of multi-scale forcing, as follows.
First, continent-scale heating over land and cooling over ocean
induce the ascent of air over the eastern parts of continents and western
parts of oceans, and descent over eastern parts of oceans and western parts
of continents. Second, local-scale sea-breeze forcing along coastal
regions enhances air descent over eastern parts of oceans and ascent over
eastern parts of continents. This leads to the formation of the well-defined
summertime subtropical LOSECOD quadruplet-heating pattern across each
continent and adjacent oceans, with long-wave radiative cooling (LO) over
eastern parts of oceans, sensible heating (SE) over western parts of
continents, condensation heating (CO) over eastern parts of continents, and
double dominant heating (D: LO+CO) over western parts of oceans. Such a
quadruplet heating pattern corresponds to a dry climate over the western
parts of continents and a wet climate over eastern parts. Third,
regional-scale orographic-uplift-heating generates poleward
ascending flow to the east of orography and equatorward descending flow to
the west.
The Tibetan Plateau (TP) is located over the eastern Eurasian continent. The
TP-forced circulation pattern is in phase with that produced by
continental-scale forcing, and the strongest monsoon and largest deserts are
formed over the Afro-Eurasian Continent. In contrast, the Rockies and the Andes
are located over the western parts of their respective continents, and
orography-induced ascent is separated from ascent due to continental-scale
forcing. Accordingly, the deserts and monsoon climate over these continents
are not as strongly developed as those over the Eurasian Continent.
A new mechanism of positive feedback between diabatic heating and
vorticity generation, which occurs via meridional transfer of heat
and planetary vorticity, is proposed as a means of explaining the formation
of subtropical desert and monsoon. Strong low-level longwave radiative
cooling over eastern parts of oceans and strong surface sensible heating on
western parts of continents generate negative vorticity that is balanced by
positive planetary vorticity advection from high latitudes. The equatorward
flow generated over eastern parts of oceans produces cold sea-surface
temperature and stable stratification, leading in turn to the formation of
low stratus clouds and the maintenance of strong in situ longwave radiative
cooling. The equatorward flow over western parts of continents carries cold,
dry air, thereby enhancing local sensible heating as well as moisture
release from the underlying soil. These factors result in a dry desert
climate. Over the eastern parts of continents, condensation heating
generates positive vorticity in the lower troposphere, which is balanced by
negative planetary vorticity advection of the meridional flow from low
latitudes. The flow brings warm and moist air, thereby enhancing local
convective instability and condensation heating associated with rainfall.
These factors produce a wet monsoonal climate. Overall, our results
demonstrate that subtropical desert and monsoon coexist as a consequence of
multi-scale forcing along the subtropics. |
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