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| Titel |
Scale interaction between the MJO and the diurnal cycle of precipitation over the Maritime Continent |
| VerfasserIn |
Simon Peatman, Adrian Matthews, David Stevens |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250073711
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| Zusammenfassung |
| The Maritime Continent is a highly-populated region of many islands and shallow oceans,
located in the oceanic warm pool, between the Indian and Pacific Oceans. A strong diurnal
cycle of precipitation exists due to onshore breezes causing strong convergence of moist air –
enhanced by topographic effects – over the land during the day time, peaking during
afternoon-evening. The respective diurnal cycle over the ocean is far weaker and does not
peak until early in the morning.
On intra-seasonal time-scales the greatest source of variability in the tropics is the
Madden-Julian Oscillation (MJO). The convectively active part of the MJO propagates slowly
(~5Â ms-1) eastward through the warm pool from the Indian Ocean to the western Pacific,
followed by the convectively suppressed part. The complex topography of the Maritime
Continent means the exact nature of the propagation through this region is unclear. Model
simulations of the MJO are often poor over the region, leading to errors in latent heat release
and, subsequently, global errors in medium-range weather prediction and climate
simulation.
Using 14 northern hemisphere winters of high-resolution satellite data it is shown that,
over regions where the diurnal cycle is strong, more than 80% of the variance in precipitation
during an MJO cycle is accounted for by changes in the amplitude of the diurnal cycle.
A canonical view of the MJO is of smooth eastward progression of a large-scale
precipitation envelope over the warm pool. However, by computing “MJO harmonics”
it is shown that the leading edge of the precipitation envelope advances over the
islands of the Maritime Continent approximately 6Â days or 2000Â km ahead of the
main body. This behaviour can be accommodated within existing theories of MJO
propagation.
When the active convective MJO envelope is over the eastern Indian Ocean, frictional
moisture convergence and topographic blocking in the easterlies of the equatorial Kelvin
wave response supply moisture to the islands of the Maritime Continent. When combined
with the relatively clear skies and strong short-wave flux, the low thermal inertia of the
islands allows a rapid response in the diurnal cycle which rectifies onto the lower-frequency
MJO. Hence, an accurate representation of the diurnal cycle and its scale interaction
appear to be necessary ingredients for models to simulate the MJO successfully. |
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