Transcript Osilasi Tropis (MJO)

Osilasi tropis
Osilasi Madden Julian(MJO)
ATMS 373
C.C. Hennon, UNC Asheville
Introduction
• MJO adalah osilasi intraseasonal
Lebih lama dari skala sinoptik (2-5 hari),
lebih pendek dari satu musim (~ 90 hari)
Umumnya memiliki periode 7-70 hari
MJO adalah osilasi intraseasonal utama di
daerah tropis
Lainnya telah diidentifikasi
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temuan
• 1970-an: Roland Madden dan Paul Julian
(NCAR) menemukan osilasi 40-50 hari
dalam angin zonal tropis
MJO (seperti yang kemudian dikenal)
adalah gelombang merambat timur di
atmosfer
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Overview
• Aktivitas MJO selama bulan Juni-Oktober dan Desember 2004-Januari 2005
menghasilkan variabilitas yang signifikan di atmosfer (angin, tekanan) dan Samudera
Pasifik
(permukaan
dan
suhu
bawah
permukaan).
Kegiatan ini dikaitkan dengan periode melemahnya signifikan dari tingkat rendah angin
timur di atas Pasifik ekuatorial dan terkait dengan inisiasi timur-laut menyebarkan
gelombang
Kelvin.
Baratan yang terkait dengan area kuasi-stasioner dari konveksi ditingkatkan dekat
garis tanggal memprakarsai gelombang Kelvin yang kuat selama akhir Januari 2005.
Pada saat ini ada ketidakpastian tentang kemungkinan pengaruh gelombang Kelvin
terbaru pada permukaan dan kondisi bawah permukaan di Pasifik ekuatorial bagian
timur.
Saat
ini,
MJO
tetap
sangat
lemah.
Model prakiraan statistik menunjukkan bahwa aktivitas MJO akan tetap lemah selama
periode 6 - 10 hari
The Madden-Julian Oscillation (MJO)
•
Madden-Julian Oscillation (MJO) adalah pola perjalanan khatulistiwa curah hujan anomali yang dalam
skala planet. Mekanisme dan penyebab MJO adalah saat ini belum dipahami dengan baik dan
merupakan subjek penelitian yang sedang berlangsung.
Para MJO dicirikan oleh perkembangan ke arah timur daerah besar curah hujan tropis baik
ditingkatkan dan ditekan, diamati terutama di atas Samudera Hindia dan Samudera Pasifik. Anomali
curah hujan biasanya pertama jelas di atas Samudera Hindia bagian barat, dan tetap jelas seperti
merambat di atas perairan laut sangat hangat dari Pasifik tropis barat dan tengah. Pola curah hujan
tropis maka umumnya menjadi sangat mencolok ketika bergerak di atas perairan laut yang lebih
dingin dari Pasifik timur, tetapi muncul kembali di atas tropis Atlantik dan Samudera Hindia. Fase
basah dan curah hujan konveksi ditingkatkan diikuti oleh fase kering di mana konveksi ditekan. Setiap
siklus berlangsung sekitar 30-60 hari.
Schematic for the 40-50 day Oscillation
A Kelvin Wave
Another view of the oscillation
Characteristics of the MJO
• Wavenumber 1
– Symmetrical and circular in shape
• Amplitude varies as wave travels around the globe
– Has been observed to extend as much as 20°-30° away from the
equator
• Can be identified by a maximum in the upper level
divergence field (200 mb)
• Frequently accompanied by convection
– East of the Dateline, convection becomes uncoupled from the
wave
• Average phase speed = 10 m/s
– Moves slower (5 m/s) between 60°E and the Dateline
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Characteristics of the MJO
• Dipole of upper level divergence between
Indian Ocean (60°E - 120°E) and the
central Pacific/SPCZ region
• Upper level divergence field moves around
the globe
– Convection tends to maximize in the Indian
Ocean, dissipate, then reform near 160°E
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Schematic of OLR evolution for 28-72 day time scales.
A cycle of cloudiness goes from 1 to 2 to 3 to 4 to 1.
OLR anomalies at 1/3 and 2/4 tend to be out-of-phase
(From Weickmann et al. 1985 – Copyright American Meteorological Society)
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Observed Structure of
MJO
• As wave approaches,
easterly trades enhanced
• After passage of
convection, westerly wind
anomalies weaken or
reverse easterly trades
• Convection dissipates over
Pacific
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MJO Propagation
200 mb
Day 0
850 mb
Indian Ocean
Pacific Ocean
Atlantic Ocean
Indian Ocean
Pacific Ocean
Atlantic Ocean
Indian Ocean
Pacific Ocean
Atlantic Ocean
200 mb
Day 10
850 mb
200 mb
Day 20
850 mb
Example of MJO passage in OLR
~40 day spacing
between waves
Time-longitude section of the OLR anomalies for the MJO-filtered
band averaged for the latitudes from 10°S to 2.5°N. The zero
contour has been omitted. Light shading for positive anomalies
and dark shading for negative anomalies
(From Wheeler and Kiladis 1999 – Copyright American Meteorological Society
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Importance of MJO
• Affects weather across the tropics
– wind, SST, cloudiness, rainfall, oceanic
effects
• Has been connected to enhanced or
suppressed times for tropical
cyclogenesis, esp. in the eastern Pacific
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Real-time MJO Monitoring
http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/amaps.all.50to20.gif
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850-hPa Vector Wind Anomalies (m s-1)
Note that shading
denotes the magnitude of
the anomalous wind
vectors.
Westerly anomalies
continue to be observed in
the vicinity of the date line.
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MJO and Tropical Cyclones
• Maloney and Hartmann (2001) found that
tropical cyclone formation in the EPAC
was enhanced during the westerly wind
phase of the MJO
– Positive phase (westerly 850 mb zonal wind
anomalies
• Strength of EPAC tropical cyclones was
also higher during the MJO positive phase
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MJO and Tropical Cyclones
• Why are TCs enhanced during positive
MJO phase?
– Enhanced cyclonic vorticity north of westerly
wind anomalies
– Enhanced convective activity
– Enhanced low-level convergence
– Near zero vertical wind shear
• Easterly MJO phase creates enhanced
low-level divergence and higher shear
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Empirical Forecast Based on the Real-time
Multivariate MJO index
MJO activity is
forecasted to be
weak over the next
6-10 day period.
Enhanced convective activity
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Enhanced Low-level Convergence
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Enhanced Cyclonic Vorticity
Note anomalous cyclonic
circulation
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Summary
• MJO activity during June-October and December 2004-January 2005 resulted in
significant variability in the atmosphere (wind, pressure) and Pacific Ocean (surface and
subsurface temperature).
• This activity was associated with periods of significant weakening of the low-level
easterly winds over the equatorial Pacific and was related to the initiation of eastwardpropagating oceanic Kelvin waves.
• Westerlies associated with the quasi-stationary area of enhanced convection near the
date line initiated a strong Kelvin wave during late January 2005. At this time there is
uncertainty concerning the possible influence of this latest Kelvin wave on the surface
and subsurface conditions in the eastern equatorial Pacific. Currently, the MJO remains
very weak.
• Statistical model forecasts indicate that MJO activity will remain weak over the next 610 day period.