atms 373c.c. hennon, unc asheville tropical oscillations madden-julian oscillation (mjo)
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ATMS 373 C.C. Hennon, UNC Asheville
Tropical Oscillations
Madden-Julian Oscillation (MJO)
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ATMS 373 C.C. Hennon, UNC Asheville
Introduction
• MJO is an intraseasonal oscillation– Longer than synoptic-scale (2-5 days), shorter
than a season (~90 days)– Generally have periods of 7-70 days
• MJO is the primary intraseasonal oscillation in the tropics– Others have been identified
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ATMS 373 C.C. Hennon, UNC Asheville
Real-time MJO Monitoring
http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/amaps.all.50to20.gif
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ATMS 373 C.C. Hennon, UNC Asheville
Introduction
• Early 1970’s: Roland Madden and Paul Julian (NCAR) discovered a 40-50 day oscillation in the tropical zonal wind
• MJO (as it came to be known) is an easterly propagating wave in the atmosphere
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ATMS 373 C.C. Hennon, UNC Asheville
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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ATMS 373 C.C. Hennon, UNC Asheville
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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ATMS 373 C.C. Hennon, UNC Asheville
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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ATMS 373 C.C. Hennon, UNC Asheville
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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ATMS 373 C.C. Hennon, UNC Asheville
Example of MJO passage in OLR
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
~40 day spacingbetween waves
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ATMS 373 C.C. Hennon, UNC Asheville
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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ATMS 373 C.C. Hennon, UNC Asheville
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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ATMS 373 C.C. Hennon, UNC Asheville
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ATMS 373 C.C. Hennon, UNC Asheville
Negative phasePositive phase
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ATMS 373 C.C. Hennon, UNC Asheville
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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ATMS 373 C.C. Hennon, UNC Asheville
Maloney and Hartmann (2000) Journal of Climate
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ATMS 373 C.C. Hennon, UNC Asheville
Enhanced convective activity
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ATMS 373 C.C. Hennon, UNC Asheville
Enhanced Low-level Convergence
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ATMS 373 C.C. Hennon, UNC Asheville
Enhanced Cyclonic Vorticity
Note anomalous cycloniccirculation
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ATMS 373 C.C. Hennon, UNC Asheville
Zero shear line farther south duringnegative MJO phase
Less north-south shear gradient in negative MJO phase (shown to be more unfavorable for genesis)
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ATMS 373 C.C. Hennon, UNC Asheville
Creation of Eddy Kinetic Energy (EKE) by MJO phase (Maloney and Hartmann 2001)Allows for growth of disturbances
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ATMS 373 C.C. Hennon, UNC Asheville
MJO and North American Monsoon
June – September precipitation in normally arid regions
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ATMS 373 C.C. Hennon, UNC Asheville
MJO and North American Monsoon
• Lorenz and Hartmann (2006) found that positive zonal wind anomalies (more westerly) lead to above-normal precipitation in northwest Mexico and Arizona up to a week later
• MJO contributes moisture surges up Gulf of California– Westerly MJO phase amplifies easterly waves
off coast of Mexico
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ATMS 373 C.C. Hennon, UNC Asheville
Simulation of MJO in Climate Models
• Previous climate models (Community Climate Model 3 (CCM3)) could only simulate a weak MJO that moved in the opposite direction– Weakness in convective scheme?
• Recent research (Zhang and Mu 2005) has resulted in a more accurate simulation– zonal wind, precipitation, OLR match closely– Period of oscillation shorter (~30 days) than observed
MJO (30-60 day)