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Tropical Oscillations

Madden-Julian Oscillation (MJO)

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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

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

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

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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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)

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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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

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

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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Negative phasePositive 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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Maloney and Hartmann (2000) Journal of Climate

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Enhanced convective activity

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Enhanced Low-level Convergence

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Enhanced Cyclonic Vorticity

Note anomalous cycloniccirculation

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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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Creation of Eddy Kinetic Energy (EKE) by MJO phase (Maloney and Hartmann 2001)Allows for growth of disturbances

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MJO and North American Monsoon

June – September precipitation in normally arid regions

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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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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)