ATMS 373C.C. Hennon, UNC Asheville Tropical Oscillations Madden-Julian Oscillation (MJO)

ATMS 373 C.C. Hennon, UNC Asheville

Tropical Oscillations

Madden-Julian Oscillation (MJO)


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


Real-time MJO Monitoring

http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/amaps.all.50to20.gif


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


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


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


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)


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


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


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


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



Negative phasePositive phase


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


Maloney and Hartmann (2000) Journal of Climate


Enhanced convective activity


Enhanced Low-level Convergence


Enhanced Cyclonic Vorticity

Note anomalous cycloniccirculation


Zero shear line farther south duringnegative MJO phase

Less north-south shear gradient in negative MJO phase (shown to be more unfavorable for genesis)


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


MJO and North American Monsoon

June – September precipitation in normally arid regions


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


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)

Documents

ATMS 373C.C. Hennon, UNC Asheville Tropical Oscillations Madden-Julian Oscillation (MJO)