What is the MJO? Large-scale disturbance of deep convection and winds that controls up to half of the variance of tropical convection in some regions Now

What is the MJO?

• Large-scale disturbance of deep convection and winds that controls up to half of the variance of tropical convection in some regions

• Now known to be a major propagator of weather systems

A Recently Discovered Coupled Event over 10000 miles and 5 weeks

• A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., the phase speed of the wave crests is equal to the group speed of the wave energy for all frequencies. This means that it retains its shape in the alongshore direction over time.

• A Kelvin wave (fluid dynamics) is also a long scale perturbation mode of a vortex in superfluid dynamics; in terms of the meteorological or oceanographical derivation, one may assume that the meridional velocity component vanishes (i.e. there is no flow in the north–south direction, thus making the momentum and continuity equations much simpler). This wave is named after the discoverer, Lord Kelvin (1879).[1][2]

2

http://en.wikipedia.org/wiki/Wave

The Madden-Julian Oscillation (MJO)

• Salient features:– Convectively coupled

disturbance that propagates eastward across the Tropics

– 5-10 m s-1 propagation speed in the Indian and west Pacific, where MJO convective variability is strongly coupled to the large-scale flow.

– Simple baroclinic wind structure, with 850 hPa and 200 hPa wind perturbations 180o out of phase

– Characteristic timescales of 30-90 days.

Madden and Julian 1972

MJO- an intraseasonal event

• Prior to 1971, it was thought that virtually all variability in the weather conditions within a given season in the Tropics was random.

• There were indications of interseasonal variations, such as the Southern Oscillation

• Studies of Tropical rainfall and pressure changes showed additional oscillations

The MJO - A Description

• A 30-60 day oscillation in the coupled Tropical ocean-atmosphere system

• An eastward progression of enhanced and suppressed convection

• Low level and upper level wind patterns show distinct anomalies

• Strong year to year variability in MJO that is related to ENSO cycle

Kelvin Waves in the Ocean

Convective Kelvin Wave

H L

Convection removesSome of the accumulating mass, slows propagation

Propagation speed: less than 20 ms-1

z

x

How MJO was discovered

In the 1960s and 1970s, new computer power created the ability to look for patterns in meteorological observations.

Could only look for patterns in TIME (at a location), not SPACE (at a given time).

Tended to detect OSCILLATIONS.

What They Found

• Surface pressure oscillates with a period of 40-50 days.

• ZONAL winds in the lower and upper troposphere also oscillate at this frequency, but 180° out-of-phase.

• The signal was limited to the deep tropics.• They found little signal in the meridional

wind, or in zonal winds in the midtroposphere.

Their Interpretation

MJO is a region of low-level convergence and convection.

Propagating eastward only.

Circumnavigates the globe in 40-50 days.

Their Interpretation

Pressure is LOW in the region of strongest convection.

Upper-level outflow is only in the zonal direction.

Kelvin Waves in the Atmosphere

Kelvin Waves in the Atmosphere

Convection

Features:Enhanced tradewinds AHEAD of the convection.

Features:Weak tradewinds—maybe even westerlies—behind the convection.

Features:Alternating areas of high and low pressure.

H L H

Features:Alternating areas of high and low OLR values!

H L H

Features:Notice that there shouldn’t be much signal in the midtropospheric winds!

Features:An area of upper-tropospheric DIVERGENCE, best seen in the VELOCITY POTENTIAL.

Circumnavigating the Globe?

• Madden and Julian originally believed that this area of convection propagated all the way around the world every 40-50 days.

• But this isn’t exactly right.


• Rather, the convection is TRIGGERED in the eastern Indian Ocean (typically by intruding midlatitude systems).

• Convection dies out in the eastern Pacific due to cold SSTs


• However, the region of upper-level divergence WILL generally travel all the way around the world as a Kelvin Wave.

Kelvin Waves and the Walker Circulation


Initially OPPOSES the Walker Circulation

Later ENHANCES the Walker Circulation!


Later, it OPPOSES the Walker Circulation and the trade winds in the Pacific!


However, these tradewinds are what maintained the high sea surface temperatures and heights of the western Pacific Warm Pool…

El Nino and the MJO

• MJO events can TRIGGER El Nino events by weakening the trade winds (or even having a WESTERLY WIND BURST).

El Nino and the MJO

• Why doesn’t EVERY MJO trigger an El Nino event?– CHARGE/DISCHARGE THEORY:– MJO is the TRIGGER—it happens much more

often than the El Nino event itself.– Not every trigger is exactly right.– Even when the trigger is right, maybe ocean

conditions are not yet right.– A partial explanation for the timing of ENSO.

An MJO that’s a little off the equator

A very short MJO

An MJO from the southwest

Triggering MJO Events

An MJO event in the Indian Ocean upsets the SSTs.

It takes time for the SSTs to recover.

Any intruding midlatitude systems during this period will FAIL to trigger an MJO event.

Only when the environment is ready (another 40-50 days) will the next midlatitude system be able to trigger a new MJO!

What are MJO events?

• Described by “Nakazawa’s Hierarchy of Convection”


• Each MJO event is actually composed of a small number of “super cloud clusters”--SCCs


• SCCs:– Move EASTWARD– Last a day or two


• What are SCCs?– Made of Cloud Clusters (CCs)


• What are CCs?– Small groups of thunderstorms– Last less than a day– Move WESTWARD

1 MJO

4 SCCs

Many CCs

Overview of Madden-Julian Oscillations (MJOs) 1. major and complex disturbances of the global tropical atmosphere-

ocean system2. propagating intraseasonal (~ 1-2 months) oscillations3. usually start in tropical Indian - W Pacific region4. have largest amplitude in tropical Indian - Pacific region5. propagate E-ward through the tropics6. may propagate around globe, especially as UL disturbance7. period 30-60 days 45 days8. zonal wave length Earth’s circumference9. occur throughout the year10. may have large impacts on global tropics and extratropics11. have impacts on midlatitude climate 12. strong atmosphere-ocean coupling makes them difficult to analyze

and model

Evidence of MJOsSea Level Pressure, Equator and 180E, May – Oct 1979

Figure from R. Madden, 31 Aug 2005

45 days

Evidence of MJOsTemperature, Upper Ocean, Equator and 155W, Aug 1991 – Jul 1992

45 days

T 125 m

T 150 m

= positive heating anomaly = energy propagation through wave train

Modeled Tropospheric Response to Western Tropical PacificPositive Heating Anomaly in Northern Winter

MJOs and Teleconnections

H

H

HLL

= dry air advection

= moist air advection


Figure from: http://www.cpc.ncep.noaa.gov/products/intraseasonal/intraseasonal_faq.html#what

cf. Higgins and Mo, J Clim, 1997

Relationships Between Propagating Tropical Positive Convection Anomaly and North Pacific – North American Circulation and Precipitation Anomalies

Z 200 Anomalies, Dec 1996 – Jan 1997


During Dec 96 - Jan 97:1. Weak La Nina conditions in tropical Pacific2. Intense MJO activity in Indian Ocean – western tropical Pacific3. Anomalously heavy precipitation and flooding in N CA, OR, WA4. Anomalously low precipitation in SW US5. Extratropical wave train similar to expected for MJO convection in

tropical E IO – W Pacific

Other examples of MJO impacts on west coast precipitation?Jan 92, Feb 93, Jan 95, Oct–Nov 03, Dec 04 – Jan 05, Dec 05 - Jan 06

NPNA response to MJO affected by season. Effects can be dramatic. Likely causes: seasonal changes in location of convection and subsidence, and in strength, location,

shear of east Asia - North Pacific jet.

OND JFM

ONDJFM

Composites for: Phase 3, All Amplitudes, and all Background States, by Season

Favorable / Unfavorable Conditions for MJO-AssociatedAnomalously Wet Conditions in CA

Favorable Unfavorable

1. Early or late phases of the MJO 2. OND or JFM3. El Niño or neutral background state 4. Wave train from Asia with anomalouslow north and west of CA5. Southwest to northeast tilt to the anomalous low off CA

1. La Niña background state2. Middle phases of the MJO

Favorable / Unfavorable Conditions for MJO-AssociatedAnomalously Dry Conditions in CA

Favorable Unfavorable

1. Middle or late phases of the MJO 2. JFM3. La Niña background state4. Anomalous high over northeastern Pacific

1. Early phases of the MJO 2. El Nino background state

Favorable and Unfavorable Factors for Wet and Dry Conditions in California

Corresponding results for PNW, BC, and AK regions (not shown)

The Satellite View of MJO• The MJO is noted

by a cluster of thunderstorms drifting eastward along the equatorial Indian and Pacific oceans.

Simplified Madden-Julian Oscillation Composite OLR from A.J. Matthews, 2000.

FormationRegion

Decay Region

Active Convection

Active Convection

EnhancedEasterlies

Active Convection

Cold air outbreak enhancement

Active Convection

Energy Build-up

Deflected Jet Stream

MJO - A Modeler’s Nightmare

• GCM simulation of convection (CPS)

• SST variations not well simulated

• Change of phase speed from eastern to western hemispheres

• Handling of very low wave number

• Recent modifications-– increased vertical

resolution– better

parameterization of:• radiation• convection• cloud formation• precipitation• surface convergence

Theory: Moisture Modes and the MSE Budget

Peters and Bretherton (2006)

• A moisture mode instability can result if large-scale divergent motions associated with convection import MSE into the column (e.g. Raymond et al. 2009, negative gross moist stability)

• Alternatively, export by divergent motions can be positive, but MSE sources such as latent heat flux and cloud-radiative feedbacks overcompensate to produce MSE increases as a result of convection.

• Such instability is manifest for WTG as strong positive moisture-convection feedbacks

Madden-Julian OscillationMadden-Julian Oscillation (a.k.a. Intraseasonal, 40-50, 30-60 Day Oscillation)(a.k.a. Intraseasonal, 40-50, 30-60 Day Oscillation)

Madden & Julian, 1972

1987/88

• Intraseasonal Time Scale: ~40-60 days• Planetary-Scale: Zonal Wavenumbers 1-3• Baroclinic Wind Structure• Eastward Propagation

E. Hem: ~5 m/s, Surf.+Conv.+Circ. InteractionsW. Hem: ~ > 10 m/s, ~Free Tropospheric Wave

• Tendency to be Equatorially Trapped• Strong Seasonal Dependence:

NH Winter: Eastward Propagation NH Summer: ~Northeast Propagation

• Significant Interannual Variability• Potential Role of Ocean/SST Feedback• Convection Has Multi-Scale Structure• Significant Remote and Extra-Tropical Impacts

U200

U850

CloudyLow OLR

ClearHigh OLR

Rainfall

Typical Variables Used for MJO Analysis

• Composite rainfall maps derived from merged satellite and in-situ measurements are separated by 10 days.

• Rainfall anomalies propagate in a eastward fashion and mainly affect the Tropical eastern hemisphere.

• These anomalies are accompanied by anomalies in wind, solar radiation, sea surface temperature, etc.

A Typical A Typical MJO in MJO in

N.H. WinterN.H. Winter

• Composite rainfall maps derived from merged satellite and in-situ measurements are separated by 10 days.

• Rainfall anomalies propagate in a northeast fashion and mainly affect the Tropical eastern hemisphere.

• These anomalies are accompanied by anomalies in wind, solar radiation, sea surface temperature, etc.

A Typical A Typical MJO in MJO in

N.H. SummerN.H. Summer

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Boreal Summer Complex Propagation & Boreal Summer Complex Propagation & Multi-Scale OrganizationMulti-Scale Organization

Kemball-Cook & Wang, 2001

Eastward PropagatingConvective Envelope

~40-50 days

Northward PropagationOf Rossby-Wave Convection(twisting, SST, moisture feedback)

Westward PropagatingRossby-Waves~ 10-20 day;

Modulated by 40-50day

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Documents

What is the MJO? Large-scale disturbance of deep convection and winds that controls up to half of the variance of tropical convection in some regions Now