The Madden-Julian Oscillation

SO442 Lecture 9/30/2013B. Barrett

http://blogs.wpri.com/2013/08/21/why-hurricane-season-may-start-to-pick-up/

First, atmospheric oscillations

• The weather we notice here in Annapolis today is really the combination of many atmospheric oscillations– Diurnal, synoptic-scale (passage of fronts, for example), intraseasonal,

seasonal, interannual (El Niño, for example)• Owing to the superposition principle, sometimes these oscillations

combine with the same sign (“constructive interference”), other times with the opposite sign (“destructive interference”)– Example: a cold front passes during a clear afternoon. The “cold air” doesn’t

seem so cold because it’s heated by the sun

Adapted from http://www.ipod.org.uk

Example of superposition:• El Niño/La Niña (top curve)• Front passages (middle curve)• Day-night cycle (bottom curve)

Real life atmospheric superposition

http://www.erh.noaa.gov/er/lwx/climate/cliplot/KBWI2013plot.png

Temperatures (top panel), rainfall (middle panel), and snowfall (bottom panel), at BWI from Jan-Sept 2013

Note the variability: diurnal, synoptic, and seasonal

How much, if any, of the variability in temperature, rainfall, and snowfall is due to the MJO?

What is the MJO?• An eastward-propagating, equatorially

trapped, planetary-scale circulation envelope that drives (and is driven by) tropical convection on timescales of 30-60 days

http://www.met.reading.ac.uk/~pete/mjo.html

What is the MJO?

• Eastward propagating– Moves to the east– Moves east and not

west because of the rotation of the Earth

http://www.calclim.dri.edu/ccw200606.html

What is the MJO?

• Equatorially trapped– Coriolis accelerations are zero at the equator, and increase poleward

(so beta, the derivative of Coriolis, is large)• Air moving to the north is deflected to the right (east and south)• Air moving to the south is deflected to the left (still to the east!, and north)

– Thus, any air parcels embedded in the MJO near the equator that move poleward as they head toward the east will return toward the equator due to Coriolis

Idealized example of an equatorially trapped wave.

Note the trough (L) and ridge (H) structure of the wave

http://www.rsmas.miami.edu/users/isavelyev/GFD-2/EquatorialWaveTheory.pdf

What is the MJO?

• Planetary scale– Covers the entire planet

• Contrast to synoptic scale (a 500-mb trough the size of the U.S.), meso-scale (a hurricane the size of the mid-Atlantic states), local-scale (land-sea breezes at the Severn River of Annapolis)

– “Wavenumber 1”• As you go around the planet, there would be one ridge and one

trough

Example of wavenumber-4 height patterns (red for the ridges, blue for the troughs. As you go around the planet, there are 4 red and 4 blue sections in each hemisphere. This time around, the waves are also “mixed” across the equator

http://www.sciencedirect.com/science/article/pii/S1364682611003191

What is the MJO?

• Drives (controls) areas of enhanced and suppressed tropical thunderstorm activity– “Areas” are huge: i.e., ½

of the equator zone would fall under “enhanced” and the other half under “suppressed”

From Madden and Julian (1972)

What is the MJO?

• Intraseasonal time scale: 30-60 days– Takes the

wavenumber-1 wave between 30-60 days to make one circumnavigation around the world at the equator

Detecting and quantifying the MJO

• Madden and Julian (1972) divided the MJO into eight “phases” (lettered A-H) based on where thunderstorms were favored

• Wheeler and Hendon (2004) went about a much more rigorous definition of where is the MJO?– Their daily index regresses

Outgoing Longwave Radiation (OLR) and 250-mb winds along the equator

– Regression gives 2 numbers: RMM1 and RMM2. WH04 developed a nifty phase-space diagram to use RMM1 and RMM2 to locate the center of active MJO

Effects of the MJO

• The clearest effect of MJO is on tropical convection around the equator.– But what actually

causes there to be more (or elss) thunderstorms?

– MJO affects upper-level wind and sea level pressure

http://www-das.uwyo.edu/~geerts/cwx/notes/chap12/mjo.html

http://www.meted.ucar.edu/climate/mjo/print.htm

Sea level pressure 100-mb wind

Some current questions

• Can the MJO modify the northern hemisphere wave train (aka the jet stream)?– If so, what would

the impacts be?• How about the

southern hemisphere?

http://www.climas.arizona.edu/feature-articles/february-2012

Some current questions

MJO effects on surface pressure and circulation, around the globe

Phase-by-phase impacts on U.S. wintertime surface temperature

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/Composites/Temperature/

Task for Wednesday 10/2

• In groups of about 3 students:– Summarize effects of the MJO on a variety of

sensible weather (weather that we humans, at the surface of the Earth, notice)

– Presentations of about 10 minutes each – Primary source for each study: one peer-reviewed

article– Article manuscripts hosted at:

http://www.usna.edu/Users/oceano/barrett/tropical.htm