SO442 Lecture 9302013 B Barrett httpblogswpricom20130821whyhurricaneseasonmaystarttopickup First atmospheric oscillations The weather we notice here in Annapolis today is really the combination of many atmospheric oscillations ID: 336065
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Slide1
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/
Slide2
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
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www.ipod.org.uk
Example of superposition
:
El Niño/La Niña (top curve)
Front passages (middle curve)
Day-night cycle (bottom curve)Slide3
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?Slide4
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
Slide5
What is the MJO?
Eastward propagatingMoves to the eastMoves
east and not west because of the rotation of the Earth
http://www.calclim.dri.edu/ccw200606.html
Slide6
What is the MJO?
Equatorially trappedCoriolis 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
Slide7
What is the MJO?
Planetary scaleCovers the entire planetContrast 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
Slide8
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)Slide9
What is the MJO?
Intraseasonal time scale: 30-60 daysTakes the wavenumber-1 wave between 30-60 days to make one circumnavigation around the world at the equatorSlide10
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 equatorRegression gives 2 numbers: RMM1 and RMM2. WH04 developed a nifty phase-space diagram to use RMM1 and RMM2 to locate the center of active MJOSlide11
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 windSlide12
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
Slide13
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/
Slide14
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 articleArticle manuscripts hosted at: http://www.usna.edu/Users/oceano/barrett/tropical.htm Slide15Slide16Slide17Slide18Slide19
http://www.nature.com/nature/journal/v455/n7212/images/nature07286-f2.2.jpgSlide20