Tropical intraseasonal oscillations Adam Sobel DEES Noon Balloon, September 19 2011.
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Transcript of Tropical intraseasonal oscillations Adam Sobel DEES Noon Balloon, September 19 2011.
Tropical intraseasonal oscillations
Adam Sobel
DEES Noon Balloon, September 19 2011
Outline
• What is an intraseasonal oscillation?
a) monsoons
b) their fluctuations within a season
• Dynamics - why do they exist?
• Role in long-range weather forecasting.
• Upcoming field program.
Monsoon: the winds change direction with the season
Monsoon: the winds change direction with the season
July winds June-Sep rainfall (cm)(Gadgil 2003)
Westerlies are associated with the rainy season
It’s not only India. Much of the tropics is monsoonal.
Annual range in rain rate (June-Augminus Dec-Feb)
Annual meanrain rate
It’s not only India. Much of the tropics is monsoonal.
Annual range in rain rate (June-Augminus Dec-Feb)
Annual meanrain rate
Equatorial regions are monsoonal too, just with different phase
Within a monsoon season, the rain is not steady
Wang et al. 2006
The rainy and dry phases are coherent and propagatefrom south to north as well as west to east
In southern sumer, the propagation is mostly west-east andconfined closer to the equator
time
longitude
Equatorial (15S-15N) outgoing longwave radiation, a measure of deep, high cloudiness (shading) – annual cycle & ENSO removed
Figure courtesyAustralian Bureau of Meteorology
The “Madden-Julian oscillation” (MJO) propagates eastwardin a belt around the equator
Statistical composite MJO in outgoing longwave radiation and lower tropospheric wind (Wheeler and Hendon 2004)
time
longitude
latitude
The “Madden-Julian oscillation” (MJO) propagates eastwardin a belt around the equator
Statistical composite MJO in outgoing longwave radiation and lower tropospheric wind (Wheeler and Hendon 2004)
time
longitude
latitude
Note raincoincides withwesterlies, justlike in normal monsoon
The intraseasonal oscillations modulate tropical cyclones
Maloney and Hartmann 2000 Leroy and Wheeler 2008
Questions
• Why do intraseasonal oscillations exist?• What is the energy source? • What sets the scales: spatial scale, and frequency or phase
speed?• We focus here on the energy source.
There is no agreement on the basic mechanisms despite ~3 ½ decades of study
Surface pressure spectrum,Nauru Island, tropical Pacific
Madden and Julian 1994
Helium spectral lines as seen through diffraction grating
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/atspect.html
Intraseasonal rain variance
NorthernSummer
SouthernSummer
Variance of rainfall on intraseasonal timescales indeed maximizes over ocean
Sobel, Maloney, Bellon, and Frierson 2008: Nature Geosci., 1, 653-657.
Intraseasonal rainfall variance, nov-apr
Climatological mean rainfall, nov-apr
Climatological patterns resemble variance, exceptthat the mean doesn’t have localized minima over land
oceanland
Net = 0 W/m^2
Over land, there can be no significant net surface flux variations, because surface can’t store heat - so there seems to be correspondence between flux and convection
Sobel et al. 2010, J. Adv. Model Earth Sys.
There is a definite suggestion that better MJO simulation corresponds to larger role for surface fluxes
control
No-WISHE(const sfcwind speed)
Simulations with NOAAGFDL AM2 climate model
But then why does it go eastward? Strongest fluxes areto the west of convection, and would tend to make it go westward
Observed cloudiness and wind from TOGA COARE
Strongest winds and fluxes are in phase with orlag precipitation, and lie in westerlies
Chen, Houze and Mapes 1996
A personal anecdote about MJO prediction
The Australian monsoon
February AugustDarwin
Melbourne
September February
Florence Falls, Litchfield Nat’l Park, Northern Territory, Australia
I bought my plane ticket 2 weeks ahead of time, wanting to see it rain in Darwin
Feb. 11
I got to Darwin just in time for the rain to start
Total rainfall from 11-22 Feb. = 495 mm
Daily rainfall at Darwin airport
Ed Lorenz taught us that because of chaos, there is aninherent limit to how far ahead we can predict the weather, around 2 weeks…
Apparently sometimes we can beat that in the tropics! E.g., with a statistical forecast assuming the MJO will evolve in the typical way, once it has started.
Hovmoellerplot and statisticalforecast courtesyMatt Wheeler
But it’s much harder to forecast the start of an MJO event
So we have a big field program this fall to study MJO initiation.
Climate models’ simulations of intraseasonal variability are flawed, but improving
Lin et al. 2006
The MJO is a translation of the planetary-scale zonal overturning (Walker) circulation
Madden and Julian 1971