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