Tropical intraseasonal oscillations Adam Sobel DEES Noon Balloon, September

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

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-Aug minus Dec-Feb) Annual mean rain rate

It’s not only India. Much of the tropics is monsoonal. Annual range in rain rate (June-Aug minus Dec-Feb) Annual mean rain rate Equatorial regions are monsoonal too, just with different phase

Within a monsoon season, the rain is not steady

Wang et al The rainy and dry phases are coherent and propagate from south to north as well as west to east

In southern sumer, the propagation is mostly west-east and confined closer to the equator time longitude Equatorial (15S-15N) outgoing longwave radiation, a measure of deep, high cloudiness (shading) – annual cycle & ENSO removed Figure courtesy Australian Bureau of Meteorology

The “Madden-Julian oscillation” (MJO) propagates eastward in 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 eastward in a belt around the equator Statistical composite MJO in outgoing longwave radiation and lower tropospheric wind (Wheeler and Hendon 2004) time longitude latitude Note rain coincides with westerlies, just like 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 astr.gsu.edu/hbase/quantum/atspect.html

Intraseasonal rain variance Northern Summer Southern Summer Variance of rainfall on intraseasonal timescales indeed maximizes over ocean Sobel, Maloney, Bellon, and Frierson 2008: Nature Geosci., 1,

Intraseasonal rainfall variance, nov-apr Climatological mean rainfall, nov-apr Climatological patterns resemble variance, except that the mean doesn’t have localized minima over land

ocean land 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 sfc wind speed) Simulations with NOAA GFDL AM2 climate model

But then why does it go eastward? Strongest fluxes are to 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 or lag precipitation, and lie in westerlies Chen, Houze and Mapes 1996

A personal anecdote about MJO prediction

The Australian monsoon FebruaryAugust Darwin 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 Feb. = 495 mm Daily rainfall at Darwin airport

Ed Lorenz taught us that because of chaos, there is an inherent 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. Hovmoeller plot and statistical forecast courtesy Matt 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