Prof. Eric D. Maloney
Department of Atmospheric Science
Colorado State University
"Intraseasonal Variability in an Aquaplanet General Circulation Model"
The Madden‐Julian oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics, characterized by eastward‐propagating coherent wind and precipitation fluctuations at 30‐60 day timescales. The MJO strongly modulates tropical cyclones and monsoon variability, and occasionally initiates ENSO events. While the existence of the MJO has been known since the early 1970's and is well characterized in observations, an understanding of the underlying dynamics of the MJO have to this point remained elusive. We still do not have a fundamental understanding of the spatial and temporal scale selection of the MJO, its slow 5 m s‐1 eastward propagation, or its seasonality. I present general circulation modeling evidence that suggests the MJO is destabilized by wind‐evaporation feedbacks and propagated eastward by horizontal moisture advection, and resembles an intraseasonal moisture mode.
An aquaplanet atmospheric general circulation model simulation with a robust intraseasonal oscillation is analyzed. The SST boundary condition resembles the observed December‐April average, although with the meridional SST gradient reduced to be one‐quarter of that observed poleward of 10o latitude. Slow, regular eastward propagation at 5 m s‐1 in winds and precipitation with amplitude greater than observed is clearly identified in unfiltered fields. Tropical precipitation rates are a strongly increasing function of column precipitable water, as in observations. Sensitivity experiments that remove wind‐evaporation feedbacks, and analysis of the moisture and moist static energy budgets, elucidate the importance of windevaporation feedback and horizontal moisture advection to the instability and propagation characteristics of model MJO. At the time of peak moistening to the east of MJO convection, the mean plus perturbation zonal wind near 850 hPa is 5 m s‐1, an advective velocity that may be responsible for the slow eastward‐propagating variability in the model.
Observational support for the modeling results shown here is also reviewed, as well as the sensitivity of the model MJO to different basic state SSTs.
Host: Adam Sobel
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