Intraseasonal Variability in an Aquaplanet General Circulation Model

Abstract

An aquaplanet atmospheric general circulation model simulation with a robust intraseasonal oscillation is analyzed. The SST boundary condition resembles the observed December-April average with continents omitted, although with the meridional SST gradient reduced to be one-quarter of that observed poleward of 10˚latitude. Slow, regular eastward propagation at 5 m s 21 in winds and precipitation with amplitude greater than that in the observed MJO is clearly identified in unfiltered fields. Local precipitation rate is a strongly non-linear and increasing function of column precipitable water, as in observations. The model intraseasonal oscillation resembles a moisture mode that is destabilized by wind-evaporation feedback, and that propagates eastward through advection of anomalous humidity by the sum of perturbation winds and mean westerly flow.A series of sensitivity experiments are conducted to test hypothesized mechanisms. A mechanism denial experiment in which intraseasonal latent heat flux variability is removed largely eliminates intraseasonal wind and precipitation variability. Reducing the lower-troposphere westerly flow in the warm pool by reducing the zonal SST gradient slows eastward propagation, supporting the importance of horizontal advection by the low-level wind to eastward propagation. A zonally symmetric SST basic state produces weak and unrealistic intraseasonal variability between 30 and 90 day timescales, indicating the importance of mean low-level westerly winds and hence a realistic phase relationship between precipitation and surface flux anomalies for producing realistic tropical intraseasonal variability.

Analysis of MJO Wind-Flux Feedbacks in the Indian Ocean Using RAMA Buoy Observations

“…This result was reproduced by Maloney and Esbensen (2005) for the east Pacific warm pool. In GCM mechanism denial experiments, Sobel et al (2008Sobel et al ( , 2010 and Maloney et al (2010) demonstrated that removing intraseasonal flux variability strongly diminished tropical intraseasonal variability. Also, when examining the vertically integrated intraseasonal MSE budget in a GCM, Maloney (2009) found that LHFLXs and horizontal advection of MSE were the two dominant terms although the importance of LHFLX as a destabilization mechanism is strongly model dependent (e.g., Andersen and Kuang 2012).…”

“…Narrow deep convective clouds along with shallow convection give way to wide (more organized) convection followed by anvil clouds (Morita et al 2006;Riley et al 2011;Powell and Houze 2013;Barnes and Houze 2013;Zuluaga and Houze 2013;Xu and Rutledge 2014). While general circulation models (GCMs) have been able to reproduce these observed characteristics of the MJO with varying degrees of success (e.g., Grabowski 2003;Maloney and Sobel 2004;Benedict and Randall 2009;Kim et al 2009;Maloney et al 2010;Chikira 2014), no consensus exists on a theory that completely explains the spatial scale, initiation and propagation of the MJO. As such, the MJO remains an ideal target phenomenon to further understanding convective and large-scale interactions and the importance of air-sea coupling.…”

The parametric sensitivity of CAM5's MJO

“…Previous authors have variously emphasized the role of low-level heating in the buildup phase or the high-level heating associated with stratiform precipitation during the active phase of convection, or its combined effect of an east-to-west tilt in the level of maximum diabatic heating. Recent studies have also emphasized the role of variations in horizontal advection in moistening and drying the column during an MJO lifecycle [Maloney et al, 2010;Andersen and Kuang, 2012;Kim et al, 2014a;Pritchard and Bretherton, 2014].…”

A general theoretical framework for understanding essential dynamics of Madden–Julian oscillation

“…In addition to theoretical modeling, various processes and mechanisms have been identified or speculated to play significant roles in MJO dynamics based on numerical model experiences and observations, including shallow convection-BL circulation interaction (Johnson et al 1999;Kikuchi and Takayabu 2004;; stratiform cloud-wave interaction (Mapes 2000;Khouider and Majda 2006;Kuang 2008;Fu and Wang 2009;Seo and Wang 2010;Holloway et al 2013); cloud-radiation interaction (Lee et al 2001;Raymond 2001;Bony and Emanuel 2005;Andersen and Kuang 2012); moisture-convection feedback (Woolnough et al 2001;Grabowski and Moncrieff 2004;Benedict et al 2014), and moisture transport (Maloney 2009;Maloney et al 2010;Andersen and Kuang 2012;Hsu and Li 2012;Kim et al 2014;Pritchard and Bretherton 2014). More details were reviewed by Zhang (2005) and Wang (2005Wang ( , 2012.…”