Moisture Modes and the Madden–Julian Oscillation

Abstract

Moisture mode instability is thought to occur in the tropical oceanic atmosphere when precipitation is a strongly increasing function of saturation fraction (precipitable water divided by saturated precipitable water) and when convection acts to increase the saturation fraction. A highly simplified model of the interaction between convection and large-scale flows in the tropics suggests that there are two types of convectively coupled disturbances: the moisture mode instability described above and another unstable mode dependent on fluctuations in the convective inhibition. The latter is associated with rapidly moving disturbances such as the equatorially coupled Kelvin wave.A toy aquaplanet beta-plane model with realistic sea surface temperatures produces a robust Madden-Julian oscillation-like disturbance that resembles the observed phenomenon in many ways. Convection in this model exhibits a strong dependence of precipitation on saturation fraction and does indeed act to increase this parameter in situations of weak environmental ventilation of disturbances, thus satisfying the criteria for moisture mode instability. In contrast, NCEP's closely related Global Forecast System (GFS) and Climate Forecast System (CFS) models do not produce a realistic MJO. Investigation of moist entropy transport in NCEP's final analysis (FNL), the data assimilation system feeding the GFS, indicates that convection tends to decrease the saturation fraction in these models, precluding moisture mode instability in most circumstances. Thus, evidence from a variety of sources suggests that the MJO is driven at least in part by moisture mode instability.

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

“…; the moisture mode theory (Raymond and Fuchs 2009;Maloney 2012, 2013;Benedict et al 2014;Adames and Kim 2015); the MJO skeleton driven by synoptic wave activity (Majda and Biello 2004;Majda and Stechmann 2009); the MJO modified by interactive upscale transfer of eddy momentum, heat and moisture (Wang and Liu 2011;Wang 2012a, b, 2013); the multi-cloud model of MJO Majda 2006, 2007); the MJO wave packet driven by the interference pattern of a narrow frequency band of mixed Rossby-gravity waves (Yang and Ingersoll 2011) or westward and eastward inertia-gravity waves (Yang and Ingersoll 2013); and the air-sea interaction theory (Flatau et al 1997;Wang and Xie 1998;Wang and Zhang 2002;Fu and Wang 2004).…”

Influence of the Madden–Julian Oscillation and Intraseasonal Waves on Surface Wind and Convection of the Tropical Atlantic Ocean

“…Recent model experiments suggest that the MJO is driven at least in part by moisture mode instability (e.g., Majda and Stechmann 2009;Raymond and Fuchs 2009;Maloney et al 2010). A moisture mode is a balanced disturbance, whose large-scale circulations are well described by the weak temperature gradient approximation (e.g., Held and Hoskins 1985;Sobel et al 2001) and involve those processes that control the tropical moisture field, such as surface latent heat flux and horizontal advection (e.g., Sobel et al 2001;Majda and Klein 2003;Maloney et al 2010).…”

“…A moisture mode is a balanced disturbance, whose large-scale circulations are well described by the weak temperature gradient approximation (e.g., Held and Hoskins 1985;Sobel et al 2001) and involve those processes that control the tropical moisture field, such as surface latent heat flux and horizontal advection (e.g., Sobel et al 2001;Majda and Klein 2003;Maloney et al 2010). MJO is a moisture mode destabilized by increasing moisture anomalies, and the propagation of the disturbances is governed by processes that make moisture anomalies move horizontally (e.g., Sobel et al 2001;Raymond and Fuchs, 2009;Maloney et al 2010). The model results of Majda and Stechmann (2009) also offer an explanation for the observed dominance of eastward-propagating intraseasonal variability that the eastward-propagating modes are more strongly coupled with the equatorial moist convective processes as convectively coupled moisture mode instability.…”

A Review of the Nonlinear Dynamics of Intraseasonal Oscillations

“…For further details about the basic ideas, successes and limitations of these theories and hypotheses and outstanding problems for improving the presented theories and models, see the discussion in Wang (2005). Recent observational and model evidence suggest that WISHE plays an important role in the MJO's dynamics (Sobel et al, 2008;Raymond and Fuchs, 2009). Marshall et al (2008) proposed an enhanced moisture convergenceevaporation feedback (EMCEF) mechanism for the MJO.…”

“…The EMCEF mechanism reconciles traditionally opposing ideas on the roles of frictional wave-CISK (conditional instability of second kind)and evaporation-wind feedback. The theoretical and modeling work in (Raymond and Fuchs, 2009) suggests that the MJO is driven by moisture mode instability. However, much remains to be revealed about the detailed workings of moisture mode dynamics in this prominent but elusive phenomenon.…”