The Intermittency of Precipitation and the MJO

The conventional view is that the Madden-Julian Oscillation (MJO) is a large-scale, low-frequency mode of the tropical climate system.. Support for this view comes in part from the fact that making deep convection sensitive to free troposphere moisture improves the MJO simulations in general circulation models (GCMs). The MJO is therefore considered a moisture mode—a large-scale oscillation in which convective activity is controlled by the large-scale, low-frequency distribution of moist static energy.

Yang and Ingersoll (2013, J. Atmos. Sci., 70, 2476–2486) simulate the MJO with a global shallow water model, in which triggered convection is employed. Different from the conventional view, we propose that the MJO is a large-scale, long-lasting envelope of small-scale inertia-gravity (IG) waves. We then seek the minimum recipe of the MJO, and present a novel 1D β-plane model that successfully simulates the MJO with the same governing mechanism as in our 2D model (Yang and Ingersoll 2014, submitted). Using this 1D model, we derive a scaling for the MJO horizontal wavenumber k ~ (S_c/c)^1/2, where S_c is the spatial-temporal frequency of convection events and c is the Kelvin wave speed. The eastward speed of the MJO arises from the difference between the eastward and westward IG waves.

Both the 1D and 2D studies support the idea that the intermittency of deep convection is key to simulating the MJO. We extend this idea to 3D using a GCM that employs a simplified Betts-Miller convection scheme. With no modifications, this GCM does not simulate MJO signals. We modify the GCM convection criteria in three different ways, and robust MJO signals are observed in all cases. The common feature of these different convection criteria is to make the precipitation more vigorous and intermittent. Instead of being slaved to the moisture budget, the MJO feels both the strength of convection and its intermittency. Our model predicts how the propagation speed and zonal wavenumber of the MJO might vary in a changing climate