Abstract
The Madden–Julian Oscillation (MJO) is the chief source of tropical intra-seasonal variability, but is simulated poorly by most state-of-the-art GCMs. Common errors include a lack of eastward propagation at the correct frequency and zonal extent, and too small a ratio of eastward- to westward-propagating variability. Here it is shown that HiGEM, a high-resolution GCM, simulates a very realistic MJO with approximately the correct spatial and temporal scale. Many MJO studies in GCMs are limited to diagnostics which average over a latitude band around the equator, allowing an analysis of the MJO’s structure in time and longitude only. In this study a wider range of diagnostics is applied. It is argued that such an approach is necessary for a comprehensive analysis of a model’s MJO. The standard analysis of Wheeler and Hendon (Mon Wea Rev 132(8):1917–1932, 2004; WH04) is applied to produce composites, which show a realistic spatial structure in the MJO envelopes but for the timing of the peak precipitation in the inter-tropical convergence zone, which bifurcates the MJO signal. Further diagnostics are developed to analyse the MJO’s episodic nature and the “MJO inertia” (the tendency to remain in the same WH04 phase from one day to the next). HiGEM favours phases 2, 3, 6 and 7; has too much MJO inertia; and dies out too frequently in phase 3. Recent research has shown that a key feature of the MJO is its interaction with the diurnal cycle over the Maritime Continent. This interaction is present in HiGEM but is unrealistically weak.
Highlights
The Madden–Julian Oscillation (MJO; Madden and Julian 1971, 1972, 1994; Zhang 2005) is the greatest source of variability on intra-seasonal time scales throughout the tropics
The Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3, forerunner to HadGEM1) used a mass flux convection scheme based on Gregory and Rowntree (1990); the HadGEM1 scheme is a revised version of that used in HadCM3 with new parameterizations, new thermodynamic closures, and the diagnosis of deep and shallow convection as detailed in Table 1 of Martin et al (2006)
We investigate whether the diurnal cycle is modulated by the MJO, and whether any modulation is similar to that seen in observations
Summary
The Madden–Julian Oscillation (MJO; Madden and Julian 1971, 1972, 1994; Zhang 2005) is the greatest source of variability on intra-seasonal time scales throughout the tropics. It consists of alternate large-scale envelopes of active and suppressed convection propagating slowly (∼5 m s−1) eastwards from the Indian Ocean to the Pacific Ocean, and an associated planetary-scale circulation. The circulation anomalies of these waves together act to enhance convection to the east of the active MJO envelope and shut off convection to the west, shifting the convective region slowly eastward (Matthews 2000)
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