by Matt Wheeler
Could MJO Predictions help for forecasting the Australian monsoon?
The MJO is typically at its strongest and most coherent during the southern hemisphere summer. During this season, the MJO displays greatest variability of low-level winds and convection across the eastern Indian Ocean and western Pacific between the equator and about 15°S (e.g., Fig.1). This region encompasses the Australian and related Indonesian summer monsoons. Thus great potential for MJO-induced variability of these monsoon systems exists.
The climatological peak of the Australian monsoon occurs in January and February, while monsoon onset typically occurs around mid to late December. Considerable intraseasonal variability of the monsoon rainfall and circulation occurs with each monsoon season being composed of a number of “bursts” and “breaks”, each lasting from a few days to a week or more. A greater portion of this variability can be ascribed to the MJO than to any other intraseasonal mode. During years of strong MJO variability, a large-scale envelope of convection sweeps eastward across the region every 30-90 days, together with changes in low-level winds from easterly to westerly (Fig.1). The precipitation variation associated with a typical MJO event has amplitude of up to 6 mm/day in the far northern coastal region (Fig.2, Phases 5 and 6 versus Phase 2), which is comparable to the standard deviation of weekly rainfall.
The MJO modulates the large-scale conditions associated with onset of the Australian monsoon, but it does not set the exact date of onset. For instance, at Darwin (12.5°S, 130.9°E) onset rarely occurs during the convectively suppressed phases of the MJO (Phases 8, 1, 2, and 3; Fig. 3), while onset tends to occur in a wide range of the convectively active phases of the MJO (Phases 4, 5, 6, and 7). Thus, while the MJO appears to act as a modulator of the large-scale conditions associated with onset of the monsoon, onset in a particular year is set by a smaller scale event such as a tropical cyclone or passage of a midlatitude synoptic-scale trough embedded in the larger-scale envelope.
The robust impact of the MJO on the Australian monsoon, together with demonstrated predictability of the MJO at lead times approaching ~3 weeks, implies the possibility of useful monsoon predictions (e.g., onset date or periods of above normal and below normal rainfall accumulation) at these lead times.
In the short-term, extended range prediction of the large-scale dynamical fields associated with the MJO needs to be translated into predictions of parameters of direct relevance to agriculture and hydrology. In the long-term, improvements in the representation of the MJO in dynamical forecast models should allow for prediction of the full range of MJO-related monsoon variability.
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Hendon, H.H. and B. Liebmann, 1990b: The intraseasonal (30-50 day) oscillation of the Australian summer monsoon. J. Atmos. Sci., 47, 2909-2923.
Wheeler, M.C. and H.H. Hendon, 2004: An all-season real-time multivariate MJO index: Development of an index for monitoring and prediction. Mon. Wea. Rev., 132, 1917-1932.