Factors controlling the seasonality of the Madden-Julian Oscillation |
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| Institution: | 1. Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico;2. Environmental Physics LABoratory (EPhysLab), CIM-UVIGO, University of Vigo, Ourense, Spain;3. Oceanography Department, U.S. Naval Academy, Annapolis, Maryland, USA;2. CSIRO Oceans and Atmosphere, Aspendale, Victoria, Australia;3. George Mason University, Fairfax, VA, United States;1. Regional Meteorological Center, India Meteorological Department, Chennai 600006, India;2. Department of Atmospheric Sciences, School of Marine Sciences, Cochin University of Science and Technology, Lakeside Campus, Cochin 682016, India;3. Advanced Centre for Atmospheric Radar Research, Cochin University of Science and Technology, Cochin 682016, India;4. Faculty of Science and Natural Resources, Universiti Malaysia Sabah, Malaysia;5. Indian Institute of Tropical Meteorology, Pashan, Pune 411008, India |
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| Abstract: | By analyzing observational data, previous studies have indicated that the tropical Madden-Julian Oscillation (MJO) is active during the boreal winter but relatively weak during the boreal summer. However, the factors that control seasonal MJO variation are not clear. To quantitatively understand the relative contributions of the occurrence frequency of enhanced MJO events and their averaged strength and lifespan to seasonal MJO amplitude, we defined the MJO events of 1979–2014 and analyzed their features in different seasons by using the Real-time Multivariate MJO (RMM) index and the newly proposed RMM-r index. The results indicate that the MJO events show a higher frequency of occurrence, a stronger intensity and a longer duration during the boreal winter (Dec.–Feb.) and spring (Mar.–May). However, the frequency, strength and lifespan of MJO events are all reduced during the boreal summer (Jun.–Aug.) and autumn (Sep.–Nov.). The enhanced MJO events in winter–spring also show a large ratio of variance for eastward to westward components. To elucidate how large-scale background fields affect seasonal MJO variation, a series of sensitivity experiments was conducted by using a 2.5-layer model that can simulate MJO-like features. It is found that the variation in low-level moisture (vertical wind shear) is the key large-scale factor affecting the seasonal variation in MJO strength (in propagation). In comparison with the summer–autumn seasons when the MJO is relatively weakened, the relatively abundant low-level moisture near the equator during boreal winter–spring may strengthen the development of MJO convection and circulation, whereas the relatively weak easterly shear (or the westerly shear anomaly) is conducive to the enhancement of an eastward-propagating MJO component. |
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| Keywords: | Seasonality of Madden-Julian Oscillation Real-time Multivariate MJO (RMM) index RMM-r index 2 5-layer model Moisture Vertical wind shear |
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