Madden-Julian Oscillation (MJO) Short Notes

Lecture: Madden-Julian Oscillation (MJO) 1. Definition: An equatorial traveling pattern of anomalous rainfall or other weather features (strong wind, outgoing long-wave radiation, and air pressure,etc.) from west to east. (1). The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean. (2). The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. (3). This pattern of tropical rainfall then generally becomes very nondescript as it moves over the cooler ocean waters of the eastern Pacific but reappears over the tropical Atlantic and Indian Ocean. (4). The wet phase of enhanced convection and precipitation is followed by a dry phase where convection is suppressed at a given location. Each cycle lasts approximately 30-60 days (intraseasonal oscillation). ENSO is an interannual oscillation (year-to-year variability) 2. Relationship between MJO and ENSO (1). Although MJO is an intraseasonal variation it shows interannual variablity. (2). In the Pacific, strong MJO activity is often observed 6 - 12 months prior to the peaks of El Niño episodes (1997-98), while weak MJO activity is typically associated with La Niña years (1996-97 active MJO vs 1997-98 El Nino). 3. MJO effect on the US weather (1). The strongest impacts of intraseasonal variability (MJO) on the United States occur during the winter months (rainy season) over the western U.S. (2). It is known that extreme precipitation events (floods) can occur at all phases of the ENSO cycle, but the largest fraction of these events occur during La Niña episodes and during ENSO-neutral winters. (3). During La Niña episodes much of the Pacific Northwest (Washington, Oregon, and Northern California) experiences increased storminess, increased precipitation and more overall days with measurable Precipitation. (4). Strong MJO activity is often observed during weak La Niña years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Niño episodes. (5). A recent example is the winter of 1996-97, which featured heavy flooding in California and in the Pacific Northwest (estimated damage costs of $2.0-3.0 billion at the time of the event) and a very active MJO. Such winters are also characterized by relatively small sea surface temperature anomalies (SSTA) in the tropical Pacific compared to stronger warm and cold episodes. In these winters there is a stronger linkage between the MJO events and extreme west coast precipitation events. (7). Pineapple Express (a MJO event) A. A significant amount of the deep tropical moisture traverses the Hawaiian Islands on its way towards western North America. B. Evolution of a pineapple express leading to a heavy west coast precipitation event (see figure) 4. Wintertime blocking activity over the North Pacific and western North America (1). Definition of blocking activity Atmospheric circulation features (High, Low, ridge, or trough) that persist near the same location for several days or more. (2). MJO is a contributor to blocking activity. 5. Tropical storm development (1). The strongest tropical cyclones tend to develop when the MJO favors enhanced precipitation. (2). As the MJO progresses eastward, the favored region for tropical cyclone activity also shifts eastward from the western Pacific to the eastern Pacific and finally to the Atlantic basin. (3). The MJO is one of many factors that contribute to the development of tropical cyclones. For example, it is well known that SSTs must be sufficiently warm and vertical wind shear must be sufficiently weak for tropical disturbances to form and persist. 6. Summer monsoon (1). During the Northern Hemisphere summer season the MJO-related effects on the Indian summer monsoon are well documented. (2). MJO-related effects on the North American summer monsoon also occur, though they are relatively weaker. (3). the relative influences of ENSO and the MJO on the summer precipitation regime of North America are not well understood. 7. MJO indice The MJO analyses are often displayed in time-longitude format so as to reveal the propagation, amplitude and location of the MJO-related features. (1). Outgoing Longwave Radiation, which is a satellite-derived measure of tropical convection and rainfall. (2). Velocity potential, which is a derived quantity that isolates the divergent component of the wind at upper levels of the atmosphere. (3). Upper-level and lower-level wind anomalies. (4). 500-hPa height anomalies to represent the atmospheric responses in midlatitudes.