Interdecadal Changes in El Niño Onset in the Last Four Decades

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Abstract
The characteristics of the onset of the Pacific basin-wide warming have experienced notable changes since the late 1970s. The changes are caused by a concurrent change in the background state on which El Niño evolves. For the most significant warm episodes before the late 1970s (1957, 1965, and 1972), the atmospheric anomalies in the onset phase (November to December of the year preceding the El Niño) were characterized by a giant anomalous cyclone over east Australia whose eastward movement brought anomalous westerlies into the western equatorial Pacific, causing development of the basin-wide warming. Meanwhile, the trades in the southeastern Pacific (20°S–0°, 125°–95°W) relaxed back to their weakest stage, resulting in a South American coastal warming, which led the central Pacific warming by about three seasons. Conversely, in the warm episodes after the late 1970s (1982, 1986–87, and 1991), the onset phase was characterized by an anomalous cyclone over the Philippine Sea whose intensification... Abstract The characteristics of the onset of the Pacific basin-wide warming have experienced notable changes since the late 1970s. The changes are caused by a concurrent change in the background state on which El Niño evolves. For the most significant warm episodes before the late 1970s (1957, 1965, and 1972), the atmospheric anomalies in the onset phase (November to December of the year preceding the El Niño) were characterized by a giant anomalous cyclone over east Australia whose eastward movement brought anomalous westerlies into the western equatorial Pacific, causing development of the basin-wide warming. Meanwhile, the trades in the southeastern Pacific (20°S–0°, 125°–95°W) relaxed back to their weakest stage, resulting in a South American coastal warming, which led the central Pacific warming by about three seasons. Conversely, in the warm episodes after the late 1970s (1982, 1986–87, and 1991), the onset phase was characterized by an anomalous cyclone over the Philippine Sea whose intensification...

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