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Atlantic Equatorial mode
The Atlantic Equatorial Mode or Atlantic Niño is a quasiperiodic interannual climate pattern of the equatorial Atlantic Ocean. It is the dominant mode of year-to-year variability that results in alternating warming and cooling episodes of sea surface temperatures accompanied by changes in atmospheric circulation. The term Atlantic Niño comes from its close similarity with the El Niño-Southern Oscillation (ENSO) that dominates the tropical Pacific basin. The Atlantic Niño is not the same as the Atlantic Meridional (Interhemispheric) Mode that consists of a north-south dipole and operates more on decadal timescales.〔Zebiak, 1993, p.1570〕 The equatorial warming and cooling events associated with the Atlantic Niño are known to be strongly related to atmospheric climate anomalies, especially in African countries bordering the Gulf of Guinea. Therefore, understanding of the Atlantic Niño (or lack thereof) has important implications for climate prediction in those regions. Although the Atlantic Niño is an intrinsic mode to the equatorial Atlantic, there may be a tenuous causal relationship between ENSO and the Atlantic Niño in some circumstances.
== Background and Structure ==
Global tropical variability is dominated by ENSO in the equatorial Pacific. This phenomenon results from air-sea interaction, producing a coupled atmosphere-ocean system that oscillates with periods on the order of three to five years. However, the physical basis for this oscillation is not limited strictly to the Pacific basin, and indeed, a very similar mode of variability exists in the equatorial Atlantic, albeit on a smaller scale.
The Atlantic Niño is characterized by a sea surface temperature anomaly centered on the equator between 0° and 30°W. Unlike its Pacific counterpart, the Atlantic Niño does not have sea surface temperature anomalies that switch sign from east to west, but rather a single basin-wide anomaly. Additionally, the amplitude of the Atlantic Niño tends to be about half that of El Niño. Not surprisingly, this sea surface temperature anomaly is closely related to a change in the climatological trade winds. A warm anomaly is associated with relaxed trade winds across a large swath of the equatorial Atlantic basin, while a cool anomaly is associated with enhanced easterly wind stress in the same region. These trade wind fluctuations can be understood as the weakening and strengthening of the Atlantic Walker circulation. This is strikingly similar to the wind stress anomalies seen in the Pacific during El Niño (or La Niña) events, although centered farther west in the Atlantic basin. A major difference between El Niño and the Atlantic Niño is that the sea surface temperature anomalies are strictly constrained to the equator in the Atlantic case, while greater meridional extent is observed in the Pacific.〔Zebiak, 1993, p.1568-1572〕
While the spatial characteristics of the mature Atlantic Niño are quite similar to its Pacific counterpart, its temporal variability is somewhat different. The Atlantic Niño varies on interannual timescales like El Niño, but also shows more variance on seasonal and annual timescales. That is to say, the Atlantic Niño explains a smaller portion of the total variance in the equatorial Atlantic than does El Niño in the equatorial Pacific. This is due to the fact that there are seasonal climate events that are superimposed on the interannual variability. The Atlantic Niño typically reaches mature phase in boreal summer (though there are exceptions), while El Niño matures in boreal winter. The development of the Atlantic Niño tends to be marked by emerging stationary patterns centered mid-basin. This is in stark contrast to El Niño, which can often develop as warm sea surface temperature anomalies that migrate west from the coast of South America or migrate east from the central Pacific.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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