| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 106 === This is an observational analysis aimed to classify mechanisms for maintaining anomalous anticyclone (AAC) in western North Pacific (WNP) associated with El Niño events, especially in the El Niño decaying summer JJA(1) following the peak phase in winter D(0)JF(1). El Niño events from 1958-2016 are categorized into fast-decay, slow-decay, and prolonged types based on their Oceanic Niño Indices (ONI) in JJA(1) that are negative, positive, positive and followed by another El Niño event in the following winter, D(1)JF(2), respectively. Composite analysis is applied to anomalous fields of Sea Surface Temperature (SST), 850hPa stream function and vertically integrated apparent heat source 〈Q1〉 of the three types of El Niño events.
In the composite fields of the fast-decay type, the development of warm SST Anomaly (SSTA)/heating over equatorial Pacific is associated with an evolution of SSTA over Indian Ocean (IO) from a dipole pattern in SON(0) to basin wide warming in D(0)JF(1) and MAM(1). The heating associated with the IO SST evolution causes an eastward propagation of equatorial easterly anomalies (equatorial part of AAC) from eastern IO in SON(0)/ D(0)JF(1) to western Pacific in MAM(1) that results in a rapid phase transition into La Niña condition in JJA(1). The most evident maintenance mechanism for the WNP AAC is the wind-evaporation-SST (WES) feedback in D(0)JF(1) and MAM(1), and the strong La Niña heating contrast in JJA(1). The IO capacitor is only evident in MAM(1). The La Niña SST distribution and associated heating over the Maritime Continent in JJA(1) drives an overturning circulation with subsidence over WNP.
The composite fields of the slow-decay type and the prolonged type of El Niño events are generally similar except that the WNP AAC associated with the prolonged type emerges late in MAM(1). The WES feedbacks is the most evident mechanism maintaining WNP AAC in D(0)JF(1) and MAM(1), and IO capacitor is evident in MAM(1) for the slow-decay type but not for the prolonged type. For both types of events, the AAC persists in JJA(1) with a broad zonal extent in the tropics (10°-30°N) from the dateline to the Bay of Bengal. Associated with the AAC for the slow-decay type, a zonal band of anomalous heating (cooling) exists between equator and 10°N (10°-20° N), and warm SSTA and anomalous heating also exist over northern IO. For the prolonged type, the heating over IO and western Pacific is weaker and more localized. The AAC in JJA(1) for the two types of El Niño is similar to the AAC found in Wang et al. (2013), Kosaka et al. (2013) and Li et al. (2017). The AAC appears to be maintained by a broader scale air-sea interaction involving extra tropical influence.
|
|---|
