| Using the monthly mean Met Office Hadley Center sea surface temperature (SST) analyses data, Global Ocean Data Assimilation System ocean temperature data, Japan Meteorological Agency ocean temperature data and NCEP circulation reanaly-sis datasets, the main ocean-atmosphere coupled processes over the tropical Pacific Ocean of the canonical El Nino and El Nino Modoki events were investigated. Two new indices based on upper-ocean heat content (HC) were proposed to quantify the two "flavors" of El Nino and their different couplings. Different evolution patterns are identified for the two types of El Nino based on the evolution of SST anomalies across the equator(5°—5°N) of the canonical El Nino and El Nino Modoki events. Moreover, we discussed the evolution features of the subsurface ocean temperature of different evolution patterns and their different impacts on Summer Rainfall over China in the decaying phases. Major conclusions can be summarized as following:(1) El Nino comes in different’"flavors" naturally being associated with various air-sea coupled progresses. The circulation anomalies during El Nino Modoki move westwards dramatically, compared with that during the canonical El Nino events. In contrast to previously reported El Nino indices based on SST or SLP, the El Nino signal is represented by HC here, and new indices based on HC are defined for the two types of events and their coupled features. Compared with the previous SST-based indices, these two new HC-based indices can not only effectively distin-guish the two types of El Nino events, e.g. the1993canonical El Nino and2006El Nino Modoki event, but also exhibit better performance in capturing the dipole pat-tern and the triple pattern of ocean-atmosphere elements anomalies during the types of El Nino, respectively. The HC-based indices have the better correlation with the intensity of the western North Pacific summer monsoon, the intensity of spring per-sistent rains over southeastern China, as well as the onset of Meiyu. It provides a new supplementary tool for short-term climate prediction using El Nino signals.(2) Two type of decaying patterns are identified for the canonical El Nino.①an evolution pattern that SST anomalies along the equator decaying from east to west (E—WD). The positive SST anomalies which are larger than0.5℃first disappear in offshore of South America and the disappearance extends to the west.②an evolu-tion pattern that SST anomalies along the equator decaying from west to east (W—ED). The positive SST anomalies which are larger than0.5℃first disappear in the central Pacific and the disappearance extends to the east.The development (decaying) of the SST anomalies of E—WD pattern is attributed to the combined effects of the positive (negative) subsurface temperature anomalies in eastern (western) Pacific. As for W—DE pattern, the eastward and upward propagation of the positive (negative) subsurface temperature anomalies in western and central Pacific contribute to their development (decaying).(3) The decaying evolution of El Nino Modoki is classified into three types[1]:①a symmetric-decaying (SD) pattern whose SST anomalies grow and decay symmet-rically with respect to a peak phase,②a prolonged-decaying (PD) pattern that is followed by a canonical El Nino type,③an abrupt-decaying (AD) pattern that is followed by a canonical La Nina type. The thermal anomalies in the subsurface lay-ers also play great part in the development and decaying evolution of El Nino Mo-doki. The decaying of SD and AD pattern both is accosiated with the combination of the cold anomalies in the equatorial eastern Pacific and eastward cold anomalies from the western Pacific. For AD pattern, the eastward cold anomalies from the western Pacific is more dramatical so that this pattern terminates rapidly after the peak. During the evolution of PD pattern, the eastward cold anomalies from the western Pacific lead to the decaying of the warm anomalies in the central and eastern Pacific and the occurance of a warm center in the equatorial eastern Pacific.(4) The relationships between different decaying types of the canonical El Nino and El Nino Modoki and the following summer rainfall anomalies over China are dis-cussed. During the summer of the two types of decaying patterns of the canonical El Nino, there are almost completely opposite rainfall anomalies in North China, South China and the region between the Yangtze River and the Yellow River Valley. In the the following summer for E—WD pattern, suppressed rainfall appears in the Yangtze—Huaihe River Valley, while wet conditions to the south of the Yangtze River and in North China. However, there are significantly positive anomalies in the region of the Yangtze River and the Yellow River Valley with the center of positive anomalies lo-cated along the Yangtze River, but the negative rainfall anomalies in most region of South China and North China for W—ED pattern. As for three decaying patterns of El Nino Modoki, different inpacts on summer rainfall over China are also remarkable: the positive anomalies are present in the the region between the Yangtze River and the Yellow River Valley in the decaying phase of SD pattern, in the Yangtze River Valley during the following summer of PD pattern, while in the Lower Reaches of the Yellow River for AD pattern. Rainfall in the Northeast China, especially in the nornthern of Northeast China is below normal for SD and AD pattern, while above normal for PD pattern. In the Southweat China, wet signal appears for SD pattern, while dry signal appears for AD and PD pattern. Different distribution of summer rainfall anomalies in the decaying phase of El Nino is closely associated with their different decaying patterns. When inspectinging the impacts of the two types of El Nino on the following summer rainfall, it will be better to consider their different decaying patterns in view of the larger significant area of rainfall anomalies. And it will provide the important reference for the short-term climate prediction. |
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