엘린뇨 남방진동을 유도할 수 있는 Seasonal Footprinting Mechanism의 호조건

Abstract

Through the comparison between two groups of years with the strong NPO signal during winter (One is followed by El Ni? and the other is not followed by the El Ni? in the following winter), a favorable condition for the seasonal footprinting mechanism was examined. Twelve years are selected for the NPO years during 61 years from 1949 to 2009. The five years among them feature the El Ni? in the following winter, while the NPO pattern in the rest seven years are not followed by El Ni?. Considering the occurrence ratio of El-Ni? to normal years, this result may indicate the role of the SFM as a stochastic forcing that initiates an El Ni? event. Conditional composites of each group of years (the NPO-El Ni? years and the NPO-normal years) show that in the northeast Pacific region two groups show slight different features in the wintertime NPO pattern as well as in the corresponding wind speed and net heat flux. For example, the wintertime wind speed and net heat flux patterns for NPO-El Ni? years are similar to those of SFM presented by Vimont et al. (2001), while there seem to be a discrete pattern of the same variables for NPO-normal years in the eastern-north Pacific. It is found that these differences are caused from the NPO structure in each group of years, and this different structure seems to affect whether the NPO-driven SST anomalies can persist until the summer or not. It is because that in the eastern-north Pacific the elongated lower cell of NPO for the NPO-normal case may disturb the NPGO-like SST warming through two processes, the decreased surface net heat flux by stronger wind speed and the ocean upwelling by increased wind curl. These factors can suppress the warming of the NPGO-like SST pattern in winter, and consequently, the difference in the composite patterns for NPO-El Ni? years and for NPO-normal years becomes stronger during summer. Finally an El Ni?-like warming is induced for the NPO-El Ni? years in the following winter but not for