1. Introduction

During El Niño events the climate in Northeast Asia is generally cool and wet in summer, and warm and calm in winter (Kurihara and Kimura). In the 1998 summer, near the end of 1997/98 El Niño, the East China Sea and southern part of the Japan Sea were covered with abnormally low saline water. This was the result of the huge amounts of fresh water that were discharge from the Yangtze River and caused poor year classes of Japanese common squid.During the recent cold regime that persisted between 1976/77 and 1987/88 in the North Pacific, Japanese sardine, Sardinops melanostictus, maintained a higher stock level, whereas stocks of anchovy, Engraulis spp., remained low (Kasai; Yasuda and Nakata).To clarify the features of this biological response associated with El Niño events and climate regime shifts, in this paper we provide evidence of several environmental and biological responses in the western and central North Pacific. First, we review the linkage between ENSO and the Asian Monsoon. Second, we present data on the extra-tropical effects of El Niño and La Niña on marine ecosystems and the ocean environment. Finally, we describe the modification of extra-tropical effects of ENSO by interdecadal variations in the ocean and the atmosphere.

2. Linkage between Asian monsoon and ENSO

2.1. Features of Asian monsoon and its role in ENSO

Climate of the western North Pacific is dominated by monsoon winds and precipitation. In summer, the southeast monsoon develops between the Tibetan Low and the North Pacific Subtropical High (Fig. 1a). When the summer monsoon encounters the Japanese mountain range, it produces a considerable amount of precipitation on the Pacific side of Japan. In winter, however, the northwesterly monsoon develops between the Siberian High and the Aleutian Low superimposed on the westerly wind (Fig. 1b).