Slow modulation of interannual variability and its relationship to El Niño/Southern Oscillation (ENSO) is investigated for the period of 1870-2007 using ship-board surface meteorological observations along a frequently travelled track across the North Indian Ocean (NIO; from the Gulf of Eden through Malacca Strait) and the South China Sea (to Luzon Strait). During the decades in the late 19th-early 20th century and in the late 20th century, the El Niño-induced NIO warming persists longer than

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... ring the 1910s-mid 1970s, well into the summer following the peak of El Niño. During the epochs of the prolonged NIO warming, rainfall drops and sea level pressure rises over the tropical Northwest Pacific in summer following El Niño. Conversely during the period when the NIO warming dissipates earlier, these atmospheric anomalies are not well developed. This supports the Indian Ocean capacitor concept as a mechanism prolonging El Niño influence into summer through the persistent Indian Ocean warming after El Niño itself has dissipated. The above centennial modulation of ENSO teleconnection to the Indo-Northwest Pacific region is reproduced in an atmospheric general circulation model forced by observed SST. The modulation is correlated not with the Pacific Decadal Oscillation but rather with the ENSO variance itself. When ENSO is strong, its effect in the Indo-Northwest Pacific strengthens, and vice versa. The fact that enhanced ENSO teleconnections occurred 100 years ago during the late 19thearly 20th century indicates that the recent strengthening of ENSO correlation over the Indowestern Pacific may not owe entirely to global warming but reflect natural variability.