Interdecadal Variations in ENSO Teleconnection to the Indo-Western Pacific for 1870-2007

Slow modulation of interannual variability and its relationship to El Nino-Southern Oscillation (ENSO) is investigated for the period of 1870-2007 using shipboard surface meteorological observations along a frequently traveled track across the north Indian Ocean (NIO; from the Gulf of Aden through Malacca Strait) and the South China Sea (to Luzon Strait). During the decades in the late nineteenth early twentieth century and in the late twentieth century, the El Nino-induced NIO warming persists longer than during the 1910s-mid-1970s, well into the summer following the peak of El Nino. During the epochs of the prolonged MO warming, rainfall drops and sea level pressure rises over the tropical northwest Pacific in summer following El Nino. 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 Nino influence into summer through the persistent Indian Ocean warming after El Nino 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 nineteenth-early twentieth century indicates that the recent strengthening of the ENSO correlation over the Indo-western Pacific may not entirely be due to global warming but reflect natural variability.