ORIGIN OF UPPER-OCEAN WARMING AND EL-NINO CHANGE ON DECADAL SCALES INTHE TROPICAL PACIFIC-OCEAN

The cause of decadal-scale variability in the tropical Pacific Ocean-s uch as that marked by the 1976-77 shift in the El Nino/Southern Oscill ation(1-7)-is poorly understood. Unravelling the mechanism of the rece nt decade-long warming in the tropical upper ocean is a particularly i mportant challenge, given the link to El Nino variability, but establi shing the hypothesized interannual/decadal oceanic connections between middle latitudes and tropics has proved elusive(8). Here we present o bservational evidence that Pacific upper-ocean warming and decadal cha nges in the Fl Nino/Southern Oscillation after 1976 may originate from decadal mid-latitude variability, In the middle 1970s the North Pacif ic Ocean is observed to have undergone a dear phase-transition; a 'see -saw' subsurface temperature anomaly pattern that rotates clockwise ar ound the subtropical gyre. At middle latitudes a subsurface warm anoma ly formed in the early 1970s from subducted surface-waters and penetra ted through the subtropics and into the tropics, thus perturbing the t ropical thermocline and driving the formation of a warm surface-water anomaly that may have influenced El Nino in the 1980s. The identificat ion of this teleconnection of extratropical thermal anomalies to the t ropics, through a subsurface ocean ''bridge'', may enable improved pre diction of decadal-scale climate variability.