Origin of upper-ocean warming and El Nino change on decadal scales in the tropical Pacific Ocean

The cause of decadal-scale variability in the tropical Pacific Ocean - such as that marked by the 1976-77 shift in the El Nino/Southern Oscillation - is poorly understood. Unravelling the mechanism of the recent decade-long warming in the tropical upper ocean is a particularly important challenge, given the link to El Nino variability, but establishing the hypothesized inter-annual/decadal oceanic connections between middle latitudes and tropics has proved elusive. Here we present observational evidence that Pacific upper-ocean warming and decadal changes in the El Nino/Southern Oscillation after 1976 may originate from decadal mid-latitude variability. In the middle 1970s the North Pacific Ocean is observed to have undergone a clear phase- transition; a 'see-saw' subsurface temperature anomaly pattern that rotates clockwise around the subtropical gyre. At middle latitudes a subsurface warm anomaly formed in the early 1970s from subducted surface-waters and penetrated through the sub-tropics and into the tropics, thus perturbing the tropical thermocline and driving the formation of a warm surface-water anomaly that may have influenced El Nino in the 1980s. The identification of this teleconnection of extratropical thermal anomalies to the tropics, through a subsurface ocean 'bridge', may enable improved prediction of decadal-scale climate variability.