Persistent record-high temperatures in the North Pacific in 2014/2015: a climate hypothesis

The 2014-15 warm “blob” in the northeast Pacific Ocean has featured record-high sea surface temperature anomalies (> 3°C). It developed during the winter of 2013/2014 in the Gulf of Alaska and later spread along the North American coast in the following fall, winter and spring of 2014/15. Northeast Pacific SST anomalies resembled the patterns of the North Pacific Gyre Oscillation (NPGO) in 2014 and of the Pacific Decadal Oscillation (PDO) early in 2015. We suggest that the generation, persistence, and spatial evolution of the warm anomalies over the winters of 2013/14 and 2014/15 was driven by large-scale climate teleconnections between the tropics and extra-tropics that are typical of El Niño precursor dynamics. More specifically, we show that the strong atmospheric ridge that forced the warm “blob” is linked to the activity of the North Pacific Oscillation (NPO), a well know pattern of atmospheric variability that acts as a stochastic driver for ENSO. Following a strong extra-tropical NPO forcing during winter/spring, tropical El Niño conditions typically develop in the summer. Continued El Niño conditions in the subsequent fall and winter typically excite atmospheric teleconnections to the Aleutian Low (AL) that that carry the signal back to the North Pacific Ocean in ways that favor the positive phase of the Pacific Decadal Oscillation (PDO). This teleconnection dynamic from extra-tropics to tropics (winter year 0) to extra-tropics (winter year +1) is a source of significant interannual persistence of North Pacific SST anomalies as they evolve from an NPO-like (e.g. NPGO) to an AL-like pattern (e.g. PDO). Even though a strong tropical El Niño did not develop during the fall/winter of 2014/2015, we show that this tropical/extra-tropical coupling played an important role in the generation, persistence and evolution of the 2014/2015 warm “blob”. Given that previous studies suggest that greenhouse forcing will energize the NPO precursor dynamics, it is important to establish if the 2014/2015 temperature extreme, and its widespread marine ecosystem impacts, will become more frequent under increasing anthropogenic climate change.