Presenter: Antonietta Capotondi (University of Colorado/CIRES and NOAA/PSL)

Description:
Starting in Winter 2013/14, the Northeast Pacific experienced extremely warm sea surface temperatures (SSTs) that began in the center of the Gulf of Alaska and subsequently extended along the U.S. West Coast, where they persisted into 2016 with evolving intensity and spatial pattern. Forecasting these evolving anomalies has proved challenging for operational systems. Yet, empirical approaches, i.e., Linear inverse Models (LIMs), indicate the existence of large-scale climate influences on Northeast Pacific MHWs. Building on those results, a LIM constructed from SST and sea surface height (SSH) monthly anomalies is used to determine the large-scale conditions that can optimally lead to the development of Northeast Pacific MHWs at different lead times. Our analysis identifies initial precursors anomalies characterized by weak SST anomalies in the Northeast Pacific, subtropical anomalies reminiscent of the Pacific Meridional Mode, as well as warm conditions in the central equatorial Pacific, and by SSH anomalies that achieve their largest values in the tropical Pacific and are consistent with off-equatorial Rossby wave dynamics. These optimal initial conditions develop into Northeast Pacific MHWs after two-to-three seasons, in conjunction with the development of Central Pacific (CP) El Niño conditions in the tropical Pacific. Diagnosis of the key ENSO eigenmodes determined from the LIM dynamical operator, which represent different aspects of evolving ENSO diversity, is used to examine their respective roles in the subsequent growth of the MHW. This analysis demonstrates a key role for ocean dynamics and especially for tropical CP El Niño conditions, in sustaining that growth. It also identifies key large-scale links that must be captured by models used for MHW prediction, or to force regional systems.