Seasonal Prediction of Wintertime North Pacific Blocking: What Are We Capturing and Missing?

During boreal winter, atmospheric blocking, which is characterized by persistent, quasi-stationary high-pressure anomalies, often drives the occurrence of cold extremes in remote regions through cold horizontal advection and anomalous surface radiative flux. Because of considerable adverse impacts on environmental and socioeconomic systems by cold extremes, seasonal prediction of blocking and the associated cold extremes has recently received much attention. However, due to the chaotic nature of the extratropical atmospheric circulation and errors in state-of-the-art climate models in the simulation of blocking, the prediction of blocking remains a challenging task. In this study, we focus on the Pacific-North America sector to examine North Pacific oceanic blocking, which is relatively less studied compared to the Euro-Atlantic blocking. By leveraging both observational data and a state-of-the-art seasonal prediction model developed at the Geophysical Fluid Dynamics Laboratory, SPEAR (Seamless System for Prediction and Earth System Research), we investigate the prediction skill of North Pacific wintertime blocking frequency and its linkage to downstream cold extremes.

The observational results show that the climatological blocking frequency during boreal winter has a local maximum over the central North Pacific Ocean and that the occurrence of North Pacific blocking drives significant cold anomalies over northwestern North America within a week. These observed features of North Pacific blocking are well reproduced by the model. Regarding the blocking prediction skill, the model results show that the frequency of the western North Pacific blocking nearby the subtropical jet exit region can be skillfully predicted at the shortest forecast lead, but skill drops off rapidly with lead time. In observation, these western North Pacific blocking events are triggered by localized tropical convection anomaly over the tropical Indian Ocean and central tropical Pacific Ocean, which excites a poleward propagating Rossby wave train and decelerates the subtropical jet accompanied by its equatorward shift. The SPEAR model simulates this chain of processes associated with the atmospheric teleconnection and the observed blocking-cold extreme linkage well, but SPEAR also exhibits errors in simulating the details of the tropical-extratropical interaction. Our results indicate that an improvement in the seasonal prediction skill of winter North Pacific blocking frequency may be achieved by the representation of the links among sea surface temperature anomalies, tropical convection, and the ensuing tropical-extratropical interaction that initiates North Pacific atmospheric blocking.