Recent Extremes in North Pacific Climate and the 2019 Alaskan Heatwave

Over the last 5-10 years the North Pacific has experienced a sequence of extreme events associated with the consecutive occurrence of land and marine heatwaves affecting the US West Coast and the Alaskan peninsula. The physical and ecological consequences of these extremes include unprecedented record high temperatures and a dramatic alteration of the marine food web that led to die offs of birds and mammals. While several studies point to a link between these trends in extremes and the rising temperature associated with climate change, the role of internal climate variability on decadal and multi-decadal timescales is still debated. Several large-ensembles of climate models forced with anthropogenic forcing scenarios reveal that internal variability can influence substantially not only regional trends (e.g. warming/cooling, drying/wetting) but also the distribution of extreme events. By combining available observations with a 100-member ensemble of a climate model simulation of the SPS8.5 warming scenario, we explore the role of internal modes of variability in the occurrence of the extreme events over the Northeast Pacific during the last decade. Specifically, we show evidence that prolonged co-occurrence of the warm phases of both Pacific Decadal Variability (PDV) and the Atlantic Multi-decadal Oscillation (AMO) have had a key influence in determining the probability distribution of extremes events over the Northern Hemisphere. While part of these fluctuations are natural, some of the recent trends in the AMO may be related to changes in aerosol forcing. Better understanding the influence of the AMO and aerosol forcing on the dynamics and PDFs of North Pacific temperature may provide a more robust estimate of how the Alaskan peninsula and the US West Coast are responding to forced and natural climate variability.