Extreme Aleutian Low (AL) events have been associated with major ecosystem reorganisations and unusual weather patterns in the Pacific region, with serious socio-economic consequences. Yet, their future evolution and impacts on atmosphere–ocean interactions remain uncertain. Here, a large ensemble of historical and future runs from the Community Earth System Model is used to investigate the evolution of AL extremes. The frequency and persistence of AL extremes are quantified and their connection with climatic variables is examined. AL extremes become more frequent and persistent under the RCP8.5 scenario, associated with changes in precipitation and air temperature patterns over North America. Future changes in AL extremes also increase the variability of the sea surface temperature and net heat fluxes in the Kuroshio Extension, the most significant heat and energy flux region of the basin. The increased frequency and persistence of future AL extremes may potentially cause substantial changes in fisheries and ecosystems of the entire Pacific region as a knock-on effect.

Regime shifts are abrupt changes in an ecosystem that may propagate through multiple trophic levels and have pronounced effects on the biotic and abiotic environment, potentially resulting in ecosystem reorganization. There are multiple mechanisms that could cause such abrupt events including natural and anthropogenic factors. In the North Pacific, a major shift in the physics of the system, including a sudden increase in sea surface temperature, was reported in 1977 with a prominent biological response in the lower trophic levels and subsequent effects on the fisheries and economy of the region. Here we investigate the statistics of physical processes that could have triggered and maintained the late 1970s shift. The hypothesis of an extreme sea level pressure event abruptly changing the oceanic conditions in winter 1976–1977, which was maintained by long‐term changes in air‐sea interaction processes, is tested. Using dynamical proxies, we show the occurrence of an extreme atmospheric event, specifically a persistent Aleutian Low during winter 1976–1977, which constitutes a substantial part of the triggering mechanism of the regime shift. Subsequent sudden changes in the net heat flux occurred in the western North Pacific, particularly in the Kuroshio Extension region, which contributed to the maintenance of the new regime.

Over the last two decades, unusual warm events have been observed in the global ocean, significantly affecting our environment and society (Frölicher & Laufkötter, 2018). A marine heatwave (MHW) is commonly defined as a "prolonged discrete anomalously warm water event that can be described by its duration, intensity, rate of evolution, and spatial extent" (Hobday et al., 2016). Thus far, understanding and monitoring MHWs have received substantial scientific and public attention (Holbrook et al., 2019(Holbrook et al., , 2020Oliver et al., 2021). Recently, the most severe MHWs were attributed to anthropogenic climate change (Laufkötter et al., 2020), highlighting the necessity for motivated efforts to limit global warming. In the North Pacific, the largest MHW ever recorded, dubbed the Warm Blob, occurred between 2013 and 2015 with maximum sea surface temperatures that reached 6°C above average in some areas along the coast of Southern California (Bond et al., 2015). Impacts on marine ecosystems include harmful algal blooms, shifts in species range, and even local extinctions (Smale et al., 2019). Consequences have also been reported in the economic sector, since aquaculture and important fisheries are vulnerable to MHW events. For example, both commercial and recreational fisheries faced major challenges and loss of millions of dollars after the 2013-2015 northeast Pacific MHW (Cavole et al., 2016). Furthermore, the largest ever-recorded harmful algal bloom in the region, caused by the extreme temperatures, produced toxins that contaminated valuable shellfish and crab fisheries (McCabe et al., 2016). Increasing sea surface temperatures may also impact regional weather by affecting storms, precipitation, air temperature and droughts, the latter posing risks for potential wildfire events (Chikamoto et al., 2017). For example, anomalously high ocean temperatures in the northeast Pacific was the key forcing for the extensive dry winters in California during 2011-2014

We introduce a new methodology to study marine heat waves, extreme events in the sea surface temperature
(SST) of the global ocean. Motivated by previously large and impactful marine heat waves and by theoretical 
expectation that the dominant heating processes coherently affect large regions of the ocean, we introduce a 
methodology from computer vision to construct marine heat wave systems (MWHSs) -- the
collation of SST extrema in dimensions of area and time. We identify \nmhws~MHWSs in the 37~year
period (1983-2019) of daily SST records and find that the duration \tdur\ (days), maximum area \maxa\ (km$^2$), and
total ``volume'' \nvox\ (days km$^2$) for the majority of MHWSs are well-described by power-law distributions: $\tdur^{-3}, \, \maxa^{-2}$ and $\nvox^{-2}$. These characteristics confirm SST extrema exhibit strong spatial coherence that define the formation
and evolution of marine heat waves. Furthermore, the most \classc\ MHWSs deviate from these power-laws and are the dominant
manifestation of marine heat waves: extrema in ocean heating are driven by the $\sim 200$ systems with largest area and
duration. We further demonstrate that the previously purported rise in the incidence of marine heat wave events over the past 
decade is only significant in these \classc\ systems. A change point analysis reveals a rapid increase in days under a severe MHW in most regions of the global ocean over the period of 2000-2005. Understanding the origin and impacts of marine heat waves in the current and future ocean, therefore, should focus on the production and evolution of the largest-scale and longest-duration heating phenomena.

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