Marine heatwaves (MHWs), prolonged periods of extreme ocean warming, pose significant threats to marine ecosystems. Their frequency and intensity are increasing globally due to anthropogenic climate change, causing widespread impacts such as coral bleaching, mass mortality of marine animals, and disruptions to fisheries. Numerous severe MHW events have been documented worldwide in recent decades, underscoring the urgency of understanding their underlying mechanisms. The Northeast Pacific Ocean has experienced particularly intense MHWs, famously known as "the Blob." These events have been linked to anomalous high sea level pressures associated with the North Pacific Oscillation (NPO) and persistent low-pressure anomalies in the Gulf of Alaska, suppressing heat loss from the ocean and resulting in extreme warm sea surface temperature (SST) anomalies. Reduced surface winds and cloud cover have also been identified as crucial factors. This study focuses on the potential contribution of Arctic warming to the increased number of MHW days in the Northeast Pacific. The substantial reduction in Arctic sea ice since the late 1990s has led to significant Arctic warming, influencing mid-latitude weather patterns. The research investigates whether accelerated Arctic warming plays a role in modifying North Pacific atmospheric circulation and thus contributing to the observed increase in Northeast Pacific MHW days.

Existing research has established a clear link between global warming and the increasing frequency and intensity of marine heatwaves. Studies have highlighted the diverse impacts of MHWs on marine ecosystems, ranging from coral bleaching and mass mortality events to disruptions in fisheries and shifts in species distribution. Previous work has identified various drivers of MHWs, including local atmosphere-ocean coupled processes, warm ocean advection, and large-scale climate variability patterns. The role of atmospheric teleconnections, particularly the influence of remote forcing on MHW occurrences, has also been investigated. Recent studies have suggested the involvement of the Atlantic meridional overturning circulation slowdown in modulating MHWs in both the North Atlantic and North Pacific. While the long-term trend of oceanic warming is globally significant, regional variations in the mechanisms driving MHW occurrences exist. In the Northeast Pacific, the "Blob" events have been studied extensively, linking them to specific atmospheric circulation patterns and changes in heat fluxes. However, the potential influence of high-latitude atmospheric forcing, particularly from the Arctic region, on Northeast Pacific MHWs during boreal summer remains an understudied area.

This study utilizes a multi-faceted approach combining observational data and idealized climate model experiments. Satellite-derived daily sea surface temperature (SST) data from NOAA Optimum Interpolation (OI) SST v2.1 were used to identify MHW events using a commonly used MHW definition (SST exceeding the 90th percentile of the seasonally varying daily climatology for at least five consecutive days). Sea ice concentration (SIC) data were obtained from NOAA's National Snow and Ice Data Center. Atmospheric circulation and heat flux data were sourced from the European Centre for Medium-Range Weather Forecasts' ERA5 reanalysis dataset. A composite analysis was performed for years with a high number of MHW days (>10 days during boreal summer) to examine the relationship between Arctic SIC, sea level pressure (SLP), low-level cloud cover (LCC), and MHW occurrence. The North Pacific Oscillation (NPO) index and a low-level cloud cover (LCC) index were calculated to quantify their variations and correlations with SST anomalies. To isolate the effect of Arctic warming, an idealized coupled general circulation model experiment (ART_Exp) was conducted using the GFDL CM2.1 model. In this experiment, historical SSTs in the Arctic (north of 65°N) were restored while maintaining a fixed CO2 concentration (353 ppm, corresponding to 1989 levels). Thirty ensemble members were simulated to account for different initial conditions and minimize the influence of internal variability. The simulated atmospheric and oceanic variables were analyzed to determine the response to Arctic warming, focusing on atmospheric circulation changes, low-level cloud fraction, and SST trends.

The analysis revealed a significant upward trend in the number of MHW days in the Northeast Pacific during boreal summer (June-July-August, JJA), with an average increase of 6.23 days per decade since 1982. This trend correlates strongly with a similar upward trend in JJA SST (0.39 °C per decade). The spatial pattern correlation coefficient between the trends in mean SST and MHW days is 0.59. The time series of area-averaged SST and MHW days during JJA show a strong positive correlation (r = 0.75, detrended, p < 0.01). The most significant SST warming trend (0.65 °C per decade) is observed during the period 2000-2019, coinciding with a substantial increase in MHW days. Composite analysis of years with an unusually high number of MHW days (>10 days in JJA) since the late 1990s shows a clear association with reduced Arctic SIC during May-June-July (MJJ), indicative of a lagged effect of Arctic warming on the North Pacific atmospheric circulation. These years are characterized by dipole-like SLP anomalies resembling a positive phase of the NPO (+NPO), consistent with weakening surface westerlies and reduced surface evaporation. The resulting decrease in latent heat loss leads to surface layer warming. Furthermore, a significant reduction in LCC during MJJ is observed preceding the MHW events, suggesting that increased solar radiation reaching the ocean surface contributes to the warming. Trend analysis reveals an increasing +NPO-like SLP pattern and a decreasing trend in LCCs over 1982-2019, which likely contribute to the rise in MHW days. The probability distribution of daily Northeast Pacific SST anomalies shifted towards warmer extremes during 2000-2019 compared to 1982-1999, further supporting the observed increase in MHW days. The idealized model experiment (ART_Exp) confirms that Arctic warming related to sea ice loss contributes to a +NPO-like atmospheric circulation pattern and decreased low-level cloud cover. This results in an upward trend in simulated SST over the Northeast Pacific and a shift of the probability distribution of daily SST anomalies towards warmer extremes, consistent with the observed increases in MHW days. While the magnitudes of trends in the model simulation are smaller than observed, this is likely due to the reduced internal variability and isolation of the Arctic warming effect in the ensemble mean.

This study provides strong evidence linking accelerated Arctic warming and sea ice loss to an increase in Northeast Pacific MHW days during boreal summer. The results demonstrate that Arctic warming alters the atmospheric circulation pattern, creating conditions favorable for MHWs. Reduced latent heat loss and increased solar radiation due to decreased low-level cloud cover contribute to sustained SST warming in the Northeast Pacific. The idealized model experiment successfully isolates the effect of Arctic warming, providing further support for the hypothesis. The observed changes in atmospheric circulation, reduced latent heat loss, increased downward shortwave radiation, and ultimately increased SST and MHW days demonstrate the crucial role of Arctic warming in modulating MHWs in the region. These findings highlight the complex interplay between high-latitude climate change and mid-latitude extreme weather events. The study adds to the growing body of research emphasizing the long-range teleconnections and cascading effects of Arctic warming. The increasing frequency and intensity of MHWs in the Northeast Pacific pose serious threats to the region's marine ecosystems, emphasizing the need for comprehensive climate change adaptation and mitigation strategies.

This research demonstrates a clear link between accelerated Arctic warming and increased marine heatwave days in the Northeast Pacific. The reduction in Arctic sea ice leads to changes in atmospheric circulation, reducing low-level clouds and increasing solar radiation, thereby increasing sea surface temperatures and the frequency of marine heatwaves. The findings underscore the significant and far-reaching consequences of Arctic warming on marine ecosystems and emphasize the need for urgent global action to reduce carbon emissions and implement robust climate change adaptation plans.

The study's idealized model experiment simplifies the complex climate system, focusing on isolating the impact of Arctic warming. While this approach effectively demonstrates the influence of Arctic warming, it may not fully capture the intricate interactions with other climate drivers, such as greenhouse gas forcing and internal climate variability. The model also exhibits some discrepancies with observed SST trends and patterns, particularly in the spatial distribution of warming. Further research using multiple climate models with larger ensembles is recommended to address these limitations and enhance the robustness of the findings. Additionally, the focus on boreal summer MHWs may limit the generalizability of the findings to other seasons.