The winter of 2013/2014 witnessed an exceptionally warm period in the Northeast Pacific Ocean, known as "The Blob," which severely impacted marine ecosystems. This event, characterized by sea surface temperature (SST) anomalies reaching 2.5°C above normal, prompted extensive research into the origins, persistence, and potential for similar future occurrences. The Blob's anomalies spread along the western North American coast, causing significant coastal warming and marine ecosystem disruption. While considerable research focused on Blob 1.0 (2013-2015), its mechanisms for development during winter are not fully transferable to the summer. In summer 2019, a similar event, termed Blob 2.0, emerged and intensified during the summer season, a period less understood in terms of the physical drivers behind such heatwaves. Unlike Blob 1.0, which originated in winter due to a persistent atmospheric ridge weakening surface winds, Blob 2.0 intensified in summer, a season where the North Pacific High (a dominant atmospheric ridge) typically prevails. An anomalous summertime atmospheric ridge might be expected to strengthen the existing circulation, increasing evaporation and ocean mixing and thus cooling the ocean. While remote SST forcing played a role in Blob 1.0's persistence, its importance in summer events was less clear. The potential role of local air-sea feedbacks involving marine stratocumulus clouds—pervasive in boreal summer—remained unclear. This study aims to examine the physical mechanisms behind Blob 2.0, investigating the influence of both remote and local SST forcing to enhance our understanding of summertime marine heatwaves in the Northeast Pacific.

Previous research has extensively studied the 2013-2015 marine heatwave (Blob 1.0), attributing its formation to a persistent atmospheric ridge over the Northeast Pacific. This ridge weakened the Aleutian Low and associated surface winds, reducing the Ekman transport of cold water and wind-driven mixing, which allowed surface heat fluxes to warm the upper ocean. Studies have also highlighted the role of remote SST forcing through atmospheric teleconnections in the persistence of these wintertime marine heatwaves. However, the influence of remote SST forcing on the North Pacific's ocean-atmosphere variability during summer is less understood. While air-sea feedbacks weren't considered a key factor in Blob 1.0's persistence, their potential significance in Blob 2.0, given the prevalence of marine stratocumulus clouds in boreal summer, is notable. The impacts of Blob 1.0 and the need to understand the mechanisms behind similar summer events highlight a gap in our knowledge, particularly regarding the role of remote and local SST forcing during the summer months.

This study uses gridded reanalysis products and satellite-derived observations to analyze the physical drivers of the summer 2019 Northeast Pacific SST anomalies (Blob 2.0). Analysis began by examining June-August (JJA) 2019 averaged SST anomalies, revealing a pattern similar to Blob 1.0, with peak anomalies exceeding 2.5°C. The study then assessed the North Pacific atmospheric circulation during this period, identifying a dipole pattern of sea level pressure anomalies resembling the North Pacific Oscillation (NPO), indicative of a weakened North Pacific High and reduced surface winds. A North Pacific High Intensity index was created to quantify this weakening, and its correlation with SSTAs in the Blob 2.0 region was determined. A mixed layer heat budget was calculated for the Blob 2.0 region to determine the factors that contributed to the extreme warming in the area during this period. The relationship between wind speed and mixed layer depth, as a measure of atmospheric power input into the ocean for turbulent mixing, was also investigated. To study remote SST forcing, the study conducted several SST-forced atmospheric general circulation model (AGCM) simulations with observed SST data from January 2018 to August 2019. Three sets of experiments were performed: global SST forcing, tropical (10°S–10°N) SST forcing, and North Pacific (>15°N) SST forcing. These simulations provided insights into the contributions of tropical and North Pacific SSTs to the weakened North Pacific High. The impact of local SST forcing and air-sea feedbacks, specifically low-cloud feedback, were investigated by comparing satellite-derived low-cloud fraction anomalies with results from the North Pacific-only AGCM experiments.

The study's key findings are as follows: 1. **Weakened North Pacific High:** Blob 2.0's primary driver was an anomalous weakening of the North Pacific High-Pressure System. This resulted in reduced surface winds, leading to decreased evaporative cooling and wind-driven upper ocean mixing. The North Pacific High Intensity index showed the weakest summer circulation in 40 years, strongly correlating with JJA-averaged SSTAs in the Blob 2.0 region. 2. **Record Shallow Mixed Layer:** The weakened North Pacific High caused a record-shallow mixed layer depth (62% shallower than average), confining the strong downward surface heat fluxes to a very thin volume. This resulted in unusually high warming (7.9°C increase from May to August in 2019), largely due to exceptionally positive SSTAs in August. 3. **Remote SST Forcing:** Atmospheric model simulations demonstrated that remote SST forcing from both the central equatorial Pacific (consistent with CP El Niño conditions) and, surprisingly, the subtropical North Pacific (consistent with the Pacific Meridional Mode – PMM) contributed to the weakening of the North Pacific High. While internal atmospheric variability primarily determined the magnitude of the weakening, remote SST forcing significantly influenced the spatial pattern and persistence of the anomalies. The analysis revealed a strong influence of CP El Niño events on the weakening of the North Pacific High during boreal summer. Furthermore, the PMM-driven shift in the Intertropical Convergence Zone (ITCZ) generated an atmospheric circulation response influencing the North Pacific High. 4. **Local Air-Sea Feedback:** Satellite observations and AGCM simulations demonstrated a reduction in low-cloud fraction in the North Pacific, closely following the warmest SSTAs. This reduction led to increased net shortwave radiation, suggesting a significant positive low-cloud feedback mechanism amplifying and sustaining the heatwave. This feedback appeared particularly important in July and August once SSTAs exceeded 1°C. The peak latent heat flux anomalies preceded the largest shortwave anomalies, possibly acting as a trigger mechanism. Furthermore, reduced low-cloud cover induced anomalous diabatic heating which further weakened the North Pacific High through the hydrostatic effect on atmospheric pressure, creating a feedback loop. 5. **Seasonal Differences:** The study highlighted the distinct mechanisms driving marine heatwaves in different seasons. The deeper wintertime mixed layer in Blob 1.0 dampened the influence of surface heat fluxes. The shallower summertime mixed layer in Blob 2.0 facilitated rapid warming but potentially shorter-lived impacts. The persistence of Blob 2.0 into winter and spring would depend on continued atmospheric forcing and air-sea interactions. 6. **Multidecadal Trends:** Several time series displayed multidecadal trends: positive trends in JJA-averaged SSTAs, surface heat flux, and a negative trend in mixed layer depths—possibly linked to anthropogenic global warming. A significant increase in interannual variance of JJA-averaged SSTAs was also observed after 2000, potentially related to anthropogenic forcing and/or the Pacific Decadal Oscillation (PDO).

This study successfully identified the key mechanisms driving the summer 2019 North Pacific marine heatwave (Blob 2.0). The weakening of the North Pacific High, a consequence of both internal atmospheric variability and remote SST forcing from the tropical and subtropical Pacific, played a crucial role. This weakening reduced wind-driven mixing and evaporation, leading to anomalously warm SSTs. The shallow mixed layer depth further amplified the warming effect. The positive feedback loop involving reduced low-cloud cover and increased shortwave radiation further sustained the heatwave. The study highlights seasonal variations in the mechanisms behind marine heatwaves, emphasizing the need to consider seasonal differences for better prediction and management. The findings emphasize the complex interplay of internal climate variability and anthropogenic climate change, with long-term trends requiring further research to disentangle the effects of internal climate variability and anthropogenic forcing on decadal-scale changes in the Northeast Pacific. The results have significant implications for fisheries and wildlife management, which rely on accurate seasonal forecasts for decision-making.

This research reveals the complex interplay of atmospheric circulation, ocean dynamics, and air-sea feedbacks driving the exceptional summer 2019 Northeast Pacific marine heatwave (Blob 2.0). The weakening of the North Pacific High, driven by both internal variability and remote SST forcing, was central to the event. The shallow mixed layer depth enhanced warming, while a positive low-cloud feedback maintained the heatwave's intensity. The study's findings underscore the need for further investigation into the relative contributions of internal and forced variability to summertime decadal variability in the Northeast Pacific. These results are crucial for predicting and managing the ecological consequences of future marine heatwaves, particularly in the context of a changing climate. Future research could focus on improving the representation of cloud feedback processes in atmospheric models and investigating the potential role of other physical factors, such as sea ice loss in high-latitude regions, in the generation of marine heatwaves.

The study's analysis relies on reanalysis products, which have inherent uncertainties. The heat budget calculations might contain errors due to the use of GODAS reanalysis, potentially including non-physical heat sources and sinks. The SST data used to force the atmospheric model (OISSTv2) has known biases, which could influence results although their impact on large-scale variability is considered minimal. Further research is needed to assess the robustness of the results using different AGCMs and observational data sets. The focus on large-scale ocean-atmosphere variability might limit the detection of finer-scale processes that could contribute to marine heatwave events.