The Arctic is experiencing rapid warming and sea ice reduction, impacting global climate and ecosystems. Arctic amplification, where Arctic temperatures warm at a rate two or more times faster than the global average, is a significant concern. While the ice-albedo feedback is a primary driver of Arctic amplification, other factors like low-level clouds and oceanic heat transport from lower latitudes play crucial roles. The Pacific Ocean's contribution to Arctic heat is relatively smaller than the Atlantic's but significantly influences sea ice extent due to its shallower entry point through the Bering Strait. This study focuses on the Chukchi Sea, known for warming trends, reduced sea ice cover, and high primary production. The sea ice covered period in the Chukchi Sea is highly correlated with oceanic heat inflow from the Pacific Ocean, a complex process influenced by factors like Bering Strait volume transport and Pacific water temperatures. This research uses satellite SST data (AMSR-E and AMSR2), atmospheric reanalysis data (ERA5), and direct observations from the R/V Mirai Arctic Expedition to comprehensively investigate the interannual variation of Pacific Arctic Ocean conditions during sea ice formation months, focusing on the exceptionally high sea surface temperatures (SSTs) in autumn 2018.

Existing literature highlights the rapid decline of Arctic sea ice extent and its far-reaching implications, including altered air-sea interactions and negative impacts on marine species. The ice-albedo feedback, where decreased sea ice leads to increased solar absorption and further warming, is recognized as a key driver of Arctic amplification. Other contributing factors include low-level clouds, turbulent heat fluxes from the ocean, and the transport of atmospheric heat, oceanic heat, and moisture from lower latitudes. Studies have documented substantial heat inflow from the Atlantic and Pacific Oceans, with the Pacific inflow exerting a significant influence despite its smaller volume, due to its proximity to the surface layer. Previous research has demonstrated the impact of seawater temperature anomalies on early winter sea ice in the Barents and Bering Seas, and the potential for positive feedback mechanisms to amplify sea ice reduction in the western Arctic basin. The Chukchi Sea, the area of focus, has been the subject of several studies documenting warming trends, reduced sea ice cover, and increased primary production. The link between sea ice cover duration in the Chukchi Sea and Pacific oceanic heat inflow has also been established, highlighting the complexity of the system.

This study employed a multi-faceted approach utilizing various datasets to understand the 2018 Pacific Arctic Ocean conditions. **Sea Surface Temperature (SST) and Sea Ice Concentration (SIC):** Daily 1/10° resolution SST and SIC data from AMSR-E and AMSR2 (2002-2018) were obtained from the Arctic Data archive System (ADS). Monthly and spatially averaged SST and SIC were calculated for the Chukchi and Bering Seas. **Pacific Decadal Oscillation (PDO):** The PDO index, representing North Pacific SST variability, was downloaded from Dr. Mantua's website. Annual mean PDO indices were calculated. **Atmospheric Conditions:** ERA5 reanalysis data (1979-2018) were used to analyze winds, air-sea heat flux (latent heat flux, sensible heat flux, net solar and thermal radiation), geopotential height anomalies at 500 hPa, and mean sea level pressure anomalies. **Argo Float Data:** Argo float #4902926 data from the central Chukchi Sea in 2018 were used to study the mixed layer depth and upper ocean heat conditions. **R/V Mirai Arctic Expedition (2018):** During the R/V Mirai cruise MR18-05C (November 4-24, 2018), continuous measurements of near-surface atmospheric and oceanographic variables were conducted using the Shipboard Oceanographic and Atmospheric Radiation (SOAR) system and ADCP. CTD measurements determined mixed layer depth. Data were obtained from JAMSTEC's Data and Sample Research System. **Sea Ice Extent from SAR Images:** Level-2 SAR data (normalized radar cross-section) from Sentinel-1A/B were used to estimate sea ice extent. Data were obtained from NOAA CoastWatch.

The study revealed several key findings: 1. **Record High SSTs:** November 2018 saw the highest recorded monthly mean SSTs in the Chukchi and Bering Seas since 2002. This occurred despite a near-zero PDO index, suggesting other factors were at play. 2. **Correlation with PDO:** November monthly SSTs in both the Chukchi and Bering Seas showed a strong correlation (r ≈ 0.7) with the annual PDO index, but this correlation increased when 2018 was excluded, emphasizing the anomaly of that year. 3. **Anomalous SST Stability:** In 2018, the SST remained unusually stable from August to September, unlike the typical seasonal decrease. This indicated a persistent warm water intrusion. 4. **Episodic Atmospheric Blocking:** An atmospheric blocking high-pressure system formed over the Bering Sea in September 2018, leading to a significant (~5 m/s) southerly wind anomaly over the Bering Strait. This anomaly enhanced the warm Pacific water inflow into the Chukchi Sea. 5. **Delayed Sea Ice Advance:** The R/V Mirai expedition observed a ~40m thick warm surface mixed layer, delaying sea ice advance by an estimated 74 days. Despite sustained below-freezing winds, the warm water prevented significant ice formation. Horizontal warm water advection further contributed to maintaining high SST near the marginal ice zone. 6. **Satellite Observations:** Satellite data confirmed lower sea ice concentration (SIC) from October 2018 and significantly slowed sea ice advance from November 13 to December 4, 2018, despite increased air-sea heat flux favorable for sea ice formation. 7. **Minor Role of Air-Sea Heat Flux:** The analysis indicates that while air-sea heat fluxes did contribute, the strong southerly winds were the primary factor driving the anomalous warm Chukchi Sea conditions and delayed sea ice formation. 8. **Independent Blocking High System:** The study found that the atmospheric blocking high system over the Bering Sea in September 2018 was independent of the PDO index. However, if such a blocking event were to occur during a PDO positive phase in the future, the increase in SST could be substantially greater, leading to a dramatic reduction in Arctic sea ice extent.

The study's findings highlight the significant role of episodic atmospheric blocking in modulating Pacific Ocean inflow to the Arctic and its impact on sea ice advance. While the long-term correlation between Chukchi Sea SST and the PDO index is strong, the record-high SSTs of 2018 demonstrate the influence of shorter-term, high-impact events. The anomalous southerly winds driven by the blocking high system effectively transported warm Pacific water into the Chukchi Sea, leading to the observed delay in sea ice formation. This underscores the importance of considering both long-term climate oscillations and episodic weather events when predicting Arctic sea ice extent. The study's findings also have far-reaching implications, impacting marine ecosystems, ship navigation, and mid-latitude weather systems. Future research should focus on understanding the interactions between oceanic eddies, advective fluxes, and their combined effect on heat transport, improving predictions of sea ice distribution and its consequences.

This study provides compelling evidence of the significant role of episodic atmospheric blocking in the Pacific Arctic region in driving record-high seawater temperatures and delaying sea ice advance. The combination of satellite data, atmospheric reanalysis, and in-situ observations from the R/V Mirai revealed the strong influence of this event, independent of the PDO. While the long-term correlation between SST and PDO remains important, this study emphasizes the need to account for the influence of transient atmospheric events when forecasting Arctic sea ice. Future research should focus on improving the understanding and prediction of these episodic events, particularly concerning their interplay with climate oscillations, and their broader environmental and societal consequences.

The study's reliance on satellite data for SST and SIC creates limitations due to data gaps and potential inaccuracies in sea ice covered areas. The use of reanalysis data for atmospheric conditions also entails uncertainties associated with model limitations. While the R/V Mirai expedition provided valuable in-situ measurements, the relatively short duration of the observations limits the generalizability of certain findings. Furthermore, the study focuses on a single year with record-breaking conditions, limiting the ability to draw broad conclusions about the frequency and long-term impacts of similar events.