The western North Pacific experienced an unprecedented marine heatwave (MHW) in the middle-high latitudes during the summer of 2022, particularly impacting the Okhotsk and Western Bering Seas (OSWBS). Sea surface temperature (SST) anomalies reached as high as 5 °C, categorizing the MHW as 'Extreme'. While previous research has explored MHW mechanisms in the Northeast Pacific, the western North Pacific's MHWs and their underlying causes remain less understood. MHW occurrences are generally linked to atmospheric forcings, oceanic dynamics, and climate variability factors like El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Atmospheric blocking high-pressure systems often play a crucial role by reducing cloud cover, increasing shortwave radiation, and suppressing surface winds, thereby minimizing latent heat loss. The July 2022 OSWBS MHW was also driven by an atmospheric blocking high-pressure system, but its intensity showed an interesting connection to subtropical precipitation in the Indian summer monsoon region. Similar connections have been noted in past events, such as the 2013-2014 South American droughts and South Atlantic MHWs. In 2022, southern Pakistan experienced record-breaking monsoon rainfall, linked to heatwaves in China's Yangtze River basin through a stationary Rossby wave pattern. However, the broader consequences of this extreme rainfall event, particularly its potential impact on high-latitude MHWs, remained largely unexplored. This study investigates the physical processes connecting extreme Indian summer monsoon precipitation to the unprecedented OSWBS MHW in July 2022, aiming to improve the predictive capability for such events in the region and illuminating how subtropical extremes might fuel high-latitude events via atmospheric teleconnections.

Existing literature extensively documents the role of atmospheric forcings and oceanic dynamics in the formation of marine heatwaves (MHWs). Studies highlight the importance of atmospheric blocking high-pressure systems in reducing cloud cover, increasing solar radiation, and suppressing surface winds, leading to elevated sea surface temperatures (SSTs) and the formation of MHWs. Climate variability, particularly ENSO and IOD, has also been shown to contribute to MHW occurrences. Several studies have focused on MHW events in the Northeast Pacific, like the "Blob 1.0" and "Blob 2.0," demonstrating the significant role of atmospheric forcing in these phenomena. However, relatively little attention has been directed towards MHWs in the middle-high latitudes of the western North Pacific, with the underlying physical mechanisms remaining inadequately understood. The teleconnection between tropical convection and extratropical weather patterns has been observed in other regions. For instance, the link between tropical deep convection over the Indian Ocean (associated with the Madden-Julian Oscillation) and severe droughts in South America coupled with South Atlantic MHWs in 2013-2014 has been established, illustrating how remote events can influence regional extremes through atmospheric pathways. The specific connection between the Indian summer monsoon and high-latitude MHWs in the western North Pacific, however, requires further investigation.

This study utilized multiple datasets to analyze the 2022 OSWBS MHW and its connection to the Indian summer monsoon. Daily SST data were obtained from the NOAA Optimum Interpolation SST dataset version 2.1 (OISST v2.1), with validation using ESA SST and ERA5 SST datasets. Monthly atmospheric data from ERA5 (geopotential, wind, temperature, humidity, heat fluxes) and monthly precipitation data from the Global Precipitation Climatology Project (GPCP) were incorporated. To compute the ocean mixed layer heat budget, daily data from the Estimating the Circulation and Climate of the Ocean Phase II (ECCO2) dataset (surface heat fluxes, mixed layer depth, temperature, velocities) were used. Satellite-derived data (surface shortwave and longwave radiation, cloud cover) from CERES were also employed. MHW events were identified using the Hobday et al. method, defining events as SSTs exceeding the 90th percentile for at least five consecutive days. MHW severity categories (Moderate, Strong, Severe, Extreme) were assigned based on the multiple of the difference between SST anomaly and the threshold. Rossby wave activity flux was calculated using the Takaya and Nakamura formulation. Daily mixed layer heat budget calculations considered the balance between temperature tendency, surface net heat flux (shortwave, longwave, latent, sensible heat fluxes), and horizontal and vertical advection. The latent heat release index was defined as the regional average vertical integral of divergence of latent heat flux in the Indian summer monsoon region. Atmospheric diabatic heating was calculated using the Yanai formulation. To test the hypothesis, numerical model experiments using the Community Atmospheric Model version 5 (CAM5) were performed, with one control run and a sensitivity experiment tripling the observed diabatic heating rate in southern Pakistan during July 2022. Statistical significance was assessed using two-tailed Student's t-tests, accounting for autocorrelation. Different climatological periods were used depending on data availability, ranging from 1979-2022 to 1982-2022 and 2000-2022.

The study confirmed the unprecedented nature of the July 2022 OSWBS MHW, characterized by high intensity (SST anomalies up to 5 °C), long duration (168 days), and extreme severity. Analysis of the ocean mixed layer heat budget showed that enhanced surface shortwave radiation was the primary driver of the SST increase, with minimal contribution from oceanic advection. A persistent atmospheric blocking high-pressure system over the OSWBS region was observed, reducing cloud cover and increasing shortwave radiation. Analysis of Rossby wave activity flux revealed a train of stationary waves originating from Central Asia, splitting into branches, with the northern branch contributing to the blocking system formation over the OSWBS. Record-breaking precipitation and latent heat release were observed in the Indian summer monsoon region in July 2022, particularly in southern Pakistan. Regression analysis demonstrated a strong link between the latent heat release index and the atmospheric circulation pattern, including the OSWBS blocking high. CAM5 model experiments confirmed that increased diabatic heating in southern Pakistan induced a robust atmospheric blocking system similar to that observed in July 2022. The correlation between Indian summer monsoon precipitation and OSWBS SST was found to be insignificant before 2011, but a strong positive correlation was observed thereafter.

This research successfully establishes a physical link between extreme Indian summer monsoon precipitation and the unprecedented OSWBS MHW in July 2022. The extreme MHW event, driven predominantly by enhanced shortwave radiation resulting from atmospheric blocking, highlights the crucial role of atmospheric teleconnections in propagating extreme events across vast distances. The findings indicate that the Indian Ocean's role in modulating the Indian summer monsoon significantly influences MHW intensity and duration in the western North Pacific, emphasizing the importance of inter-basin interactions in extreme weather events. While the study focuses on a single extreme event, the strong correlation observed after 2011 between monsoon precipitation and OSWBS SST suggests a potentially broader relationship warranting further investigation. The increasing influence of the Indian summer monsoon on OSWBS SST could have significant implications for marine ecosystems and weather forecasting in the region. The study's findings contribute to our understanding of how climate variability in one region can influence extreme events in another, ultimately enhancing the predictive capabilities of future MHW forecasts.

This study provides compelling evidence of a teleconnection between extreme Indian summer monsoon precipitation and the unprecedented marine heatwave in the western North Pacific's Okhotsk and Western Bering Seas in July 2022. The findings demonstrate the crucial role of atmospheric blocking high-pressure systems, amplified by latent heat release from the monsoon, in driving the rapid increase in sea surface temperatures. Future research should explore the broader applicability of this relationship beyond the 2022 event, investigating the long-term dynamics of the interaction between the Indian summer monsoon and OSWBS MHWs and the potential influence of climate change on these interactions. Improved understanding of these connections will enhance the accuracy and lead-time of MHW predictions and contribute to better management of marine ecosystems.

This study primarily focuses on a single extreme MHW event in 2022. While the results suggest a potential broader link between Indian summer monsoon precipitation and OSWBS SST, further research is needed to confirm this relationship across multiple years and under different climate conditions. The residual term in the heat budget analysis represents unresolved processes, potentially including observational errors, which might affect the precise quantification of the influence of different factors. The study utilizes publicly available datasets, and potential limitations related to data resolution and accuracy might influence the interpretation of the findings. Furthermore, the specific mechanisms involved in the propagation of Rossby waves and their interaction with the blocking high-pressure system warrant further exploration.