Unraveling the Indian monsoon's role in fueling the unprecedented 2022 Marine Heatwave in the Western North Pacific

The study addresses why a record-breaking marine heatwave (MHW) developed over the Okhotsk Sea and Western Bering Sea (OSWBS) in July 2022, with sea surface temperature (SST) anomalies up to 5 °C. Prior work has emphasized atmospheric forcing for Northeast Pacific MHWs (e.g., the Blobs) but mechanisms for western North Pacific mid–high-latitude MHWs remain unclear. The authors hypothesize that extreme Indian summer monsoon precipitation, particularly over southern Pakistan in 2022, released substantial latent heat that triggered quasi-stationary Rossby waves, strengthening a blocking high over the OSWBS. This blocking reduced cloud cover and enhanced shortwave radiation, rapidly warming the mixed layer and fueling the extreme MHW. The study aims to elucidate this teleconnection mechanism and its implications for prediction of OSWBS MHWs.

Existing literature indicates MHWs are driven by a combination of atmospheric forcing (e.g., blocking highs reducing cloud cover, enhancing shortwave radiation; suppressing winds and latent heat loss) and ocean dynamics, modulated by large-scale climate variability such as ENSO and the Indian Ocean Dipole. Previous case studies in the NE Pacific (2013–2015 and 2019–2020 Blobs) attribute dominant roles to atmospheric anomalies. Teleconnection pathways, including circumglobal teleconnections and Rossby wave propagation, can link tropical/subtropical convection to extratropical blocking. Analogous mechanisms have linked Indian Ocean convection (e.g., MJO) to South American drought and South Atlantic MHWs in 2013–2014. However, the western North Pacific mid–high-latitude MHW mechanisms, especially links to Indian monsoon rainfall extremes and inter-basin interactions, have been less explored.

- Data: Daily SST from NOAA OISST v2.1 (0.25°, 1982–2022), validated with ESA SST and ERA5 SST (both interpolated to 0.25°, 1982–2022). Monthly atmospheric data from ERA5 (1°, 1979–2022): geopotential, winds, temperature, humidity, surface fluxes. Precipitation from GPCP (2.5°, 1979–2022). Mixed layer budget variables from ECCO2 daily (0.25°, 1992–2022). Satellite-derived shortwave/longwave radiation and cloud cover from CERES (1°, 2000–2022). Climatology periods aligned per dataset availability. - MHW detection and categorization: Following Hobday et al., define MHWs as SST exceeding the seasonally varying 90th percentile for at least 5 consecutive days (events separated by ≤2 days merged). Severity is the ratio of SSTA to (threshold–climatology); categories: Moderate (1–2x), Strong (2–3x), Severe (3–4x), Extreme (>4x). - Atmospheric diagnostics: Identify blocking via geopotential/streamfunction anomalies at 200/500 hPa; compute Takaya–Nakamura wave activity flux (WAF) at 200 hPa to diagnose Rossby wave propagation. - Ocean mixed-layer heat budget: Compute daily budget over OSWBS using ECCO2 and OISST. SST tendency related to net surface heat flux (shortwave, longwave, latent, sensible), and horizontal/vertical advection; residual represents unresolved processes and data errors. Mixed layer depth taken as regional mean from ECCO2. - Latent heat release index: Defined as the regional average vertically integrated divergence of latent heat flux over Indian monsoon region (15°N–35°N, 60°E–80°E) using ERA5 fields, representing latent heating from condensation associated with precipitation. - Atmospheric diabatic heating: Estimate vertically integrated diabatic heating from ERA5 following Yanai’s budget framework to quantify observed heating magnitude over southern Pakistan in July 2022 (~18 J s⁻¹ m⁻² vs July climatology ~6 J s⁻¹ m⁻²). - Modeling: Community Atmosphere Model version 5 (CAM5), FC5 configuration (~2.5°×1.9°, 31 levels). Two 30-year experiments (use last 10 years): (1) CTL_run with climatological SST/sea ice and no added diabatic heating; (2) HEATING_run identical but with July anomalous diabatic heating tripled in 17°N–30°N, 60°E–73°E (southern Pakistan region) to emulate observed 2022 ratio. Analyze July mean differences in geopotential height and winds to assess teleconnection and blocking response.

- Event characterization: OSWBS experienced an unprecedented MHW in 2022 lasting 168 days, peaking in July, reaching Extreme category with maximum intensity 4.6 °C. SST anomalies reached up to 5 °C; >1.5 million km² exceeded +3 °C. Regional mean July SST anomalies exceeded 3 °C, highest on record. - Atmospheric forcing: A strong, persistent blocking high formed over OSWBS (barotropic structure at 200 and 500 hPa), part of a quasi-stationary Rossby wave train originating from Central Asia; northern branch converged over OSWBS to intensify blocking. - Surface flux mechanism: Blocking reduced cloud cover and increased surface shortwave radiation over OSWBS; July 2022 cloud fraction was the second lowest on record and shortwave radiation was record high. Cloud–shortwave correlation: r = −0.877 (p < 0.01). Mixed-layer heat budget shows SST tendency primarily driven by enhanced net shortwave radiation; correlation between shortwave and dT/dt: r = 0.66 (p < 0.01) in July; advection terms were negligible. - Monsoon rainfall link: July 2022 exhibited record-breaking precipitation over the Indian summer monsoon region, especially southern Pakistan. The latent heat release index (vertically integrated divergence of latent heat flux) was also record high and strongly correlated with rainfall (r = 0.898, p < 0.01). Regression of 200/500 hPa geopotential height and winds onto the latent heat release index reproduces observed positive height anomalies over Central Asia, Bohai Sea, and OSWBS with strong anticyclonic circulation, consistent with blocking formation. - Modeling confirmation: CAM5 sensitivity experiment with tripled July diabatic heating over southern Pakistan generates atmospheric circulation patterns resembling observations, including a robust blocking high over OSWBS, confirming causality from monsoon latent heating to extratropical blocking and MHW-forcing conditions. - Temporal change in linkage: Correlation between Indian summer monsoon precipitation and OSWBS July SST is weak before 2011 but becomes very strong after 2011 (r = 0.97, p < 0.01), indicating a recent strengthening of the teleconnection relevant for prediction.

The findings substantiate the hypothesis that extreme Indian summer monsoon precipitation can, via latent heat release, excite quasi-stationary Rossby waves that reinforce a blocking high over the OSWBS. This blocking suppresses convection, reduces cloud cover, and substantially increases shortwave radiation into the ocean mixed layer, rapidly warming SSTs and producing an extreme MHW. The heat budget analysis confirms radiative forcing dominance, with negligible advective contributions during the peak. The regression and modeling experiments consistently link monsoon diabatic heating anomalies to the extratropical circulation response, demonstrating a teleconnection pathway from subtropics to mid–high latitudes. The identified post-2011 strengthening of the precipitation–SST relationship suggests evolving background conditions or circulation regimes that may enhance predictability of OSWBS MHWs. The study underscores the importance of inter-basin interactions in modulating MHW intensity and duration, expanding understanding beyond local atmospheric–oceanic drivers.

This work reveals a mechanistic teleconnection whereby extreme Indian summer monsoon precipitation, through latent heat release, triggers Rossby waves that strengthen a blocking high over the western North Pacific OSWBS, reducing cloud cover and enhancing shortwave radiation to drive an unprecedented 2022 MHW. Quantitative diagnostics (cloud–shortwave r = −0.877; shortwave–SST tendency r = 0.66; rainfall–latent heating r = 0.898) and CAM5 heating-forced experiments corroborate the causal chain. The recent intensification (post-2011) of the monsoon rainfall–OSWBS SST linkage (r = 0.97) highlights prospects for improved MHW prediction using monsoon indicators. Future research should investigate the causes of the post-2011 regime shift, refine representation of unresolved processes in heat budgets, and integrate teleconnection predictors into operational MHW forecasting frameworks.

- The mixed-layer heat budget contains a substantial residual term reflecting unresolved processes (e.g., vertical turbulent diffusion) and observational/data assimilation errors, which may influence quantitative attribution. - The causal inference relies on a targeted idealized forcing (tripled diabatic heating) in CAM5; while effective, model dependence and simplifications could affect realism of amplitude and spatial structure. - The strong post-2011 linkage between monsoon precipitation and OSWBS SST requires further investigation to establish robustness, mechanisms, and generalizability across periods and models. - Oceanic processes beyond the mixed-layer (e.g., subsurface variability) are not fully resolved in the attribution for July 2022, though advection terms appeared small during the peak.